MODIFICATION OF BIODEGRADABLE POLYMER FILMS · Modification of biodegradable polymer films iii were...
Transcript of MODIFICATION OF BIODEGRADABLE POLYMER FILMS · Modification of biodegradable polymer films iii were...
MODIFICATION OF BIODEGRADABLE
POLYMER FILMS
Camille Fromageot
Master in Materials Science
Submitted in fulfillment of the requirements for the degree
of Doctor of Philosophy
Faculty of Science and Engineering
Queensland University of Technology
2019
Modification of biodegradable polymer films i
Keywords
1,8-Diazabicyclo[5.4.0]undec-7-ene, 2-oxepane-1,5-dione, accelerated artificial
ageing, biodegradable polymers, blends, chain scission, copolymers, crosslinking,
degradation, L-lactide, organocatalysis, photodegradation, photoprodegradant,
photooxidation, polyesters, poly(L-lactide), reactive extrusion, ring-opening
polymerisation, thermo-oxidation, tin (II) octanoate, transesterification, Ultraviolet
irradiation.
ii Modification of biodegradable polymer films
Abstract
Poly(L-lactide) (PLLA), a biodegradable and compostable polyester, features versatile
properties that support its use as a sustainable alternative to common polyolefins.
However, to reduce environmental impact of dispersed polymer fragments, PLLA-
based polymers need to be developed that can degrade within tailored life-times and
without requiring additional waste treatment. Therefore, the global objective of this
PhD project was to accelerate the photodegradation of PLLA by adding ketone
moieties into the polymer matrix. A lactone-type molecule that featured a ketone
within its structure, 2-oxepane-1,5-dione (OPD), was selected to either be blended with
poly(L-lactide) or copolymerized with L-lactide to produce photodegradable PLLA-
based materials.
OPD was first used as an additive that was blended with a commercial grade PLLA.
Films were produced with an OPD content ranging from 0 to 10 wt%, and then
artificially aged under conditions mimicking natural outdoor exposure. A faster
embrittlement was observed for films containing 4 to 10 wt% OPD compared to neat
PLLA and films with 2 wt% OPD. Gel permeation chromatography, differential
scanning calorimetry as well as spectroscopic techniques enabled the photosensitizing
role of OPD, when used as an additive, to be assessed.
Following the successful use of OPD for increasing the photodegradation rate of
PLLA, its incorporation into the backbone of PLLA was then investigated. Both in-
melt modification and copolymerisation were employed. First, transesterification
reactions between the ester groups of PLLA and OPD were investigated via reactive
extrusion in an effort to prepare poly(L-lactide-co-OPD) copolymers. However,
spectroscopic techniques demonstrated the absence of incorporated OPD while
revealing evidence of thermo-oxidative degradation. In order to limit the extent of such
degradation, ring-opening polymerisation of L-lactide and OPD were carried out under
milder conditions. Two sets of conditions were investigated: in the bulk at 110 ºC with
tin (II) octanoate as catalyst; and, in solution at room temperature using 1,8-
diazabicyclo[5.4.0]undec-7-ene (DBU) and benzyl alcohol as catalyst and initiator,
respectively. Experiments carried out in the bulk resulted in limited incorporation of
OPD and for the DBU-benzyl alcohol system, although poly(L-lactide) homopolymers
Modification of biodegradable polymer films iii
were obtained within short times, no copolymer could be obtained. In both cases, the
ketone moiety hindered the polymerisation by competing with the ester functional
groups from both monomers for reaction with the catalysts.
In order to confirm the hindering effect of the ketone moiety of OPD during
copolymerisation, a modified OPD was employed where the ketone was protected
using ethylene ketal groups. Copolymerisation reactions of this protected OPD
monomer with L-lactide were performed in the bulk using tin (II) octanoate and benzyl
alcohol as catalyst and initiator, respectively. Copolymers with various compositions
were obtained, and subsequent deprotection steps afforded poly(L-lactide-co-OPD).
Accelerated ageing of copolymers revealed increased rates of photodegradation
compared to PLLA itself, with a mechanism based on crosslinking events rather than
chain scissions.
Overall, this PhD project showed that 2-oxepane-1,5-dione could increase the
photodegradation rate of PLLA when used as an additive or when incorporated into
the PLLA backbone, with each method of incorporation showing a different
mechanism of degradation. OPD, as an additive, efficiently accelerated the
photodegradation of poly(L-lactide) to the point of embrittlement via both cross-
linking and chain scission, while only crosslinking of the copolymer was observed
when OPD was incorporated into the PLLA backbone.
iv Modification of biodegradable polymer films
Table of Contents
Keywords .................................................................................................................................. i
Abstract .................................................................................................................................... ii
Table of Contents .................................................................................................................... iv
List of Figures ......................................................................................................................... vi
List of Schemes ...................................................................................................................... xii
List of Tables ......................................................................................................................... xiv
List of Abbreviations ............................................................................................................ xvii
Statement of Original Authorship .......................................................................................... xx
Acknowledgements ............................................................................................................... xxi
Chapter 1: Literature Review ............................................................................. 1
1.1 Introduction .................................................................................................................... 1
1.2 Polylactide ...................................................................................................................... 3 1.2.1 Evolution of the Polylactide Market .................................................................... 3 1.2.2 From Lactic Acid to Polylactide .......................................................................... 4 1.2.3 Thermal Properties and Crystallinity of Polylactide .......................................... 10 1.2.4 Mechanical Properties ........................................................................................ 13
1.3 Degradation of Polylactide ........................................................................................... 14 1.3.1 Biodegradation ................................................................................................... 15 1.3.2 Thermal Degradation ......................................................................................... 17 1.3.3 Photodegradation ............................................................................................... 20
1.4 Tailoring the Degradation of Poly(L-Lactide) .............................................................. 25 1.4.1 Accelerating the Biodegradation Rate ............................................................... 25 1.4.2 Improving the Thermal Resistance and Mechanical Properties ......................... 26 1.4.3 Accelerating the Photodegradation Rate ............................................................ 28
1.5 Tailoring Degradability ................................................................................................ 30
1.6 Project Proposal ........................................................................................................... 34
1.7 List of References ........................................................................................................ 37
Chapter 2: 2-Oxepane-1,5-Dione: an Efficient Photosensitizer for Poly(L-
lactide) 47
2.1 Background .................................................................................................................. 47
2.2 Results and Discussion ................................................................................................. 49 2.2.1 2-Oxepane-1,5-Dione as a Photosensitizer ........................................................ 49 2.2.2 Initial Characteristics of the Films of Poly(L-lactide) and 2-Oxepane-1,5-
Dione .................................................................................................................. 51 2.2.3 Photodegradation of PLLA - OPD Blends ......................................................... 61 2.2.4 Influence of Temperature on the Degradation Behaviour of the Blends ........... 75 2.2.5 Mechanism of Photodegradation ....................................................................... 80
2.3 Summary ...................................................................................................................... 84
2.4 Experimental ................................................................................................................ 84
Modification of biodegradable polymer films v
2.5 List of References .........................................................................................................87
Chapter 3: Reactive Extrusion of Poly(L-lactide) With 2-Oxepane-1,5-
Dione 93
3.1 Background ...................................................................................................................93
3.2 Results and Discussion .................................................................................................94 3.2.1 Melt-Modification of Poly(L-lactide) with 2-Oxepane-1,5-Dione .....................94 3.2.2 Thermal Stability of 2-Oxepane-1,5-Dione ......................................................122
3.3 Summary .....................................................................................................................124
3.4 Experimental ...............................................................................................................124
3.5 List of References .......................................................................................................130
Chapter 4: Functionalization of Poly(L-lactide) with 2-Oxepane-1,5-
Dione 133
4.1 Background .................................................................................................................133
4.2 Results and Discussion ...............................................................................................136 4.2.1. Transition Metal-Catalysed Copolymerisation of L-lactide and 2-
Oxepane-1,5-Dione in the Bulk ........................................................................136 4.2.2. Organocatalysed Copolymerisation of L-Lactide and OPD in Solution ...........153
4.3 Summary .....................................................................................................................163
4.4 Experimental ...............................................................................................................164
4.5 List of References .......................................................................................................171
Chapter 5: Photodegradation of Functionalized Poly(L-Lactide) with 2-
Oxepane-1,5-Dione ................................................................................................. 175
5.1 Background .................................................................................................................175
5.2 Results and Discussion ...............................................................................................177 5.2.1 Synthesis of 1,4,8-Trioxaspiro[4.6]-9-Undecanone .........................................177 5.2.2 Synthesis of Poly(L-Lactide-co-TOSUO) ........................................................180 5.2.3 Synthesis of Poly(L-Lactide-co-2-Oxepane-1,5-Dione) ...................................190 5.2.4 Photodegradation of Poly(L-lactide-co-OPD) ..................................................196 5.2.5 Mechanism of Photodegradation ......................................................................210
5.3 Summary .....................................................................................................................212
5.4 Experimental ...............................................................................................................213 5.4.1 Material ............................................................................................................213 5.4.2 Methods ............................................................................................................213
5.5 List of References .......................................................................................................218
Chapter 6: Conclusions and Future Research Directions ............................ 222
6.1 Conclusions ................................................................................................................222
6.2 Future Research Directions .........................................................................................224
Appendices .............................................................................................................. 225
vi Modification of biodegradable polymer films
List of Figures
Figure 1.1. Structures of selected biodegradable polyesters: poly(ɛ-
caprolactone) (a), poly(-hydroxybutyrate) (b) and polylactide (c). ............. 2
Figure 1.2. Stereoisomers of lactic acid and lactide.30 ................................................ 5
Figure 1.3. Structure of the different stereoisomers that can be obtained from
polymerisation of lactide: (a) poly(L-lactide) (PLLA), (b) poly(D-
lactide) (PDLA) and (c) poly(D,L-lactide) (PDLLA). .................................. 11
Figure 2.1. Structure of 2-oxepane-1,5-dione. .......................................................... 48
Figure 2.2. ATR-FTIR average spectrum of 2-oxepane-1,5-dione (average of 9
spectra after baseline correction). ................................................................ 50
Figure 2.3. UV-Visible spectrum of OPD showing an absorbance maximum at
273 nm (measured in methanol at 1 mmol·L-1; a baseline spectrum
was measured in methanol). ......................................................................... 51
Figure 2.4. Visual aspects of the films of PLLA - OPD blends before
accelerated ageing. a: 0 wt%; b: 2 wt%; c: 4 wt%; d: 6 wt%; e: 8 wt%;
f: 10 wt% OPD. ............................................................................................ 52
Figure 2.5. GPC traces of PLLA - OPD 0 - 10 wt% films before UV
degradation, measured in chloroform (the percentage values
correspond to the concentration of OPD in the blends). .............................. 54
Figure 2.6. DSC thermograms from the second heating cycle for PLLA - OPD
blend films before accelerated ageing (the percentage values
correspond to the concentration of OPD in the blends). .............................. 55
Figure 2.7. ATR-FTIR average spectra of PLLA - OPD 0-10 wt% films before
degradation (average of 9 spectra per film after baseline correction
and normalization with the -CH3 bending band at 1455 cm-1). ................... 58
Figure 2.8. Carbonyl band in the ATR-FTIR spectra of the PLLA - OPD (0 -
10 wt%) blend films before ageing revealing the shoulder from 1725
to 1690 cm-1 due to the OPD ketone moiety (average of 9 spectra per
film after baseline correction and normalization with the -CH3
bending band at 1455 cm-1). ......................................................................... 59
Figure 2.9. UV-Visible spectra of PLLA - OPD films before accelerated
ageing, showing an increase in absorbance in the range 250 - 300 nm
due to the n-π* transition of the ketone moiety of OPD. ............................. 60
Figure 2.10. Effect of UV exposure on the films of PLLA - OPD (0-10 wt%)
before (top) and after 14 days (bottom) of UV exposure using a QUV
device (UV-A 340 lamps, 50 °C) with whitening and embrittlement
observed. ...................................................................................................... 62
Figure 2.11. Evolution of the GPC distributions of each film before (plain line)
and after ten irradiation days (dashed line) in the QUV, revealing a
shift towards low molecular weight for films containing OPD. .................. 63
Modification of biodegradable polymer films vii
Figure 2.12. Decrease in the number average molecular weight of the aged
PLLA - OPD films (0 - 10 wt% OPD) versus irradiation days, as
measured by GPC in chloroform (data were obtained from three
different batches of films and averaged). ..................................................... 64
Figure 2.13. Evolution of the polydispersities of the PLLA - OPD blend films
during artificial ageing. ................................................................................ 65
Figure 2.14. 1/𝑀𝑛 vs irradiation days of the PLLA - OPD films (0 - 10 wt%). ...... 66
Figure 2.15. Evolution of the number of chain scissions of the aged films of
PLLA - OPD (0 - 10 wt%) in the QUV, calculated from the 𝑀𝑛
measured by GPC in chloroform. ................................................................ 68
Figure 2.16. Evolution of the melting temperature of the PLLA - OPD films as
a function of irradiation time in the QUV. ................................................... 69
Figure 2.17. Evolution of the glass transition of the PLLA - OPD films as a
function of irradiation time in the QUV. ..................................................... 70
Figure 2.18. ATR-FTIR average spectra of PLLA film before and after one
and ten irradiation days (average of 9 spectra after baseline correction
and normalization with the -CH3 bending band at 1452 cm-1). ................... 71
Figure 2.19. ATR-FTIR average spectra of PLLA - OPD 2 wt% film before
and after one and ten irradiation days (average of 9 spectra after
baseline correction and normalization with the -CH3 bending band at
1452 cm-1). ................................................................................................... 72
Figure 2.20. ATR-FTIR average spectra of PLLA - OPD 10 wt% film before
and after one and ten irradiation days (average of 9 spectra after
baseline correction and normalization with the -CH3 bending band at
1452 cm-1). ................................................................................................... 73
Figure 2.21. UV-visible spectra from the PLLA only film as a function of
irradiation time in the QUV. ........................................................................ 74
Figure 2.22. UV-visible spectra of the PLLA - OPD 10 wt% film as a function
of irradiation time in the QUV. .................................................................... 75
Figure 2.23. Comparison of the GPC traces of neat PLLA when covered (C)
and uncovered (U) before and after ten days in the QUV. .......................... 76
Figure 2.24. Comparison of the GPC traces of PLLA - OPD 10 wt% when
covered (C) and uncovered (U) before and after ten days in the QUV. ...... 78
Figure 3.1. ATR-FTIR average spectrum of POPD revealing the ester and
ketone bands at 1723 and 1700 cm-1, respectively (nine spectra were
collected, baseline-corrected and averaged). ............................................... 95
Figure 3.2. Evolution of apparent viscosity during the extrusion of neat PLLA
at 190 °C for 10 minutes (six extrusions were performed and values of
apparent viscosity were averaged). .............................................................. 98
Figure 3.3. Evolution of apparent viscosity during the extrusion of pure PLLA,
PLLA with tin (II) octanoate and PLLA - OPD with tin (II) octanoate
formulations at 190 °C for 10 minutes (the percentages account for the
OPD initial feed; 0 wt% corresponds to the PLLA – tin (II) octanoate
formulation without OPD). .......................................................................... 99
viii Modification of biodegradable polymer films
Figure 3.4. ATR-FTIR spectra of extrudates of PLLA - OPD - tin (II)
octanoate at 190 °C for 10 minutes after one purification step (the
weight percentages represent the OPD initial feed; 9 spectra were
measured per film, baseline corrected and normalized to the -CH3
bending band at 1455 cm-1). ....................................................................... 101
Figure 3.5. Evolution of the ketone stretching shoulder at 1717 cm-1 in the
ATR-FTIR spectra of extrudates of PLLA - OPD 15 wt% - tin (II)
octanoate at 190 °C for 10 minutes with the number of purification
steps (average of 9 spectra per film after baseline correction and
normalization to the -CH3 bending band at 1454 cm-1). ............................ 102
Figure 3.6. Evolution of the 1H NMR spectra of the extrudates resulting from
the extrusion of PLLA with OPD 15 wt% at 190 ºC for 10 minutes
(top: crude extrudate; middle: extrudate after two purification steps;
bottom: extrudate after three purification steps), measured in CDCl3. ...... 103
Figure 3.7. GPC traces of purified extrudates of PLLA and OPD 0 - 15 wt% at
190 °C for 10 minutes measured in chloroform (the traces were
baseline-corrected and normalized). .......................................................... 104
Figure 3.8. DSC thermograms from the second heating cycle of purified
extrudates of PLLA - OPD (the percentages account for the OPD
initial feed; 0 wt% corresponds to the PLLA – tin (II) octanoate
formulation without OPD). ........................................................................ 107
Figure 3.9. Photographs of extrudates collected every 30 minutes of a reactive
extrusion experiment of PLLA - OPD (15.1 wt%) catalysed by tin (II)
octanoate, revealing the change of colour over time. ................................ 109
Figure 3.10. ATR-FTIR spectra of double-purified extrudates after various
residence times revealing the broadening of the carbonyl band (1820 -
1660 cm-1) and the appearance of a broad band between 3700 to 2700
cm-1 (average of 9 spectra per film after baseline correction and
normalization to the -CH3 bending band at 1454 cm-1). ............................ 110
Figure 3.11. 1H NMR spectra of double-purified extrudates of PLLA - OPD -
tin (II) octanoate after various residence times at 190 ºC, measured in
CDCl3. ........................................................................................................ 111
Figure 3.12. GPC traces of purified extrudates of PLLA - OPD 15 wt%,
collected every 20 minutes at 190 °C, measured in chloroform (the
traces were baseline-corrected and normalized). ....................................... 112
Figure 3.13. DSC thermograms from the second heating cycle of double-
purified extrudates after various residence times. ...................................... 113
Figure 3.14. Evolution of the apparent viscosity vs residence time for
extrudates of PLLA - OPD 10 wt% without and with titanium (IV)
tetrabutoxide as the transesterification catalyst. ........................................ 116
Figure 3.15. ATR-FTIR spectra of twice-purified extrudates without and with
titanium (IV) tetrabutoxide as transesterification catalyst (average of
nine spectra after baseline-correction and normalization with the –CH
stretching band at 1454 cm-1). .................................................................... 117
Modification of biodegradable polymer films ix
Figure 3.16. 1H NMR spectra of twice-purified extrudates: using titanium (IV)
tetrabutoxide (top) and without any transesterification catalyst
(bottom), measured in CDCl3. ................................................................... 118
Figure 3.17. GPC traces of PLLA - OPD 10 wt% with or without
transesterification catalyst, measured in chloroform (the traces were
baseline-corrected and normalized). .......................................................... 119
Figure 3.18. DSC thermograms of purified extrudates of PLLA - OPD 10 wt%
with and without titanium (IV) tetrabutoxide as the transesterification
catalyst. ...................................................................................................... 121
Figure 3.19. TGA trace of 2-oxepane-1,5-dione measured from 0 to 1000 °C
under nitrogen showing a decomposition step around 160 ºC with an
inflection point at 184.1 °C. ....................................................................... 122
Figure 3.20. DSC thermogram of OPD on a nonisothermal run at a heating rate
of 10 ºC·min-1 under nitrogen showing both the melting (Tm) and
decomposition (Tdecomposition) phases. ......................................................... 123
Figure 4.1. 1H NMR spectra of various poly(L-lactide-co-OPD) with initial
OPD concentration of 5 (bottom); 10 (middle) and 20 mol% (top),
measured in CDCl3. ................................................................................... 138
Figure 4.2. (a) 1H NMR spectrum of poly(L-lactide-co-OPD); (b) PGSE NMR
spectra of poly(L-lactide-co-OPD) using 3 % magnetic field gradient
pulse; (c) PGSE NMR spectra of poly(L-lactide-co-OPD) using 95 %
magnetic field gradient pulse, measured in CDCl3. ................................... 140
Figure 4.3. ATR-FTIR spectra of the different poly(L-lactide-co-OPD)s
revealing the characteristic bands of poly(L-lactide) (the mol%
represents the concentration of incorporated OPD within the
copolymer). ................................................................................................ 141
Figure 4.4. Enlarged view of the carbonyl region of the ATR-FTIR spectra of
the different poly(L-lactide-co-OPD)s revealing the OPD shoulder at
1725 - 1700 cm-1 (the two maxima observed around 1750 cm-1 for 7
mol% OPD was due to noise resulting from the resolution (4 cm-1)
used to run the spectra and the normalization process to the band at
1453 cm-1 assigned to -CH3 bending). ....................................................... 142
Figure 4.5. GPC traces of synthesized poly(L-lactide) and poly(L-lactide-co-
OPD)s measured in chloroform (the traces were baseline-corrected
and normalized). ........................................................................................ 144
Figure 4.6. DSC thermograms of purified poly(L-lactide) 1 and poly(L-lactide-
co-OPD) 2, 3, 4 under nitrogen on a second heating run. .......................... 145
Figure 4.7. ATR-FTIR average spectrum of the product 5, resulting from the
ROP of L-lactide with an initial OPD feed of 50 mol% (average of 9
spectra per film after baseline correction and normalization with the -
CH3 bending band at 1456 cm-1). ............................................................... 147
Figure 4.8. ATR-FTIR average spectrum of the product 6, resulting from the
ROP of L-lactide with an initial OPD feed of 75 mol% (average of 9
spectra per film after baseline correction and normalization with the -
CH3 bending band at 1456 cm-1). ............................................................... 148
x Modification of biodegradable polymer films
Figure 4.9. GPC trace of products 5 and 6, resulting from the ROP of L-lactide
with an OPD initial feed of 50 and 75 mol%, respectively, measured
in chloroform (the traces were baseline-corrected and normalized). ......... 149
Figure 4.10. 1H NMR spectra of (a) purified compound 7; (b) purified
compound 8, measured in CDCl3. .............................................................. 151
Figure 4.11. Concentrations of analytes in red complexes measured by ICP-
OES, revealing tin and silicon as the main components. ........................... 152
Figure 4.12. Homopolymerisation of L-lactide in DCM at room temperature
(monomer conversion calculated from 1H NMR spectroscopy) under
the following conditions: [LLA]0 = 2.072 mol·L-1, LLA / BDU = 15,
LLA / benzyl alcohol = 87. ......................................................................... 155
Figure 4.13. Representative ATR-FTIR average spectrum of synthesized
poly(L-lactide) using DBU and benzyl alcohol as catalyst and initiator,
respectively. ............................................................................................... 156
Figure 4.14. ATR-FTIR averaged spectra of purified polymers with OPD
initial feed ranging from 0 to 77 mol% (average of 9 spectra after
baseline correction and normalization with the -CH3 bending band at
1454 cm-1). ................................................................................................. 158
Figure 4.15. Enlarged view of the carbonyl region in the ATR-FTIR spectra of
purified polymers with an OPD initial feed of 0 to 77 mol%. ................... 159
Figure 4.16. GPC traces of copolymers of L-lactide and OPD with an initial
OPD concentration of 0 to 23 mol% measured in chloroform (the
traces were baseline-corrected and normalized). ....................................... 160
Figure 4.17. Structure of 1,4,8-trioxaspiro[4.6]-9-undecanone. ............................. 161
Figure 4.18. 1H NMR spectrum of the crude product of the ROP of L-lactide
and protected OPD revealing the conversion of L-lactide only,
measured in CDCl3. .................................................................................... 163
Figure 5.1. Structure of 1,4,8-trioxaspiro[4.6]-9-undecanone. ............................... 175
Figure 5.2. 1H NMR spectrum of TOSUO, measured in CDCl3. ............................ 179
Figure 5.3. 13C NMR spectrum of TOSUO, measured in CDCl3. ........................... 179
Figure 5.4. Representative 1H NMR spectrum of poly(L-lactide-co-TOSUO),
measured in CDCl3. .................................................................................... 182
Figure 5.5. ATR-FTIR spectra of different poly(L-lactide-co-TOSUO)s
(average of 9 spectra per film after baseline correction and
normalization with the -CH3 bending band at 1455 cm-1). ........................ 184
Figure 5.6. Enlarged view of the carbonyl region (1850 – 1650 cm-1) in the
ATR-FTIR spectra of the different poly(L-lactide-co-TOSUO)s (the
two maxima observed around 1755 cm-1 were due to noise resulting
from the resolution (4 cm-1) used to run the spectra and the
normalization process). .............................................................................. 185
Figure 5.7. GPC traces of purified poly(L-lactide-co-TOSUO) measured in
chloroform (the traces were baseline-corrected and normalized). ............. 186
Modification of biodegradable polymer films xi
Figure 5.8. DSC thermograms from the second heating cycle of purified
poly(L-lactide-co-TOSUO). ....................................................................... 189
Figure 5.9. Comparison of the 1H NMR spectra of copolymer before (referred
to as A) and after deprotection of the ketones in DCM at room
temperature (referred to as B). ................................................................... 192
Figure 5.10. ATR-FTIR spectra of poly(LLA-co-TOSUO) and poly(LLA-co-
OPD) with an enlarged view of the carbonyl region (1850 – 1650 cm-
1) revealing a shoulder at 1715 cm-1 corresponding to the C=O
stretching band of the ketone moity of OPD, a shoulder at 1775 cm-1
corresponding to the carbonyl stretching of a lactone-type product
resulting from transesterification during the deprotection step. ................ 193
Figure 5.11. Comparison of the GPC traces of the copolymer before and after
deprotection of the ketones, measured in chloroform (the traces were
baseline-corrected and normalized). .......................................................... 194
Figure 5.12. Evolution of the GPC traces of poly(L-lactide-co-OPD) (5.2
mol% OPD segments) as a function of irradiation time, measured in
THF (the traces were baseline-corrected and normalized). ....................... 198
Figure 5.13. Evolution of the GPC traces of poly(L-lactide-co-OPD) (8 mol%
OPD segments) as a function of irradiation time, measured in THF
(the traces were baseline-corrected and normalized). ................................ 199
Figure 5.14. DSC thermograms from the second heating cycle of poly(L-
lactide-co-OPD) with 5.2 mol% OPD with increasing irradiation time
in the QUV. ................................................................................................ 202
Figure 5.15. Enlarged view of the glass transitions in the DSC thermograms
from the second heating cycle of poly(L-lactide-co-OPD) with 5.2
mol% OPD with increasing irradiation time in the QUV. ......................... 203
Figure 5.16. DSC thermograms from the second heating cycle of poly(L-
lactide-co-OPD) with 8 mol% OPD with irradiation days in the QUV. .... 204
Figure 5.17. ATR-FTIR average spectra of poly(L-lactide-co-OPD) (5.2 mol%
OPD) before and after ten days of UV irradiation (average of 9 spectra
after baseline correction and normalization to the -CH3 bending band
at 1454 cm-1). ............................................................................................. 207
Figure 5.18. Enlarged view of the carbonyl band in the ATR-FTIR spectra of
poly(L-lactide-co-OPD) (5.2 mol% OPD): anhydride region 1900 -
1810 cm-1 and ketone region 1740 - 1690 cm-1. ........................................ 208
Figure 5.19. ATR-FTIR average spectra of poly(L-lactide-co-OPD) (8 mol%
OPD) powder before and after irradiation (average of 9 spectra after
baseline correction and normalization with the -CH3 bending band at
1454 cm-1). ................................................................................................. 209
Figure 5.20. Enlarged view of the carbonyl band in the ATR-FTIR spectra of
poly(L-lactide-co-OPD) (8 mol% OPD): anhydride region 1900 - 1810
cm-1 and ketone region 1740 - 1690 cm-1. ................................................. 210
xii Modification of biodegradable polymer films
List of Schemes
Scheme 1.1. Microbial fermentation of L-lactic acid.13 ............................................... 4
Scheme 1.2. Synthesis of lactide: polycondensation of lactic acid followed by
depolymerisation via backbiting.31 ................................................................ 6
Scheme 1.3. Anionic ring-opening polymerisation of lactide. .................................... 8
Scheme 1.4. Cationic ring-opening polymerisation of lactide.23 ................................. 9
Scheme 1.5. Ring-opening polymerisation of L-lactide following a
coordination-insertion mechanism.23 ........................................................... 10
Scheme 1.6. Hydrolysis of an ester linkage. .............................................................. 15
Scheme 1.7. (a) Intramolecular transesterification resulting in the formation of
lactide, oligomers, acetaldehyde, and carbon monoxide; (b)
intermolecular transesterification; (c) hydrolysis.62, 82 ................................. 19
Scheme 1.8. Norrish type II mechanism for the photodegradation of PLLA
under UV-C light. ........................................................................................ 21
Scheme 1.9. Proposed mechanism of racemization occurring both at the
hydroxyl chain end (left) and the carboxyl chain end (right) during the
photodegradation of PLLA exposed to UV-C light.99 ................................. 22
Scheme 1.10. Photodegradation mechanism of PLA based on hydrogen
abstraction from the carbon in the α-position to the carbonyl group
with formation of macroradicals leading to anhydride as a main
photodegradation product (X represents chromophoric defects). ................ 24
Scheme 1.11. Photodegradation of ethylene-carbon monoxide copolymers via
Norrish type I and II. .................................................................................... 32
Scheme 1.12. Synthesis of poly(2-oxepane-1,5-dione) via the ROP of TOSUO.
Conditions and reagents: a. Al(OiPr)3, toluene, 25 °C, H3O+; b.
(C6H5)3CBF4, dichloromethane, 25 °C, 1 hour. ........................................... 33
Scheme 1.13. Synthesis of poly(ɛ-caprolactone-co-2-oxepane-1,5-dione).
Conditions and reagents: a. tin (II) octanoate, toluene, 90 °C. .................... 33
Scheme 1.14. Ring-opening polymerisation of L-lactide and 2-oxepane-1,5-
dione to afford poly(L-lactide-co-2-oxepane-1,5-dione). Conditions
and reagents: a. tin (II) octanoate, 110-160 °C, in the bulk; b. DBU,
benzyl alcohol, DCM, room temperature. .................................................... 35
Scheme 1.15. Synthesis of poly(L-lactide-co-OPD) via two steps: a. Ring-
opening polymerisation of L-lactide and TOSUO to afford poly(L-
lactide-co-TOSUO). b. Deprotection of the acetal groups of poly(L-
lactide-co-TOSUO) to afford poly(L-lactide-co-OPD). ............................... 37
Scheme 2.1. Synthesis of 2-oxepane-1,5-dione. Conditions and reagents: 1,4-
cyclohexanedione, mCPBA, DCM, 40 °C, 4 h, 45 % yield. ........................ 49
Scheme 2.2. Norrish type I and II cleavages of ketones. ........................................... 81
Modification of biodegradable polymer films xiii
Scheme 2.3. Proposed photodegradation mechanism for PLLA - OPD blends
initiated by the Norrish type I cleavage of OPD, releasing radicals that
attack the PLLA backbone leading to hydrogen abstraction. The rest
of the mechanism is based on previous reports, leading to PLLA chain
scission and anhydride formation.13, 14 ......................................................... 84
Scheme 4.1. Conversion of tin (II) octanoate into tin (II) alkoxide via reaction
with alcohol or residual protic impurities. ................................................. 134
Scheme 4.2. Ring-opening polymerisation of lactide using DBU and an alcohol
as catalyst and initiator, respectively. ........................................................ 135
Scheme 4.3. Tin (II) octanoate-catalysed ROP of L-lactide and OPD at 110 °C
in the bulk to afford poly(L-lactide-co-OPD). ........................................... 137
Scheme 5.1. Two step synthesis of poly(L-lactide-co-OPD): Conditions and
reagents: a. tin (II) octanoate, in the bulk, 110 ºC; b.
triphenylcarbenium tetrafluoroborate (TPFB), DCM, room
temperature, 2 hours................................................................................... 177
Scheme 5.2. Baeyer-Villiger oxidation of 1,4-cyclohexane monoethylene
acetal by mCPBA to afford TOSUO and the mCPBA by-product, 3-
chlorobenzoic acid. .................................................................................... 178
Scheme 5.3. ROP of L-lactide and TOSUO in the bulk at 110 °C to afford
poly(L-lactide-co-TOSUO) using tin (II) octanoate and benzyl alcohol
as the catalyst and the initiator respectively. ............................................. 180
Scheme 5.4. Conversion of tin (II) octanoate into tin (II) alkoxide via reaction
with alcohol or residual protic impurities. ................................................. 187
Scheme 5.5. Chemical structure of triphenylcarbenium tetrafluoroborate
(TPFB). ...................................................................................................... 190
Scheme 5.6. Mechanism of the deprotection using TPFB involving a hydride
abstraction from the ethylene acetal that affords an oxonium ion that is
subsequently quenched by aqueous work-up.30, 31 ..................................... 191
Scheme 5.7. Deprotection of the ketone acetal groups of poly(L-lactide-co-
TOSUO) using TPFB in DCM at room temperature to afford poly(L-
lactide-co-OPD). Conditions and reagents: a. poly(L-lactide-co-OPD),
TPFB (1.5 equivalents of ethylene ketal groups), DCM, 2 hours, room
temperature, 80 - 85 %. .............................................................................. 191
Scheme 5.8. Norrish type I and II cleavages of the ketone of ring-opened OPD. ... 212
xiv Modification of biodegradable polymer films
List of Tables
Table 1.1. Comparison of the physical properties of lactic acid and lactide
enantiomers.26, 32, 33 ........................................................................................ 7
Table 1.2. Thermal properties of random stereoisomers of PLA (neither
molecular weight values nor accuracy of the thermal transitions values
were reported in the literature).39 ................................................................. 11
Table 1.3. Characteristics of the three crystal forms of PLLA. ................................. 13
Table 2.1. Average values of 𝑀𝑛 , 𝑀𝑤 and polydispersity of three batches of
PLLA -OPD blend films (OPD: 0 - 10 wt%) before accelerated
ageing, measured by GPC in chloroform. .................................................... 54
Table 2.2. Evolution of the glass transition and melting temperature of the
films with OPD content obtained by DSC before ageing (the
measurements were performed on three different batches of films and
the values were averaged). ........................................................................... 57
Table 2.3. Comparison of the thermal properties of the transparent and opaque
sections of the PLLA - OPD 10 wt% film. .................................................. 57
Table 2.4. ATR-FTIR band assignment of poly(L-lactide) based on reported
literature.13, 47 ................................................................................................ 59
Table 2.5. r2 values and rate constants k determined from the 𝑀𝑛 measured by
GPC for the six formulation films. ............................................................... 67
Table 2.6. Comparison of the 𝑀𝑛 and the polydispersity of the uncovered and
covered films of PLLA - OPD blends (0 - 10 wt%) as a function of
irradiation days. ............................................................................................ 78
Table 3.1. Formulations of the extrusions of PLLA and OPD 0 - 15 wt% using
tin (II) octanoate as the transesterification catalyst. ..................................... 96
Table 3.2. Molecular weight of three-times-purified extrudates of PLLA with
OPD (processed via reactive extrusion for 10 minutes at 190 °C,
measured by GPC in chloroform (three measurements were performed
and values were averaged; the percentages account for the OPD initial
feed; 0 wt% corresponds to the PLLA – tin (II) octanoate formulation
without OPD). ............................................................................................ 105
Table 3.3. Thermal properties of purified extrudates obtained by DSC on a
second heating cycle (three measurements were performed and values
were averaged). .......................................................................................... 108
Table 3.4. Molecular weight of purified extrudates of PLLA - OPD 15 wt% at
190 °C collected every 20 minutes, measured by GPC in chloroform
(three measurements were performed and values were averaged)............. 112
Table 3.5. Evolution of the thermal properties of purified extrudates collected
every 20 minutes, as measured by DSC on a second heating run (three
measurements were performed and values were averaged). ...................... 114
Modification of biodegradable polymer films xv
Table 3.6. Formulations of extrudates featuring titanium (IV) tetrabutoxide as
the transesterification catalyst. ................................................................... 115
Table 3.7. Molecular weight of twice-purified extrudates of PLLA - OPD 10
wt% for 10 minutes at 190 °C with or without titanium (IV)
tetrabutoxide measured by GPC in chloroform (three measurements
were performed and values were averaged). .............................................. 119
Table 3.8. Thermal properties of extrudates of PLLA - OPD 10 wt% with and
without titanium (IV) tetrabutoxide, as measured by DSC in the
second heating run (three measurements were performed and values
were averaged). .......................................................................................... 121
Table 3.9. Formulations of the different extrudates. ............................................... 125
Table 4.1. Conditions and results of the ring-opening polymerisations of L-
lactide and OPD 0 – 20 mol% at 110 ºC catalysed by tin (II) octanoate
in the bulk. ................................................................................................. 137
Table 4.2. Molecular weight and polydispersities of PLLA and poly(L-lactide-
co-OPD)s measured in chloroform (two measurements were
performed and values were averaged). ...................................................... 144
Table 4.3. Thermal properties of purified poly(L-lactide-co-OPD) measured by
DSC on a second heating cycle (two measurements were performed
and values were averaged). ........................................................................ 146
Table 4.4. Comparison of characteristic bands in the ATR-FTIR spectra of L-
lactide and poly(L-lactide) with their assignments based on reported
literature.34, 35 ............................................................................................. 148
Table 4.5. Evolution of the conversion and number average molecular weight
over time of the ROP of L-lactide using DBU and benzyl alcohol as
catalyst and initiator, respectively, DPn 87. ............................................... 154
Table 4.6. Conditions and results of the batch polymerisations of L-lactide and
OPD in solution at room temperature with DBU and benzyl alcohol as
catalyst and initiator, respectively.............................................................. 157
Table 4.7. Molecular weight and polydispersities of the purified products of
the batch polymerisations of L-lactide and OPD using DBU as the
catalyst. ...................................................................................................... 160
Table 4.8. Conditions of batch copolymerisations in DCM at room temperature
using DBU and benzyl alcohol as catalyst and initiator, respectively. ...... 169
Table 5.1. Copolymerisation of L-lactide and TOSUO in the bulk at 110 °C
catalysed by tin (II) octanoate. ................................................................... 183
Table 5.2. Comparison of the theoretical 𝑀𝑛 and the measured values for the
different poly(L-lactide-co-TOSUO) copolymers. ..................................... 186
Table 5.3. Thermal properties of the different poly(L-lactide-co-TOSUO)
(three measurements were performed and the values were averaged). ..... 189
Table 5.4. Molecular weights of copolymers before and after deprotection, as
measured by GPC in chloroform. .............................................................. 195
xvi Modification of biodegradable polymer films
Table 5.5. Thermal transitions of poly(L-lactide-co-OPD) after the
deprotection of the ketone acetal groups (three measurements were
performed and values were averaged). ...................................................... 195
Table 5.6. Evolution of the number and weight averaged molecular weights,
polydispersity and the chain scission of poly(L-lactide-co-OPD) as a
function of irradiation time in the QUV. .................................................... 200
Table 5.7. Thermal properties of aged poly(L-lactide-co-OPD) copolymers
determined by DSC for samples before irradiation and after UV
irradiation for 2-10 days (two measurements were performed and
values were averaged). ............................................................................... 205
Table 5.8. Conditions of the ROP of L-lactide and TOSUO in the bulk at 110
°C. .............................................................................................................. 215
Modification of biodegradable polymer films xvii
List of Abbreviations
𝑀𝑒 Entanglement molecular weight (g·mol-1)
𝑀𝑛 Number average molecular weight (g·mol-1)
𝑀𝑤 Weight average molecular weight (g·mol-1)
𝑇𝑔,∞ Glass transition of polylactide having an infinite molecular weight
𝜒𝑐 Crystallinity (%)
∆𝐻𝑐𝑐 Cold crystallization enthalpy (J·g-1)
∆𝐻𝑚 Melting enthalpy (J·g-1)
∆𝐻𝑚0 Melting enthalpy of 100 % crystalline PLA sample
Al(OiPr)3 Aluminium isopropoxide
AR Analytical reagent
BnOH Benzyl alcohol
br Broad
CaSO4 Calcium sulfate
CDCl3 Deuterated chloroform
CH2Cl2 Dichloromethane
Ð Dispersity
Da Dalton
DBU 1,8-Diazabicyclo[5.4.0]undec-7-ene
DCM Dichloromethane
DMF-DMA N,N-dimethylformamide dimethyl acetal
DPn Degree of polymerisation
DSC Differential scanning calorimetry
EVOH Poly(ethylene-co-vinylalcohol)
FDA Food and Drug Administration
FTIR Fourier transform infrared
GPC Gel permeation chromatography
GWP Global warming potential
HPLC High-performance liquid chromatography
ICP-OES Inductively coupled plasma optical emission spectroscopy
IR Infrared
xviii Modification of biodegradable polymer films
LCA Life cycle assessment
LDPE Low-density polyethylene
LLA L-lactic acid
m Multiplet (NMR)
mCPBA 3-Chloroperbenzoic acid
mol% Mol percentage (%)
NIR Near-infrared
NMR Nuclear magnetic resonance
OPD 2-oxepane-1,5-dione
PBAT Poly(butylene adipate-co-terephtalate)
PCL Poly(ε-caprolactone)
PDLA Poly(D-lactic acid)
PDLLA Poly(D,L-lactic acid)
PE Polyethylene
PEG Poly(ethylene glycol)
PGSE NMR Pulsed field gradient spin-echo nuclear magnetic resonance
PHB Poly(β-hydroxybutyrate)
PhD Doctor of Philosophy
PLLA Poly(L-lactic acid)
POPD Poly(2-oxepane-1,5-dione)
PTOSUO Poly(1,4,8-trioxaspiro[4.6]-9-undecanone)
PP Polypropylene
RH Relative humidity
RH Hydrodynamic radius
ROP Ring-opening polymerisation
RT Room temperature
s Chain scission
Sn(Oct)2 Tin (II) octanoate
st Strong
TBD 1,5,7-Triazabicyclo[4.4.0]dec-5-ene
Tcc Cold crystallization temperature (ºC)
Tg Glass transition temperature (ºC)
TGA Thermal gravimetric analysis
Modification of biodegradable polymer films xix
Ti(OBu)4 Titanium (IV) tetrabutoxide
TiO2 Titanium dioxide
Tm Melting temperature (ºC)
TMPD N,N,N’,N’-tetramethyl-1,4-phenylenediamine
TOSUO 1,4,8-trioxaspiro[4.6]-9-undecanone
TPFB Triphenylcarbenium tetrafluoroborate
UV Ultra-violet
w Weak
wt% Weight percentage (%)
ZnO Zinc oxide
𝑤 Weight fraction
xx Modification of biodegradable polymer films
Statement of Original Authorship
The work contained in this thesis has not been previously submitted to meet
requirements for an award at this or any other higher education institution. To the best
of my knowledge and belief, the thesis contains no material previously published or
written by another person except where due reference is made.
Signature:
Date: ________05/07/2019_________________
QUT Verified Signature
Modification of biodegradable polymer films xxi
Acknowledgements
This PhD project would not have been possible without the following persons.
My principal supervisor, Prof. Steven Bottle, for his guidance and support. I would
like to thank my associate supervisors as well, Dr Melissa Nikolic, Dr John Colwell
and Dr Christiane Lang. Thanks to the four of them for being available to discuss,
suggest ideas or help throughout my PhD project.
The Cooperative Research Centre for Polymers for the financial support, not only as a
scholarship but also for opportunities to travel and present my work during the Annual
Meetings, as well as attending several workshops to develop my communication skills.
Prof Graeme George, for giving me advice and sharing his sound knowledge and
experience during discussions.
The Cooperative Research Centre for Polymers staff in Brisbane, including Emilie
Gautier, Jorja Cork and Michael Murphy for their great support and friendship.
Other QUT academics in the chemistry discipline, Dr Llew Rintoul, and Dr Mark
Wellard for their help and discussions.
The technical staff, Dr Chris Carvahlo, Dr Lauren Butler, Mr Peter Hegarty and Ms
Leonora Newby for their availability and assistance.
My friends for their unconditional support, regardless of the distance.
My family, thank you for your presence throughout the distance and time, your
considerable support and understanding that helped me over this PhD journey.
Chapter 1: Literature Review 1
Chapter 1: Literature Review
1.1 INTRODUCTION
The world production of polymers has evolved from 230 million tons in 2005 to 335
million tons in 2016 and is expected to reach 400 million tons in 2020.1, 2 Their
durability, ease of fabrication and low cost justify the extensive use of polymers in a
broad range of applications such as packaging, textiles, medical items, and building
materials.2 However, their versatile properties have also contributed to one of the
world’s major issues: pollution.
Common polymers, such as polypropylene and polyethylene, can persist longer than
the time range required by the application fields. An appropriate waste treatment is
therefore required. To date, plastic waste is either thermally treated, recycled, or
decomposed in landfills.2 Cumulative waste of primary and recycled polymers reached
6,300 Mt between 1950 and 2015, with 60 % accumulating in landfills. If the
worldwide production and waste management follow the same trend, about 12,000 Mt
of plastic waste are estimated to accumulate in the environment by 2050.3 Plastic
debris are increasingly gathered in oceans, in concentrations reaching up to 1.0 - 2.5
kg·km-2 in the Pacific Ocean, in the west of the United States of America, or in the
Atlantic Ocean, between South America and Africa, and between Cuba and Europe.2,4
These plastics are typically not readily degraded, and come from sources such as
shopping bags, agricultural mulch films, or fishing nets and fishing lines. This leads
not only to drastic visual pollution in the environment but also to fatal consequences
for animals including turtles and whales.5
Different approaches are emerging to solve this issue. For instance, a landfill ban has
been decided by European countries, such as Denmark, Switzerland, and Germany.2
Plastic bags have also been subjected to taxation or banned, leading to a 90 % reduction
in use.6
Biodegradable plastics may provide a renewable alternative to petroleum-based
polymers and a solution to plastic pollution. These polymers emerged in 1926 with the
first report of bio-based polymers for poly(-hydroxybutyrate) (PHB) and were
developed further in the 1970s due to a petroleum crisis.7 In 2011, biodegradable
2 Chapter 1: Literature Review
plastics represented 30.1 Mt (8.2 %) of the plastics market.1 These polymers degrade
via depolymerisation of their chains through hydrolysis, and assimilation of oligomers
by microorganisms.
Polyesters have experienced a growing interest as biodegradable polymers due to the
degradability of the ester bond by hydrolysis. Several biodegradable polyesters have
been or are being commercially developed, including poly(ɛ-caprolactone) (PCL),
poly(-hydroxybutyrate) (PHB) and polylactide (PLA) (Figure 1.1).8
Figure 1.1. Structures of selected biodegradable polyesters: poly(ɛ-caprolactone) (a),
poly(-hydroxybutyrate) (b) and polylactide (c).
Polylactide is an aliphatic, thermoplastic polyester produced from lactic acid, typically
derived from renewable resources (fermented corn starch). The mechanical properties,
tensile strength and elastic modulus, are similar to polystyrene and poly(ethylene
terephthalate).9 The biodegradability and compostability of PLA strongly support its
use as a sustainable alternative to common polyolefins. Moreover, progress in
manufacturing of the monomer from fermentation has reduced the price of PLA to
$2.50 - 3.00 per kg compared to other biodegradable polyesters, such as poly(ɛ-
caprolactone) (> $9 per kg).10 The ease of fabrication and the versatile mechanical
properties have resulted in a wide range of applications, from textiles to medical
devices, or food packaging.11, 12
The environmental impact of polylactide has been quantified by life cycle assessment
(LCA) tools, from the raw material to end-of-life scenarios. Different global warming
potentials (GWP) have been assessed for polylactide, depending on the models used
Chapter 1: Literature Review 3
in the LCA. While some studies showed little differences between PLA and their
comparable petroleum alternatives (3.0 - 3.1 CO2 eq. per kg for PLA compared to 3.5
for polystyrene), others revealed a GWP of 0.6 CO2 eq. per kg of polylactide compared
to 1.6 for polypropylene, or 2.3 for polystyrene.13 14 In terms of end-of-life scenarios,
polylactide (under dry packaging conditions) hardly degrades in landfill over a period
of hundred years, as revealed by extrapolation from collected data.15 Polylactide can
be degraded under composting conditions, at an industrial scale where conditions, such
as temperature, humidity, and microorganisms are controlled.14 However, to be
competitive to commodity polymers, PLA-based polymers need to be developed that
can degrade within tailored life-times and without requiring additional waste treatment
(e.g. in physical removal, landfill, mechanical recycling).
This literature review will focus on the different steps in the life cycle of polylactide,
except for the waste treatment phase. The production of polylactide will be reviewed
from the raw material to the polymerisation and the physico-chemical properties of the
polyester. Subsequently, three degradation pathways will be discussed, i.e. thermal,
biological and photolytic degradation, including studies that have been undertaken to
modify their rates of degradation.
1.2 POLYLACTIDE
1.2.1 Evolution of the Polylactide Market
Carothers first synthesized a low molecular weight poly(L-lactide) (PLLA) in 1932.7
Dupont (United States of America) further developed PLA manufacturing and
patented the production of a high molecular weight polyester in 1954.16 In 1966,
poly(L-lactide) was reported to have a non-toxic tissue response when implanted in
animals, resulting in developments of sutures and orthopedic applications in 1971.17
In the late 1980s, improvements in process technology for production of lactic acid
and reduction of costs broadened the application range to disposable packaging
applications, fibres for textile applications, and even more durable goods for the
automotive industry.18-22
Polylactide is now produced worldwide by different companies, such as Mitsui
Chemicals and Shimadzu Corporation in Japan, Purac Biochem in the Netherlands,
and Boeringer Ingelheim in Germany.23 NatureWorks (United States of America) is
4 Chapter 1: Literature Review
the main manufacturer with 140,000 tons produced annually. They have launched
research and development programs to produce lactic acid via commercialization of a
new fermentation process, based on methane rather than agricultural feedstocks.24 The
development of the process, the reduction of costs, and the broadening of the
applications all suggest a growing market for PLA. Production is estimated to reach
12 million tonnes by 2020; three times more than in 2011.1
The variety of applications may be explained by the diversity of processing techniques.
The processing of PLA can be achieved by extrusion, injection moulding, blown film
extrusion, thermoforming, as well as film and sheet casting.25
1.2.2 From Lactic Acid to Polylactide
1.2.2.1. Lactic Acid and Lactide
Lactic acid (2-hydroxy propionic acid) is a hydroxycarboxylic acid with a stereo-
centre, leading to two optically active forms: the L- and D- enantiomers, respectively
(Figure 1.2). It is produced by either chemical synthesis or microbial fermentation.26
The former produces a racemic mixture of the two enantiomers while the latter results
in the natural and optically pure form of L-lactic acid.7 The stereochemical purity of
lactic acid is of primary importance as it impacts the stereochemistry of polylactide,
which influences the thermal and mechanical properties (refer to sections 1.2.3 and
1.2.4). Therefore, microbial fermentation is chosen as the main production process.27
Microbial fermentation starts with a carbon source (Scheme 1.1).26 Glucose, starch or
lignocellulose are the typical substrates from which lactic acid is derived.
Scheme 1.1. Microbial fermentation of L-lactic acid.13
Chapter 1: Literature Review 5
However, the cost-efficiency of lactic acid production from starch and lignocellulose
has made them the principal carbon sources.28 Starch is obtained from corn, wheat or
potatoes. Hydrolysis of starch yields dextrose or maltose, which are subsequently
fermented by microorganisms to afford lactic acid.7 The microorganisms that produce
the lactic acid cannot hydrolyse the starch itself, making the enzymatic hydrolysis a
necessary step. Those microorganisms need to meet a few requirements such as a high
activity to reduce the fermentation time and a high conversion yield to reduce
feedstock costs. α-Amylase and glucoamylase are among the enzymes used for the
fermentation step and are commercially available.26
The choice of microorganism influences the stereochemical purity of the obtained
lactic acid. For instance, Lactobacilli (bacteria) and Rhizopus (fungus) produce the L-
lactic acid form in up to 99 % purity.27, 28 D-lactic acid can also be produced in 99.4 %
purity by microbial fermentation using special bacterial species, such as
Sporolactobacillus inulinus or Lactobacillus delbrueckii.26, 29
Polylactide polymerisation commences from a cyclic derivative of lactic acid, lactide
(3,6-dimethyl-1,4-dioxane-2,5-dione). The chirality of lactic acid leads to the
production of lactide in three stereoisomeric forms: L-lactide, D-lactide and a meso
form D,L-lactide (Figure 1.2).
Figure 1.2. Stereoisomers of lactic acid and lactide.30
6 Chapter 1: Literature Review
Lactide is obtained by condensation of lactic acid under reduced pressure (70 - 250
mbar) and high temperature (190 °C) (Scheme 1.2). Oligomers of lactic acid (degree
of polymerisation (DPn) around 10) are obtained at first. Lactide is then afforded by
the depolymerisation of the prepolymer via a backbiting process, most commonly
catalysed by tin (II) octanoate.26, 31 However, the required catalyst and high
temperatures promote racemization.31 Purification is thus necessary not only to
separate lactide from the by-products (lactic acid, oligomers and water), but also
separate different isomeric forms of lactide. Distillation, solvent crystallization or melt
crystallization are used to isolate a stereochemically pure lactide.26 The stereochemical
purity not only impacts the thermal properties of the monomers, it will also influence
the purity of the resulting polylactide.
Scheme 1.2. Synthesis of lactide: polycondensation of lactic acid followed by
depolymerisation via backbiting.31
The stereochemistry of both lactic acid and lactide impacts their thermal properties
(Table 1.1). Meso-lactic acid features a melting point of 16.8 °C instead of 53 °C for
both pure L-lactic acid or D-lactic acid. L-Lactide and D-lactide have a melting point of
Chapter 1: Literature Review 7
96 - 97 °C while the meso and racemic mixture have a melting point of 53 - 54 °C and
122 - 126 °C, respectively.
In terms of solubility, lactic acid is miscible with various solvents including
chloroform, ether, ethyl acetate, and hexanol, while lactide is soluble in benzene,
toluene, ethyl acetate, methanol, and acetone, among others.32
Table 1.1. Comparison of the physical properties of lactic acid and lactide
enantiomers.26, 32, 33
Lactic acid Lactide
Isomer L D L,D L D meso rac
Melting point
(°C)
53 53 16.8 95 - 98 95 - 98 53 - 54 122 - 126
Optical rotation
(°)
+ 2.5 - 2.5 - - 260 + 260 - -
1.2.2.2. Synthesis of Polylactide
Poly(lactic acid) is obtained by polycondensation of lactic acid while polylactide
results from the ring-opening polymerisation (ROP) of lactide.
1.2.2.2.1. Polycondensation of Lactic Acid
Polycondensation relies on the reaction of the hydroxyl and carboxylic acid groups of
lactic acid to afford poly(lactic acid) and water. The reaction is subjected to an
equilibrium; the water produced during the process needs to be removed to achieve a
better yield. The obtained molecular weight is inversely proportional to the conversion,
according to the Carothers equation:
𝐷𝑃𝑛 =
1
1 − 𝑝
With 𝐷𝑃𝑛 the average degree of polymerisation and p the conversion of monomer.34
Obtaining high molecular weight poly(lactic acid) is only achievable for high
conversions. The long reaction times and high temperatures favour side reactions such
as the formation of lactide by back-biting.35 As a result, only low molecular weight
8 Chapter 1: Literature Review
poly(lactic acid) is formed by this synthetic method, limiting the number of
applications in which it can be used.
1.2.2.2.2. Ring-Opening Polymerisation of Lactide
The ring-opening polymerisation of lactide overcomes the issues identified above for
polycondensation. For instance, high average molecular weight polymers up to
100,000 Da can be obtained by ROP.7 Carothers first reported the ROP of lactide in
1932.36 Further improvement in the purification of lactide enabled high molecular
weight PLA to be obtained and thus the use of this method on a commercial scale.7
Three different mechanisms are generally accepted depending on the type of catalyst
used: anionic, cationic, and coordination-insertion mechanism.
The anionic mechanism is triggered by the attack of a nucleophilic initiator on the
carbon of the carbonyl group, resulting in cleavage of the C-O bond and an oxygen-
centred anion on the chain end. Repeated anionic ring-opening of the cyclic monomer
yields linear polylactide (Scheme 1.3). Nucleophilic initiators include organometallic
compounds such as alkyl magnesium bromide, alkyl lithium, metal amides, as well as
alkoxides.23, 37
Scheme 1.3. Anionic ring-opening polymerisation of lactide.
Chapter 1: Literature Review 9
ROP via a cationic mechanism is initiated by the protonation of the carbonyl oxygen-
atom, resulting in an electrophilic activation of the O-CH bond. The propagation
occurs via cleavage of the alkyl-oxygen bond by the nucleophilic attack of another
lactide (Scheme 1.4). Typical electrophilic initiators are Brønsted or Lewis acids, such
as triethyloxonium tetrafluoroborate or trifluoroacetic acid.23, 37 Trifluoromethane
sulfonic acid and methyl trifluoromethane sulfonic acid have been reported as the only
electrophilic reagents capable of polymerising lactide.7
Scheme 1.4. Cationic ring-opening polymerisation of lactide.23
ROP following a coordination-insertion mechanism is the most common route,
yielding polylactides with controlled architectures and molecular weights. The
polymerisation is initiated through the temporary coordination of the exocyclic oxygen
of lactide with the metal atom of the catalyst. The increase in nucleophilicity of the
alkoxide part of the initiator and the electrophilicity of the carbonyl group of the
monomer results in the cleavage of the acyl-oxygen bond of the lactide. The resulting
lactide chain is then inserted into the metal-oxygen bond of the initiator. The
polymerisation proceeds by the opening of lactide monomers and their subsequent
insertion into the bond between the metal atom and the adjacent oxygen atom (Scheme
1.5).23, 38
Metal alkoxides or carboxylates are the most commonly used catalysts for ring-
opening polymerisation via this mechanism. Tin (II) octanoate is the catalyst most
widely used, including at an industrial scale by NatureWorks in their patented
process.31 The high catalytic activity, high solubility in lactide and approval by the
United States Food and Drug Administration (FDA) justify its wide use. 10 ppm of
10 Chapter 1: Literature Review
residual tin (II) octanoate after bulk polymerisation of lactide is considered as non-
toxic.29
Scheme 1.5. Ring-opening polymerisation of L-lactide following a coordination-
insertion mechanism.23
1.2.3 Thermal Properties and Crystallinity of Polylactide
The thermal properties of polylactide are highly related to its stereochemistry.
Polymerisation of lactide yields stereoisomers with various percentages of the different
isomers, depending on its optical purity, the catalyst used and the type of
Chapter 1: Literature Review 11
polymerisation mechanism. The common nomenclature of some stereoisomers is
illustrated in Figure 1.3.
Figure 1.3. Structure of the different stereoisomers that can be obtained from
polymerisation of lactide: (a) poly(L-lactide) (PLLA), (b) poly(D-lactide) (PDLA)
and (c) poly(D,L-lactide) (PDLLA).
Poly(L-lactide) and poly(D-lactide) present similar thermal properties with a glass
transition (Tg) in the range of 50 - 70 °C and a melting temperature (Tm) between 170
and 190 °C. However, random copolymers with higher stereocomplexity display
different thermal transitions depending on the composition. The reduction of L-units
results in lowering both the Tg and Tm (Table 1.2).39
Table 1.2. Thermal properties of random stereoisomers of PLA (neither molecular
weight values nor accuracy of the thermal transitions values were reported in the
literature).39
Copolymer ratio Glass transition (°C) Melting temperature (°C)
100 / 0 (L/D,L)-PLA 63 178
95 / 5 (L/D,L)-PLA 59 164
90 / 10 (L/D,L)-PLA 56 150
85 / 15 (L/D,L)-PLA 56 140
12 Chapter 1: Literature Review
In addition to the tacticity, the glass transition is also sensitive to the molecular
weight.40 Fambri et al.41 reviewed the evolution of Tg and Tm as a function of number
average molecular weights ranging from 2 to 110 kDA for PLLA samples. As the
molecular weight increased, the values for the glass transitions evolved from about 25
to 60 °C, and for the melting temperatures from 130 to 180 °C. The differences were
attributed to limited chain mobility of the amorphous phase caused by the crystalline
regions (no correlation with an increase in crystallization was reported in the review).41
Polylactide is generally described as a semi-crystalline polymer. However, the
crystallinity is also influenced by the stereochemistry. Both PLLA and PDLA are
semi-crystalline polymers with a degree of crystallinity of about 37 %. The spherulite
radius is 100 - 1,000 µm for films obtained by solution-casting.42 PDLLA is
amorphous because of the irregularity of the structure.41 Such differences in
crystallinity between stereoisomers strongly affect the degradation kinetics, either the
oxidation or the hydrolysis rates. Indeed, both of which will preferentially occur in the
amorphous regions due to the limited permeation of oxygen or water diffusion,
respectively, in the crystalline regions (refer to section 1.3.1).
Three crystal forms (α, β and γ) were reported for PLLA depending on the
crystallization conditions (Table 1.3). The α form is the most common as it develops
from the melt or solution casting process, thus at industrial conditions.43, 44 The β and
γ forms require specific conditions to develop. The β form is produced during melt
solution or solution electrospinning whereas the γ form occurs through epitaxial
crystallization.45, 46
In addition to these three forms, a distorted crystal form was reported as α’.48 This
crystal form requires temperatures ranging from 90 to 120 °C to crystallize along with
the α form. Below 100 °C, only the α’ form crystallizes whereas from 100 to 120 °C,
both α and α’ forms crystallize and coexist.47, 49 At 150 °C, a solid-solid phase
transition α’-α occurs.49 The less ordered chain packing of the α’ crystal impacts the
mechanical properties of PLLA by decreasing the Young’s modulus and increasing
the elongation at break.50
Chapter 1: Literature Review 13
Table 1.3. Characteristics of the three crystal forms of PLLA.
Crystal
form
Crystal system Chain
conformation
Cell parameters References
α Pseudo-
orthorombic
103 helical a = 1.07 nm
b = 0.645
nm
c = 2.78 nm
α = 90 °
β = 90 °
γ = 90 °
43, 47
β Orthorombic 31 helical a = 1.031
nm
b = 1.821
nm
c = 0.90 nm
α = 90 °
β = 90 °
γ = 90 °
46, 47
γ Orthorombic 31 helical a = 0.995
nm
b = 0.625
nm
c = 0.88 nm
α = 90 °
β = 90 °
γ = 90 °
45, 47
1.2.4 Mechanical Properties
Polylactide is characterized by mechanical properties that enable its use in applications
where conventional polymers are usually chosen, for example in packaging. The
modulus of elasticity ranges from 3 to 4 GPa, the tensile strength from 50 to 70 MPa
and the elongation at break from 2 to 5 %.51, 52 In comparison, polystyrene and
poly(ethylene terephthalate) feature elastic moduli of 3.2 and 2.8 - 4.1 GPa and
elongations at break ranging between 3 and 300 %, respectively (no accuracy on these
values was reported).53 The mechanical properties depend on a number of intrinsic
properties of polylactide, such as stereochemistry, crystallinity and molecular weight.
Concerning the effect of stereochemistry on mechanical properties, semi-crystalline
PLLA presents a tensile strength ranging from 50 to 70 MPa; whereas, amorphous
PDLLA is characterized by values ranging from 40 to 53 MPa.54 In terms of
crystallinity, an annealed PLLA (molecular weight 20,000 Da) with a degree of
crystallinity of 70 % featured a modulus of elasticity of 4,100 MPa and a tensile
14 Chapter 1: Literature Review
strength of 47 MPa. A non-treated PLLA sample of similar molecular weight (23,000
Da) with a degree of crystallinity of 9 % presented a modulus of elasticity of 3,550
MPa and a tensile strength of 59 MPa (no accuracy on these values were reported).54
Regarding the influence of the molecular weight on the mechanical properties, a
general trend is observed to increase the values of the tensile properties (yield strength,
tensile strength, yield elongation, elongation at break and modulus of elasticity) with
increasing molecular weight. For instance, an increase from 50 to 100 kg·mol-1 for
PLLA doubled the values of tensile strength and modulus of elasticity. However, the
flexural strength was reported to reach a plateau for a molecular weight of 35,000
g·mol-1 for both amorphous PDLLA and PLLA, and 55,000 g·mol-1 for semi-
crystalline PLLA.54 Regarding very low molecular weights, the polymer loses its
strength and starts to embrittle when the molecular weight reaches a critical value, the
chain entanglement molecular weight (𝑀𝑒 ), with the polymer strength being inversely
proportional to the molecular weight. When the polymer strength reaches zero, the 𝑀𝑛
corresponds to 𝑀𝑒 .55 𝑀𝑒
was reported to be in the range of 8 to 10 kg·mol-1 for
polylactide.56 A PLLA with 98:2 L:D enantiomer content presented a value of 9 kg·mol-
1.57
A factor that was shown to have no influence on the mechanical properties was the
method used to produce polylactide. The mechanical properties of PLA samples of
similar molecular weight synthesised by the direct condensation polymerisation of
lactic acid and the ring-opening polymerisation of lactide did not differ.58
To summarize, semi-crystalline poly(L-lactide) features appropriate mechanical
properties for packaging applications compared to amorphous poly(D,L-lactide).
Molecular weight substantially impacts the mechanical properties. 𝑀𝑛 should exceed
the entanglement molecular weight (9 - 10 kg·mol-1) to produce films while lower
values result in the loss of polymer strength and embrittlement.
1.3 DEGRADATION OF POLYLACTIDE
Degradation is defined as “a deleterious change in the chemical structure, physical
properties or appearance of a polymer, which may result from chemical cleavage of
the macromolecules forming a polymeric item regardless of the mechanism of chain
cleavage” according to the Standards PD CEN/TR 155351:2006 and ASTM D883.59,60
Degradation involves different processes depending on the causal factors: thermal
Chapter 1: Literature Review 15
degradation, mechanochemical degradation, photodegradation, catalytic degradation,
or biodegradation.61 Three main degradation pathways will be reviewed here:
biodegradation, thermal degradation, and photodegradation. Biodegradation, a biotic
process, results in the disappearance of the polymer by fragmentation and assimilation
by micro-organisms. Whereas thermal degradation and photodegradation are both
abiotic processes. Thermal degradation can already occur at early stages of the
polymer life cycle during processing, resulting in negative consequences, such as the
loss of molecular weight and mechanical properties. Photodegradation occurs when
the polymer is exposed to UV irradiation at any point during its lifecycle.62 While
thermal degradation during processing and manufacture should be minimized to
broaden the application range, both photodegradation and biodegradation may be used
advantageously to avoid pollution by polymer residues. These three degradation
pathways will be described and studies undertaken to monitor their rates will be
discussed.
1.3.1 Biodegradation
Biodegradation is divided into three steps: deterioration, fragmentation and
assimilation. These steps overlap to gradually decrease the molecular weight, resulting
in the loss of mechanical properties and embrittlement of the polyester.63 Deterioration
and fragmentation steps rely on hydrolysis of the ester linkage of polylactide (Scheme
1.6).
Scheme 1.6. Hydrolysis of an ester linkage.
The hydrolytic mechanism strongly depends on the sample thickness, with the critical
thickness Lcritical, where the mechanism changes from a bulk erosion to a surface
erosion mechanism for a thickness greater than this value. Bulk erosion occurs when
the rate of water diffusion through the polymer is higher than the rate of hydrolysis.
On the contrary, the rate of hydrolysis exceeds the rate of water diffusion in a surface
erosion mechanism resulting in a decrease in thickness.55 For polylactide, a sample
thickness lower than 0.5 - 2 mm leads to a bulk erosion mechanism, while thicknesses
16 Chapter 1: Literature Review
greater than 7.4 cm result in a surface erosion mechanism. A core-accelerated erosion
mechanism proceeds for thicknesses between those values.64
Intrinsic properties of polylactide affect the hydrolysis rate, such as stereochemistry.
For instance, comparative studies of the hydrolytic degradation between amorphous
films of PLLA, PDLA and PDLLA of similar molecular weights in a phosphate-
buffered solution at 37 °C revealed a faster reduction in the molecular weight for
PDLLA than for PLLA and PDLA films. The faster hydrolysis rate was attributed to
the difference in tacticity with lower interaction between PDLLA chains than for
PLLA or PDLA.64 The presence of the D-enantiomer within a PLLA matrix also
impacted the hydrolysis rate by lowering the optical purity, thus modifying the chain
packing, and facilitating the water diffusion between the polymer chains.63 For
instance, an amorphous PLLA with low amounts of D-units (0.2 and 1.2 %) showed a
faster decrease in molecular weight compared to optically pure amorphous PLLA
when placed in a phosphate-buffered solution at 37 °C.65 The hydrolysis rate is also
impacted by the crystallinity. Pantani et al.66 investigated the rates of water diffusion
and the global biodegradation rate (in compost conditions) between amorphous and
crystalline PLLA samples (with 4 % of D-enantiomer). Hydrolysis of both samples led
to a decrease in Tg because of the plasticization effect of the water, diffusing and
enhancing chain mobility. Moreover, both samples embrittled, suggesting a loss in
molecular weight. The crystallinity did not affect the early stages of water diffusion,
but influenced the swelling of the samples after a few days and significantly decreased
the biodegradation rate. The majority of biodegradation studies of PLLA demonstrated
that the amorphous regions are predominantly hydrolysed, forming oligomers and
monomers.67
The hydrolysis of ester linkages induces a reduction in molecular weight with the
formation of oligomers, loss of mechanical properties, and subsequent fragmentation.
Once fragmented, PLA is assimilated by microorganisms through enzymatic chain
cleavage involving proteases (e.g. Amycolatopsis) or lipases (e.g. Bacillus siniithii).68
The microorganisms attack the chain ends of the polymers, the number of end groups
being inversely proportional to the molecular weight. This assimilation process leads
to the formation of biomass, carbon dioxide and water.62, 69 The stereochemistry of
PLA influences the assimilation process. Several studies used Proteinase K, an
enzyme isolated from a fungus, Engyodontium, which is stable over a pH range of 4 -
Chapter 1: Literature Review 17
12. Proteinase K proved to efficiently cleave LL-, DL- and LD- bonds but not DD-
bonds.70 The DD- bonds could be assimilated by another thermophile called Strain 73,
from Geobacillus stearothermophilus.68, 71
Biodegradation not only depends on intrinsic properties of polylactide, it is
considerably impacted by external factors. pH dictates the mechanism type. For
instance, hydrolysis occurs via a surface erosion mechanism in alkaline media or via a
bulk erosion mechanism in acidic media.72, 73 For a hydroxyl-terminated PLLA,
cleavage preferentially occurred at the first ester bond in acidic media whereas the
second ester bond was cleaved in alkaline media.74 In terms of kinetics, the hydroxide
ions in alkaline media catalyse the cleavage of the ester linkages.75 Temperature also
impacts the rate of hydrolytic degradation. For temperatures between the glass
transition and melting temperature, water diffusion is facilitated by enhanced chain
mobility. Temperatures above the melting temperature accelerate the rate of
homogeneous hydrolysis by melting the crystalline regions.67 As a result, the influence
of external factors (temperature, pH, relative humidity) results in different
biodegradation rates depending on the environmental conditions. PLA degrades within
three to four weeks in compost where the conditions are controlled (average
temperature 60 ºC, average relative humidity 65 %, pH 7.5).76, 77 However, in landfill
conditions in Thailand, the fragmentation of PLA started only after six months.78 Its
biodegradation rate in soil, when parameters like temperature, humidity, pH and the
nature of the microorganisms are not controlled, is slower due to its Tg being around
60 °C, which is higher than the temperature under these conditions. An increase in the
environmental temperature leads to an increase in the hydrolysis rate, decreasing the
molecular weight of the polymer. This leads to a higher mobility of the polymer chains
and therefore leads to a higher rate of PLA biodegradation.79 Ho et al.76 investigated
the degradation of PLA films in two sites with different temperatures and relative
humidities. The molecular weight loss was faster in the place with the higher
temperature and humidity, suggesting the prodegradant effects of those two
parameters. However, the microbial types should significantly explain these results as
well (no precision of microbial types was given).
1.3.2 Thermal Degradation
Polylactide is subject to thermal degradation at different stages of its lifecycle.
Thermolysis occurs at high temperatures when only a limited amount of oxygen is
18 Chapter 1: Literature Review
present, for example during processing, while thermo-oxidation takes place at longer
timescales, under oxygen, and at lower temperatures during storage, for example. This
review focusses on the degradation occurring at early stages of the lifecycle during
processing. As the melting temperature ranges from 170 to 180 ºC, manufacturers
recommend processing temperatures between 185 to 250 ºC.7
McNeill et al.80, 81 investigated the thermal degradation of PLA at temperatures
ranging from 230 to 440 °C. They reported the formation of oligomers as the main
products besides acetaldehyde, water, and carbon dioxide above 230 °C, or methyl
ketene above 320 °C. It was suggested that unzipping depolymerisation (a non-radical
backbiting ester interchange with the hydroxyl chain ends) occurred in the temperature
range of 230 - 440 °C. These transesterification reactions were further confirmed as
the dominant pathway for degradation at temperatures above 200 ºC (Scheme 1.7 (a)).
Other reactions were suggested, such as cis-elimination forming acrylic acid and
acyclic oligomers, radical reactions and depolymerisation catalysed by residual tin (II)
octanoate.82 Since the temperatures required for processing do not exceed 250 ºC, non-
radical transesterification reactions are considered to be the predominant reason for
the observed loss in molecular weight. This reduction is promoted by higher
temperatures (Scheme 1.7 (b)).83 The transesterification reactions were reported to be
catalysed by transition metals. Tin (II) octanoate, a catalyst for the ROP of PLA, also
catalyses PLA thermal degradation. An increase in tin (II) octanoate content was found
to lead to a decrease in the degradation temperature of PLLA, and the greater the
amount of tin (II) octanoate, the more selective the production of lactides.84 Other
transition metals, such as zinc, aluminium, iron, titanium and zirconium were found to
catalyse chain transfer, intra- and intermolecular transesterification, and
depolymerisation reactions at temperatures above 240 °C. Comparisons of their
efficiency as transesterification catalysts led to inconsistent conclusions in the
literature. A first order of efficiency was suggested: tin < zinc < aluminium < iron.85
In contrast, another order of efficiency was stated: tin > zinc > zirconium > titanium >
aluminium.86 This difference could be explained by the form in which these metals
were used or the range of their concentration. The latter order was based on the use of
the metals as alkoxides, organic acids and enolate salts while the first order stated their
use simply as metals, lacking a more precise indication of their form. Further studies
on the effects of processing parameters on the thermal degradation of PLLA at 210
Chapter 1: Literature Review 19
and 240 ºC revealed the strong impact of residual moisture and residence time in the
extruder. The molar mass loss was higher for longer residence times and residual
moisture levels, which provoked chain scission by hydrolysis of the ester linkages
(Scheme 1.7 (c)).87
Scheme 1.7. (a) Intramolecular transesterification resulting in the formation of
lactide, oligomers, acetaldehyde, and carbon monoxide; (b) intermolecular
transesterification; (c) hydrolysis.62, 82
20 Chapter 1: Literature Review
Thermal degradation of PLA follows first-order law kinetics with an apparent
activation energy ranging from 22 to 28.5 kcal·mol-1 in air.81, 88 However, recent
studies demonstrated better fitting second-order law kinetics with an apparent
activation energy around 25 kcal·mol-1.89
Racemization is a consequence of thermal degradation of PLLA. For instance, the
pyrolysis of PLA between 400 and 600 °C afforded racemization due to the production
of L-lactide.90 For lower temperatures (250 - 290 °C), PLLA samples sealed under
reduced pressure confirmed the depolymerisation process to afford L-lactide at first.
However, the increase in both temperature and degradation time resulted in the
formation of D- and meso-lactide (1 and 8 % after 5 hours to 10 and 33 % after 15
hours, respectively) and a decrease in L-lactide (from 91 % after 5 hours to 57 % after
15 hours).91
The loss in molecular weight during thermo-oxidation of PLLA at 70, 100, 130, and
150 ºC provoked a decrease in the glass transitions and the mechanical properties, such
as the strain at break. For instance, the strain at break was reduced from 20 % before
thermal ageing to about 0 % after 500 hours at 100 ºC, or 150 hours at 150 ºC,
respectively.92
To summarize, the temperature range required for PLA extrusion already induces
thermal degradation to a certain extent, where transesterification reactions are the main
pathway of the degradation. These reactions can be catalysed by transition metals such
as tin, aluminium or zinc.
1.3.3 Photodegradation
In some applications, polymers may be exposed to UV irradiation. While the product
should feature UV stability during its use, UV exposure could be used to accelerate
the degradation at the end of the useful life and result in a faster removal of the polymer
from the environment. Poly(L-lactide) features a carbonyl group C=O in its repeating
unit. This group absorbs UV irradiation at 280 nm via the n-π* transition with the
corresponding extinction coefficient ɛ at that wavelength of less than 100 L·mol-1·cm-
1.93 Although ɛ is low, inducing a relative stability towards UV, PLA does undergo
structural and mechanical changes upon UV irradiation. The UV spectrum is divided
into three domains: UV-C 100 - 280 nm totally absorbed by the Earth’s atmosphere;
Chapter 1: Literature Review 21
UV-B 280 - 315 nm partially blocked by the ozone layer; UV-A 315 - 400 nm, which
is the largest portion of natural UV light, not blocked by the ozone layer.94 Two main
photooxidation mechanisms have been discussed in the literature for polylactide,
depending on the type of UV light used.
Ikada et al.95-97 investigated the photodegradation of PLLA using light sources in the
UV-C domain. They observed an increase in the absorbance bands in the IR spectra at
3290 cm-1 and 990 cm-1, which were assigned to carboxylic acid and C=C double
bonds, respectively. An increase in the number of chain scissions as a function of
irradiation time was also observed based on gel permeation chromatography (GPC)
analysis. Photocleavage via a Norrish type II mechanism was then suggested to occur
at the carbonyl group via UV absorption at the ester linkage (n-π* transition) (Scheme
1.8). A comparative study of solvent-casted films of amorphous and melt-crystallized
PLLA irradiated with UV-C ( < 300 nm, 255 W·m-2) revealed that the
photodegradation proceeded as a bulk process in both, the amorphous and the
crystalline regions.98
Scheme 1.8. Norrish type II mechanism for the photodegradation of PLLA under
UV-C light.
Racemization converted L-lactic acid units into D-units as revealed by the loss of
optical purity during UV-C (254 nm) irradiation of PLLA films.99 The authors
suggested a racemization occurring at both hydroxyl and carboxyl chain ends, with
one D-lactate unit being formed for every chain scission (Scheme 1.9).
22 Chapter 1: Literature Review
Scheme 1.9. Proposed mechanism of racemization occurring both at the hydroxyl
chain end (left) and the carboxyl chain end (right) during the photodegradation of
PLLA exposed to UV-C light.99
Another photodegradation mechanism was proposed based on studies using UV-A
light, which is more relevant to natural outdoor conditions. Bocchini and coworkers
studied the photooxidation of PLA at wavelengths above 300 nm and followed the
structural changes using infrared spectroscopy.100 The appearance of a shoulder at
1845 cm-1 on the carbonyl band, assigned to anhydrides, was noticeable during ageing.
The photodegradation mechanism proposed included initiation by chromophoric
impurities in the polymer and hydrogen abstraction from the carbon in the α-position
to the carbonyl group on the polymer backbone as the main degradation pathway. The
formed macroradicals then reacted with oxygen to afford hydroperoxides. Subsequent
photolysis of those hydroperoxides was proposed to form anhydrides as the main
photodegradation product (Scheme 1.10). Gardette et al.101 confirmed this mechanism
and observed the occurrence of chain scission, resulting in a reduction in molecular
weight. However, the molecular weight decreased only slowly from 80,000 to about
70,000 g·mol-1 after 400 hours of UV irradiation (using a Sepap 12.24 unit, under
Chapter 1: Literature Review 23
normal atmosphere at 60 °C), revealing a certain stability of polylactide towards UV
light.
External factors, such as temperature and relative humidity, have been reported to
enhance the photooxidation of PLA when exposed to UV light (315 nm) by increasing
the rate of hydrolysis.102 The combined increase in temperature and relative humidity
(RH) resulted in faster molecular weight loss compared to films stored in the dark.
When exposed to UV light, at 100 % RH and 60 °C, the molar mass loss was faster,
with values reaching 4 % of the initial value after fifteen weeks compared to 15 %
when stored in the dark.102 Reduction in molecular weight and loss of mechanical
properties (stiffness and strength) was confirmed by other researchers when PLA was
exposed to UV light in the range of 295 - 400 nm.103
Biodegradation, thermal and photo-degradation occur at different stages of the life
cycle of PLA via different mechanisms, but all result in lowering the molecular weight.
Consequently, the mechanical properties are altered and embrittlement starts. The rates
of the different types of degradation also differ. Thermal degradation during
processing results in a loss of mechanical or thermal properties. This process needs to
be minimized to obtain PLLA that meets commercial mechanical property
requirements for each intended application. Both photodegradation and biodegradation
are beneficial pathways in terms of degrading the polymer at the end of its life.
Although the biodegradability is also influenced by intrinsic properties of PLLA
(stereochemistry, crystallinity), external factors, including temperature, humidity, and
soil composition play a critical role in the rate of degradation. Regarding
photodegradation, the reported loss of molecular weight and occurrence of chain
scissions happen rather slowly. Similarly to biodegradation, both intrinsic properties
of PLLA and external conditions impact the degradation rate. While external factors
can hardly be controlled in outdoor applications, poly(L-lactide) can be modified to
accelerate the degradation rate. The following section of this review will focus on
studies aiming at either increasing or minimizing the effects of the three degradation
pathways by modifying the structure of the polyester.
24 Chapter 1: Literature Review
Scheme 1.10. Photodegradation mechanism of PLA based on hydrogen abstraction
from the carbon in the α-position to the carbonyl group with formation of
macroradicals leading to anhydride as a main photodegradation product (X
represents chromophoric defects).
Chapter 1: Literature Review 25
1.4 TAILORING THE DEGRADATION OF POLY(L-LACTIDE)
Poly(L-lactide) degrades via three main processes whose rates can be challenging to
predict. For instance, PLLA degrades within three or four weeks in compost but the
degradation time can increase to up to six months under landfill conditions (refer to
section 1.3.1). However, the degradation behaviour can be tailored either by
modifying the polymer backbone via copolymerisation or by blending with suitable
polymers or additives.
1.4.1 Accelerating the Biodegradation Rate
Copolymerisation or blending with poly(ɛ-caprolactone) was extensively investigated
to enhance the biodegradation of poly(L-lactide). L- and D,L-lactide were
copolymerized with ɛ-caprolactone (40 / 60; 20 / 80) and hydrolysed in phosphate-
buffered solution at pH 7.0 at 23 and 37 °C. Kinetic studies based on the loss in
molecular weight revealed higher rate constants for the copolymers than for neat PLLA
and PCL, respectively. Higher temperatures and a greater D-unit content significantly
increased the rate constant.104 When hydrolysed in a phosphate-buffered solution at
pH 7.4 and at 37 °C, films of PLLA / PCL blends (50 / 50 and 75 / 25) revealed the
highest rate constants for hydrolysis compared to neat PLLA, as well as faster weight
loss, molecular weight reduction and reduction in tensile strength. This enhanced
hydrolytic degradation was attributed to the chain-end carboxylic group of poly(ɛ-
caprolactone).105 Moreover, biodegradation in soil was impacted by poly(ɛ-
caprolactone) segments, which were preferentially attacked by selected enzymes at pH
values ranging between 7.3 and 6.8 (total carbon content 5.7 %, study conducted in
Japan for twenty months).106
The hydrolysis of polylactide was also accelerated using additives as nanofillers. For
instance, hydrolysis of a PLA melt-blended with 3 wt% of an organically modified or
a non-modified Montmorillonite (Cloisite®), respectively, proceeded faster in the bulk
than neat PLA in a phosphate-buffered solution for five months. Such acceleration was
attributed both to their hydrophilicity and their morphology facilitating or not the water
diffusion (clay platelets or intercalated).107 Compost studies of melt-blended PLA
nanocomposite with similar Montmorillonite (Cloisite and Nanofil, 5 wt%)
demonstrated visual signs of degradation (surface modification, whitening) after three
weeks. The number average molecular weight decreased by 55, 79, and 41 % for neat
26 Chapter 1: Literature Review
PLA, PLA - Cloisite and PLA - Nanofil, respectively, after seventeen weeks. The
observed differences were suggested to arise from the heterogeneous dispersion of the
nanofillers in the PLLA matrix. The enhanced biodegradation in compost was
attributed to the presence of hydroxyl groups in the silicate layers.108 The dispersion
of the nanofillers influences the rate as the hydrolysis was reported to occur at the
interface between the PLA matrix and the nanofiller.109
1.4.2 Improving the Thermal Resistance and Mechanical Properties
As reviewed in section 1.3.2, thermal degradation proceeds through various reactions,
such as hydrolysis and back-biting transesterification. Moisture present during
processing results in random chain scission as a result of hydrolysis. The molecular
weight is reduced and, consequently, the mechanical properties of the processed
polymers are altered. NatureWorks recommends processing PLLA with a moisture
content less than 250 ppm to prevent viscosity degradation.24 Back-biting
transesterifications slowly reduce the molecular weight by depolymerisation and
random chain scission starting from the carboxyl and hydroxyl chain-end of
polylactide. End-group modification of polylactide has proved to successfully prevent
those reactions from occurring. For instance, acetylation of hydroxyl-end groups has
been performed on PLLA samples. Thermogravimetric analysis revealed that
acetylated PLLA presented a degradation profile starting at 360 °C instead of 260 °C
for neat PLLA. However, the acetylation of hydroxyl groups involved elimination of
residual traces of tin, which contributed even more to enhance the thermal stability.110
Modification of hydroxyl end-groups into cinnamate esters proved to be more efficient
than acetylation for improving the thermal stability of polylactide. Hydroxyl end-
groups of PLLA were reacted with cinnamoyl chloride to afford cinnamate esters as
end-groups. 10 % weight loss for the modified PLLA occurred at 320 °C instead of
240 °C for the unmodified samples. The modified PLLA still contained a quite high
amount of tin after treatment (600 ppm instead of 1000 ppm before treatment,
measured by Inductively Coupled Plasma Atomic Emission spectroscopy).111
On an industrial scale, additives are used to improve the thermal resistance of PLA
during processing. For instance, Dupont commercialized several additives (Biomax®)
which improve the thermal stability to 95 °C when mixed at 2 - 4 wt% concentration
range.112 Acrylic-based additives, commercialized by Arkema, enhanced the melt-
Chapter 1: Literature Review 27
strength of PLA during processing by forming networks via entanglement of acrylic
and PLA chains, impeding chain scissions.112
Polylactide is characterized by high values for tensile strength and tensile modulus,
but it is a glassy and brittle polymer with low tensile toughness and elongation at break
(refer to section 1.2.4). Toughening polylactide can be achieved by plasticization. This
method improves the processability by increasing the flexibility and the elongation at
break of polylactide. The glass transition is lowered as a result of the blending of the
plasticizer with the polylactide.53
Lactide is an extensively used plasticizing agent for polylactide, because of their
similar chemical structures.53 Citrate esters have also proved to increase the elongation
at break (610 % with 30 wt% of triethyl citrate compared to 7 % for neat PLA) and
reduce the glass transition temperature.113, 114 However, small molecules, such as
lactide, tend to migrate within the polymer matrix. For instance, lactide was reported
to migrate to the surface, then out of the matrix, leading to stiffening over time.53 To
overcome this issue, oligomers of higher molecular weight have been investigated as
plasticizers for polylactide. Oligomers of lactic acid with carboxyl or hydroxyl end-
groups (𝑀𝑛 of 1,179 and 1,050 Da, respectively) were employed as plasticizers to
increase the ductility of polylactide. A single glass transition measured by Differential
Scanning Calorimetry (DSC) proved the miscibility between both types of oligomers
and polylactide. The elongation at break increased from 5 % for neat PLA to 430 %
and 480 %, respectively, when blended with 20 wt% of each type of oligomer.115
Other oligomers, such as poly(ethylene glycol) (PEG), poly(propylene glycol) or
poly(diethylene adipate) efficiently plasticize polylactide.53, 116 However,
concentrations exceeding 20 wt% are often required, leading to phase separation and
alteration of the mechanical properties. To overcome these limitations, acrylated-
poly(ethylene glycol) was successfully grafted onto the PLLA backbone using radical
initiators via reactive blending, to improve the miscibility and avoid phase
separation.117 Another study involved the maleation of PLLA via reactive extrusion
using radical initiator, followed by the esterification between the anhydride functions
with hydroxyl-terminated PEG to improve the compatibility of the PLLA / PEG
blends.118 Reactive blending of PLLA and poly(butylene adipate-co-terephtalate)
28 Chapter 1: Literature Review
(PBAT) were successfully performed using transesterification reactions to synthesise
low amounts of PLLA - PBAT copolymer to increase interfacial adhesion between
PLLA and PBAT.119, 120
At an industrial scale, impact modifiers have been developed to improve the toughness
of polylactide. BlendexTM 338 additive (NatureWorks LLC), an acrylonitrile-
butadiene-styrene terpolymer, increased the notched Izod impact strength from 26.7
J·m-1 of notch to 518 J·m-1 of notch and the elongation at break from 10 % to 281 %
when blended at 20 % with polylactide.53 Dupont commercialized Biomax® Strong,
an ethylene-butyl acrylate copolymer, which aimed to improve the toughness without
altering the transparency of polylactide. The Spencer impact, which is a measure of
resistance to impact-puncture penetration in a film, increased from 1250 g·mm-1 to
3500 g·mm-1 for blends of poly(L-lactide) with 2 wt% of Biomax® Strong 100.53
1.4.3 Accelerating the Photodegradation Rate
A number of nanofillers have been used to reinforce the mechanical or thermal
properties of polylactide and thus to extend the application range. Their low cost and
ease of processability enable the production of composite films by melt-blending or
solvent-casting techniques. However, accelerated weathering studies of such films
revealed prodegradant effects of the nanofillers on the PLA matrix. For instance, melt-
blended films of PLLA with Montmorillonite, Sepiolite and fumed silica, 5 wt% of
fillers, were aged using a SEPAP ageing device (Atlas) under oxidative conditions (
> 300 nm, 60 °C, under air). Infrared (IR) spectroscopy of the irradiated samples
revealed the disappearance of the -CH2 stretching bands at 2922 and 2853 cm-1 of the
nanofillers without an induction period. Moreover, a linear increase in the absorbance
at 1845 cm-1, assigned to anhydride, was observed with irradiation time. The anhydride
formation was faster for the composites than for neat PLLA.100, 121 Therias and
coworkers investigated the photooxidation of extruded PLLA with a thermal stabilizer
(Ultranox 626A, 0.3 wt%) and ZnO nanofillers in a content range of 0 - 3 wt% using
a SEPAP ageing device (Atlas) ( > 300 nm, 60 °C, under air). The monitoring of the
films photodegradation by IR spectroscopy demonstrated similar results, with a faster
formation of anhydrides as a function of irradiation time for the composite films
compared to neat PLLA. Moreover, GPC measurements revealed a decrease in
molecular weight of about 20 % for neat PLLA, 30 % for PLLA 1 wt% with ZnO, and
Chapter 1: Literature Review 29
60 % for PLLA with 3 wt% ZnO, after 300 hours in the Sepap, suggesting the
occurrence of chain scissions.122 In another study, PLLA was melt-blended with
halloysite nanotubes in the range of 0 - 12 wt% and the resulting films were aged using
the same device and conditions as above. Similar results could be observed, with an
increase in anhydride formation for the modified PLLA.123 Gardette et al.101 melt-
compounded PLLA with a different nanocomposite filler, CaSO4 (content range 0 - 40
wt%), obtained films by compression moulding, and subsequently aged using the
above-mentioned ageing device under the same conditions. The IR monitoring
revealed the formation of anhydrides and faster kinetics for the modified PLLA than
for the neat one. GPC measurements confirmed a drop in the molecular weight due to
random chain scissions. All studies featured similar outcomes, that is a faster
anhydride formation with increasing concentration of fillers, resulting in random chain
scissions throughout the ageing process. The presence of transition metals as
chromophoric impurities in the different fillers were suggested to accelerate the
photodegradation without any induction period, by catalysing the hydroperoxide
decomposition from which anhydrides were formed. The type of nanofillers, their
concentration and degree of dispersion influenced the extent of anhydride
formation.100, 101, 121-123
The same effect could be shown with TiO2 as the nanofiller. Solvent-cast films of
PLLA with 0.5, 1, 2, 5 and 10 wt% TiO2 were artificially aged under UV-A light (365
nm) and their weight loss was recorded over time. A linear relationship between weight
loss and irradiation days was demonstrated, as well as a faster degradation with
increased concentration of TiO2. TiO2 induced photodegradation by generating active
oxygen species that subsequently attacked the polymer chains and resulted in chain
scission.124 However, this photocatalytic effect could be inhibited by the ability of TiO2
to block UV light at the surface of the films.125 Another key factor was the surface area
of the nanofiller compared to the PLLA matrix, as observed in a comparative
photodegradation study ( = 254 nm) of solvent-casted films of PLLA with either TiO2
nanoparticles or nanowires. The larger surface area of TiO2 nanowires favoured
recombination of electron-hole pairs in the bulk and reduced the probability of the
generation of reactive oxygen species.126
Photosensitizers were investigated as prodegradants for PLLA as well, such as
N,N,N’,N’-tetramethyl-1,4-phenylenediamine (TMPD). Sakai and coworkers aged
30 Chapter 1: Literature Review
solvent-casted films of PLLA - TMPD 0.4 wt% under UV-A light (356 nm) and
observed a decrease in molecular weight by GPC. The photoionization of TMPD
enabled the release of a free electron, leading to the formation of radicals and
subsequent chain scissions.127 Tsuji et al.128 compared the efficiency of TMPD on the
photodegradation of amorphous and crystallized PLLA films with an initial TMPD
concentration in the range of 0 - 1 wt%. GPC measurements of the irradiated films
(using UV-C) confirmed the reduction in molecular weight, with a larger decrease with
higher TMPD content. Both amorphous and crystallized PLLA films underwent chain
scissions, in contrast to hydrolytic degradation (refer to section 1.3.1).
1.5 TAILORING DEGRADABILITY
Bio- and photo-degradation are processes that can degrade polymer fragments during
and after the service life. Accelerating one of either biodegradation or
photodegradation will impact the overall degradation rate. Concerning biodegradation,
external conditions play a considerable role in the degradation of both neat and
modified poly(L-lactide). Thus, accelerating the biodegradation rate requires finding
the optimum external conditions to obtain the highest rate possible. However, the rate
of photodegradation could be substantially increased by adding prodegradants that
initiate the degradation and result in chain scissions. Therefore, the project introduced
in this thesis aims at accelerating the photodegradation rate of PLLA by modifying the
backbone via the addition of chromophores.
Similar strategies have been employed for polyolefins. Photodegradable polyolefins
are classified into two categories: photoinduced photodegradable and intrinsically
photodegradable. The first category includes polymers without chromophores on their
backbone, in contrast to the second category where polymers contain chromophores
within their structures.129
Photoinduced photodegradable polyolefins require prodegradants to catalyse their
degradation. Polypropylene (PP) or polyethylene (PE) are typical polyolefins lacking
chromophores on their backbones. Extensive research on understanding the effects of
antioxidants and light stabilizers on the formation of hydroperoxides led to the
development of the Scott-Gilead photo-biodegradation process. This process involved
formulations based on PP or PE containing a balance of anti- and pro-oxidants that
catalyse the fragmentation after a predictable induction time. Iron, manganese or
Chapter 1: Literature Review 31
nickel are typical transition metals composing the pro-oxidant salts, while cobalt or
zinc are part of the UV light stabilizer complexes.130-132 UV irradiation of Plastor films,
a commercial PE with iron dithiocarbamate, resulted in the combined formation of
carboxylic acids as products of hydroperoxide decomposition and embrittlement of the
film due to reduction of molecular weight.133 Not only do the transition-metal salts
accelerate photodegradation, the level of oxidation catalysed by the transition metals
also dictates the hydrophilic nature of the film that enables the microorganisms to
interact with the polymer matrix.134-136 These modified polymers are referred to as
oxobiodegradable.
Intrinsically photodegradable polymers feature chromophores in their backbone that
can interact with light and do not require additional prodegradants to degrade. Ketones
are among the most efficient chromophores characterized by an accessible n-π*
transition, relevant for photochemical studies in outdoor conditions (UV-A). They
undergo chemical change via various reactions, such as -cleavage, known as Norrish
type I and intramolecular elimination, referred to as Norrish type II. The Norrish type
I reaction yields free radicals whereas type II comprises an intramolecular
rearrangement and results in a methyl ketone and a C=C double bond as the
fragmentation products.137 Guillet et al.138 investigated the photodegradation of
ethylene-carbon monoxide copolymers and reported a Norrish type II reaction
mechanism at room temperature and a Norrish type I mechanism at elevated
temperatures (Scheme 1.11). Dupont patented a polyketone-type polymer based on
ethylene with carbon monoxide in the concentration range of 0.5 - 1.6 wt%.129 Shell
commercialized aliphatic polyketones named Carilon, alternating copolymers of
ethylene and carbon monoxide featuring small amounts of incorporated polypropylene
units.129 Photodegradation studies undertaken in outdoor conditions (in Italy) revealed
a degradation mechanism based on Norrish type I, involving the formation of radicals
that subsequently reacted with oxygen to form hydroperoxides. Decomposition of
hydroperoxides resulted in chain scission, and embrittlement of the irradiated films.139,
140 Guillet and coworkers further investigated ketones as chromophores for
poly(ethylene terephthalate) (PET) by copolymerising it with in-chain and side-chain
ketone-containing molecules and irradiating the copolymers under UV-A light. A
faster decrease in molecular weight was observed for the copolymers featuring the
ketone in the backbone compared to the side-chain ketone copolyesters, demonstrating
32 Chapter 1: Literature Review
the efficient prodegradant effect of the ketone on the photodegradation of the
polyester.141
Scheme 1.11. Photodegradation of ethylene-carbon monoxide copolymers via
Norrish type I and II.
Following the polyketone-type polymer strategy, Tian and coworkers investigated the
synthesis of novel aliphatic polyesters with photodegradable potential, specifically
poly(2-oxepane-1,5-dione) (POPD).142 POPD presents a similar structure to poly(-
caprolactone), but features a ketone to increase the photodegradation potential. To
achieve this goal, they initially synthesised poly(1,4,8-trioxaspiro[4.6]-9-undecanone)
(PTOSUO) and subsequently deprotected the ketone to yield POPD (Scheme 1.12).
The UV-visible spectra of POPD revealed a broad absorption band ranging from 230
to 300 nm due to the ketone whereas PCL was characterized by a narrow band around
240 nm assigned to the ester functional group. Latere et al.143 subsequently
demonstrated that OPD, when copolymerized with ɛ-caprolactone at 30 mol%,
accelerated the hydrolytic degradation compared to neat PCL due to higher
hydrophilicity (Scheme 1.13). However, in their work, no UV ageing studies were
undertaken to evaluate the prodegradant potential of OPD or POPD.
Chapter 1: Literature Review 33
Scheme 1.12. Synthesis of poly(2-oxepane-1,5-dione) via the ROP of TOSUO.
Conditions and reagents: a. Al(OiPr)3, toluene, 25 °C, H3O+; b. (C6H5)3CBF4,
dichloromethane, 25 °C, 1 hour.
Scheme 1.13. Synthesis of poly(ɛ-caprolactone-co-2-oxepane-1,5-dione). Conditions
and reagents: a. tin (II) octanoate, toluene, 90 °C.
34 Chapter 1: Literature Review
1.6 PROJECT PROPOSAL
Adding chromophores to the polymer structure effectively enhances the
photodegradation of a polymer by undergoing cleavage and release of radicals that
attack the polymer chains. This PhD project focusses on using ketones as efficient
chromophores to accelerate the photodegradation of PLLA. This form of PLA features
appropriate crystallinity and mechanical properties for film production and packaging
applications. Following the work of Tian and coworkers, 2-oxepane-1,5-dione (OPD)
was chosen as the chromophoric compound to modify poly(L-lactide). OPD not only
features a ketone in its structure, it also presents an ester linkage which can react with
PLLA via intermolecular transesterification in the melt, e.g. via reactive extrusion.
Alternatively, it can be incorporated by copolymerisation with L-lactide via a ring-
opening mechanism (refer to section 1.5). Therefore, the structure of OPD broadens
the possible strategies for modification of PLLA to potentially accelerate its
photodegradation. The thesis will be structured as follows.
Chapter 2 of this thesis presents an investigation into the prodegradant potential of 2-
oxepane-1,4-dione on the photodegradation of commercial grade poly(L-lactide). This
compound was blended with poly(L-lactide) in chloroform in various concentrations
(0 - 10 wt%). The high molecular weight of PLLA impacted its solubility and thus
limited the choice of solvent. Different films were obtained by solvent-casting and
subsequently artificially aged under UV-A light using a QUV accelerated weathering
tester (Q-lab, Ohio). The visual aspects, molecular weights, thermal properties and
chemical structures of the different films were evaluated to determine the effect of
OPD on the properties of PLLA. The films were characterized using GPC, DSC,
Attenuated Total Reflectance Fourier Transform Infrared (ATR-FTIR) spectroscopy,
and UV-visible spectroscopy. The morphology and embrittlement behaviour of the
films were visually monitored during the ageing process while the evolution of
molecular weight and thermal properties were monitored via GPC and DSC
measurements. The chemical modifications were monitored over time by ATR-FTIR
and UV-visible spectroscopies to gain insight into the prodegradant behaviour of OPD.
A photodegradation mechanism was derived from the obtained observations.
Following the investigation of the prodegradant potential of OPD when used as an
additive, Chapter 3 explores the potential of OPD as a photoprodegradant when
incorporated into the poly(L-lactide) backbone. First attempts focussed on melt-
Chapter 1: Literature Review 35
modification of commercially available PLLA based transesterification during
reactive extrusion. The OPD initial feed, the residence time and the transesterification
catalyst were investigated to achieve the incorporation of OPD into the polymer
backbone. The extrudates were characterized by 1H NMR, ATR-FTIR spectroscopies,
as well as GPC and DSC. The characterization data revealed unsuccessful
incorporation of OPD, while identifying products of thermo-oxidative degradation.
Chapter 4 presents a second strategy involving the incorporation of OPD into PLLA
using milder conditions than during reactive extrusions via copolymerisation of L-
lactide and OPD. Ring-opening polymerisations were performed using two sets of
conditions (Scheme 1.14).
Scheme 1.14. Ring-opening polymerisation of L-lactide and 2-oxepane-1,5-dione to
afford poly(L-lactide-co-2-oxepane-1,5-dione). Conditions and reagents: a. tin (II)
octanoate, 110-160 °C, in the bulk; b. DBU, benzyl alcohol, DCM, room
temperature.
Firstly, copolymerisations were performed in the bulk at 110 ºC using tin (II) octanoate
as the catalyst. The chemical structures of the resulting polymers were characterized
36 Chapter 1: Literature Review
by 1H NMR and ATR-FTIR spectroscopies, while their molecular weights and thermal
properties were assessed by GPC and DSC, respectively. Poly(L-lactide-co-OPD)
copolymers were successfully synthesised but only featured very low amounts of OPD
segments. Another set of conditions was then selected to increase the level of OPD
incorporation into the copolymer, in solution at room temperature using 1,8-
diazabicyclo[5.4.0]undec-7-ene (DBU) and benzyl alcohol, as the catalyst and the
initiator, respectively. In both cases, a competing action of the ketone moiety of OPD
with the ester groups of L-lactide and OPD towards the catalysts were observed,
accounting for the low level of OPD incorporation achieved.
Chapter 5 further explores the synthesis of poly(L-lactide-co-OPD) with increased
OPD concentration within the copolymer. To achieve such goal, a modified OPD
featuring an ethylene ketal protecting group was copolymerised with L-lactide, in the
bulk at 110 ºC using tin (II) octanoate and benzyl alcohol as the catalyst and the
initiator, respectively. The acetal protecting groups in the TOSUO segments were
subsequently removed to reveal the ketone of the OPD (Scheme 1.15).
Chapter 1: Literature Review 37
Scheme 1.15. Synthesis of poly(L-lactide-co-OPD) via two steps: a. Ring-opening
polymerisation of L-lactide and TOSUO to afford poly(L-lactide-co-TOSUO). b.
Deprotection of the acetal groups of poly(L-lactide-co-TOSUO) to afford poly(L-
lactide-co-OPD).
Copolymers were synthesised with variable incorporation of OPD ranging from 4.8 to
12.7 mol%. The success of the copolymerisation and the deprotection step were
assessed by 1H NMR and ATR-FTIR spectroscopies, while the molecular weights and
thermal properties were analysed by GPC and DSC. Photodegradation studies were
undertaken of two poly(L-lactide-co-OPD)s with different OPD concentration to
evaluate the prodegradant behaviour of the incorporated OPD.
Chapter 6 summarises the project and provides recommendations for future work.
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Chapter 2: 2-Oxepane-1,5-Dione: an Efficient Photosensitizer for Poly(l-lactide) 47
Chapter 2: 2-Oxepane-1,5-Dione: an Efficient
Photosensitizer for Poly(L-lactide)
2.1 BACKGROUND
Poly(L-lactide) is a biodegradable and biocompostable polyester with high mechanical
performance and low toxicity. The versatile properties of PLLA have attracted
significant research interest with broad applications including biomedical,
pharmaceutical and packaging.1-3 Although PLLA readily degrades under commercial
composting conditions (~ 60 °C), the biodegradation rate under less controlled
conditions remains slow because of variations in temperature, humidity, pH, types of
microorganisms. The combination of increased in applications of PLLA in conjunction
with its slow degradation rate may lead to the accumulation of plastic fragments in the
environment.4-6 To minimize such accumulation in the environment, it is desirable to
design new poly(L-lactide)-based polymers with accelerated degradation rates.
Photodegradation provides an attractive method to achieve such degradation.7 For
instance, previous studies on the photodegradation of neat PLLA have demonstrated
significant chain scissions leading to film embrittlement.8, 9
Enhancing the photodegradation rate of PLLA can be achieved by two methods: the
incorporation of chromophores into the polymer backbone, or blending the polymer
with photoinitiators.10 Previous studies on accelerating the photodegradation rate of
PLLA focused on using additives. Photosensitizers such as N,N,N,N-tetramethyl-1,4-
phenylenediamine have been extensively used and shown to enhance the
photodegradation rate of PLLA by producing radicals that subsequently attacked the
polymer backbone.11, 12 Inorganic additives, such as montmorillonite, sepiolite or
calcium sulfate have been successfully employed as nanoreinforcement materials and
were revealed to accelerate PLLA photodegradation by acting as chromophoric fillers,
which initiated the degradation.13-15 Although nanofillers were readily incorporated
into the PLLA matrix, their dispersion remained challenging. The possible formation
of aggregates could decrease the interfacial area between the nanofillers and PLLA,
altering the prodegradant potential.16, 17 Further treatment via surface modification of
48 Chapter 2: 2-Oxepane-1,5-Dione: an Efficient Photosensitizer for Poly(l-lactide)
the nanoparticles was investigated to overcome the dispersion issue, which involved
extra steps in the preparation of the materials.18, 19
To overcome those limitations, our approach here was to use a molecule that
dissociates to give free-radicals upon UV irradiation and could be either physically
mixed with or chemically incorporated into the PLLA backbone. Among
chromophores, ketones are of interest for photochemistry due to their n-π* transition
and have been extensively reported to initiate and propagate the photooxidation
processes when incorporated in polyolefins.20-23 Here, 2-oxepane-1,5-dione (OPD,
Figure 2.1), a lactone-type molecule with a ketone functional group, was chosen and
investigated as a photosensitizer for PLLA. Previous work using OPD has focused on
the versatility of the ketone moiety to add functionality to biodegradable polyesters
through copolymerisation to achieve controlled architectures for biomaterials
applications such as tissue engineering and drug delivery. The hydrophilicity of OPD
was found to accelerate the hydrolytic degradation of the resulting copolymers and the
reduction of the ketone to hydroxyl moieties facilitated the binding with biological
molecules such as peptides.24-30 In contrast, the strategy described in this Chapter is an
investigation of the photodegradability of the OPD controlled by reactions of the
ketone that accelerate the photodegradation of poly(L-lactide) film.
Figure 2.1. Structure of 2-oxepane-1,5-dione.
In the work undertaken here, PLLA and OPD were physically blended, with OPD
concentrations varying from 2 to 10 wt%. The obtained films were subsequently aged
in the laboratory using UV lamps that mimicked natural outdoor conditions. Visual,
chemical and physical changes were monitored using various spectroscopic
techniques, gel permeation chromatography and differential scanning calorimetry. The
OPD-containing films were compared to neat PLLA films to investigate the effect of
OPD on the rate and mechanism of the photodegradation of PLLA.
Chapter 2: 2-Oxepane-1,5-Dione: an Efficient Photosensitizer for Poly(l-lactide) 49
2.2 RESULTS AND DISCUSSION
2.2.1 2-Oxepane-1,5-Dione as a Photosensitizer
In the study undertaken here, OPD was synthesised following procedures previously
reported (Scheme 2.1).27, 29, 31 OPD was recovered as white crystals after
recrystallization from cyclohexane and ethyl acetate at 80 °C in 40 % yield in
agreement with the literature.
Scheme 2.1. Synthesis of 2-oxepane-1,5-dione. Conditions and reagents: 1,4-
cyclohexanedione, mCPBA, DCM, 40 °C, 4 h, 45 % yield.
The 1H NMR spectra matched the spectra reported in the literature, and so did the
melting point (110-113 °C for a reported range of 110-112 °C) (refer to appendices).31
The Attenuated Total Reflection Fourier Transform Infrared (ATR-FTIR) spectra
revealed a strong C=O stretching band at 1703 cm-1 with a shoulder at 1717 cm-1
supporting the presence of the ketone and the formation of the ester group (Figure
2.2). UV-Visible spectroscopic analysis of OPD revealed a broad absorption band with
a maximum at 273 nm due to the n-π* transition of the ketone (Figure 2.3). The
purified OPD was stored at 10 °C under inert atmosphere, in the dark, to avoid
degradation.
OPD was thus synthesised via a single step from commercially available reagents with
reproducible yields and purity. The ketone on the lactone ring conferred its
photodegradability potential (refer to section 2.1). Once fully characterized, OPD was
subsequently blended to PLLA and some films were obtained. They were
characterized to evaluate the influence of OPD on the polymer performance before
ageing.
50 Chapter 2: 2-Oxepane-1,5-Dione: an Efficient Photosensitizer for Poly(l-lactide)
Figure 2.2. ATR-FTIR average spectrum of 2-oxepane-1,5-dione (average of 9
spectra after baseline correction).
Chapter 2: 2-Oxepane-1,5-Dione: an Efficient Photosensitizer for Poly(l-lactide) 51
Figure 2.3. UV-Visible spectrum of OPD showing an absorbance maximum at 273
nm (measured in methanol at 1 mmol·L-1; a baseline spectrum was measured in
methanol).
2.2.2 Initial Characteristics of the Films of Poly(L-lactide) and 2-
Oxepane-1,5-Dione
2.2.2.1. Morphology of the Films
Blending PLLA with molecules or polymers represents a cost-effective processing
technique to successfully improve thermal and mechanical properties as well as to
accelerate the degradation rate.32-35 PLLA blend films have been commonly obtained
by solvent-casting, which involves the dissolution of the polymer in a solvent, the
casting of the solution onto a substrate and solvent evaporation.36
Following this procedure, poly(L-lactide) and 2-oxepane-1,5-dione were dissolved in
chloroform and cast into film. The choice of solvent matters as it can affect the chain
scission process by promoting hydrogen abstraction from the macromolecule chains.7
However, the high molecular weight of PLLA limited the choice of solvent and the
52 Chapter 2: 2-Oxepane-1,5-Dione: an Efficient Photosensitizer for Poly(l-lactide)
effect of solvent could be taken out of consideration because the films were obtained
using the same procedure for each OPD concentration used.
The OPD concentration was limited to the range 0 - 10 wt% in order to preserve the
homogeneity and mechanical performance of the resulting PLLA - OPD films. The
solutions were homogeneous, clear and transparent suggesting a complete dissolution
of both compounds. Films of PLLA - OPD blends were subsequently solvent-casted
and dried under vacuum to remove traces of solvent. The visual aspects of the different
films before degradation are shown in Figure 2.4.
Figure 2.4. Visual aspects of the films of PLLA - OPD blends before accelerated
ageing. a: 0 wt%; b: 2 wt%; c: 4 wt%; d: 6 wt%; e: 8 wt%; f: 10 wt% OPD.
The films looked homogeneous and transparent except for the formulation with 10
wt% of OPD. This film showed randomly dispersed white spherulites, possibly
attributed to OPD crystals, which will be further discussed in section 2.2.2.3. The
morphology differences were repeatedly observed for every batch of film produced.
The heterogeneity of the PLLA - OPD 10 wt% film confirmed the choice of 10 wt%
as the upper concentration limit.
The thicknesses of the films ranged from 41 to 46 μm. Film thickness has been reported
to influence the photodegradation process by enabling or limiting the oxygen diffusion
within the film.37 For instance, the photodegradation of LDPE films of thickness
greater than 200 μm was dependent on the rate of oxygen diffusion.38 In the work
undertaken here, the films were thin enough to ensure an homogeneous photooxidation
throughout the film thickness.39 No translation study to bulk materials was carried out
Chapter 2: 2-Oxepane-1,5-Dione: an Efficient Photosensitizer for Poly(l-lactide) 53
in this work (in this case, oxygen starvation could occur, impacting the degradation
process).
In order to further investigate the influence of OPD on the properties of the blends
before ageing, the molecular weights of the different films were analysed by GPC.
2.2.2.2. Initial Molecular Weight of the PLLA - OPD Blend Films
OPD was used as an additive with the aim of accelerating the photodegradation rate of
PLLA without altering its initial properties. The blends should still meet the property
requirements for packaging applications, such as mechanical stability. Such properties
are directly linked to the molecular weight of the polymer. Figure 2.5 shows the GPC
traces for PLLA - OPD films, measured in chloroform. The neat PLLA was
characterized by a broad unimodal distribution. The PLLA - OPD blends (2 - 10 wt%
OPD) presented similar distributions with no significant shift towards low molecular
weight. The average values of number, weight average molecular weights and
polydispersities for three batches of films before UV exposure are shown in Table 2.1
(values were calibrated against polystyrene narrow-molecular-weight-distribution
standards). The 𝑀𝑛 values for the six films ranged from 104,300 ± 10,900 Da to
125,900 ± 11,100 Da with polydispersities ranging from 1.5 to 2.5. No trend between
the OPD amount and the polydispersity could be observed.
Perego et al.40 measured the inherent viscosities in chloroform of PLLA and converted
the results into molecular weights with the Mark-Houwink equations in the following
form:
𝜂 = 5.45 × 10−4𝑀𝜈 0.73
They reported 55,000 g·mol-1 as the molecular weight for which mechanical properties
reached a plateau, with tensile strength of 58,000 MPa and modulus of elasticity of
3,750 MPa. As the PLLA used here was of commercial grade, the initial molecular
weights of the different films were unsurprisingly above this critical value suggesting
sufficient mechanical properties for film and packaging applications.
54 Chapter 2: 2-Oxepane-1,5-Dione: an Efficient Photosensitizer for Poly(l-lactide)
Figure 2.5. GPC traces of PLLA - OPD 0 - 10 wt% films before UV degradation,
measured in chloroform (the percentage values correspond to the concentration of
OPD in the blends).
Table 2.1. Average values of 𝑀𝑛 , 𝑀𝑤
and polydispersity of three batches of PLLA -
OPD blend films (OPD: 0 - 10 wt%) before accelerated ageing, measured by GPC in
chloroform.
OPD content (wt%) 𝑀𝑛 (Da) 𝑀𝑤
(Da) Ð
0 124,200 ± 5,600 237,100 ± 8,600 1.94 ± 0.04
2 108,800 ± 7,300 212,700 ± 2,400 2.46 ± 0.11
4 125,900 ± 11,100 217,400 ± 12,500 1.51 ± 0.15
6 104,600 ± 22,000 196,100 ± 13,400 2.33 ± 0.31
8 104,300 ± 10,900 196,000 ± 4,900 2.12 ± 0.15
Chapter 2: 2-Oxepane-1,5-Dione: an Efficient Photosensitizer for Poly(l-lactide) 55
10 102,200 ± 4,100 190,600 ± 1,400 1.99 ± 0.09
Once the molecular weights of the films were determined, the influence of OPD on the
thermal properties and degrees of crystallinity were subsequently investigated.
2.2.2.3. Initial Thermal Properties and Crystallinity
The initial thermal properties and degrees of crystallinity of the PLLA - OPD films
were determined using DSC on a second heating run to erase the thermal history of the
samples. Figure 2.6 shows the DSC thermograms.
Figure 2.6. DSC thermograms from the second heating cycle for PLLA - OPD blend
films before accelerated ageing (the percentage values correspond to the
concentration of OPD in the blends).
All films were characterized by a glass transition (Tg), a small exothermic peak
assigned to cold crystallization (Tcc) and a single endothermic peak corresponding to
a melting temperature (Tm). This unique melting peak suggested that OPD was
completely miscible in the PLLA matrix. The addition of OPD did not significantly
56 Chapter 2: 2-Oxepane-1,5-Dione: an Efficient Photosensitizer for Poly(l-lactide)
modify the melting peak. However, the glass transition shifted to slightly lower
temperatures with increasing OPD content.
Table 2.2 shows the thermal properties of the different films. The degree of
crystallinity was calculated according to the following equation:
𝜒𝑐(%) = ∆𝐻𝑚 + ∆𝐻𝑐𝑐
∆𝐻𝑚0 × 100
With ∆𝐻𝑚 the melting enthalpy, ∆𝐻𝑐𝑐 the cold crystallization enthalpy and ∆𝐻𝑚0 the
melting enthalpy of 100 % crystalline PLA sample (93.7 J·g-1).41, 42
Neat PLLA film displayed a glass transition at 59.8 ± 0.8 °C and a subsequent melting
peak at 149.1 ± 1.1 °C in agreement with literature.43, 44 A small endothermic peak at
127.1 °C preceded the melting peak. That peak corresponded to cold crystallization
and was attributed to a recrystallization of imperfect crystals into the α form.43 The
films with OPD displayed modified thermal properties. The glass transition gradually
shifted to lower temperatures with the addition of OPD. The PLLA - OPD 10 wt%
film displayed the lowest glass transition (49.4 ± 3.3 °C). The shift in Tg was possibly
due to a plasticization effect from the OPD on the PLLA matrix. The Tg values were
higher than the temperature used during the ageing in the QUV (50 °C) except for the
PLLA - OPD 10 wt% film. Ageing at temperatures below Tg limits oxygen diffusion
within the polymer matrix because the chain mobility is much lower than can be
observed in a rubbery state above Tg. The melting temperature proportionally
decreased with the concentration of OPD, from 148.6 ± 0.9 to 144.3 ± 3 °C with 2 and
10 wt% OPD, respectively. The cold crystallization temperature decreased, while the
degree of crystallinity increased with increasing OPD concentration. Such behaviours
highlighted that the presence of OPD accelerated the crystallization rates because of
its nucleation effect.
As discussed in section 2.2.2.1, the film containing 10 wt% OPD displayed evidence
of heterogeneity with both transparent and opaque sections.
Table 2.3 presents the thermal properties of those two sections (refer to appendices
for the DSC thermograms). The transparent section was characterized by a Tg of 55.6
°C, which is lower than neat PLLA and suggests the presence of OPD as a plasticizer.
The opaque section displayed much lower values of Tg and Tm than neat PLLA, as
well as a significant higher degree of crystallinity compared to any of the values
Chapter 2: 2-Oxepane-1,5-Dione: an Efficient Photosensitizer for Poly(l-lactide) 57
reported in Table 2.2. These results suggested that 10 wt% OPD enhanced the
crystallization in a heterogeneous manner during the solvent-casting process.
Table 2.2. Evolution of the glass transition and melting temperature of the films with
OPD content obtained by DSC before ageing (the measurements were performed on
three different batches of films and the values were averaged).
OPD
(wt%)
Tg
(°C)
Tcc
(°C) ∆𝐻𝑐𝑐 (J.g-1)
Tm
(°C) ∆𝐻𝑚 (J·g-1)
𝜒𝑐 (%)
0 59.8 ± 0.8 127.1 ± 1.5 4.69 ± 0.6 149.1 ± 1.1 3.50 ± 0.8 8.7
2 58.4 ± 1.9 126.6 ± 0.9 4.01 ± 1.7 148.6 ± 0.9 2.81 ± 1.2 5.9
4 54.6 ± 2.1 126.4 ± 2.4 4.09 ± 2.2 147.3 ± 1.9 3.87 ± 1.8 8.5
6 53.4 ± 3.3 125.0 ± 2.9 5.88 ± 5.6 146.4 ± 1.7 2.67 ± 1.3 12.7
8 50.5 ± 5.8 124.6 ± 4.3 6.41 ± 1.6 145.5 ± 2.3 5.84 ± 1.3 13.1
10 49.4 ± 3.3 121.6 ± 6.3 8.76 ± 4.7 144.3 ± 2.9 7.76 ± 4.2 17.6
Table 2.3. Comparison of the thermal properties of the transparent and opaque
sections of the PLLA - OPD 10 wt% film.
Tg
(°C)
Tcc
(°C) ∆𝐻𝑐𝑐 (J.g-1)
Tm
(°C) ∆𝐻𝑚 (J·g-1)
𝜒𝑐 (%)
Transparent section 55.9 - - 147.6 4.6 4.9
Opaque section 46.4 114.3 14.0 141.0 12.4 28.2
The chemical structures of the films were subsequently investigated by ATR-FTIR and
UV-visible spectroscopies to study their chemical structures and identify the presence
of OPD in the blends.
2.2.2.4. Chemical Characterization of the Films
ATR-FTIR spectroscopy was used as a non-destructive surface analysis technique to
further characterize the different films and evaluate their homogeneity.45 For PLA and
diamond as the ATR crystal, the depth penetration of the IR beam calculated from the
Harrick equation was reported to be 0.30 µm.15 Here, the film thicknesses ranged from
41 to 46 µm, confirming that information on the surface composition films was
produced by this ATR-FTIR analysis in this section.
58 Chapter 2: 2-Oxepane-1,5-Dione: an Efficient Photosensitizer for Poly(l-lactide)
Nine spectra were collected for each film on three different positions. Figure 2.7
shows the average spectra of the six films after being baseline corrected and
normalized by comparison with the band at 1455 cm-1 (due to -CH3 bending) to
suppress any effect from differences in contact with the ATR crystal and depth of
penetration of the IR beam. The spectra all revealed similar characteristic bands: the
C=O stretching band at 1747 cm-1, the –C-O- stretching bands at 1180 and 1085 cm-1,
and the –CH3 bending at 1455 cm-1.
Figure 2.7. ATR-FTIR average spectra of PLLA - OPD 0-10 wt% films before
degradation (average of 9 spectra per film after baseline correction and normalization
with the -CH3 bending band at 1455 cm-1).
The sharp band at 754 cm-1 was assigned to residual chloroform.46 This band was
consistently observed for all the film batches. The characteristic bands for neat PLLA
are given in Table 2.4 based on reported literature.13, 43 Every band obtained in the
spectra matched those of neat PLLA. The addition of OPD did not lead to any shift of
wavelength of the different bands. However, the carbonyl region (1800 - 1675 cm-1)
noticeably differed for the formulations containing OPD. The ester band of the PLLA
Chapter 2: 2-Oxepane-1,5-Dione: an Efficient Photosensitizer for Poly(l-lactide) 59
presented an additional shoulder from 1725 to 1690 cm-1 compared to neat PLLA
(Figure 2.8). This shoulder was assigned to the ketone of OPD.45
Figure 2.8. Carbonyl band in the ATR-FTIR spectra of the PLLA - OPD (0 - 10
wt%) blend films before ageing revealing the shoulder from 1725 to 1690 cm-1 due
to the OPD ketone moiety (average of 9 spectra per film after baseline correction and
normalization with the -CH3 bending band at 1455 cm-1).
Table 2.4. ATR-FTIR band assignment of poly(L-lactide) based on reported
literature.13, 47
Band position (cm-1) Assignment
2996 -CH- stretch (asymmetric)
2945 -CH- stretch (symmetric)
1747 -C=O carbonyl stretch
1455 -CH3 bend
60 Chapter 2: 2-Oxepane-1,5-Dione: an Efficient Photosensitizer for Poly(l-lactide)
Films were also characterized by UV-Visible spectroscopy before ageing. Figure 2.9
presents the spectra of the six film types.
Figure 2.9. UV-Visible spectra of PLLA - OPD films before accelerated ageing,
showing an increase in absorbance in the range 250 - 300 nm due to the n-π*
transition of the ketone moiety of OPD.
The spectrum of PLLA presented the lowest absorbance, which was expected
considering that the PLLA structure does not contains a UV chromophore. The
addition of OPD led to a proportional increase in absorbance in the range 250 - 300
nm. This increase was attributed to the n-π* transition of the OPD ketone. The increase
1384; 1359 -CH- deformation (symmetric and
asymmetric bend)
1182; 1085 -C-O- stretch
1043 -OH bend
926, 868 -C-C- stretch
Chapter 2: 2-Oxepane-1,5-Dione: an Efficient Photosensitizer for Poly(l-lactide) 61
in OPD content led to higher absorbance at higher wavelength, with the highest
absorbance reached with 10 wt% OPD. As discussed in section 2.2.2.1, the neat PLLA
film was transparent whereas the PLLA - OPD 10 wt% film presented some white
spherulites due to a heterogeneous crystallization. This difference in morphology led
to light scattering and thus increase in the baseline absorbance.
Spectroscopic methods enabled the characterization of the films before degradation
with accurate identification of OPD. OPD’s presence in the blends was characterized
by a shoulder at 1725 - 1690 cm-1 and an increase in absorbance between 250 - 300
nm in ATR-FTIR and UV-Visible spectroscopies, respectively. These regions were
monitored during ageing to assess chemical changes to OPD during artificial ageing.
2.2.3 Photodegradation of PLLA - OPD Blends
The reason for adding OPD to the PLLA matrix was to accelerate the photodegradation
of the polyester. A secondary goal was to tune the photodegradation rate through
modification of OPD concentration. Exposing the polymer directly under natural
outdoor exposure represents the most straightforward ageing approach. All external
factors such as fluctuation in the temperature, humidity and external stress are thus
taken into consideration. However, this type of experiment also includes a long
observation time range, up to several months or more. Artificially accelerated ageing
enables the assessment of polymer lifetimes within shorter study times. Devices such
as Sepap (Atlas) and QUV accelerated weathering testers (Q-lab, Ohio) are suitable
for mimicking natural outdoor exposure by controlling parameters such as UV light
intensity and wavelengths, temperature, humidity and time of each cycle.39 The
degradation times can then be correlated to the time taken to degrade during natural
outdoor exposure.
In the present work, films of PLLA - OPD blends (0 - 10 wt% OPD range) were
artificially aged using a QUV device by irradiating samples with UV-A light relevant
for natural outdoor exposure. 24 hours cycles were performed at 50 °C. Changes in
visual aspects, molecular weight, thermal properties and chemical structures were
monitored throughout the QUV ageing process.
2.2.3.1. Visual Changes of the Films
The visual aspect of the different films was observed after every cycle to evaluate the
deterioration for each formulation. The six films were transparent and homogeneous
62 Chapter 2: 2-Oxepane-1,5-Dione: an Efficient Photosensitizer for Poly(l-lactide)
before degradation, except for the film with 10 wt% OPD as previously discussed
(refer to section 2.2.2.1). As the degradation proceeded, whitening, opacity and
brittleness of the films significantly increased for films with 6, 8 and 10 wt% OPD
(Figure 2.10). Films with 4 to 10 wt% OPD embrittled in the time range 8 - 12 days.
After 27 days in the QUV, the films of neat PLLA and PLLA - OPD 2 wt% hadn’t
embrittled yet. These visual observations suggested that OPD had a prodegradant
effect during photodegradation, especially when the OPD content was greater than 4
wt%.
Figure 2.10. Effect of UV exposure on the films of PLLA - OPD (0-10 wt%) before
(top) and after 14 days (bottom) of UV exposure using a QUV device (UV-A 340
lamps, 50 °C) with whitening and embrittlement observed.
Changes in the sample opacity, colour (yellowing, whitening due to the disorganisation
of the matrix structure and to the enhance of the scattered light) or brittleness are visual
signs of degradation.48 Whitening, here, is caused by an increase in crystallinity as a
result of degradation firstly occurring in the amorphous phase of the polymer. New
crystal segments are formed, increasing the degree of crystallinity and thus the film
opacity.49-51
The prodegradant potential of OPD was investigated by assessing the molecular
weight changes throughout the photodegradation study.
2.2.3.2. Evolution of Molecular Weight
Aged film samples were characterized by GPC to investigate the evolution of the
molecular weights of the different formulations throughout the ageing process. Figure
2.11 presents the GPC distributions of each film before and after twelve days of UV
exposure in the QUV after baseline correction. The PLLA only sample showed a broad
unimodal distribution with a small shift towards lower molecular weight after ten days
in the QUV. The distributions of the PLLA - OPD (2 - 10 wt%) films presented more
Chapter 2: 2-Oxepane-1,5-Dione: an Efficient Photosensitizer for Poly(l-lactide) 63
significant shifts towards lower molecular weight within ten days of UV exposure. The
increase in OPD concentration led to greater shifts, suggesting a greater decrease in
the molecular weight.
Figure 2.11. Evolution of the GPC distributions of each film before (plain line) and
after ten irradiation days (dashed line) in the QUV, revealing a shift towards low
molecular weight for films containing OPD.
This observation was confirmed by calculating values of 𝑀𝑛 (Figure 2.12). The initial
values of the films presented a large variation, possibly due to an initial degradation
during the samples preparation due to OPD. The 𝑀𝑛 values of neat PLLA decreased
after four days in the QUV but a trend that could describe the decrease was not
observed. Ageing of PLLA under similar conditions (UV-A light, 60 °C) led a rapid
decrease in molecular weight after 100 hours of irradiation (18 % loss of molecular
weight) followed by slower rates for longer irradiation times.14
64 Chapter 2: 2-Oxepane-1,5-Dione: an Efficient Photosensitizer for Poly(l-lactide)
Each of the OPD-containing films showed a drastic decrease in 𝑀𝑛 after two days in
the QUV (approximately 50 %). The 𝑀𝑛 continued to slightly decrease until four days.
The values reach a plateau after that time in the QUV.
Figure 2.12. Decrease in the number average molecular weight of the aged PLLA -
OPD films (0 - 10 wt% OPD) versus irradiation days, as measured by GPC in
chloroform (data were obtained from three different batches of films and averaged).
Concerning the polydispersities of the six films, they remained within the range 2 - 2.5
independently of the UV exposure days (Figure 2.13).
Chapter 2: 2-Oxepane-1,5-Dione: an Efficient Photosensitizer for Poly(l-lactide) 65
Figure 2.13. Evolution of the polydispersities of the PLLA - OPD blend films during
artificial ageing.
A comparative kinetic study was performed based on the molecular weight data
obtained by GPC. The photodegradation of PLLA has been associated with both first
and second order kinetic laws defined by the following equations, respectively:
ln 𝑀𝑛(𝑡) = ln 𝑀𝑛(0)
− 𝑘𝑡 (1)
1
𝑀𝑛(𝑡)
= 1
𝑀𝑛(0)
+ 𝑘𝑡 (2)
With 𝑀𝑛(𝑜) the intial molecular weight, 𝑀𝑛(𝑡)
the molecular weight at an irradiation
time t and k the rate constant of photodegradation.9, 50 However, the first order kinetic
law was found using UV-C light while the second order kinetic law used UV-A light,
relevant for mimicking natural outdoor conditions.
Based on those findings, the inverse of the measured 𝑀𝑛 values were plotted against
irradiation time and demonstrated linear relationships (r2 ranging from 0.80 to 0.98)
(Figure 2.14, Table 2.5). The photodegradation rates significantly increased for the
66 Chapter 2: 2-Oxepane-1,5-Dione: an Efficient Photosensitizer for Poly(l-lactide)
films containing from 4 to 10 wt% OPD (Table 2.5). Those results suggested that the
presence of OPD accelerated the reduction of molecular weight at early stages of the
photodegradation process without showing an induction time.
The photodegradation of PLLA was previously studied under artificial and natural
conditions and followed by GPC.9, 15, 50, 51 Both types of ageing studies revealed
reductions in molecular weight and broadening of the polydispersities. In particular,
accelerated ageing under similar conditions as the present work (UV-A light, 60 °C)
revealed a rapid decrease in molecular weight after 100 hours of irradiation (18 % loss
of molecular weight) followed by slower rates for longer irradiation times.15 These
results suggested that chain scission occurs under these conditions at random locations
along the macromolecular chains.22, 52 In the work presented here, the combined
decrease in molecular weight and relatively constant polydispersity strongly suggests
similar process.8, 53
Figure 2.14. 1/𝑀𝑛 vs irradiation days of the PLLA - OPD films (0 - 10 wt%).
Chapter 2: 2-Oxepane-1,5-Dione: an Efficient Photosensitizer for Poly(l-lactide) 67
Table 2.5. r2 values and rate constants k determined from the 𝑀𝑛 measured by GPC
for the six formulation films.
Film formulation r2 k (Da·days-1)
PLLA 0.80 1.79.10-7 ± 6.66.10-8
PLLA - OPD 2 wt% 0.95 1.19.10-6 ± 1.90.10-7
PLLA - OPD 4 wt% 0.98 3.02.10-6 ± 3.32.10-7
PLLA - OPD 6 wt% 0.97 4.24.10-6 ± 5.03.10-7
PLLA - OPD 8 wt% 0.95 3.25.10-6 ± 5.25.10-7
PLLA - OPD 10 wt% 0.93 3.05.10-6 ± 6.05.10-7
To determine the number of chain scissions s, the following equation was used:
𝑠 =𝑀𝑛(0)
𝑀𝑛(𝑡)
− 1
With 𝑀𝑛(0) and 𝑀𝑛(𝑡)
the number average molecular weights before UV exposure and
after a time, t, in the QUV, respectively.54 As shown in Figure 2.15, the films presented
different behaviours. Neat PLLA only showed a small number of chain scissions after
four days in the QUV and this value remained small. The film with OPD also started
to show the occurrence of chain scissions after four days in the QUV. The films with
2 wt% OPD demonstrated the lowest values of chain scission compared to other
formulation film with OPD. The number of chain scissions increased for those films
approximately with irradiation time. However, a plateau seems to be reached after 4
days, which is in agreement with the 𝑀𝑛 values previously reported. The formulation
with 4 wt% OPD seemed to be the ideal amount as adding more OPD did not
significantly increase the number of chain scissions. This suggested that 4 wt% could
be the solubility limit of OPD in PLLA, having a real effect on the PLLA
photodegradation. Adding more OPD tended to separate phases and produce OPD rich
phases, which was in agreement with visual observations made in section 2.2.3.1.
These random chain scissions led to embrittlement. From the visual observations
previously discussed (refer to section 2.2.3.1), embrittlement of PLLA - OPD at 4 - 10
wt% OPD occurred from 8 - 10 days.
68 Chapter 2: 2-Oxepane-1,5-Dione: an Efficient Photosensitizer for Poly(l-lactide)
Figure 2.15. Evolution of the number of chain scissions of the aged films of PLLA -
OPD (0 - 10 wt%) in the QUV, calculated from the 𝑀𝑛 measured by GPC in
chloroform.
The molecular weight of a polymer directly influences its mechanical properties, with
critical values that define the existence of mechanical properties on one hand (e.g.
entanglement molecular weight), and the degradation extent on the other hand.
Rasselet et al.52 investigated the relationship between the molecular weight and
mechanical properties changes during oxidative degradation of PLA. They linked the
embrittlement behaviour to the 𝑀𝑛 and defined a critical molecular weight for
complete embrittlement at 40,000 g·mol-1. The strain at break reached a plateau value
of 1 % at this critical molecular weight. The 𝑀𝑛 of the formulations containing OPD
reached 40,000 g·mol-1 at different times and continued to decrease with increasing
time of exposure. The PLLA - OPD film containing 4 to 10 wt% OPD reached a 𝑀𝑛
of < 40,000 g·mol-1 after four days of UV exposure. The PLLA and PLLA with 2 wt%
OPD films did not reach 40,000 g·mol-1 after ten days of UV exposure. The neat PLLA
film presented a 𝑀𝑛 higher than 80,000 g·mol-1 after ten days in the QUV.
Chapter 2: 2-Oxepane-1,5-Dione: an Efficient Photosensitizer for Poly(l-lactide) 69
The GPC measurements suggested that photodegradation led to random chain
scissions with accelerated rates with increasing OPD concentration. In addition to
affecting mechanical properties, the molecular weight of the samples are directly
linked to the thermal properties.
2.2.3.3. Thermal Properties and Crystallinity Modifications
The thermal properties and degrees of crystallinity were determined by DSC
throughout the course of the ageing studies. Figure 2.16 presents the evolution of the
melting temperatures of the PLLA - OPD films during their ageing. The films
containing OPD revealed a decrease in the Tm with bigger shifts for the films with 6,
8 and 10 wt% OPD. The decrease may be attributed to the reduction in molecular
weight that led to structural changes of the crystalline regions. For instance, the
occurrence of chain scission led to lattice disorder in the crystalline regions.55
Figure 2.16. Evolution of the melting temperature of the PLLA - OPD films as a
function of irradiation time in the QUV.
Figure 2.17 shows the evolution of the glass transitions of the PLLA - OPD blends as
a function of irradiation time.
70 Chapter 2: 2-Oxepane-1,5-Dione: an Efficient Photosensitizer for Poly(l-lactide)
Figure 2.17. Evolution of the glass transition of the PLLA - OPD films as a function
of irradiation time in the QUV.
The ageing of neat PLLA did not significantly change the glass transition which only
slightly increased from 60 to 60.5 °C after ten days in the QUV. This behaviour was
reported by Tsuji et al.11 for a degraded PLLA with glass transition increasing from
59.9 to 65.4 °C after 400 hours of photodegradation. They attributed this increase to a
low-temperature annealing affect that stabilized the chain packing in the amorphous
phase of the PLLA. In contrast, the glass transition of the films containing OPD shifted
to lower temperatures. This decrease was more noticeable for the films containing 4 to
10 wt% OPD. These observations were consistent with the chain scission events
revealed by the GPC measurements, since the glass transition temperature and the
number average molecular weight are linked according to the Fox-Flory relationship
as follows:56
𝑇𝑔 = 𝑇𝑔,∞ −𝑘
𝑀𝑛
With k as the Flory-Fox constant, 𝑇𝑔,∞ the glass transition of polylactide having an
infinite molecular weight (reported value of 55 °C for PLA52) and 𝑀𝑛 is the number
average molecular weight. The reduction in 𝑀𝑛 thus provoked a decrease in Tg.
Chapter 2: 2-Oxepane-1,5-Dione: an Efficient Photosensitizer for Poly(l-lactide) 71
Moreover, the chain scissions likely released low molecular weight photoproducts,
such as oligomers. Oligomers can have a plasticization effect on the polymer matrix
leading to the lowering of Tg.47
2.2.3.4. Chemical Structural Modifications
Chemical changes resulting from the ageing process of the films were monitored by
ATR-FTIR spectroscopy. Figure 2.18, Figure 2.19 and Figure 2.20 show the spectra
of PLLA, PLLA - OPD 2 wt% and PLLA - OPD 10 wt% respectively, before and after
UV exposure (one and ten days) in the QUV (refer to appendices for the spectra of the
other formulations).
Figure 2.18. ATR-FTIR average spectra of PLLA film before and after one and ten
irradiation days (average of 9 spectra after baseline correction and normalization
with the -CH3 bending band at 1452 cm-1).
Every spectrum was normalized with the band at 1454 cm-1 (due to –CH3 bending) to
eradicate any effect from differences in contact with the ATR crystal and depth of
72 Chapter 2: 2-Oxepane-1,5-Dione: an Efficient Photosensitizer for Poly(l-lactide)
penetration of the IR beam. The main noticeable difference between aged and unaged
spectra was the decrease in the OPD shoulder at 1725 - 1690 cm-1 during ageing.
For both PLLA films containing 2 and 10 wt% OPD, the shoulder of the OPD ketone
started to disappear after one day in the QUV. This shoulder continued to noticeably
disappear throughout the ageing process in the QUV. This suggested that the OPD
ketone reacted to UV light at early stages of the irradiation process, without any
induction period.
Figure 2.19. ATR-FTIR average spectra of PLLA - OPD 2 wt% film before and
after one and ten irradiation days (average of 9 spectra after baseline correction and
normalization with the -CH3 bending band at 1452 cm-1).
Chapter 2: 2-Oxepane-1,5-Dione: an Efficient Photosensitizer for Poly(l-lactide) 73
Figure 2.20. ATR-FTIR average spectra of PLLA - OPD 10 wt% film before and
after one and ten irradiation days (average of 9 spectra after baseline correction and
normalization with the -CH3 bending band at 1452 cm-1).
UV-visible spectroscopy was also used to monitor changes during the
photodegradation of the films. The PLLA film revealed a very weak absorption in the
region 250 - 400 nm before degradation that slightly decreased after one day in the
QUV, but was not further affected with increased ageing time (Figure 2.21). The
PLLA - OPD 10 wt% revealed an increase in the absorbance that could be attributed
to the increase in whitening of the film throughout ageing (Figure 2.22) (refer to
appendices for the spectra of the other formulations).
74 Chapter 2: 2-Oxepane-1,5-Dione: an Efficient Photosensitizer for Poly(l-lactide)
Figure 2.21. UV-visible spectra from the PLLA only film as a function of irradiation
time in the QUV.
Chapter 2: 2-Oxepane-1,5-Dione: an Efficient Photosensitizer for Poly(l-lactide) 75
Figure 2.22. UV-visible spectra of the PLLA - OPD 10 wt% film as a function of
irradiation time in the QUV.
All the data obtained above were collected under accelerated photo-oxidative
conditions, involving UV-A light and heat (50 °C). To isolate the effects of UV, the
subsequent section of this work was undertaken to evaluate the influence of the
temperature on the photodegradation mechanism.
2.2.4 Influence of Temperature on the Degradation Behaviour of the
Blends
In photodegradation, pure photochemical effect and pure thermal effect are intimately
imbricated. In this section, the effect of pure thermo-oxidation was investigated with
the same temperature conditions in which photodegradation effectively induced the
degradation of the blends. Duplicates of each film were produced and simultaneously
aged in the QUV. For each formulation, one film was exposed to UV while the other
was covered to be protected from the light. Samples were collected every two days in
76 Chapter 2: 2-Oxepane-1,5-Dione: an Efficient Photosensitizer for Poly(l-lactide)
the QUV and characterized by GPC. Figure 2.23 and Figure 2.24 respectively show
a comparison of the evolution of the GPC distributions before and after ten irradiation
days in the QUV for the PLLA and PLLA - OPD 10 wt% films.
Figure 2.23. Comparison of the GPC traces of neat PLLA when covered (C) and
uncovered (U) before and after ten days in the QUV.
The neat PLLA did not present any significant shift towards low molecular weight
after ten days in the QUV for neither the uncovered nor covered film. These
observations were expected from previous experiments, with no significant decrease
in the molecular weight for the PLLA film detected as described earlier (refer to
section 2.2.3.2).
However, the PLLA - OPD 10 wt% films revealed interesting results. Only the
distribution of the uncovered, UV-exposed film shifted towards lower molecular
weight. The covered film (without UV exposure, only heat exposure) showed very
little change in distribution before and after ten days in the QUV, suggesting that no
chain scission occurred during the thermooxidation process (Figure 2.24).
Chapter 2: 2-Oxepane-1,5-Dione: an Efficient Photosensitizer for Poly(l-lactide) 77
The number average molecular weights and polydispersities of both uncovered and
covered films were compared as a function of irradiation days (Table 2.6). The
uncovered and covered films presented two opposite behaviours. Concerning the
uncovered films, the 𝑀𝑛 continuously and drastically decreased for the films
containing OPD, which is consistent with previously discussed results (refer to section
2.2.3.2). The covered films on the other hand presented a much lower degree of
decrease in 𝑀𝑛 . Only the neat PLLA film demonstrated similar behaviour for the
uncovered and covered films with values of 𝑀𝑛 remaining approximately unchanged
for both. The evolution of polydispersity values with the irradiation time were too
random to observe a particular trend.
These results firstly demonstrated that the absence of light prevented the decrease in
molecular weight for OPD containing samples. The temperature alone did not lead to
any significant degradation. However, the combined action of UV and temperature led
to reduction of molecular weight, confirming that the OPD acts as a photoprodegradant
under these ageing conditions. So, degradation of the OPD-containing films require
both UV and heat to proceed. Based on these findings and those previously reported,
a photodegradation mechanism is proposed in the following section.
78 Chapter 2: 2-Oxepane-1,5-Dione: an Efficient Photosensitizer for Poly(l-lactide)
Figure 2.24. Comparison of the GPC traces of PLLA - OPD 10 wt% when covered
(C) and uncovered (U) before and after ten days in the QUV.
Table 2.6. Comparison of the 𝑀𝑛 and the polydispersity of the uncovered and
covered films of PLLA - OPD blends (0 - 10 wt%) as a function of irradiation days.
Sample
𝑀𝑛 (g·mol-1) Ð
Days Covered Uncovered Covered Uncovered
PLLA 0 128,400 126,300 1.86 1.63
2 121,400 118,900 1.86 1.93
4 116,700 113,200 1.94 1.90
6 125,300 116,500 1.87 1.80
8 137,800 121,000 1.72 1.81
10 131,700 93,000 1.78 2.13
Chapter 2: 2-Oxepane-1,5-Dione: an Efficient Photosensitizer for Poly(l-lactide) 79
PLLA - OPD 2 wt% 0 117,100 105,900 1.84 2.01
2 113,100 101,200 1.86 1.91
4 111,700 62,300 1.93 2.34
6 119,000 71,600 1.77 1.84
8 124,600 54,600 1.71 2.15
10 104,900 45,700 1.82 2.16
PLLA - OPD 4 wt% 0 117,500 121,600 1.78 1.73
2 102,800 56,100 1.68 2.20
4 98,800 39,900 1.80 2.11
6 98,400 42,200 1.82 1.84
8 98,500 35,300 1.79 2.03
10 84,600 32,000 1.91 1.95
PLLA - OPD 6 wt% 0 100,000 124,300 1.96 1.69
2 106,900 68,200 1.80 1.78
4 85,400 38,500 1.99 2.16
6 95,300 42,300 1.78 1.69
8 93,100 29,300 1.81 1.99
10 83,900 14,200 1.85 2.96
PLLA - OPD 8 wt% 0 104,400 115,200 1.89 1.74
2 88,400 59,700 1.83 1.93
4 100,900 41,520 1.86 1.95
6 99,300 37,410 1.77 2.03
8 104,100 28,730 1.74 2.21
10 78,600 29,980 1.99 1.81
PLLA - OPD 10 wt% 0 103,600 105,400 1.83 1.80
2 108,000 75,700 1.64 1.66
80 Chapter 2: 2-Oxepane-1,5-Dione: an Efficient Photosensitizer for Poly(l-lactide)
4 91,300 40,400 1.93 2.17
6 93,500 39,300 1.99 1.96
8 92,800 35,600 1.93 1.96
10 91,800 31,300 1.82 2.03
2.2.5 Mechanism of Photodegradation
Six film formulations based on PLLA and OPD were artificially aged using conditions
that mimicked natural outdoor conditions. GPC analysis of PLLA without additives
revealed little change in molecular weight after ten irradiation days (refer to section
2.2.3.2). The polydispersity did not significantly change as well with values remaining
around 2 before and after irradiation. However, the presence of OPD in PLLA led to
more significant modifications. For all the films containing OPD, the molecular
weights decreased and the Ð reached values around 2 at early stages of the ageing
process. Those results supported random chain scission process occurring during
ageing. The chain scissions led to a decrease in mechanical properties and
embrittlement of the films (refer to section 2.2.3.1). Along with the reduction in
molecular weight, there was the fast disappearance of the OPD shown by ATR-FTIR
spectroscopy (refer to section 2.2.3.4). The results suggested that the OPD ketone was
lost due to chain cleavage at early stages of the photodegradation.
Different mechanisms have been discussed for the photodegradation of poly(L-
lactide). The first mechanism was based on main chain scission according to Norrish
type II photocleavage occurring at the carbonyl group in the ester linkage via a n-π*
transition.57 An alternative mechanism was proposed by Bocchini and coworkers,
confirmed by Gardette et al., based on studies using UV-A, relevant to natural outdoor
conditions.13, 14 This mechanism included initiation of degradation by impurities,
which form radicals upon UV exposure. The radicals formed subsequently abstract a
tertiary hydrogen from the PLLA chains, thus forming macroradicals. The
macroradicals subsequently react with oxygen to form peroxide radicals that propagate
the degradation by abstracting other hydrogens and forming hydroperoxides. The
photolysis of hydroperoxides leads to anhydrides as the most stable photo-products.13,
14
Chapter 2: 2-Oxepane-1,5-Dione: an Efficient Photosensitizer for Poly(l-lactide) 81
The physical blending of OPD with PLLA was attempted in order to promote the
photodegradation rate of the blends. Indeed, ketones have proved to efficiently
accelerate the photodegradation of polyolefins when used as either in-chain or side-
chain groups.20 Such ketones undergo chain scission via Norrish type I and II processes
(Scheme 2.2).
Scheme 2.2. Norrish type I and II cleavages of ketones.
The Norrish type I process is a photochemical α-cleavage of the ketone into two free
radical intermediates, including an acyl radical. Acyl radicals can eliminate carbon
monoxide to give alkyl radicals. The Norrish type II process involves an
intramolecular hydrogen abstraction from the carbon in the γ position relative to the
carbonyl group. This abstraction is followed by the cleavage of the α-β C-C bond,
82 Chapter 2: 2-Oxepane-1,5-Dione: an Efficient Photosensitizer for Poly(l-lactide)
resulting in the formation of an enol and a terminal C=C double bond. The enol
subsequently tautomerizes to the more stable ketone.22
In the work described here, the OPD ketone was expected to undergo photocleavage
via Norrish type reactions. The absence of a hydrogen on the carbon in the γ position
relative to the ketone reduced the likelihood of a contribution of a Norrish type II
mechanism to the photodegradation.22 Thus OPD cleavage via a Norrish Type I
mechanism should lead to the formation of intermediate radicals followed by the
elimination of carbon monoxide. The loss of the ketone was confirmed by ATR-FTIR
spectroscopy with the disappearance of the shoulder between 1725 and 1690 cm-1
(refer to section 2.2.3.4). The resulting alkyl radicals could subsequently attack the
PLLA chains by abstracting a tertiary hydrogen in the α-position relative to the ester
group. The rest of the mechanism should correspond to the one reported in the
literature, resulting in PLLA chain scission (Scheme 2.3). Those chain scissions were
observed for the PLLA - OPD blends based on the GPC measurements (refer to section
2.2.3.2). However, no photoproducts were identified in the present work, probably
because of the little ageing time range needed to embrittle the PLLA - OPD films.
Chapter 2: 2-Oxepane-1,5-Dione: an Efficient Photosensitizer for Poly(l-lactide) 83
84 Chapter 2: 2-Oxepane-1,5-Dione: an Efficient Photosensitizer for Poly(l-lactide)
Scheme 2.3. Proposed photodegradation mechanism for PLLA - OPD blends
initiated by the Norrish type I cleavage of OPD, releasing radicals that attack the
PLLA backbone leading to hydrogen abstraction. The rest of the mechanism is based
on previous reports, leading to PLLA chain scission and anhydride formation.13, 14
2.3 SUMMARY
OPD was investigated as an additive to enhance the photodegradation rate of poly(L-
lactide). Blends of PLLA and OPD were produced by film-casting with an OPD
content ranging from 0 to 10 wt%. The films were artificially aged in a QUV device
using UV-A at 50 °C. Films containing OPD in the range 4 - 10 wt% rapidly embrittled
compared to neat PLLA and PLLA - OPD films with 2 wt% OPD. From the chain
scissions calculated based on GPC measurements, 4 wt% seemed to be the solubility
limit of OPD in the PLLA-OPD formulations, making this value ideal to accelerate the
photodegradation of PLLA. This behaviour was attributed to the drastic decrease in
the molecular weight of the films at early stages of the ageing process. Spectroscopic
techniques revealed the disappearance of OPD at early stages of the photodegradation
process. A mechanism was proposed based on these observations. The
photodegradation was suggested to be initiated by the cleavage of OPD via a Norrish
type I mechanism, leading to the formation of initiating radicals. These radicals may
then attack the PLLA backbone ultimately leading to chain scission and embrittlement
of the films. As a conclusion, OPD was revealed to be an efficient photosensitizer for
PLLA. Further investigations as described in the following chapters were subsequently
undertaken to incorporate OPD into the PLLA backbone to evaluate the
photodegradation potential of the copolymers.
2.4 EXPERIMENTAL
2.4.1. Materials
Poly(L-lactide) 4043D grade was purchased from NatureWorks LLC and dried 4 h at
80 °C under vacuum in a dried oven prior to use. Sigmacote®, 1,4-cyclohexanedione
(98 %) and 3-chloroperbenzoic acid (≤ 77 %) were purchased from Sigma-Aldrich
and used as received. Anhydrous sodium sulfate, dichloromethane, diethyl ether,
cyclochexane and ethyl acetate (AR grades) were purchased from ChemSupply and
Chapter 2: 2-Oxepane-1,5-Dione: an Efficient Photosensitizer for Poly(l-lactide) 85
used as received. Chloroform (HPLC grade) was purchased from Merck and filtered
prior to use.
2.4.2. Methods
2.4.2.1. Synthesis of 2-Oxepane-1,5-Dione
m-Chloroperbenzoic acid (5 g, 29.0 mmol, 1.6 equiv.) was dissolved in DCM (40 mL)
and dried over anhydrous Na2SO4. The solution was filtered into a 100 mL round
bottom flask. 1,4-Cyclohexanedione (2 g, 17.8 mmol) was slowly added to the mCPBA
solution and the reaction mixture was stirred at 40 °C for 3 h. The resulting mixture
was cooled to room temperature while stirring for 15 h and then concentrated under
reduced pressure. The crude product was washed with Et2O, filtered, and recrystallized
from cyclohexane and EtOAc (5/3 vol/vol). OPD was recovered as white crystals in
40% yield. Mp. 111-113 °C (Lit., 110-112 °C31). 1H NMR (CDCl3, 600 MHz), δ ppm
= 4.42 (t, 2 H, CH2O), 2.85 (t, 2 H, CH2-COO), 2.65 (m, 4 H, CH2CO).13C NMR
(CDCl3, 600 MHz), δ ppm = 204.9 (CO), 173.3 (COO), 63.4 (CH2O), 44.8 (CH2CO),
38.7 (CH2CO), 28.0 (CH2COO). ATR-FTIR: ʋ max= 2969 (w, -CH2), 1740 (s, -C=O
of lactone), 1702 (s, -C=O of ketone), 1455, 1437 and 1392 (-CH2) cm-1. All
characterization data obtained were consistent with literature.31
2.4.2.2. Film Casting of Poly(L-lactide) and OPD
Poly(L-lactide) grade 4043D was purchased from NatureWorks LLC and used as
received. Poly(L-lactide) and OPD (0, 2, 4, 6, 8 and 10% wt.) were dissolved in
chloroform (10 mL). 1.5 mL of each solution was cast into a Petri dish (5 cm diameter),
previously coated with SigmaCote (Sigma-Aldrich). The dishes were covered to
ensure slow evaporation and avoid an orange-peel effect. Films were then vacuum
dried until a constant weight was reached, and stored under nitrogen at -15 °C in the
dark.
86 Chapter 2: 2-Oxepane-1,5-Dione: an Efficient Photosensitizer for Poly(l-lactide)
2.4.2.3. Accelerated Photo Ageing
PLLA - OPD (0-10 wt%) films were mounted onto 35 mm aluminium slide holders
and exposed to UV-A 340 lamps at an irradiance of 0.68 W/m2 at 340 nm in a QUV
accelerated weathering tester (Q-lab, Ohio). Water was present in the QUV in order to
maintain maximum and consistent levels of humidty for the degradation study. The
QUV was operated at a black panel temperature of 50 °C and cycles lasted for 24 h.
The irradiance sensors were calibrated every 500 hours.
2.4.2.4. Accelerated Thermal Ageing
Films of each formulation were mounted onto 35 mm aluminium slide holders and
covered with aluminium foil. The films were aged in the dark in the QUV using the
same conditions as shown above (refer to section 2.4.2.3).
2.4.3. Measurements
2.4.3.1. Films Thickness
The films thickness was measured on a Teclock Upright Stand Type US-16B. Three
measurements were performed on each film and averaged.
2.4.3.2. Gel Permeation Chromatography
Molecular weights of synthesised polymers were studied by gel permeation
chromatography with a Waters gel permeation chromatography system equipped with
a Waters 1515 isocratic HPLC pump, Waters 2707 autosampler with a 100 µL
injection loop, column heater (30oC) and a Waters 2487 dual wavelength absorbance
detector (analysis at 254 and 273 nm, corresponding to the absorbance of OPD) in
series with a Waters 2414 refractive index detector (analysis temperature, 30 oC) was
used for GPC analysis. Three consecutive Waters Styragel columns (HR5, HR4, and
HR1, all 7.8x300 mm, 5 µm particle size), preceded by a Waters Styragel guard
column (WAT054405, 4.6x30 mm, 20 µm particle size) were used during analysis.
Chloroform was used as the eluent at a flow rate of 1 mL·min-1. The molecular weight
separation ranges for the columns, relative to polystyrene are: HR5 – 50,000-4,000,000
Da; HR4 – 5,000-600,000 Da; HR1 - 100-5,000 Da; Guard column - 100-10,000 Da.
Samples were typically prepared at a concentration of 2.5 - 5 mg·mL-1 and
determination of molecular weight performed by calibration against polystyrene
narrow-molecular-weight-distribution standards.
Chapter 2: 2-Oxepane-1,5-Dione: an Efficient Photosensitizer for Poly(l-lactide) 87
2.4.3.3. Differential Scanning Calorimetry
The thermal properties of the samples were recorded by Differential Scanning
Calorimetry on a TA DSC Q100. Heat / cool / heat runs were performed on
temperature ranges that depended on the samples run (0 - 160 °C for OPD; 0 - 230 °C
for PLA-based polymers). Heating and cooling rates were set at 10 °C·min-1. The
transition temperatures reported were from the second heating cycle. The cycles were
performed on samples of 2 - 3 mg under nitrogen atmosphere, at a flow rate of 50
mL·min-1.
2.4.3.4. Fourier Transform Infrared Spectroscopy
Fourier Transform Infrared spectroscopy was performed on a Thermo Nicolet 5700
FTIR spectrometer, using OMNIC 7.2a software. 64 scans were run with a 4 cm-1
resolution. 9 spectra were collected on three different pieces of the samples, baseline-
corrected, normnalized and averaged. Spectral analysis was undertaken using OMNIC
7.2a and GRAMS software.
2.4.3.5. UV-visible Spectroscopy
UV-visible spectra of 2-oxepane-1,5-dione were collected on a Varian Cary 50 UV-
Visible spectrometer using quartz cells. OPD was dissolved in methanol (liquid
chromatography grade) at 1 mmol·L-1. A baseline spectrum was measured using
methanol (liquid chromatography grade) for solutions analysis and a baseline
correction was applied to the spectra. For the analysis of PLLA-OPD blends, films
were mounted into film holders. The baseline spectrum was measured out of air, and
a baseline correction was applied to the spectra. For all analysis, the range was 700 -
200 nm and a scan rate of 600 nm·min-1 was applied.
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23. B. G. Ranby and J. F. Rabek, Photodegradation, photo-oxidation, and
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28. E. L. Prime, J. J. Cooper‐White and G. G. Qiao, Macromolecular bioscience 7
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31. J.-P. Latere, P. Lecomte, P. Dubois and R. Jérôme, Macromolecules 35 (21),
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34. M. Harada, K. Iida, K. Okamoto, H. Hayashi and K. Hirano, Polymer
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and P. Dubois, Polymer International 63 (9), 1724-1731 (2014).
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90 Chapter 2: 2-Oxepane-1,5-Dione: an Efficient Photosensitizer for Poly(l-lactide)
40. G. Perego, G. D. Cella and C. Bastioli, Journal of Applied Polymer Science 59
(1), 37-43 (1996).
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Chapter 2: 2-Oxepane-1,5-Dione: an Efficient Photosensitizer for Poly(l-lactide) 91
Chapter 3: Reactive Extrusion of Poly(l-lactide) With 2-Oxepane-1,5-Dione 93
Chapter 3: Reactive Extrusion of Poly(L-
lactide) With 2-Oxepane-1,5-
Dione
3.1 BACKGROUND
In the preceding chapter (Chapter 2), the photodegradation of poly(L-lactide) was
accelerated via blending with a ketone-containing molecule, 2-oxepane-1,5-dione.
Films of PLLA with 4 - 10 wt% OPD embrittled after eight to twelve days of artificial
ageing, as opposed to neat PLLA that underwent little decrease in molecular weight.
Once the photosensitizing potential of OPD was assessed as an additive, the next step
was to investigate its prodegradant effect when incorporated onto the PLLA backbone.
The strategy to reach such goal was to perform in-melt modification of PLLA via
reactive extrusion. Reactive extrusion is a continuous, solvent-free and cost-effective
technique used to blend, polymerize or graft polymers in the melt. Modified polymers
are obtained within short times and can easily be recovered. However, the high
processing temperature required can result in degradation via chain scissions or
crosslinking.1
Thermal degradation of PLLA mainly proceeds through transesterification reactions,
resulting in reduction of molecular weight and broadening of polydispersity.2, 3 While
transesterifications are usually considered a drawback, they have great potential for
modifying the polymer backbone. For instance, transesterifications were used to
synthesise copolymers of polylactide with small molecules such as ricinoleic acid, -
caprolactone (ε-CL) in solution and in the bulk 4, 5, poly(ethylene-co-vinylalcohol)
(EVOH), catalysed by tin (II) octanoate and 1,5,7-triazabicyclo[4.4.0]dec-5-ene
(TBD) 6, or poly(butylene adipate-co-terephtalate) (PBAT), catalysed by titanium (IV)
butoxide.7 From the similarity between ε-CL and OPD, similar experiments could
possibly be conducted and successfully incorporate OPD into PLLA. If it cannot be
done as a monomer, transesterification of polymerized OPD and PLLA could be tried,
as done in the PBAT / PLA work.7
94 Chapter 3: Reactive Extrusion of Poly(l-lactide) With 2-Oxepane-1,5-Dione
This chapter explored transesterification reactions between a commercial grade PLLA
and OPD in reactive extrusion experiments. Firstly, POPD was investigated to
potentially be incorporated into PLLA in the melt. Due to handling difficulties of
POPD, reactive extrusion experiments focused on OPD as a monomer. Reactive
extrusions were performed by considering the effect of the OPD initial feed, the
residence time and the transesterification catalyst. The incorporation of OPD was
determined by ATR-FTIR and proton NMR spectroscopies, while the extent of
thermo-oxidative degradation due to processing was monitored by GPC and DSC.
3.2 RESULTS AND DISCUSSION
3.2.1 Melt-Modification of Poly(L-lactide) with 2-Oxepane-1,5-Dione
Prior to exploring the transesterification strategy, polymerisations of OPD were
conducted, with the aim to subsequently incorporate POPD into PLLA via reactive
extrusions.
3.2.1.1. Polymerisation of 2-Oxepane-1,5-Dione
2-Oxepane-1,5-dione was polymerised in toluene at 90 °C and in the bulk at 120 °C,
both catalysed by tin (II) octanoate. In solution, a yellow precipitate formed after 2
hours and was recovered by filtration. The precipitate was washed with methanol,
filtered, dried under vacuum and recovered as a white powder in 40.9 % yield. When
carried out in the bulk at 120 °C, an increase in viscosity of the mixture was observed
during the reaction. The reaction was thermally quenched after 5 h, followed by the
recovery of a yellow and viscous product in 26 % yield. The compounds were dried
under vacuum prior to characterization.
The products were insoluble in common organic solvents (including acetone,
methanol, chloroform and dimethyl sulfoxide), thus the recovery after the reaction was
difficult, lowering the yield. Moreover, the low solubility hindered the characterization
by solution NMR spectroscopy. The ATR-FTIR spectrum of a typical polymer sample
revealed both an intense absorption band at 1723 cm-1 assigned to the ester –O-C=O
bond stretch and a less intense vibrational band at 1179 cm-1 assigned to the ester bond
(Figure 3.1). The presence of the ketone moiety was evidenced by a strong band at
1700 cm-1. Thermal analysis of the product, measured by DSC, revealed a broad
Chapter 3: Reactive Extrusion of Poly(l-lactide) With 2-Oxepane-1,5-Dione 95
endothermic peak from 25 to 125 °C, from which no melting temperature could be
deduced, possibly due to different crystalline forms (refer to appendices).
Polymerisation of OPD was previously carried out in solution. Latere et al.8
polymerised OPD in dry toluene with 1-phenyl-2-propanol and tin (II) octanoate as
initiator and catalyst, respectively. POPD was obtained as a semicrystalline polymer,
featuring a glass transition of 37 °C and a melting temperature of 147 ºC. Tian and
coworkers reported the 1H NMR spectrum of POPD in trifluoroacetic anhydride /
deuterated chloroform (1 / 5 vol / vol).9 However, the insolubility of POPD and P(ε-
CL – OPD) copolymers was reported in common organic solvents, especially for
polymers with OPD contents higher than 50 mol%.8, 10
Figure 3.1. ATR-FTIR average spectrum of POPD revealing the ester and ketone
bands at 1723 and 1700 cm-1, respectively (nine spectra were collected, baseline-
corrected and averaged).
Although reported procedure for the synthesis of POPD exists, the insolubility of this
compound hindered the handling and the purification steps. OPD as a monomer, on
the contrary, was soluble in commonly used organic solvents, enabling complete
96 Chapter 3: Reactive Extrusion of Poly(l-lactide) With 2-Oxepane-1,5-Dione
characterization. With an ester functional group on its structure, transesterifications
could still be performed between the OPD monomer and PLLA. Therefore, further
modification experiments of PLLA were conducted using OPD as a monomer.
3.2.1.2. Incorporation of 2-Oxepane-1,5-Dione into Poly(L-Lactide) by
Reactive Extrusion
The incorporation of OPD into PLLA was attempted via reactive extrusion by
investigating the following factors: the OPD initial feed, the residence time and the
transesterification catalyst. For each factor, the extrudates were purified and
characterized by GPC and DSC to assess their molecular weights and their thermal
properties, respectively. The potential incorporation of OPD into PLLA was
investigated by ATR-FTIR and 1H NMR spectroscopies.
3.2.1.2.1. Influence of OPD Initial Feed
A series of reactive extrusions were carried out using a laboratory-scale Haake Minilab
extruder. A backflow channel enabled the recirculation of the extrudates, while
sampling and recovering of the material was performed through a die. In terms of
weight, the Minilab extruder allowed the extrusion of samples ranging from 3 to 6 g.
Samples of a constant weight of 4 g of PLLA were mixed with appropriate amounts of
OPD and tin (II) octanoate (Table 3.1).
Table 3.1. Formulations of the extrusions of PLLA and OPD 0 - 15 wt% using tin
(II) octanoate as the transesterification catalyst.
OPD feed
(wt%)
OPD
(wt%)
Sn(Oct)2
(wt%)
Recovered mass
(g)
PLLA - - 1.1716
0 - 1.85 1.1265
5 5.0 1.86 1.4958
10 10.2 1.86 1.8252
15 14.9 1.71 1.6450
Chapter 3: Reactive Extrusion of Poly(l-lactide) With 2-Oxepane-1,5-Dione 97
In order to remove residual moisture and minimize degradation via hydrolysis, OPD
was dried and stored under vacuum while poly(L-lactide) was dried under vacuum at
80 °C for 4 h prior to extrusion. The temperature of extrusion was optimized to ensure
the complete melting of compounds without favouring thermal degradation. The
minimum temperature that satisfied those conditions was 190 °C. The rotation speed
was set to 20 rpm during the feeding / melting step and to 100 rpm during the test to
efficiently mix the compounds. A residence time of 10 minutes was selected for each
extrusion. Dried PLLA without added OPD was extruded prior to each reactive
extrusion to clean the extruder. After 10 minutes, extrudates were flushed out and
purified by dissolution - reprecipitation using chloroform and methanol as solvent and
non-solvent, respectively. Polymers were recovered as white materials and dried under
vacuum prior to characterization. Only a limited amount of extrudates could be flushed
out of the extruder, explaining the low recovered masses reported (Table 3.1).
3.2.1.2.1.1. Torque and Relative Viscosity
The evolution of torque and apparent viscosity were recorded over time. The torque
values remained equal to zero during 10 minutes for every extrusion experiment. The
evolution of torque is linked to the changes in the melt viscosity of the extrudates and
is an indication of the success of grafting reactions during reactive extrusion For
instance, the extrusion of physical blends of polylactide and poly(ɛ-caprolactone)
revealed a continuous decrease in torque, while their compatibilized blends via
transesterification using triphenyl phosphite resulted in an increase in torque.11 A
similar increase in torque was observed in reactive extrusion of polylactide with
glycidol, a chain extender.12 The results obtained in this section did not allow for
monitoring the efficiency of the transesterifications between PLLA and OPD, as values
remained equal to zero throughout the extrusions, which could be explained by the
detection limit of the extruder.
On the contrary, the apparent viscosity values evolved throughout the extrusions. For
instance, the apparent viscosity of extruded PLLA continuously decreased over the 10
minutes, as illustrated by the averaged values of six different extrusions in Figure 3.2.
The PLLA - tin (II) octanoate formulation exhibited lower values of apparent
viscosities than neat PLLA, while the addition of OPD 5 - 15 wt% resulted in values
equal to zero during the extrusions (Figure 3.3). These experiments were performed
98 Chapter 3: Reactive Extrusion of Poly(l-lactide) With 2-Oxepane-1,5-Dione
solely once, but based on the error calculated from the extrusions of neat PLLA, the
values obtained here could be close to each other. Moreover, they were so low that
they were below the signal-to-noise limit.
Polylactide grades of 𝑀𝑤 ~ 100,000 g·mol-1 feature melt viscosities in the range 5 -
10 kPa·s at shear rates of 600 - 3,000 rpm.13 However, a few factors impact the melt
viscosity, such as the initial 𝑀𝑤 and the shear rate or the type of processing.14 The
decrease in apparent viscosity of PLLA was previously reported to be emphasized by
higher residence times and temperature.15, 16 However, even short residence time (5
minutes) at 180 ºC was enough to reduce the melt viscosity.16
Figure 3.2. Evolution of apparent viscosity during the extrusion of neat PLLA at 190
°C for 10 minutes (six extrusions were performed and values of apparent viscosity
were averaged).
Chapter 3: Reactive Extrusion of Poly(l-lactide) With 2-Oxepane-1,5-Dione 99
Figure 3.3. Evolution of apparent viscosity during the extrusion of pure PLLA,
PLLA with tin (II) octanoate and PLLA - OPD with tin (II) octanoate formulations at
190 °C for 10 minutes (the percentages account for the OPD initial feed; 0 wt%
corresponds to the PLLA – tin (II) octanoate formulation without OPD; the
formulations with 5; 10 and 15 wt% of OPD initial feed gave values of apparent
viscosity equal to 0).
The continuous decrease observed during the extrusion of PLLA alone suggested that
chain scission occurred. Indeed, the apparent viscosity is linked to molecular weight,
and hence chain scission events, according to the following relationships:
𝜂 = 𝑘𝑀𝑤 3.4
𝑠 =𝑀𝑤0
𝑀𝑤
− 1
With 𝜂 the apparent viscosity, 𝑠 the number of chain scissions, 𝑀𝑤0 the initial weight
average molecular weight and 𝑀𝑤 the weight average molecular weight.17 During
processing, the polymer is subjected to shearing forces, high temperatures and residual
traces of oxygen or water. These conditions favour mechanical, thermal, or oxidative
100 Chapter 3: Reactive Extrusion of Poly(l-lactide) With 2-Oxepane-1,5-Dione
degradation as well as hydrolysis of ester moieties. As a result, the relative viscosity
is lowered due to scission of polymer chains.18, 19 The risk of hydrolysis can be
minimized by drying the materials. For instance, NatureWorks recommends
processing PLLA with a moisture content less than 250 ppm to prevent viscosity
degradation.20 Drying of PLA at 70 °C for 4 hours under vacuum has been shown to
reduce the moisture content to less than 190 ppm, as determined by Karl Fisher
titration.21
3.2.1.2.1.2. Analysis of Extrudates by Spectroscopic Techniques
After reprecipitation, purified extrudates were analysed by spectroscopic techniques
to investigate the potential incorporation of OPD into the poly(L-lactide) backbone.
ATR-FTIR average spectra of extrudates after one purification step are shown in
Figure 3.4. All spectra were baseline corrected and normalized to the band at 1455
cm-1 (due to -CH3 bending) to suppress any effect from differences in contact with the
ATR crystal and depth of penetration of the IR beam. The spectra revealed the
characteristic bands of PLLA: the C=O stretching band at 1756 cm-1, the –C-O-
stretching bands at 1183 and 1088 cm-1, and the –CH3 bending at 1455 cm-1.14 The
carbonyl band (within the region 1800 - 1675 cm-1) displayed an extra shoulder around
1715 cm-1, that could potentially correspond to the ketone moity of OPD (characterized
by a shoulder in the range 1725 - 1690 cm-1, refer to Chapter 2).22 However, the
spectra of PLLA-tin (II) octanoate also featured that extra shoulder on the carbonyl
band. This shoulder could potentially be assigned to carboxylic acids, as thermo-
oxidative degradation products. To further investigate whether OPD was incorporated
into the backbone of the PLLA, or simply blended with the PLLA, more purification
steps were performed and the evolution of this shoulder was monitored by ATR-FTIR
spectroscopy. After three purification steps, the shoulder seemed to show a relative
increase (Figure 3.5). The broadening of the carbonyl band of PLLA during thermo-
oxidative degradation was previously reported at 150 ºC and attributed to new carbonyl
compounds.19
Chapter 3: Reactive Extrusion of Poly(l-lactide) With 2-Oxepane-1,5-Dione 101
Figure 3.4. ATR-FTIR spectra of extrudates of PLLA - OPD - tin (II) octanoate at
190 °C for 10 minutes after one purification step (the weight percentages represent
the OPD initial feed; 9 spectra were measured per film, baseline corrected and
normalized to the -CH3 bending band at 1455 cm-1).
102 Chapter 3: Reactive Extrusion of Poly(l-lactide) With 2-Oxepane-1,5-Dione
Figure 3.5. Evolution of the ketone stretching shoulder at 1717 cm-1 in the ATR-
FTIR spectra of extrudates of PLLA - OPD 15 wt% - tin (II) octanoate at 190 °C for
10 minutes with the number of purification steps (average of 9 spectra per film after
baseline correction and normalization to the -CH3 bending band at 1454 cm-1).
To further investigate the potential incorporation of OPD into the PLLA, the purified
products were examined using 1H NMR spectroscopy. As an example for all
compositions, the formulation of PLLA and 10 wt% OPD will be discussed. The
spectrum of the crude extrudate featured a quartet at 5.16 ppm and a doublet at 1.58
ppm assigned to the methine protons (-CH) and the methyl group protons (-CH3) of
poly(L-lactide).23 A multiplet at 2.75 ppm and a quartet at 4.37 ppm were observed as
well (Figure 3.6).
Chapter 3: Reactive Extrusion of Poly(l-lactide) With 2-Oxepane-1,5-Dione 103
Figure 3.6. Evolution of the 1H NMR spectra of the extrudates resulting from the
extrusion of PLLA with OPD 15 wt% at 190 ºC for 10 minutes (top: crude extrudate;
middle: extrudate after two dissolution-reprecipitation steps; bottom: extrudate after
three dissolution-reprecipitation steps), measured in CDCl3.
Ring-opened OPD is characterized in 1H NMR spectroscopy by a multiplet at 2.6 ppm
from the protons adjacent to the carbonyl moiety (-COCH2), a doublet at 2.7 - 2.8 ppm
assigned to the protons adjacent to the ketone (-CH2COCH2) and a quartet at 4.3 - 4.4
ppm from protons next to the oxygen of the ester moiety (-O-CH2-).10 Therefore, the
peaks observed at 2.75 and 4.37 ppm could be assigned to ring-opened OPD. However,
the spectra of the products from different purification steps revealed the disappearance
of those peaks, to solely display the quartet at 5.16 ppm and the doublet at 1.58 ppm
of PLLA. The 1H NMR spectra of other formulations revealed the same results. The
104 Chapter 3: Reactive Extrusion of Poly(l-lactide) With 2-Oxepane-1,5-Dione
OPD could either not have reacted via transesterification reactions with PLLA, or only
in very low concentration that could not be detected by NMR spectroscopy.
3.2.1.2.1.3. Molecular Weights of Extrudates
Three-times-purified extrudates were analysed by GPC in chloroform to investigate
the extent of degradation during the extrusions. The GPC traces of the extrudates are
shown in Figure 3.7. After extrusion, PLLA alone featured a broad unimodal
distribution. The extrudates of the other formulations displayed broad unimodal
distributions with a shift towards low molecular weight, suggesting a decrease in the
molecular weight.
Figure 3.7. GPC traces of purified extrudates of PLLA and OPD 0 - 15 wt% at 190
°C for 10 minutes measured in chloroform (the traces were baseline-corrected and
normalized).
This observation was confirmed by the measured values of 𝑀𝑛 (Table 3.2). The
extruded PLLA displayed a 𝑀𝑛 of 38,800 ± 1,000 g∙mol-1 and a polydispersity of 1.80
Chapter 3: Reactive Extrusion of Poly(l-lactide) With 2-Oxepane-1,5-Dione 105
± 0.05. The extrusion of PLLA with tin (II) octanoate at 190 °C for 10 minutes resulted
in a decrease in 𝑀𝑛 and a simultaneous increase in polydispersity from 1.80 ± 0.05 to
2.57 ± 0.23. The decrease in 𝑀𝑛 was greater when OPD was added to the PLLA with
tin (II) octanoate. The number of chain scissions increased with the initial OPD feed,
which is in agreement with the constant decrease in the relative viscosity described
earlier (refer to section 3.2.1.2.1.1).
Table 3.2. Molecular weight of three-times-purified extrudates of PLLA with OPD
(processed via reactive extrusion for 10 minutes at 190 °C, measured by GPC in
chloroform (three measurements were performed and values were averaged; the
percentages account for the OPD initial feed; 0 wt% corresponds to the PLLA – tin
(II) octanoate formulation without OPD).
Compound 𝑀𝑛 (g·mol-1) 𝑀𝑤
(g·mol-1) Ð Chain
scissions
Virgin PLLA 107,300 ± 4,900 209,300 ± 800 1.96 ± 0.09 -
PLLA 38,800 ± 1,000 69,900 ± 600 1.80 ± 0.05 2
0 9,500 ± 900 24,100 ± 100 2.57 ± 0.23 10
5 5,500 ± 100 12,100 ± 100 2.20 ± 0.01 19
10 3,100 ± 100 6,400 ± 0 2.12 ± 0.01 34
15 5,500 ± 100 11,400 ± 100 2.07 ± 0.04 19
3.2.1.2.1.4. Thermal Properties of Extrudates
The thermal properties and degrees of crystallinity of the three-times-purified
extrudates were determined using DSC with a heat / cool / heat cycle. When cooled
down from the melt at 10 ºC·min-1, the extrudates only exhibited vitrification due to
Tg and no crystallization (refer to appendices for the thermograms). The absence of
crystallization is usually observed for commercial PLLAs, whose crystallization rates
are slower than the cooling rate used in this work.24 After a second heating step (where
the thermal history of the samples had been previously erased), all extrudates were
characterized by a glass transition (Tg), an exothermic peak assigned to cold
106 Chapter 3: Reactive Extrusion of Poly(l-lactide) With 2-Oxepane-1,5-Dione
crystallization (Tcc) and an endothermic peak corresponding to melting temperature
(Tm) (Figure 3.8).
The glass transition values of extruded formulations containing PLLA, OPD and tin
(II) octanoate shifted towards lower values compared to extruded PLLA (Table 3.3).
The differences in glass transition values were expected based on the GPC
measurements since Tg is linked to 𝑀𝑛 according to the Flory-Fox relationship:
𝑇𝑔 = 𝑇𝑔,∞ −𝑘
𝑀𝑛
Where k is the Flory-Fox constant, 𝑇𝑔,∞ the glass transition of polylactide having an
infinite molecular weight (reported value of 55 °C for PLA19) and 𝑀𝑛 is the number
average molecular weight.25 The thermo-oxidative degradation caused a reduction in
molecular weight due to chain scissions, thus inducing a drop in glass transition
temperature. Moreover, much lower values of 𝑀𝑛 were observed for extrudates of
PLLA - OPD with tin (II) octanoate compared to neat PLLA, accounting for the
difference in Tg values.
Regarding the melting peak, extruded PLLA featured a small exothermic peak before
the melting transition. This small exothermic peak was reported to arise from the
transition of the disorded α’ and to the ordered α phase (refer to Chapter 1).26 Products
from extrusion of PLLA - OPD (0 - 15 wt%) with tin (II) octanoate shifted to lower
values compared to the starting material and displayed a double melting peak.
Regarding the shift to lower values, processing induced thermo-mechanical
degradation, yielding chain scissions. The enhanced mobility due to shorter
macromolecular chains accounted for the decrease in melting temperatures.27 The
double melting behaviour was previously reported for polylactide and was attributed
to several events. It could be caused by a melt-recrystallization process, involving the
melting of original crystals, recrystallization and subsequent melting of recrystallized
crystals.28, 29 Melting of crystals featuring different lamellar thicknesses or melting of
the crystalline phases α and α’ are also reported explanations.30
Chapter 3: Reactive Extrusion of Poly(l-lactide) With 2-Oxepane-1,5-Dione 107
Figure 3.8. DSC thermograms from the second heating cycle of purified extrudates
of PLLA - OPD (the percentages account for the OPD initial feed; 0 wt%
corresponds to the PLLA – tin (II) octanoate formulation without OPD).
Eventually, the degree of crystallinity was calculated according to the following
equation:
𝜒𝑐(%) = ∆𝐻𝑚 + ∆𝐻𝑐𝑐
∆𝐻𝑚0 × 100
With ∆𝐻𝑚 being the melting enthalpy, ∆𝐻𝑐𝑐 the cold crystallization enthalpy and ∆𝐻𝑚0
the melting enthalpy of 100 % crystalline PLLA sample (93.7 J∙g-1).31, 32 The degrees
of crystallinity did not significantly differ between each extrudate. Although the
addition of OPD and tin (II) octanoate led to a higher number of chain scissions, as
revealed by GPC measurements, no increase in the 𝜒𝑐 was obtained. Crystallinity is
usually enhanced by degradation occurring during processing. Indeed, chain scissions
release new chain segments that crystallize due to an enhanced chain mobility above
the Tg. This phenomenon is known as chemi-crystallization, and was previously
observed for PLLA subjected to thermo-oxidative degradation.19, 24
108 Chapter 3: Reactive Extrusion of Poly(l-lactide) With 2-Oxepane-1,5-Dione
Table 3.3. Thermal properties of purified extrudates obtained by DSC on a second
heating cycle (three measurements were performed and values were averaged).
OPD
feed
(wt%)
Tg
(°C)
Tcc
(°C)
∆𝐻𝑐𝑐
(J·g-1)
Tm1
(°C)
Tm2
(°C
)
∆𝐻𝑚
(J·g-1)
𝜒𝑐
(%)
PLLA 60.8 ±
0.2
98.4 ±
0.4
18.5 ±
0.5
- 171.4
± 0.3
48.7 ±
0.3
71.6 ±
0.2
0 52.5 ±
2.1
94.8 ±
1.6
27.5 ±
2.4
- 164.6
± 1.1
39.2 ±
3.2
71.2 ±
5.9
5 49.3 ±
2.6
93.7 ±
2.1
30.5 ±
2.3
146.3 ±
1.0
159.3
± 1.9
38.9 ±
5.3
74.1 ±
8.0
10 50.1 ±
1.6
94.5 ±
1.5
30.8 ±
0.5
147.3 ±
2.6
158.1
± 1.7
39.1 ±
2.1
74.5 ±
1.7
15 51.3 ±
2.4
95.7 ±
2.9
27.2 ±
1.1
- 161.4
± 1.9
34.6 ±
3.3
65.8 ±
2.4
The absence of incorporation of OPD into the PLLA backbone could be due to the
short residence time (10 minutes). Subsequent reactive extrusions were performed for
longer residence times.
3.2.1.2.2. Influence of Residence Time
The residence time was increased to investigate its influence on transesterification
during reactive extrusion of PLLA and OPD. A first extrusion was performed at 190
°C for 2 hours with PLLA, 15.1 wt% OPD and 1.98 wt% tin (II) octanoate. Samples
were collected every 30 minutes to assess the extent of degradation. Increasing the
residence time from 30 minutes to 1 hour at 190 °C resulted in polymer degradation,
as characterized by a change of colour from yellow to brown. The colour of the
subsequent samples evolved from brown to almost black, indicating further
degradation over time (Figure 3.8).
Chapter 3: Reactive Extrusion of Poly(l-lactide) With 2-Oxepane-1,5-Dione 109
A second experiment was performed for 1 h 20 min in which samples were collected
every 20 minutes to limit the extent of degradation. Sampling lowered the mass of
extruded sample inside the extruder, thus impacting the apparent viscosity
measurements. Therefore, no apparent viscosity data was reported here. The four
extrudates were purified twice using the dissolution - reprecipitation technique.
Purified extrudates with 20 and 40 minutes residence time were recovered as white
powders. However, the ones after 60 and 80 minutes were orange powders, suggesting
thermal degradation. Extrudates were dried under vacuum prior to characterization.
Figure 3.9. Photographs of extrudates collected every 30 minutes of a reactive
extrusion experiment of PLLA - OPD (15.1 wt%) catalysed by tin (II) octanoate,
revealing the change of colour over time.
Thermal degradation was confirmed by ATR-FTIR spectroscopy, as revealed by the
broadening of the carbonyl band (1820 - 1660 cm-1) as well as the appearance of a
broad band from 3700 to 2700 cm-1 after 40 minutes of extrusion (Figure 3.10). These
changes suggested the formation of carboxylic acid groups. Carboxylic acid was
previously reported as a product of the pyrolysis of the ester moieties of poly(ε-
caprolactone).33 The formation of alkene end-groups was also observed during the
thermal degradation of hydrogen β-substituted esters, including poly(ε-
caprolactone).33, 34 Pyrolysis of random copolymers of -caprolactone and OPD
revealed that the ketone moiety of OPD at 150 °C accelerated the pyrolysis compared
to PCL, resulting in chain scission and release of alkene end-groups.34
30 min 1 h 1 h 30 2 h
110 Chapter 3: Reactive Extrusion of Poly(l-lactide) With 2-Oxepane-1,5-Dione
1H NMR spectroscopy revealed the presence of the quartet at 5.16 ppm and a doublet
at 1.58 ppm assigned to the methine and methyl protons of PLLA, respectively.23 A
doublet at 1.50 ppm was present after 20 minutes, featuring an increased in integration
with residence time. All four samples displayed both a multiplet at 4.37 ppm, and
peaks in the region between 2.88 - 2.55 ppm (Figure 3.11). The integration of the
multiplet at 4.37 ppm increased with residence time. The doublet at 1.50 ppm
corresponded to oligomers of polylactide.35 Oligomers are among the thermo-
oxidative degradation products reported for polylactide exposed to temperatures above
200 ºC.36-38 The multiplet at 4.36 ppm suggested the presence of –CH(CH3)OH end-
groups, as reported in the literature.39 As revealed by the GPC measurements, the
increase in residence time resulted in chain scissions, accounting for the increase in
end-groups.
Figure 3.10. ATR-FTIR spectra of double-purified extrudates after various residence
times revealing the broadening of the carbonyl band (1820 - 1660 cm-1) and the
Chapter 3: Reactive Extrusion of Poly(l-lactide) With 2-Oxepane-1,5-Dione 111
appearance of a broad band between 3700 to 2700 cm-1 (average of 9 spectra per film
after baseline correction and normalization to the -CH3 bending band at 1454 cm-1).
Therefore, the increased residence time led to chain scissions of the PLLA matrix,
resulting in oligomers as thermo-oxidative degradation products. OPD may have also
undergone degradation, as revealed by the formation of carboxylic acids. Thermo-
oxidative degradation of the materials could account for the absence of incorporation
of OPD into PLLA.
Figure 3.11. 1H NMR spectra of double-purified extrudates of PLLA - OPD - tin (II)
octanoate after various residence times at 190 ºC, measured in CDCl3.
Samples were analysed by GPC in chloroform to assess the evolution of molecular
weights and polydispersities. The evolution of the GPC traces of the purified
extrudates collected every 20 minutes, after baseline correction and normalization, are
shown in Figure 3.12. The extrudates exhibited broad unimodal distributions, with a
shift towards lower molecular weight with increased residence time. The shift was
greater with longer residence times, suggesting a greater decrease in molecular
weights. Values of 𝑀𝑛 confirmed that observation (Table 3.4). The 𝑀𝑛
decreased from
107,300 ± 4,900 to 2,900 ± 100 g·mol-1 between the starting material and after 20
112 Chapter 3: Reactive Extrusion of Poly(l-lactide) With 2-Oxepane-1,5-Dione
minutes of extrusion, respectively. The 𝑀𝑛 continuously decreased with increased
residence time. As a result, the chain scissions increased from 36 to 97 events between
20 and 80 minutes of extrusion.
Figure 3.12. GPC traces of purified extrudates of PLLA - OPD 15 wt%, collected
every 20 minutes at 190 °C, measured in chloroform (the traces were baseline-
corrected and normalized).
Table 3.4. Molecular weight of purified extrudates of PLLA - OPD 15 wt% at 190
°C collected every 20 minutes, measured by GPC in chloroform (three measurements
were performed and values were averaged).
Residence time
(min)
𝑀𝑛 (g·mol-1) 𝑀𝑤
(g·mol-1) Ð Chain
scissions
Virgin PLLA 107,300 ± 4,900 209,300 ± 800 1.96 ± 0.09 -
Chapter 3: Reactive Extrusion of Poly(l-lactide) With 2-Oxepane-1,5-Dione 113
20 2,900 ± 100 6,600 ± 100 2.29 ± 0.02 36
40 2,000 ± 100 4,100 ± 0 2.03 ± 0.10 53
60 1,200 ± 100 2,400 ± 0 2.11 ± 0.20 88
80 1,100 ± 0 2,100 ± 100 1.89 ± 0.01 97
The decrease in molecular weight was expected to affect the thermal properties of the
double-purified extrudates. These properties were assessed by DSC using a heating /
cooling / heating cycle. All extrudates only exhibited vitrification when cooled down
from the melt at 10 ºC·min-1, the absence of crystallization being explained in Section
3.2.1.2.1.4 (refer to appendices for thermograms). After a second heating run, all
extrudates featured a glass transition, a cold crystallization and a double melting peak
(Figure 3.13).
Figure 3.13. DSC thermograms from the second heating cycle of double-purified
extrudates after various residence times.
114 Chapter 3: Reactive Extrusion of Poly(l-lactide) With 2-Oxepane-1,5-Dione
Both glass transition and melting temperatures shifted towards lower temperatures
with increased residence time (Table 3.5). These decreases indicated reductions in
molecular weights, resulting from chain scissions due to thermal degradation.
Table 3.5. Evolution of the thermal properties of purified extrudates collected every
20 minutes, as measured by DSC on a second heating run (three measurements were
performed and values were averaged).
Time
(min)
Tg
(°C)
Tcc
(°C)
∆𝐻𝑐𝑐
(J·g-1)
Tm1
(°C)
Tm2
(°C)
∆𝐻𝑚
(J·g-1)
𝜒𝑐
(%)
20 49.2 ±
1.7
89.2 ±
1.4
8.7 ± 0.5 147.9 ±
0.3
153.5 ±
1.0
49.6 ±
2.2
62.2 ±
1.9
40 50.3 ±
0.3
95.8 ±
0.5
28.9 ±
1.8
144.6 ±
0.2
148.9 ±
0.2
48.4 ±
4.0
82.4 ±
6.0
60 39.1 ±
2.1
101.2 ±
1.3
26.6 ±
2.0
122.8 ±
1.7
136.3 ±
1.1
26.5 ±
1.7
56.7 ±
3.9
80 39.3 ±
1.5
104.9 ±
0.6
18.4 ±
1.4
122.9 ±
0.3
135.1 ±
1.0
19.0 ±
0.8
39.9 ±
2.4
Increasing the residence time did not result in successful OPD incorporation. Instead,
thermo-oxidative degradation seemed to occur, as revealed by an increase in chain
scissions and the formation of oligomers and carboxylic acids. The next extrusions
employed another transesterification catalyst.
3.2.1.2.3. Influence of Transesterification Catalyst
Incorporation of OPD onto PLLA was attempted by varying the OPD initial feed and
the residence time, which did not result in successful modification of PLLA. Regarding
experimental factors, both temperature and transesterification catalyst remained
unchanged in the previous extrusions. However, the temperature was previously
optimized and set at 190 ºC to ensure the melting of PLLA and to limit the extent of
thermal degradation. The final attempts to perform transesterification reactions
between PLLA and OPD investigated the role of the catalyst. Several catalysts, such
as zinc or titanium-based catalysts, have been found to improve blend
Chapter 3: Reactive Extrusion of Poly(l-lactide) With 2-Oxepane-1,5-Dione 115
compatibilization of polyesters via transesterification. Comparative research to
compatibilize polyethylene terephthalate with functionalized polyethylene using zinc
acetate and titanium (IV) tetrabutoxide improved the transesterification yield.40
Titanium (IV) tetrabutoxide was also successfully used to compatibilize blends of
PLLA and poly(butylene adipate-co-terephthalate) (PBAT) in the melt at 200 ºC.7
Therefore, this catalyst was selected and extrusions of PLLA and 10 wt% OPD were
performed at 100 rpm and 190 ºC for 10 minutes with 0.07 wt% of catalyst (Table
3.6).
The torque and the relative viscosity were recorded over time. A control experiment
was carried out using the same conditions without transesterification catalyst. The
extrudates were flushed out from the extruder, purified twice as previously explained
and were obtained as white polymers. They were dried under vacuum prior to
characterization.
Table 3.6. Formulations of extrudates featuring titanium (IV) tetrabutoxide as the
transesterification catalyst.
Catalyst PLLA
(g)
OPD
(mg)
Catalyst
(wt%)
Mass
recovered (g)
None 4.0130 415.0 - 0.7918
Ti(OBu)4 4.0084 390.99 0.21 1.2869
The torque values remained equal to zero throughout both extrusions, potentially due
to the detection limit as reported for the previous extrusions. The apparent viscosity
values remained higher in the presence of titanium (IV) tetrabutoxide than for the
control extrusion (Figure 3.14). In both cases, the values decreased over time, with a
faster decrease in the extrusion containing the catalyst. However, based on the error
calculated from the extrusion of PLLA alone, the values obtained here could more
relatively close to each other (refer to section 3.2.1.2.1.1). Higher apparent viscosities
were also observed for PLLA with 0.07 wt% Ti(OBu)4 compared to neat PLLA in the
case of PLLA-PBAT compatibilization study cited earlier.7 The decrease in relative
viscosity suggested the occurrence of chain scissions, as explained in section
3.2.1.2.1.3.
116 Chapter 3: Reactive Extrusion of Poly(l-lactide) With 2-Oxepane-1,5-Dione
Figure 3.14. Evolution of the apparent viscosity vs residence time for extrudates of
PLLA - OPD 10 wt% without and with titanium (IV) tetrabutoxide as the
transesterification catalyst.
The potential incorporation of OPD into PLLA was investigated by ATR-FTIR and
1H NMR spectroscopies. The ATR-FTIR spectra of extrudates in both cases displayed
the characteristic bands of PLLA (Figure 3.15). No difference was observed between
the two extrudates. An additional shoulder on the carbonyl band could demonstrate
either the presence of incorporated OPD or new carbonyl-containing products resulting
from the thermo-oxidative degradation. The 1H NMR spectra of both extrudates solely
showed the characteristics peaks of PLLA, without any trace of ring-opened OPD
(Figure 3.16), suggesting that no incorporation of OPD onto PLLA occurred during
the extrusion experiments.
Chapter 3: Reactive Extrusion of Poly(l-lactide) With 2-Oxepane-1,5-Dione 117
Figure 3.15. ATR-FTIR spectra of twice-purified extrudates without and with
titanium (IV) tetrabutoxide as transesterification catalyst (average of nine spectra
after baseline-correction and normalization with the –CH stretching band at 1454 cm-
1).
118 Chapter 3: Reactive Extrusion of Poly(l-lactide) With 2-Oxepane-1,5-Dione
Figure 3.16. 1H NMR spectra of twice-purified extrudates: using titanium (IV)
tetrabutoxide (top) and without any transesterification catalyst (bottom), measured in
CDCl3.
The molecular weight of the extrudates was analysed by GPC using chloroform as the
eluent (Figure 3.17). No significant shift was observed towards the low molecular
weight when titanium (IV) tetrabutoxide was employed during the extrusion. Values
of 𝑀𝑛 and 𝑀𝑤
that did not significantly differ with or without transesterification
catalyst (Table 3.7). The distribution of PLLA – Ti(OBu)4 solely broadened,
suggesting an increase in polydispersity. Indeed, the polydispersity evolved from 1.84
± 0.01 to 1.94 ± 0.03 without and with catalyst, respectively (Table 3.7).
Chapter 3: Reactive Extrusion of Poly(l-lactide) With 2-Oxepane-1,5-Dione 119
Figure 3.17. GPC traces of PLLA - OPD 10 wt% with or without transesterification
catalyst, measured in chloroform (the traces were baseline-corrected and
normalized).
Table 3.7. Molecular weight of twice-purified extrudates of PLLA - OPD 10 wt%
for 10 minutes at 190 °C with or without titanium (IV) tetrabutoxide measured by
GPC in chloroform (three measurements were performed and values were averaged).
Compound 𝑀𝑛 (g·mol-1) 𝑀𝑤
(g·mol-1) Ð
Virgin PLLA 107,300 ± 4,900 209,300 ± 800 1.96 ± 0.09
None 29,000 ± 200 53,700 ± 50 1.84 ± 0.01
Ti(OBu)4 27,600 ± 700 53,600 ± 500 1.94 ± 0.03
120 Chapter 3: Reactive Extrusion of Poly(l-lactide) With 2-Oxepane-1,5-Dione
Thermal properties of extrudates were analysed by DSC in the second heating cycle,
after erasing the thermal history of the samples. Both formulations were semi-
crystalline, as demonstrated by the presence of a glass transition, an exothermic peak
assigned to cold crystallization, and a small exothermic peak prior to an endothermic
peak corresponding to the melting temperature (Figure 3.18). This small exothermic
peak was reported to arise from the transition of the disorded α’ and to the ordered α
phase. Indeed, at relevant crystallization temperatures, the α’ phase is preferentially
formed, as reviewed in Chapter 1.26
As a conclusion, similar results to those previously obtained were observed by 1H
NMR and FTIR spectroscopic analyses. Indeed, only the typical peaks in NMR and
absorption bands in FTIR spectroscopies of PLLA were observed for the crude and
purified extrudates. The nature of transesterification catalyst did not seem to influence
a successful incorporation of OPD into the PLLA backbone, under the set of conditions
used.
Chapter 3: Reactive Extrusion of Poly(l-lactide) With 2-Oxepane-1,5-Dione 121
Figure 3.18. DSC thermograms of purified extrudates of PLLA - OPD 10 wt% with
and without titanium (IV) tetrabutoxide as the transesterification catalyst.
Table 3.8. Thermal properties of extrudates of PLLA - OPD 10 wt% with and
without titanium (IV) tetrabutoxide, as measured by DSC in the second heating run
(three measurements were performed and values were averaged).
Catalyst Tg
(°C)
Tcc
(°C)
∆𝐻𝑐𝑐
(J·g-1)
Tm
(°C)
∆𝐻𝑚
(J·g-1)
𝜒𝑐
(%)
None 59.8 ± 1.2 99.2 ± 1.2 15.2 ± 2.8 170.2 ± 0.2 51.1 ± 0.6 70.7 ± 2.8
Ti(OBu)4 60.4 ± 0.2 97.5 ± 0.2 13.1 ± 1.1 170.1 ± 0.2 51.6 ± 0.8 69.1 ± 1.7
122 Chapter 3: Reactive Extrusion of Poly(l-lactide) With 2-Oxepane-1,5-Dione
3.2.2 Thermal Stability of 2-Oxepane-1,5-Dione
Attempts to incorporate OPD into PLLA was investigated through varying the
extrusion factors (OPD initial feed, residence time, transesterification catalyst).
However, none of the extrusions performed resulted in successful incorporation of
OPD. The thermal stability of OPD could possibly explain the lack of incorporation.
Therefore, the thermal degradation of OPD was assessed by Thermal Gravimetric
Analysis (TGA). One decomposition step occurred around 160 °C, with an inflection
point at 184.1 ºC, accounting for a mass loss of 96.3 % (Figure 3.19).
A nonisothermal DSC run of OPD up to 250 ºC confirmed the decomposition of OPD
at high temperature. Indeed, the DSC thermogram exhibited both an endothermic peak
at 113.6 ± 0.65 ºC and an exothermic peak starting from 175 ºC with a maximum at
196.9 ± 0.55 ºC (Figure 3.20). The transition at 113 ºC was identified as the melting
of OPD crystals, while the exothermic peak corresponded to decomposition.
Figure 3.19. TGA trace of 2-oxepane-1,5-dione measured from 0 to 1000 °C under
nitrogen showing a decomposition step around 160 ºC with an inflection point at
184.1 °C.
Chapter 3: Reactive Extrusion of Poly(l-lactide) With 2-Oxepane-1,5-Dione 123
The combined TGA and DSC measurements confirmed the hypothesis of the OPD
degradation with increased temperatures. As the reactive extrusions were performed
at 190 °C, OPD probably underwent thermal degradation, from which carboxylic acids
and alkene end-groups were formed, as revealed by spectroscopic techniques (refer to
section Error! Reference source not found.). Regarding the thermal stability of p
olylactide, the temperature at which 5 wt% of the total mass of unprocessed PLA was
volatilized was determined to be 331 ºC.41, 42
The degradation of OPD over time would inhibit the occurrence of transesterification
reactions with PLLA, explaining the absence of ring-opened OPD on the PLLA
backbone. As the PLLA employed in this work did not allow lower extrusion
temperatures (refer to section 3.2.1.2.1), thermal degradation of OPD could not be
avoided.
Figure 3.20. DSC thermogram of OPD on a nonisothermal run at a heating rate of 10
ºC·min-1 under nitrogen showing both the melting (Tm) and decomposition
(Tdecomposition) phases.
124 Chapter 3: Reactive Extrusion of Poly(l-lactide) With 2-Oxepane-1,5-Dione
3.3 SUMMARY
In this chapter, a reactive extrusion route was explored for incorporating ketone
moieties into poly(L-lactide) via transesterification reactions using tin (II) octanoate as
the catalyst by considering the effects of the OPD initial feed and the residence time.
ATR-FTIR and 1H NMR spectroscopies of purified extrudates revealed the absence of
ring-opened OPD, while demonstrating carboxylic acids and alkene-end groups as
proof of thermo-oxidative degradation. Changing the transesterification catalyst did
not enable to successfully incorporate OPD onto PLLA with the set of conditions used.
Eventually, TGA and DSC measurements of OPD demonstrated its thermal instability
at the high temperatures required for PLLA processing. In order to limit the extent of
thermal degradation, the next chapter focused on copolymerisation experiments of L-
lactide and OPD, using milder conditions.
3.4 EXPERIMENTAL
3.4.1. Materials
Poly(L-lactide) 4003D grade was purchased from NatureWorks LLC and stored under
nitrogen. OPD was synthesised as previously reported (refer to Chapter 2) and dried
under vacuum prior to use. Methanol and chloroform, AR grades, were purchased from
ChemSupply and used as received. Chloroform (HPLC grade) used for GPC analysis
was purchased from Merck and filtered prior to use. Deuterium chloroform used for
NMR spectroscopy analysis was also purchased from Merck and stored at 10 °C.
3.4.2. Methods
3.4.2.1. Haake Minilab Extruder
Laboratory-scale extrusions were performed on a Thermo-Haake Minilab Rheomex
CTW5 laboratory-scale extruder (Thermo-Electron). The extruder was equipped with
conical twin screw of diameters of 5 - 14 mm and a length of 109.5 mm and co-rotation
was applied in every experiment. The extrudates recirculated thanks to a backflow
channel. The apparent viscosity was determined from the pressure difference between
two pressure sensors located in the backflow channel. A die enabled the sampling and
flushing out of the material.
3.4.2.2. Reactive Extrusion of Poly(L-Lactide) and OPD
Chapter 3: Reactive Extrusion of Poly(l-lactide) With 2-Oxepane-1,5-Dione 125
Poly(L-lactide) 4003D grade was dried under vacuum for 4 hours while OPD was
vacuum-dried for 24 hours prior to extrusion. In a typical reactive extrusion
experiment, poly(L-lactide) 4003D grade, OPD and the transesterification catalyst
were mixed together. The Haake Minilab extruder was pre-heated at 190 °C and the
torque and screw speed were calibrated. The reactants were introduced into the Haake
extruder via the feeding arm, and melted at 190 °C with a screw speed at 20 rpm. Once
the reactants were completed melted, the speed screws automatically increased to 100
rpm, marking the start of the extrusion. After a determined residence time, the
extrudate was flushed out of the Haake, and purified by reprecipitation using
chloroform and cold methanol (0 - 1 °C; vol:vol 1:10) as the solvent and non-solvent,
respectively.
Table 3.9. Formulations of the different extrudates.
PLLA
(g)
OPD
(g)
Sn(Oct)2
(mg)
Ti(OBu)4
(mg)
OPD initial feed (wt%)
PLLA alone 4.0125 - - -
0 4.0054 - 74.1 -
5 4.0032 200.8 74.6 -
10 4.0057 407.2 74.5 -
15 4.0086 597.1 68.6 -
Residence time
2 h 4.0047 600.7 68.1 -
1 h 20 4.0064 605.2 71.6 -
Transesterification catalyst
None 4.0130 415.0 - -
Ti(OBu)4 4.0084 391.0 - 8.49
OPD initial feed
126 Chapter 3: Reactive Extrusion of Poly(l-lactide) With 2-Oxepane-1,5-Dione
PLLA alone, after three reprecipitation steps:
White polymer. 1H NMR (CDCl3, δ ppm): 5.16 (q, H, CHL-LA), 1.58 (d, 3H, CH3L-LA).
ATR-FTIR: ʋ max = 2995 (w, -CH2 stretching, asymmetric), 2948 (w, -CH2 stretching,
symmetric), 1753 (s, -C=O carbonyl stretching), 1454 (-CH3 bending), 1383 and 1359
(-CH- deformation, symmetric and asymmetric), 1182, 1130 and 1085 (-C-O-
stretching), 1044 (-OH bending), 871 (w, -C-C- stretching). GPC (Refractive Index
detector): 𝑀𝑛 (Ð) = 38,800 ± 1,000 g·mol-1 (1.80 ± 0.05). DSC (second heating cycle):
Tg = 60.8 ± 0.2 ºC, Tcc = 98.4 ± 0.4 ºC (∆𝐻𝑐𝑐 = 18.5 ± 0.5 J·g-1), Tm = 171.4 ± 0.3 ºC
(∆𝐻𝑚 = 48.7 ± 0.3 J·g-1).
PLLA - OPD 0 wt% - tin (II) octanoate, after three reprecipitation steps:
White polymer. 1H NMR (CDCl3, δ ppm, subscripts L-LA and OPD denote each
repeating units): 5.17 (q, H, CHL-LA), 4.44 (t, 2H, -CH2OOPD), 4.35 (m, H, -
CH(CH3)OHL-LA), 2.74 (m, 2H, CH2C=OOPD), 2.66 (m, 2H, C=OCH2OPD), 1.59 (d, 3H,
CH3L-LA). ATR-FTIR: ʋ max = 2996 (w, -CH2 stretching, asymmetric), 2948 (w, -CH2
stretching, symmetric), 1755 (s, -C=O carbonyl stretching) with a shoulder from 1730
to 1695 cm-1, 1455 (-CH3 bending), 1383 and 1359 (-CH- deformation, symmetric and
asymmetric), 1181, 1130 and 1085 (-C-O- stretching), 1044 (-OH bending), 871 (w, -
C-C- stretching). GPC (Refractive Index detector): 𝑀𝑛 (Ð) = 9,500 ± 900 g·mol-1 (2.57
± 0.23). DSC (second heating cycle): Tg = 52.5 ± 2.1 ºC, Tcc = 94.8 ± 1.6 ºC (∆𝐻𝑐𝑐 =
27.5 ± 2.4 J·g-1), Tm = 164.6 ± 1.1 ºC (∆𝐻𝑚 = 39.2 ± 3.2 J·g-1).
PLLA - OPD 5 wt% - tin (II) octanoate, after three reprecipitation steps:
White polymer. 1H NMR (CDCl3, δ ppm, subscripts L-LA and OPD denote each
repeating units): 5.17 (q, H, CHL-LA), 4.44 (t, 2H, -CH2OOPD), 4.35 (m, H, -
CH(CH3)OHL-LA), 2.74 (m, 2H, CH2C=OOPD), 2.66 (m, 2H, C=OCH2OPD), 1.59 (d, 3H,
CH3L-LA). ATR-FTIR: ʋ max = 2996 (w, -CH2 stretching, asymmetric), 2948 (w, -CH2
stretching, symmetric), 1755 (s, -C=O carbonyl stretching) with a shoulder from 1730
to 1695 cm-1, 1454 (-CH3 bending), 1383 and 1359 (-CH- deformation, symmetric and
asymmetric), 1182, 1130 and 1086 (-C-O- stretching), 1044 (-OH bending), 871 (w, -
C-C- stretching). GPC (Refractive Index detector): 𝑀𝑛 (Ð) = 5,500 ± 100 g·mol-1 (2.20
± 0.015). DSC (second heating cycle): Tg = 49.3 ± 2.6 ºC, Tcc = 93.7 ± 2.1 ºC (∆𝐻𝑐𝑐 =
30.5 ± 2.3 J·g-1), Tm1 = 146.3 ± 1.0 ºC, Tm2 = 159.5 ± 1.9 ºC (∆𝐻𝑚 = 38.9 ± 5.3 J·g-1).
PLLA - OPD 10 wt% - tin (II) octanoate, after three reprecipitation steps:
Chapter 3: Reactive Extrusion of Poly(l-lactide) With 2-Oxepane-1,5-Dione 127
White polymer. 1H NMR (CDCl3, δ ppm, subscripts L-LA and OPD denote each
repeating units): 5.17 (q, H, CHL-LA), 4.44 (t, 2H, -CH2OOPD), 4.35 (m, H, -
CH(CH3)OHL-LA), 2.74 (m, 2H, CH2C=OOPD), 2.66 (m, 2H, C=OCH2OPD), 1.59 (d, 3H,
CH3L-LA). ATR-FTIR: ʋ max = 2996 (w, -CH2 stretching, asymmetric), 2948 (w, -CH2
stretching, symmetric), 1750 (s, -C=O carbonyl stretching) with a shoulder from 1730
to 1695 cm-1, 1454 (-CH3 bending), 1384 and 1359 (-CH- deformation, symmetric and
asymmetric), 1181, 1130 and 1084 (-C-O- stretching), 1043 (-OH bending), 871 (w, -
C-C- stretching). GPC (Refractive Index detector): 𝑀𝑛 (Ð) = 3,100 ± 100 g·mol-1 (2.12
± 0.01). DSC (second heating cycle): Tg = 50.1 ± 1.6 ºC, Tcc = 94.5 ± 1.5 ºC (∆𝐻𝑐𝑐 =
30.8 ± 0.5 J·g-1), Tm1 = 147.3 ± 2.6 ºC, Tm2 = 158.1 ± 1.7 ºC (∆𝐻𝑚 = 39.1 ± 2.1 J·g-1).
PLLA - OPD 15 wt% - tin (II) octanoate, after three reprecipitation steps:
White polymer. 1H NMR (CDCl3, δ ppm, subscripts L-LA and OPD denote each
repeating units): 5.17 (q, H, CHL-LA), 4.44 (t, 2H, -CH2OOPD), 4.35 (m, H, -
CH(CH3)OHL-LA), 2.74 (m, 2H, CH2C=OOPD), 2.66 (m, 2H, C=OCH2OPD), 1.59 (d, 3H,
CH3L-LA). ATR-FTIR: ʋ max = 2996 (w, -CH2 stretching, asymmetric), 2948 (w, -CH2
stretching, symmetric), 1751 (s, -C=O carbonyl stretching) with a shoulder from 1730
to 1666 cm-1, 1453 (-CH3 bending), 1384 and 1358 (-CH- deformation, symmetric and
asymmetric), 1181, 1128 and 1084 (-C-O- stretching), 1043 (-OH bending), 870 (w, -
C-C- stretching). GPC (Refractive Index detector): 𝑀𝑛 (Ð) = 5,500 ± 100 g·mol-1 (2.07
± 0.04). DSC (second heating cycle): Tg = 51.3 ± 2.4 ºC, Tcc = 95.7 ± 2.9 ºC (∆𝐻𝑐𝑐 =
27.2 ± 1.1 J·g-1), Tm = 161.4 ± 1.9 ºC (∆𝐻𝑚 = 34.6 ± 3.3 J·g-1).
Residence time
20 minutes, after two reprecipitation steps:
White powder. 1H NMR (CDCl3, δ ppm): 5.17 (q, H, CHL-LA), 4.36 (m, H, -
CH(CH3)OHL-LA), 1.58 (d, 3H, CH3L-LA). ATR-FTIR: ʋ max = 2997 (w, -CH2
stretching, asymmetric), 2948 (w, -CH2 stretching, symmetric), 1752 (s, -C=O
carbonyl stretching) with a shoulder from 1730 to 1695 cm-1, 1454 (-CH3 bending),
1384 and 1359 (-CH- deformation, symmetric and asymmetric), 1181, 1129 and 1085
(-C-O- stretching), 1044 (-OH bending), 871 (w, -C-C- stretching). GPC (Refractive
Index detector): 𝑀𝑛 (Ð) = 2,900 ± 0 g·mol-1 (2.29 ± 0.02). DSC (second heating cycle):
Tg = 49.2 ± 1.7 ºC, Tcc = 89.2 ± 1.4 ºC (∆𝐻𝑐𝑐 = 8.7 ± 0.5 J·g-1), Tm1 = 147.9 ± 0.3 ºC,
Tm2 = 153.5 ± 1.0 ºC (∆𝐻𝑚 = 49.6 ± 2.2 J·g-1).
128 Chapter 3: Reactive Extrusion of Poly(l-lactide) With 2-Oxepane-1,5-Dione
40 minutes, after two reprecipitation steps:
White powder. 1H NMR (CDCl3, δ ppm): 5.17 (q, H, CHL-LA), 4.36 (m, H, -
CH(CH3)OHL-LA), 1.57 (d, 3H, CH3L-LA). ATR-FTIR: ʋ max = 3500 (b, w, -COOH
stretching), 2997 (w, -CH2 stretching, asymmetric), 2948 (w, -CH2 stretching,
symmetric), 1755 (s, -C=O carbonyl stretching) with a shoulder from 1730 to 1695
cm-1, 1455 (-CH3 bending), 1384 and 1359 (-CH- deformation, symmetric and
asymmetric), 1182, 1130 and 1087 (-C-O- stretching), 1044 (-OH bending), 871 (w, -
C-C-stretching). GPC (Refractive Index detector): 𝑀𝑛 (Ð) = 2,000 ± 100 g·mol-1 (2.03
± 0.10). DSC (second heating cycle): Tg = 50.3 ± 0.3 ºC, Tcc = 95.8 ± 0.5 ºC (∆𝐻𝑐𝑐 =
28.9 ± 1.8 J·g-1), Tm1 = 144.6 ± 0.2 ºC, Tm2 = 148.9 ± 0.2 ºC (∆𝐻𝑚 = 48.4 ± 4.0 J·g-1).
60 minutes, after two reprecipitation steps:
Yellow powder. 1H NMR (CDCl3, δ ppm): 5.17 (q, H, CHL-LA), 4.37 (m, H, -
CH(CH3)OHL-LA), 1.59 (d, 3H, CH3L-LA). ATR-FTIR: ʋ max = 3500 (b, w, -COOH
stretching ), 2997 (w, -CH2 stretching, asymmetric), 2948 (w, -CH2 stretching,
symmetric), 1748 (s, -C=O carbonyl stretching) with a shoulder from 1730 to 1660
cm-1, 1455 (-CH3 bending), 1384 and 1359 (-CH- deformation, symmetric and
asymmetric), 1181, 1128 and 1085 (-C-O- stretching), 1043 (-OH bending), 871 (w, -
C-C- stretching). GPC (Refractive Index detector): 𝑀𝑛 (Ð) = 1,200 ± 100 g·mol-1 (2.11
± 0.20). DSC (second heating cycle): Tg = 39.1 ± 2.1 ºC, Tcc = 101.2 ± 1.3 ºC (∆𝐻𝑐𝑐 =
26.6 ± 2.0 J·g-1), Tm1 = 122.8 ± 1.7 ºC, Tm2 = 136.3 ± 1.1 ºC (∆𝐻𝑚 = 26.5 ± 1.7 J·g-1).
80 minutes, after two reprecipitation steps:
Orange powder. 1H NMR (CDCl3, δ ppm): 5.16 (q, H, CHL-LA), 4.37 (m, H, -
CH(CH3)OHL-LA), 1.58 (d, 3H, CH3L-LA). ATR-FTIR: ʋ max = 3500 (b, w, -COOH
stretching), 2997 (w, -CH2 stretching, asymmetric), 2948 (w, -CH2 stretching,
symmetric), 1748 (s, -C=O carbonyl stretching) with a shoulder from 1730 to 1660
cm-1, 1455 (-CH3 bending), 1384 and 1359 (-CH- deformation, symmetric and
asymmetric), 1181, 1128 and 1085 (-C-O- stretching), 1043 (-OH bending), 871 (w, -
C-C- stretching). GPC (Refractive Index detector): 𝑀𝑛 (Ð) = 1,100 ± 0 g·mol-1 (1.89
± 0.01). DSC (second heating cycle): Tg = 39.3 ± 1.5 ºC, Tcc = 104.9 ± 0.6 ºC (∆𝐻𝑐𝑐 =
18.4 ± 1.4 J·g-1), Tm1 = 122.9 ± 0.3 ºC, Tm2 = 135.1 ± 1.0 ºC (∆𝐻𝑚 = 19.0 ± 0.8 J·g-1).
Transesterification catalyst
Chapter 3: Reactive Extrusion of Poly(l-lactide) With 2-Oxepane-1,5-Dione 129
PLLA - OPD 10 wt% - no transesterification catalyst, after two reprecipitation steps:
Orange powder. 1H NMR (CDCl3, δ ppm): 5.17 (q, H, CHL-LA), 1.58 (d, 3H, CH3L-LA).
ATR-FTIR: ʋ max = 2997 (w, -CH2 stretching, asymmetric), 2948 (w, -CH2 stretching,
symmetric), 1749 (s, -C=O carbonyl stretching) with a shoulder from 1730 to 1695
cm-1, 1454 (-CH3 bending), 1383 and 1359 (-CH- deformation, symmetric and
asymmetric), 1181 and 1085 (-C-O- stretching). GPC (Refractive Index detector): 𝑀𝑛
(Ð) = 29,100 ± 200 g·mol-1 (1.84 ± 0.01). DSC (second heating cycle): Tg = 58.8 ± 1.2
ºC, Tcc = 99.2 ± 1.2 ºC (∆𝐻𝑐𝑐 = 15.2 ± 2.8 J·g-1), Tm1 = 158.6 ± 0.9 ºC, Tm2 = 170.2 ±
0.2 ºC (∆𝐻𝑚 = 51.1 ± 0.6 J·g-1).
PLLA - OPD 10 wt% - titanium (IV) tetrabutoxide, after two reprecipitation steps:
Orange powder. 1H NMR (CDCl3, δ ppm): 5.17 (q, H, CHL-LA), 1.59 (d, 3H, CH3L-LA).
ATR-FTIR: ʋ max = 2996 (w, -CH2 stretching, asymmetric), 2948 (w, -CH2 stretching,
symmetric), 1749 (s, -C=O carbonyl stretching) with a shoulder from 1730 to 1695
cm-1, 1453 (-CH3 bending), 1383 and 1359 (-CH- deformation, symmetric and
asymmetric), 1181 and 1084 (-C-O- stretching). GPC (Refractive Index detector): 𝑀𝑛
(Ð) = 27,600 ± 700 g·mol-1 (1.94 ± 0.03). DSC (second heating cycle): Tg = 60.4 ± 0.2
ºC, Tcc = 97.5 ± 0.2 ºC (∆𝐻𝑐𝑐 = 13.1 ± 1.1 J·g-1), Tm1 = 158.2 ± 0.5 ºC, Tm2 = 170.1 ±
0.2 ºC (∆𝐻𝑚 = 51.6 ± 0.8 J·g-1).
3.4.2.3. Gel Permeation Chromatography
Please refer to section 2.4.3.2 (Chapter 2).
3.4.2.4. Differential Scanning Calorimetry
Please refer to section 2.4.3.3 (Chapter 2).
3.4.2.5. Fourier Transform Infrared Spectroscopy
Please refer to section 2.4.3.4 (Chapter 2).
3.4.2.6. Proton Nuclear Magnetic Resonance Spectroscopy
Proton Nuclear Magnetic Resonance spectra were recorded on a 600 MHz Bruker
spectrometer with 32 scans. 1 mg.mL-1 solutions in deuterated chloroform were used
for NMR analyses. The spectra were calibrated with the CDCl3 peak at 7.26 ppm.
3.4.2.7. Thermogravimetric Analysis
130 Chapter 3: Reactive Extrusion of Poly(l-lactide) With 2-Oxepane-1,5-Dione
Thermogravimetric analysis was conducted on a TA instruments Q500
thermogravimetric analyser. Approximately 30 mg of sample were heated in a
platinum crucible in the temperature range 25 - 1000 °C at 5 °C·min-1, under nitrogen.
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26. S. Saeidlou, M. A. Huneault, H. Li and C. B. Park, Progress in Polymer Science
37 (12), 1657-1677 (2012).
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132 Chapter 3: Reactive Extrusion of Poly(l-lactide) With 2-Oxepane-1,5-Dione
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Chapter 4: Functionalization of Poly(l-lactide) with 2-Oxepane-1,5-Dione 133
Chapter 4: Functionalization of Poly(L-
lactide) with 2-Oxepane-1,5-Dione
4.1 BACKGROUND
In the preceding chapter (Chapter 3), in-melt modification of poly(L-lactide) was
undertaken to incorporate 2-oxepane-1,5-dione in order to increase the functionality.
However, such attempts remained unsuccessful due to the thermal instability of 2-
oxepane-1,5-dione. Another modification strategy under milder conditions relies on
the polymerisation of L-lactide with functionalized monomers. Such reactions enable
the design of copolymers with predictable molecular weights and controlled
architectures with well-defined end-groups. 2-Oxepane-1,5-dione features the same
structure as ε-caprolactone with an additional ketone. Copolymerisations of ε-
caprolactone and OPD were carried out in solution and in the bulk. The
copolymerisations were catalysed by various metal derivatives including aluminium
isopropoxide, dimethyl tin dimethoxide and tin (II) octanoate.1 Random copolymers
were obtained with up to 30 mol% incorporation of OPD.2 Regarding
copolymerisation with L-lactide, Prime et al.3 first synthesized a poly(L-lactide-co-
OPD) in the bulk, using tin (II) octanoate as the catalyst and butanol as the initiator.
They carried out a polymerisation with only one OPD initial feed ratio, 24.6 mol%,
which resulted in an incorporation level of 4 mol%. Dai and coworkers also employed
OPD to afford comb-type copolymers of poly(4-hydroxyl-ε-caprolactone-co-ε-
caprolactone)-g-poly(L-lactide). They first copolymerised poly(ε-caprolactone-co-
OPD) (OPD initial feed 25 mol%) in toluene at 90 ºC with tin (II) octanoate as catalyst.
Subsequent reduction of the ketone moieties to the corresponding pendent hydroxyl
groups enabled the initiation of the graft polymerisation of L-lactide, in the bulk with
tin (II) octanoate catalyst.4
In terms of catalyst, ring-opening polymerization of lactide and lactone using metal
alkoxides as catalyst has been extensively investigated.5-7 The polymerisation is
initiated by protic reagents such as alcohols or residual impurities (lactic acid, water)
that react with tin (II) octanoate to yield tin (II) alkoxide (Scheme 4.1).8
134 Chapter 4: Functionalization of Poly(l-lactide) with 2-Oxepane-1,5-Dione
Scheme 4.1. Conversion of tin (II) octanoate into tin (II) alkoxide via reaction with
alcohol or residual protic impurities.
The reaction proceeds via a coordination-insertion mechanism. The monomer is
coordinated to the Lewis-acidic metal centre, and subsequently inserted into one of the
tin (II) alkoxide bonds via nucleophilic addition of the alkoxy group on the carbonyl
carbon. The acyl-oxygen cleavage results in the opening of the ring and the insertion
of the monomer. Eventually the polymerisation is terminated by hydrolysis of the
active propagation chain. 9, 10 This catalyst affords high molecular weight polylactide
with values up to 105 - 106 Da.5
With the aim of designing polymers with predictable molecular weights, low
polydispersities and end group fidelity, extensive research has been focused on the use
of guanidines and amidines to catalyse the ROP of polyesters. Lohmeijer et al.6
demonstrated the efficiency of organocatalytic polymerisation of L-lactide with 1,8-
diaza[5.4.0]bicycloundec-7-ene (DBU) in solution at room temperature. They
suggested a mechanism based on the activation of the alcohol by DBU through
hydrogen bonding. The polymerisation proceeds through nucleophilic attack of lactide
by the activated catalyst, resulting into polylactides with predictable molecular weights
and high end group fidelity within short reaction times (Scheme 4.2). Few
transesterifications were observed as supported by their narrow polydispersities (Ð ≤
1.1). As a pseudo-living polymerisation, none or few terminating chains occur and
quenching is necessary to terminate the polymerisation. Adding an acid in the
polymeric medium results in deactivating DBU, thus quenching the reaction.7
Sn(Oct)2 + ROH Oct-Sn-OR + OctH
Oct-Sn-OR + ROH Sn(OR)2 OctH+
Chapter 4: Functionalization of Poly(l-lactide) with 2-Oxepane-1,5-Dione 135
Scheme 4.2. Ring-opening polymerisation of lactide using DBU and an alcohol as
catalyst and initiator, respectively.
This chapter extends the copolymerisation work of L-lactide and OPD in the bulk,
using tin (II) octanoate as the catalyst. Several OPD initial feeds were selected with
the aim to obtain higher level of incorporation than previously achieved by other
studies reported in the literature. Due to limited OPD incorporation, the temperature
was also investigated as a factor to increase the OPD incorporation resulting into the
formation of red precipitates. GPC, DSC and spectroscopic techniques were used to
characterize the compounds and explain the low incorporation level of OPD achieved.
In order to increase the concentration of OPD into the copolymers, another set of
conditions was employed – that uses DBU as catalyst and benzyl alcohol as the
initiator. Polymerisations were performed in dichloromethane at room temperature.
136 Chapter 4: Functionalization of Poly(l-lactide) with 2-Oxepane-1,5-Dione
4.2 RESULTS AND DISCUSSION
4.2.1. Transition Metal-Catalysed Copolymerisation of L-lactide and 2-
Oxepane-1,5-Dione in the Bulk
The copolymerisation strategy was applied to modify poly(L-lactide). Ring-opening
polymerisations of L-lactide and OPD were performed in the bulk to mimic the reactive
extrusion conditions used in Chapter 3, while avoiding thermal degradation.
4.2.1.1. Ring-Opening Polymerisation of L-Lactide and OPD 5 - 20 wt%
A series of poly(L-lactide-co-OPD) polymers were synthesized in the bulk by the
transition-metal catalysed ring-opening polymerisation with various feed monomer
ratios using tin (II) octanoate catalyst (Scheme 4.3). Prior to the reaction, both
monomers were purified by recrystallization and dried under vacuum for several days
to limit any initiation processes from residual impurities. Polymerisations were carried
out under inert atmosphere at 110 °C to ensure complete melting of both monomers
(melting temperatures of L-lactide and OPD: 95 and 110 C respectively) and lasted
until high monomer conversion (supported by an increase in viscosity of the polymeric
mixture which stops the stirring process). The copolymers were purified by
reprecipitation using chloroform as solvent and methanol as non-solvent. Depending
on the polymerisation, several precipitations were required to remove unreacted
monomers, as monitored by 1H NMR spectroscopy. The conditions, time, yields, and
molecular weights for each copolymerisation are summarized in Table 4.1.
Chapter 4: Functionalization of Poly(l-lactide) with 2-Oxepane-1,5-Dione 137
Scheme 4.3. Tin (II) octanoate-catalysed ROP of L-lactide and OPD at 110 °C in the
bulk to afford poly(L-lactide-co-OPD).
Table 4.1. Conditions and results of the ring-opening polymerisations of L-lactide
and OPD 0 – 20 mol% at 110 ºC catalysed by tin (II) octanoate in the bulk.
a Calculated from integration ratios in 1H NMR spectra of the purified polymers; b
Purification yield.
Regarding the ROP of L-lactide alone, the stirring stopped after 4 hours due to high
viscosity. The addition of OPD slowed down the kinetics of L-lactide, as revealed by
the longer reaction times needed to increase the viscosity of the polymeric mixture.
After four purification steps, the spectra of poly(L-lactide-co-OPD)s revealed a quartet
at 5.17 ppm and a doublet at 1.57 ppm which are respectively assigned to the methine
and methyl protons of poly(L-lactide).10 Two multiplets at 4.40 ppm and in the range
2.80 - 2.60 ppm correspond to protons next to the ester oxygen and the protons adjacent
to the carbonyl and ketone moieties of ring-opened OPD respectively (Figure 4.1).1,
Entry Initial feed molar
ratio (L-LA / OPD)
Time (h) Copolymer molar
ratio (PLLA / OPD) a
Yield (%) b
1 1 / 0 4 1 / 0 29.2
2 1 / 0.058 46.5 1 / 0.048 24.6
3 1 / 0.11 46 1 / 0.052 35.4
4 1 / 0.23 72 1 / 0.070 61.0
138 Chapter 4: Functionalization of Poly(l-lactide) with 2-Oxepane-1,5-Dione
11 The multiplet at 4.36 ppm represents a characteristic –CH(CH3)OH end-group.12
The molar ratio between the converted monomers was calculated based on the
integration ratios of the peaks at 5.17 and in the range 2.80 - 2.60 ppm. An
incorporation of 4.8 to 7 mol% of OPD was found for the copolymers.
Figure 4.1. 1H NMR spectra of various poly(L-lactide-co-OPD) with initial OPD
concentration of 5 (bottom); 10 (middle) and 20 mol% (top), measured in CDCl3.
The chemical shifts of OPD as a monomer or when copolymerized do not differ in the
1H NMR spectra. The only noticeable difference is the two triplets, at 4.40 and 2.80
ppm in the monomer, changing to multiplets in the ring-opened OPD.11, 13 The
presence of OPD segments in the copolymer was confirmed by Pulsed Field Gradient
Spin-Echo (PGSE) NMR spectroscopy. This technique relies on the fact that molecules
move via rotational and translational motions in solution. Translational motions are
characterized by self-diffusion coefficients which are related to molecular dimensions,
Chapter 4: Functionalization of Poly(l-lactide) with 2-Oxepane-1,5-Dione 139
specifically the hydrodynamic radius (RH). RH corresponds to the radius of the
hypothetical size of the spherical particle of a molecule in solution. Molecules can be
separated based on their diffusion coefficients by applying two different magnetic field
gradient pulses.14 This technique enables to determine polymer compositions among
others.15, 16 Poly(L-lactide-co-OPD) was dissolved in CDCl3 and subjected to two
different magnetic field gradient pulses: 3 and 95 %. At 3 % gradient, molecules of all
diffusion coefficients appear on the spectrum from short to long hydrodynamic sizes.
Both small molecules, such as unreacted monomers, and slowly diffusing polymers
will appear on the spectrum. On the contrary, only peaks for slowly diffusing polymers
can be observed when the 95 % magnetic field gradient is applied. From this spectrum,
the chemical composition of the macromolecule can be determined, with the assurance
that every peak is originated from protons in the polymer.
The spectrum at 3 % gradient shows a quartet at 5.17 ppm and a doublet at 1.58 ppm
which are assigned to the methine and methyl protons of poly(L-lactide) respectively
(Figure 4.2). The multiplets at 4.36 ppm and in the 2.80 - 2.65 ppm range are assigned
to OPD, either in its ring-opened or cyclic form, accounting for 1.2 mol% based on
integration ratios.1 The spectrum at 95 % displays the peaks assigned to PLLA as well
as smaller peaks corresponding to ring-opened OPD. The OPD segments account for
1 mol% based on integration ratios, thus suggesting a small amount of unreacted OPD
was still present.
140 Chapter 4: Functionalization of Poly(l-lactide) with 2-Oxepane-1,5-Dione
Figure 4.2. (a) 1H NMR spectrum of poly(L-lactide-co-OPD); (b) PGSE NMR
spectra of poly(L-lactide-co-OPD) using 3 % magnetic field gradient pulse; (c) PGSE
NMR spectra of poly(L-lactide-co-OPD) using 95 % magnetic field gradient pulse,
measured in CDCl3.
The chemical structures of the purified copolymers were analysed by ATR-FTIR
spectroscopy. Nine spectra were collected on three different samples of each polymer
- each baseline corrected, averaged and normalized to the band at 1453 cm-1 (assigned
to -CH3 bending) to suppress any effect from contact differences with the ATR crystal
and depth of penetration of the IR beam (Figure 4.3). The spectra all revealed similar
characteristic bands: the C=O stretching band of the ester groups at 1750 cm-1, the -C-
O- stretching band of the ester deformation bands at 1181 and 1083 cm-1, and the -CH3
bending band at 1453 cm-1. All the bands obtained in the spectra matched those of neat
PLLA. 17, 18 The addition of OPD neither led to any shift of wavelength of the different
Chapter 4: Functionalization of Poly(l-lactide) with 2-Oxepane-1,5-Dione 141
bands nor to splitting of the ester band into two distinct bands. However, an additional
shoulder appeared on the band from 1725 to 1700 cm-1 which are assigned to a ketone
functionality (Figure 4.4).19 This ketone could either be assigned to the OPD monomer
or the ring-opened comonomer in the polymer as no wavenumber shift was noticeable
between them. Besides the copolymer with 7 mol% OPD, the single ester band
observed for the copolymers indicates the randomness of the copolymers. The two
maxima observed around 1750 cm-1 with 7 mol% OPD is likely due to noise resulting
from the resolution (4 cm-1) used to run the spectra and the normalization process. Qian
et al.20 reported the ATR-FTIR spectra of random copolymers of L-lactide and ε-
caprolactone which exhibits a single absorption band at 1756 cm-1 whereas two distinct
bands at 1761 and 1732 cm-1 were observed for the block structures.
Figure 4.3. ATR-FTIR spectra of the different poly(L-lactide-co-OPD)s revealing
the characteristic bands of poly(L-lactide) (the mol% represents the concentration of
incorporated OPD within the copolymer).
142 Chapter 4: Functionalization of Poly(l-lactide) with 2-Oxepane-1,5-Dione
Figure 4.4. Enlarged view of the carbonyl region of the ATR-FTIR spectra of the
different poly(L-lactide-co-OPD)s revealing the OPD shoulder at 1725 - 1700 cm-1
(the two maxima observed around 1750 cm-1 for 7 mol% OPD was due to noise
resulting from the resolution (4 cm-1) used to run the spectra and the normalization
process to the band at 1453 cm-1 assigned to -CH3 bending).
Latere and coworkers synthesized random copolymers of ε-caprolactone and OPD in
dry toluene at 90 ºC using tin (II) octanoate catalyst. The concentration of OPD
segments in the copolymers closely matched the initial feed. This was confirmed by
1H NMR spectroscopy analysis. Copolymers with OPD concentrations up to 77 mol%
were obtained.1 However, Prime et al.11 first reported the copolymerisation of L-lactide
and OPD in the bulk at 110 °C initiated by butanol in the presence of tin (II) octanoate
catalyst. The resulting copolymer contained 4 mol% of incorporated OPD for an initial
feed of 24.6 mol%. They justified this low incorporation level to the interference of
the ketone moiety with tin (II) octanoate. No other initial OPD feed was tested to reach
higher incorporation levels. A difference in the reactivity ratios could also explain the
Chapter 4: Functionalization of Poly(l-lactide) with 2-Oxepane-1,5-Dione 143
low incorporation level of OPD when copolymerized with L-lactide compared to ε-
caprolactone. Extensive work was performed on synthesize random or block
copolymers of L-lactide and ε-caprolactone using various catalysts such as aluminium
isopropoxide and tin (II) octanoate.20-22 These studies demonstrated the preferential
conversion of L-lactide compared to ε-caprolactone leading to a lower incorporation
level of the latter than expected. The higher polymerisation enthalpy of L-lactide was
suggested to arise from the ring strain of the bond oppositions of the six-membered
ring.23, 24 By analogy with PLLA - PCL copolymers, OPD could be characterized by a
lower polymerisation enthalpy compared to L-lactide when catalysed by tin (II)
octanoate, resulting in slower conversions.
The molecular weights and polydispersities of the polymers were assessed by GPC in
chloroform. Each polymer exhibited sharp monodal distributions. A shift towards the
lower molecular weights was observed with increase in the initial OPD feed as well as
broadening of the distributions compared to neat PLLA 1 (Figure 4.5). Calculated
values of 𝑀𝑛 and polydispersities confirmed the decrease in molecular weight with
increase in the initial OPD feed as well as an increase of the polydispersity (Table
4.2). Both long reaction times and the presence of tin (II) octanoate favour thermal
degradation. This predominantly proceeds through backbiting transesterifications and
results into chain scission and increased polydispersities.25, 26 As the addition of OPD
reduced the conversion of L-lactide resulting into longer reaction times, such thermal
degradation could occur which thus explains the GPC result obtained herein. Apart
from neat PLLA 1, only copolymer 2 featured a molecular weight above the
entanglement molecular weight (𝑀𝑒 = 8 - 10 kDa).27 Attempts to prepare a film with
the solvent-casting technique resulted into recrystallization of the copolymer instead
of producing a film. The GPC calibration, performed against narrow polystyrene
standards, is known to overestimate molecular weight values for PLLA and PCL
(correction factor in DCM ~ 0.68).28, 29 Thus copolymer 2 probably displayed a lower
molecular weight than 𝑀𝑒 , thus hindering film formation.
144 Chapter 4: Functionalization of Poly(l-lactide) with 2-Oxepane-1,5-Dione
Figure 4.5. GPC traces of synthesized poly(L-lactide) and poly(L-lactide-co-OPD)s
measured in chloroform (the traces were baseline-corrected and normalized).
Table 4.2. Molecular weight and polydispersities of PLLA and poly(L-lactide-co-
OPD)s measured in chloroform (two measurements were performed and values were
averaged).
Entry 𝑀𝑛 (g·mol-1) 𝑀𝑤
(g·mol-1) Ð
1 43,000 ± 900 53,700 ± 1,500 1.25 ± 0.06
2 12,700 ± 100 16,200 ± 100 1.28 ± 0.02
3 6,900 ± 600 11,400 ± 2,200 1.64 ± 0.18
4 3,900 ± 100 5,900 ± 100 1.51 ± 0.02
Chapter 4: Functionalization of Poly(l-lactide) with 2-Oxepane-1,5-Dione 145
The OPD incorporation into the PLLA backbone was expected to modify the thermal
properties of the resulting copolymers. Such properties were determined by DSC using
a heat / cool / heat cycle. The DSC thermograms of polymers measured on a second
heating run are shown in Figure 4.6.
Figure 4.6. DSC thermograms of purified poly(L-lactide) 1 and poly(L-lactide-co-
OPD) 2, 3, 4 under nitrogen on a second heating run.
Poly(L-lactide) 1 displayed a glass transition and an endothermic peak corresponding
to the melting temperature. Poly(L-lactide-co-OPD) 2 and 3 exhibited a glass transition
and a double endothermic peak. Poly(L-lactide-co-OPD) 4 featured a glass transition,
an exothermic peak due to cold crystallization and a broad endothermic peak. The
single glass transition confirmed the random structure of the copolymers, even though
the glass transitions were weak and difficult to observe. For random copolymers, the
Tg can be predicted from the Fox relationship:
146 Chapter 4: Functionalization of Poly(l-lactide) with 2-Oxepane-1,5-Dione
1
𝑇𝑔=
𝑤1
𝑇𝑔1+
𝑤2
𝑇𝑔2
With 𝑤1, 𝑤2 the weight fraction and 𝑇𝑔1, 𝑇𝑔2 the glass transitions of PLLA and POPD
respectively.30 As POPD was reported to exhibit a Tg of 37 ºC, lower values of Tg for
the copolymers compared to neat PLLA were expected.13 Noticeable changes observed
include the appearance of a double melting behaviour as well as the shift of the melting
temperature towards the lower temperatures. The double melting behaviour of PLA
was attributed to either a melt-recrystallization process, the melting of different
lamellar thicknesses or melting of the α and α’ crystals forms.31-33 However, the double
endotherm could also arise from the presence of different crystalline structures as per
the incorporation of ring-opened OPD reported to be semi-crystalline.13 The decrease
in melting temperature was in agreement with the Fox-Flory theory. The molar mass
decrease induced structural changes in the crystalline regions. The enhanced chain
mobility facilitated crystallization resulting in the cold crystallization transition
observed for copolymer 4.
Table 4.3. Thermal properties of purified poly(L-lactide-co-OPD) measured by DSC
on a second heating cycle (two measurements were performed and values were
averaged).
Entry Tg
(°C)
Tcc
(°C)
∆𝐻𝑐𝑐
(J·g-1)
Tm1
(°C)
Tm2
(°C)
∆𝐻𝑚
(J·g-1)
𝜒𝑐
(%)
1 56.2±2.9 - - - 174.3±0.1 51.0±0.7 54.4±0.7
2 42.0±2.5 - - - 162.8±0.3 55.2±0.7 58.9±0.7
3 51.9±0.5 - - 153.3±0.2 159.5±0.4 47.9±3.4 51.1±3.6
4 39.8±2.0 88.1±1.7 7.0±0.7 137.4±0.8 - 35.1±1.2 44.8±2.0
4.2.1.2. Attempted Synthesis of Poly(L-Lactide-co-OPD) with Increased
OPD Initial Feed
Other polymerisations were performed with higher initial OPD feeds with the aim to
increase its incorporation into the copolymers. Initial OPD feeds of 50 and 75 wt%
were selected. Polymerisations were carried out for 7 hours at 100 °C in the bulk,
catalysed by tin (II) octanoate. A single purification of the products was achieved by
Chapter 4: Functionalization of Poly(l-lactide) with 2-Oxepane-1,5-Dione 147
reprecipitation using chloroform and cold hexane (0 - 1 ºC; 1:10 vol:vol). The
products, obtained as wax materials, were dried under vacuum prior to
characterization. Compounds 5 and 6 were obtained in 50 % and 48.4 % yields
respectively.
The chemical structure of the resulting polymers were analysed by ATR-FTIR
spectroscopy. Nine spectra were collected on three different samples of each polymer,
baseline corrected, averaged, and normalized to the band at 1456 cm-1. Both average
spectra displayed sharp and quite intense bands at 1241 and 934 cm-1 which were
absent on the spectrum of poly(L-lactide) (Figure 4.7 and Figure 4.8). These bands,
which are characteristics of monomeric L-lactide, confirm the conversion did not
proceed to completion in either case (Table 4.4).34
Figure 4.7. ATR-FTIR average spectrum of the product 5, resulting from the ROP of
L-lactide with an initial OPD feed of 50 mol% (average of 9 spectra per film after
baseline correction and normalization with the -CH3 bending band at 1456 cm-1).
148 Chapter 4: Functionalization of Poly(l-lactide) with 2-Oxepane-1,5-Dione
Figure 4.8. ATR-FTIR average spectrum of the product 6, resulting from the ROP of
L-lactide with an initial OPD feed of 75 mol% (average of 9 spectra per film after
baseline correction and normalization with the -CH3 bending band at 1456 cm-1).
Table 4.4. Comparison of characteristic bands in the ATR-FTIR spectra of L-lactide
and poly(L-lactide) with their assignments based on reported literature.34, 35
Band position (cm-1) Assignment
L-lactide Poly( L-lactide)
3000 2996 -CH- stretch (asymmetric)
2931 2945 -CH- stretch (symmetric)
1752 1747 -C=O carbonyl stretch
1456 1455 -CH3 bend
1355; 1325 1384; 1359 -CH- deformation (symmetric and
asymmetric bend)
1241 - -CO-O-C deformation
1145; 1094 1182; 1085 -C-O- deformation
1054 1043 -OH deformation
Chapter 4: Functionalization of Poly(l-lactide) with 2-Oxepane-1,5-Dione 149
The molecular weights of purified product were measured by GPC in chloroform. Both
traces featured a broad distribution assigned to an oligomer, and a second sharp
distribution assigned to unconverted L-lactide (Figure 4.9).
Figure 4.9. GPC trace of products 5 and 6, resulting from the ROP of L-lactide with
an OPD initial feed of 50 and 75 mol%, respectively, measured in chloroform (the
traces were baseline-corrected and normalized).
The thermal properties of the purified products were determined using DSC on a
second heating run to erase the thermal history of the samples (refer to appendices for
thermograms). Compound 5 featured a broad endothermic peak from which no real
conclusion could be made. Compound 6 featured a broad endothermic peak with a
maximum at 55.4 C corresponding to the melting temperature. Poly(L-lactide) is
932 - -CO-O- deformation
823 868 -C-C- deformation
150 Chapter 4: Functionalization of Poly(l-lactide) with 2-Oxepane-1,5-Dione
typically characterized by a glass transition ranging from 50 to 70 C and a melting
temperature from 170 to 190 C.36 However, neither of the samples featured a glass
transition. This, along with the lower melting point observed, confirmed an incomplete
conversion of L-lactide to poly(L-lactide). However, the 55.4 C melting point
corresponds to the melting temperature of meso-lactide and suggests that racemization
occurred during the reaction. Such racemisation is favoured under longer reaction
conditions.37
4.2.1.3. Attempted Synthesis of Poly(L-Lactide-co-OPD) at Higher
Temperatures
Final efforts to increase the OPD segments within the copolymers involved
investigating the effect of temperature (increasing the temperature). Polymerisations
were carried out at 150 and 170 °C with a constant OPD concentration (20 mol%) in
the bulk and catalysed by tin (II) octanoate. Some red precipitates formed during the
polymerisation. The polymeric mixtures turned yellow, orange to brown when the
polymerisation was carried out at 170 ºC. Once thermally quenched, the reaction
mixtures were reprecipitated in cold hexane to give a slightly yellow powder (7) and
an orange wax (8) in 55.3 and 49.2 % conversions respectively. Hexane was selected
as a non-solvent for the reprecipitation because the colour suggested degradation and
thus, low molecular weight polymers – known to readily reprecipitate in hexane.
The 1H NMR spectra of the purified compounds are illustrated in Figure 4.10. Along
with the methine and methyl protons of PLLA at 5.16 and 1.58 ppm respectively,
various diagnostic peaks were observed in the range 2.25 - 0.80 ppm. The singlet at
1.25 and doublet at 1.49 ppm could be assigned to residual tin (II) octanoate. The broad
peak in the range 2.00 - 1.50 ppm could arise from the coordination of the
paramagnetic tin ion.
Chapter 4: Functionalization of Poly(l-lactide) with 2-Oxepane-1,5-Dione 151
Figure 4.10. 1H NMR spectra of (a) purified compound 7; (b) purified compound 8,
measured in CDCl3.
Increasing the temperature induced the precipitation of red crystals insoluble in
common organic solvents, including dimethyl sulfoxide. The DSC thermogram of the
red precipitate revealed a broad endothermic peak, possibly due to different kind of
crystal structures (refer to appendices). The ATR-FTIR spectra showed a strong band
at 1700 cm-1 corresponding to ketone moiety, with the shoulder at 1727 cm-1 assigned
to the -C=O bond of an ester group.
The chemical composition of the product was further analysed by inductively coupled
plasma optical emission spectroscopy (ICP-OES). This technique converts a sample
in solution into an aerosol, which is subsequently vaporized within a plasma. The
atoms reach an excited state due to collisions, then relax to their ground state via the
emission of light. Elements can then be identified and quantified due to the
characteristic wavelengths and their intensities respectively.38 The red crystals were
dissolved in a mixture of hydrofluoric acid / hydrogen chloride and analysed by this
technique. The results indicate the compounds were predominantly composed of tin,
with a small percentage of iron for 11 (Figure 4.11). A tiny amount of silicon was
observed as well. However, silicon is a common contamination for solution ICP-OES
due to the handling of samples, particularly when using hydrofluoric acid. As much as
152 Chapter 4: Functionalization of Poly(l-lactide) with 2-Oxepane-1,5-Dione
a tiny amount of silicon could arise from contamination, some of it possibly originated
from the sample handling as well. Based on the ATR-FTIR and ICP-OE spectroscopic
results, these compounds seemed to be complexes formed between OPD and tin (II)
octanoate during the polymerisations. The ketone moiety of OPD possibly reacted with
tin (II) octanoate, thus hindering the conversion of both L-lactide and OPD. Latere and
coworkers reported a competition between the ketone moieties of OPD and the ester
groups of ɛ-caprolactone in the coordination process with tin (II) octanoate. Such
interaction was shown to slow down the kinetics of the polymerisation. ROP control
experiments performed with cyclohexanone and tin (II) octanoate resulted in a
decrease of the conversion of ε-caprolactone from 67 to 25 % for a concentration of
cyclohexanone of 0 to 58 mol% respectively. They concluded a hindering effect by the
ketone moiety on the polymerisation.1
Figure 4.11. Concentrations of analytes in red complexes measured by ICP-OES,
revealing tin and silicon as the main components.
In conclusion, the metal-catalysed polymerisations of L-lactide and OPD, with an
initial concentration range 0 - 20 mol%, afforded copolymers with only very low
amounts of incorporated OPD. To increase the concentration of OPD segments within
Chapter 4: Functionalization of Poly(l-lactide) with 2-Oxepane-1,5-Dione 153
the copolymer backbone, reactions were performed with higher initial OPD
concentrations and higher temperatures (150; 170 ºC). Higher OPD initial
concentrations (50; 75 mol%) seemed to block the copolymerisation by inhibiting the
conversion of L-lactide. Increasing the temperature resulted in the precipitation of a
red complex in the polymerisation medium. The precipitate was mainly composed of
tin and OPD. The ketone moiety of OPD possibly hindered the conversion of both
monomers by reacting with the tin (II) catalyst to form complexes instead of
facilitating ring-opening.
4.2.2. Organocatalysed Copolymerisation of L-Lactide and OPD in Solution
In order to increase the level of OPD incorporated into the PLLA backbone,
polymerisations were performed in solution using a different catalytic system: 1,8-
diaza[5.4.0]bicycloundec-7-ene (DBU) as catalyst and benzyl alcohol as the initiator.
4.2.2.1. Homopolymerisation of L-Lactide
The kinetics of the ROP of L-lactide were investigated using DBU and benzyl alcohol
as catalyst and initiator respectively. A molecular weight of 12,500 g·mol-1 was
targeted (degree of polymerisation (DPn) 87) to obtain polymers featuring molecular
weights above the entanglement molecular weight.39
Efforts were made to remove residual traces of oxygen and moisture. The
polymerisations were performed under nitrogen in a glovebox. Reagents and the
solvent were prepared as followed: dry dichloromethane was degassed via five freeze-
pump-thaw cycles and stored over activated molecular sieves (4 Å); DBU and benzyl
alcohol were distilled over calcium hydroxide under vacuum and degassed; L-lactide
was dissolved in dry DCM and dried over activated molecular sieves (4 Å) for at least
24 hours prior to the reaction. A solution of appropriate volumes of L-lactide, benzyl
alcohol and DBU was stirred at room temperature with samples collected over time
and quenched with excess glacial acetic acid.
The conversion of L-lactide was determined from the integration of the methylene
protons of L-lactide and PLLA at 5.02 and 5.16 ppm in the 1H NMR spectrum
respectively. A rapid consumption of L-lactide was observed with up to 98 - 99 %
conversion after 1.5 minutes of polymerisation. The presence of peaks at 7.33 ppm,
corresponding to protons of benzyl alcohol, confirmed its role as the initiator.
154 Chapter 4: Functionalization of Poly(l-lactide) with 2-Oxepane-1,5-Dione
The evolutions of molecular weight and polydispersity were assessed by GPC in
chloroform (refer to appendices). After 1.5 minutes, a number average molecular
weight of 18,500 ± 800 g∙mol-1 was obtained with a polydispersity of 1.21 ± 0.02
(Table 4.5). The calibration against narrow polystyrene standards, known to
overestimate molecular weight values for polyesters, accounts for the difference
between the targeted and measured values.28, 29 In comparison, 𝑀𝑛 were calculated
from the integration ratio of methylene protons of PLLA and protons from the benzyl
alcohol in the 1H NMR spectra, as followed:
𝐷𝑃𝑛 =𝐼(𝐶𝐻𝑃𝐿𝐿𝐴) × 5
𝐼(𝑃ℎ) × 1
The obtained values for 𝑀𝑛 after 1.5 minutes matched the expected values closely. The
small difference could be due to the resolution of the peaks in the 1H NMR spectra and
the accuracy of their integration. In both cases, however, values remained relatively
unchanged throughout the polymerisation time, suggesting few transesterification
reactions.
Table 4.5. Evolution of the conversion and number average molecular weight over
time of the ROP of L-lactide using DBU and benzyl alcohol as catalyst and initiator,
respectively, DPn 87.
Time Conversion (%) a 𝑀𝑛 (g·mol-1) a 𝑀𝑛
(Da) b Ð b
Time 0 0 - - -
1.5 min 98.5 ± 0.7 13,700 ± 100 18,500 ± 800 1.21 ± 0.02
3 min 98.5 ± 0.7 12,400 ± 2,900 19,500 ± 800 1.18 ± 0.02
4.5 min 98.5 ± 0.7 14,400 ± 0 17,900 ± 1,100 1.26 ± 0.01
6 min 98 ± 0 12,000 ± 0 17,700 ± 200 1.26 ± 0.02
7.5 min 98 ± 0 12,000 ± 0 17,100 ± 900 1.29 ± 0.01
a Determined by 1H NMR spectroscopy in CDCl3; b Measured by GPC analysis in
chloroform.
Chapter 4: Functionalization of Poly(l-lactide) with 2-Oxepane-1,5-Dione 155
Based on the conversion determined by 1H NMR spectroscopy, a kinetic study of the
polymerisation was performed. The ROP of L-lactide catalysed by DBU was reported
to follow a pseudo first-order kinetic model, according to the relationship:
ln (1
1 − 𝑥) = 𝑘𝑎𝑝𝑝𝑡
With 𝑥 the conversion of L-lactide, calculated from the 1H NMR spectra and 𝑘𝑎𝑝𝑝 the
apparent kinetic constant for given initial conditions.7, 11 This relationship was used
with the experimental values reported in Table 4.5. For experimental reasons, the first
collected sample was after 1.5 min when the conversion was complete. This explains
the shape of the curve illustrated in Figure 4.12.
Figure 4.12. Homopolymerisation of L-lactide in DCM at room temperature
(monomer conversion calculated from 1H NMR spectroscopy) under the following
conditions: [LLA]0 = 2.072 mol·L-1, LLA / BDU = 15, LLA / benzyl alcohol = 87.
In addition to 1H NMR spectroscopy, the chemical structure of the synthesised PLLA
was investigated by ATR-FTIR spectroscopy. The spectra presented similar
characteristic bands of poly(L-lactide), including the -C=O stretching band of the ester
156 Chapter 4: Functionalization of Poly(l-lactide) with 2-Oxepane-1,5-Dione
groups at 1748 cm-1, the -C-O- deformation bands of the ester at 1180 and 1082 cm-1,
and the -CH3 bending band at 1454 cm-1 (Figure 4.13).17, 18
Figure 4.13. Representative ATR-FTIR average spectrum of synthesized poly(L-
lactide) using DBU and benzyl alcohol as catalyst and initiator, respectively.
Benzyl alcohol and DBU efficiently initiated and catalysed, respectively, the ROP of
L-lactide in DCM at room temperature. Optimisation of the experimental conditions
enabled to obtain poly(L-lactide) with known end-groups and controlled molecular
weights.
4.2.2.2. Batch Polymerisations of L-Lactide and OPD
Once the experimental conditions were optimized, a series of batch polymerisations of
L-lactide and OPD were performed with an OPD initial concentration in the range 10
- 75 wt%. A reaction with pure L-lactide was simultaneously carried out as a control.
The DPn were calculated to reach a molecular weight at least equal to the entanglement
molecular weight of PLLA (Table 4.6). The polymerisations were quenched by glacial
Chapter 4: Functionalization of Poly(l-lactide) with 2-Oxepane-1,5-Dione 157
acetic acid, then concentrated under reduced pressure and purified by reprecipitation
using chloroform and hexane (1:10 vol:vol) to furnish white powders.
Table 4.6. Conditions and results of the batch polymerisations of L-lactide and OPD
in solution at room temperature with DBU and benzyl alcohol as catalyst and initiator,
respectively.
a Initial OPD feed (mol%); b Calculated from the integration ratios in the 1H NMR
spectra of crude polymers.
The conversion of L-lactide was calculated from the integrations ratio of the respective
methine protons at 5.02 ppm and at 5.16 ppm for L-lactide and PLLA in the 1H NMR
spectra. The L-lactide conversion varied depending on the initial feed of OPD (Table
4.6). When polymerized without OPD, L-lactide was completely polymerized after 20
minutes. The addition of 11 mol% of OPD to the polymerisation medium resulted in
slowing down the kinetics of L-lactide, reaching 99 % conversion only after 17 hours
instead of 20 minutes as observed without OPD. Increasing the OPD feed by > 50
mol% completely inhibited the conversion of L-lactide even after 50 hours of reaction.
The chemical structures of purified polymers were further analysed by ATR-FTIR
spectroscopy. Nine spectra were collected on three different samples of each polymer,
baseline-corrected, averaged and normalized with the -CH3 bending band at 1454 cm-
1. The spectra of polymers 13, 14 and 15 displayed similar characteristic bands: the -
C=O stretching band of ester at 1747 cm-1, the -C-O- deformation band at 1181 and
1084 cm-1 and the -CH3 bending band at 1454 cm-1 (Figure 4.14). The presence of a
Polyme
r
OPD0
a
LLA/I
n
LLA/Ca
t
OPD/I
n
OPD/Ca
t
Tim
e
Conversio
n of LLAb
Yiel
d
(%)
13 0 72 13 - - 20
min
99 18.5
14 11 70 13 8 2 17 h 99 21.6
15 23 70 13 16 3 18 h 85 21.8
16 55 72 13 44 8 50 h 0 12.4
17 77 72 13 62 12 50 h 0 10.8
158 Chapter 4: Functionalization of Poly(l-lactide) with 2-Oxepane-1,5-Dione
small band at 1647 cm-1 in the spectra of polymers 16 and 17 was assigned to the -
C=N stretching band of residual DBU (Figure 4.15).40 The carbonyl band of the
polymer 17 revealed two strong -C=O stretching bands, at 1722 and 1703 cm-1,
corresponding to the ester and ketone of unreacted OPD (refer to Chapter 2).
Figure 4.14. ATR-FTIR averaged spectra of purified polymers with OPD initial feed
ranging from 0 to 77 mol% (average of 9 spectra after baseline correction and
normalization with the -CH3 bending band at 1454 cm-1).
The incomplete conversion of monomers was expected to affect the molecular weights
of the products. The GPC traces measured in chloroform are illustrated in Figure 4.16.
As no conversion was observed for the OPD initial feeds of 55 and 77 mol%, no GPC
measurement was performed on compounds 16 and 17. Compounds 13 and 14 were
characterized by narrow monomodal distributions as expected from a full conversion
of L-lactide. On the contrary, compound 15 displayed two narrow distributions, at
about 25 and 31 minutes retention time. They correspond to the oligomer and residual
L-lactide respectively. The values of 𝑀𝑛 for 13 and 14 were in agreement with the
Chapter 4: Functionalization of Poly(l-lactide) with 2-Oxepane-1,5-Dione 159
predicted ones. 𝑀𝑛 of 15 was lower because of the low conversion of L-lactide (Table
4.7).
Figure 4.15. Enlarged view of the carbonyl region in the ATR-FTIR spectra of
purified polymers with an OPD initial feed of 0 to 77 mol%.
160 Chapter 4: Functionalization of Poly(l-lactide) with 2-Oxepane-1,5-Dione
Figure 4.16. GPC traces of copolymers of L-lactide and OPD with an initial OPD
concentration of 0 to 23 mol% measured in chloroform (the traces were baseline-
corrected and normalized).
Table 4.7. Molecular weight and polydispersities of the purified products of the
batch polymerisations of L-lactide and OPD using DBU as the catalyst.
Entry
OPD/LLA (mol%) 𝑀𝑛 (g·mol-1)a
𝑀𝑛 (Da) b
𝑀𝑤 (Da) b Ð b
13 0 10,316 9,400 ± 100 12,300 ±
100
1.31 ±
0
14 11 11,141 10,400 ±
100
13,300 ±
100
1.28 ±
0
15 23 12,166 5,800 ± 100 7,000 ± 100 1.22 ±
0
Chapter 4: Functionalization of Poly(l-lactide) with 2-Oxepane-1,5-Dione 161
a Theoretical molecular weight assuming full conversion of both monomers. b
Measured by GPC in chloroform.
Adding OPD to the polymerisation of L-lactide resulted in either slowing down or
completely hindering the conversion of L-lactide. Lohmeijer and coworkers attempted
to polymerise δ-valerolactone and ε-caprolactone in solution at room temperature
using DBU as catalyst and 4-pyrenobutanole as initiator. Although they used DBU
loadings up to 20 mol% relative to the monomer, neither of the lactones got converted.
They suggested the catalyst was not nucleophilic enough to activate the ring-opening
polymerisation of both lactones. The addition of a thiourea-based catalyst was
necessary as a dual-activation of the monomer.7
The ring-opening polymerisation of OPD by DBU could also be hindered due to the
ketone moiety, as demonstrated when tin (II) octanoate was used as catalyst. Protecting
the ketone moiety and performing a batch polymerisation with L-lactide and the
protected version of OPD could enable the investigation of the role of the ketone
moiety on the polymerisation process. In this work, this strategy was investigated due
to its similarity with the polymerizations already carried out.
4.2.2.3. Investigation on the Role of the OPD Ketone Moiety on the
Polymerisation
The potential inhibiting role of OPD on the conversion of L-lactide during the
polymerisation was subsequently studied by protecting the ketone moiety of OPD with
acetal group (Figure 4.17).
Figure 4.17. Structure of 1,4,8-trioxaspiro[4.6]-9-undecanone.
Further information on the synthesis and characterization of this protected monomer
will be discussed in Chapter 5. L-lactide and the protected OPD (25 mol%) were
162 Chapter 4: Functionalization of Poly(l-lactide) with 2-Oxepane-1,5-Dione
stirred together at room temperature. DBU and benzyl alcohol were added to the
polymeric mixture. After 10 minutes of reaction, 97 % of L-lactide was converted, as
revealed by the integration ratio of the methine protons of PLLA and residual L-lactide
at 5.16 and 5.02 ppm respectively in 1H NMR spectra. After quenching with glacial
acetic acid, the polymerisation mixture was concentrated under reduced pressure and
the crude residues purified twice by reprecipitation, using chloroform and cold hexane
(0 - 1 °C) as solvent and non-solvent respectively. The desired product was obtained
as a white powder in 39 % yield. The 1H NMR spectrum of the crude polymer revealed
a quadruplet at 5.16 ppm and a doublet at 1.57 ppm which were assigned to the methine
and methyl protons of the L-lactide repeating unit. A quadruplet at 5.02 ppm and a
doublet at 1.69 ppm provided support for residual L-lactide. A multiplet at 7.35 ppm
of benzyl alcohol end-groups confirmed its initiating role. However, the triplets at
4.29, 2.01 and 1.90 ppm, as well as the singlet at 3.99 ppm were all characteristic of
TOSUO monomer (Figure 4.18).41 This suggested that DBU solely catalysed the
polymerisation of L-lactide and remained inactive towards the protected OPD. When
L-lactide and OPD (23 mol%) were copolymerised using the same conditions, the
kinetics of L-lactide conversion was much slower (85 % after 18 hours, refer to section
4.2.2.2). The protection of the ketone moiety of OPD with acetal groups enabled to
polymerise L-lactide within 10 minutes. This result suggests the ketone inhibited the
polymerisation. The next chapter focuses on investigating the protected version of
OPD to increase the level of incorporation and study the photodegradation of the
resulting copolymers, once the ketone would be deprotected.
Chapter 4: Functionalization of Poly(l-lactide) with 2-Oxepane-1,5-Dione 163
Figure 4.18. 1H NMR spectrum of the crude product of the ROP of L-lactide and
protected OPD revealing the conversion of L-lactide only, measured in CDCl3.
4.3 SUMMARY
Following the reactive extrusions of PLLA and OPD in Chapter 3, the strategy
employed in this chapter relied on the copolymerisation of L-lactide and OPD in the
bulk in the presence of the versatile tin (II) octanoate catalyst. Several monomer feed
ratios and conditions were investigated. Although the desired copolymers were
successfully obtained, PGSE spectroscopy analysis confirmed the incorporation of
only low amounts of OPD. Another catalytic system, comprising DBU and benzyl
alcohol as catalyst and initiator, respectively, was investigated in order to increase the
incorporation level. Poly(L-lactide) was obtained within a few minutes. A good control
164 Chapter 4: Functionalization of Poly(l-lactide) with 2-Oxepane-1,5-Dione
over the molecular weight was obtained as well as narrow polydispersities (suggesting
few transesterification reactions) and chain-end fidelity. However, DBU, an
insufficient nucleophile, no OPD ring-opening was observed. Hence, none of the
desired copolymers were obtained with the DBU-benzyl alcohol system. In both
catalytic systems, the ketone moiety hindered the polymerisation to occur. Concerning
tin (II) octanoate, the ketone moiety underwent competition with the ester functions of
both monomers in the coordination process with tin. Employing DBU on the acetal
protected OPD resulted into a complete conversion of L-lactide into PLLA. The result
supported the non-nucleophilicity of DBU as well as its effect on the ketone moiety.
The next chapter further expands the synthesis of random copolymers of poly(L-
lactide-co-OPD) with the protected-OPD so as to increase the incorporation level.
Artificially ageing was subsequently undertaken on the modified PLLAs.
4.4 EXPERIMENTAL
4.4.1. Materials
L-lactide ((3s)-cis-3.6-dimethyl-1,4-dioxane-2,5-dione) (98 %) was purchased from
Sigma-Aldrich, recrystallized twice from toluene and dried under vacuum prior to use.
OPD was synthesized as previously reported (refer to Chapter 2) and dried under
vacuum prior to use. Methanol, hexane fraction and chloroform were all AR grade,
purchased from ChemSupply and used as received. Chloroform (HPLC grade) used
for GPC analysis was purchased from Merck and filtered prior to use. Deuterium
chloroform used for NMR spectroscopy analysis was also purchased from Merck and
stored at 10 °C.
4.4.2. Methods
4.4.2.1. Ring-Opening Polymerisation of L-lactide and OPD 5 - 20 wt%
L-Lactide and OPD were introduced into a flame-dried and degassed Schlenk vessel
under nitrogen atmosphere. The Schlenk vessel was then sealed under nitrogen
atmosphere and immersed into a silicone oil bath preheated at 110 °C. Once the
monomers were completely melted, tin (II) octanoate was added to the Schlenk vessel
under nitrogen atmosphere. After being thermally quenched, the polymeric mixture
was purified by dissolution - reprecipitation using chloroform and cold methanol (0 -
Chapter 4: Functionalization of Poly(l-lactide) with 2-Oxepane-1,5-Dione 165
1 ºC; 1:10 vol:vol) as solvent and non-solvent, respectively. The polymer was
recovered by filtration.
Compound 1: L-lactide (2.0467 g, 14.20 mmol), tin (II) octanoate (5 mg, 12 µmol).
White polymer. Purification yield: 29.2 %. 1H NMR (CDCl3, δ ppm): 5.17 (q, H, CH),
1.57 (d, 3H, CH3). ATR-FTIR: ʋ max = 2995 (w, -CH2 stretching, asymmetric), 2948
(w, -CH2 stretching, symmetric), 1750 (s, -C=O carbonyl stretching), 1453 (-CH3
bending), 1382 and 1359 (-CH- deformation, symmetric and asymmetric), 1181, 1127
and 1083 (-C-O- stretching), 1043 (-OH bending), 869 (w, -C-C- stretching). GPC
(Refractive Index detector): 𝑀𝑛 (Ð) = 43,000 ± 900 g·mol-1 (1.25 ± 0.06). DSC (second
heating cycle): Tg = 56.2 ± 2.9 ºC, Tm = 174.3 ± 0.1 ºC (∆𝐻𝑚 = 51.0 ± 0.7 J·g-1).
Compound 2: L-lactide (2.0014 g, 13.89 mmol), OPD (104.0 mg, 812.5 µmol), tin (II)
octanoate (5 mg, 12 µmol). White powder. Purification yield: 24.6 %. 1H NMR
(CDCl3, δ ppm, subscripts L-LA and OPD denote each repeating units): 5.17 (q, H,
CHL-LA), 4.40 (t, 2H, -CH2OOPD), 4.34 (m, H, -CH(CH3)OHL-LA), 2.81 (m, 2H,
CH2C=OOPD), 2.66 (m, 2H, C=OCH2OPD), 1.58 (d, 3H, CH3L-LA). ATR-FTIR: ʋ max =
2996 (w, -CH2 stretching, asymmetric), 2948 (w, -CH2 stretching, symmetric), 1750
(s, -C=O carbonyl stretching) with a shoulder from 1725 to 1700 cm-1, 1454 (-CH3
bending), 1384 and 1359 (-CH- deformation, symmetric and asymmetric), 1181, 1129
and 1084 (-C-O- stretching), 1043 (-OH bending), 871 (w, -C-C- stretching). GPC
(Refractive Index detector): 𝑀𝑛 (Ð) = 12,700 ± 100 g·mol-1 (1.28 ± 0.02). DSC (second
heating cycle): Tg = 42.0 ± 2.5 ºC, Tm = 162.8 ± 0.3 ºC (∆𝐻𝑚 = 55.2 ± 0.7 J·g-1).
Compound 3: L-lactide (0.9937 g, 6.894 mmol), OPD (98.9 mg, 772.7 µmol), tin (II)
octanoate (5 mg, 12 µmol). White powder. Yield: 43.5 %. 1H NMR (CDCl3, δ ppm,
subscripts L-LA and OPD denote each repeating units): 5.17 (q, H, CHL-LA), 4.41 (t,
2H, -CH2OOPD), 4.36 (m, H, -CH(CH3)OHL-LA), 2.79 (m, 2H, CH2C=OOPD), 2.66 (m,
2H, C=OCH2OPD), 1.58 (d, 3H, CH3L-LA). ATR-FTIR: ʋ max = 2996 (w, -CH2
stretching, asymmetric), 2948 (w, -CH2 stretching, symmetric), 1750 (s, -C=O
carbonyl stretching) with a shoulder from 1725 to 1700 cm-1, 1453 (-CH3 bending),
1384 and 1358 (-CH- deformation, symmetric and asymmetric), 1181, 1128 and 1083
(-C-O- stretching), 1043 (-OH bending), 870 (w, -C-C- stretching). GPC (Refractive
Index detector): 𝑀𝑛 (Ð) = 6,900 ± 600 g·mol-1 (1.64 ± 0.18). DSC (second heating
cycle): Tg = 51.9 ± 0.5 ºC, Tm1 = 153.3 ± 0.2 ºC, Tm2 = 159.5 ± 0.4 ºC (∆𝐻𝑚 = 47.9 ±
3.4 J·g-1).
166 Chapter 4: Functionalization of Poly(l-lactide) with 2-Oxepane-1,5-Dione
Compound 4: L-lactide (2.0422 g, 14.17 mmol), OPD (418.5 mg, 3.270 mmol), tin
(II) octanoate (5 mg, 12 µmol). Yellow powder. Purification yield: 61.0 %. 1H NMR
(CDCl3, δ ppm, subscripts L-LA and OPD denote each repeating units): 5.17 (q, H,
CHL-LA), 4.42 (t, 2H, -CH2OOPD), 4.35 (m, H, -CH(CH3)OHL-LA), 2.80 (m, 2H,
CH2C=OOPD), 2.67 (m, 2H, C=OCH2OPD), 1.58 (d, 3H, CH3L-LA). ATR-FTIR: ʋ max =
2996 (w, -CH2 stretching, asymmetric), 2948 (w, -CH2 stretching, symmetric), 1754
(s, -C=O carbonyl stretching) with a shoulder from 1730 to 1700 cm-1, 1455 (-CH3
bending), 1383 and 1359 (-CH- deformation, symmetric and asymmetric), 1181, 1129
and 1086 (-C-O- stretching), 1043 (-OH bending), 871 (w, -C-C- stretching). GPC
(Refractive Index detector): 𝑀𝑛 (Ð) = 3,900 ± 100 g·mol-1 (1.51 ± 0.02). DSC (second
heating cycle): Tg = 39.8 ± 2.0 ºC, Tcc = 88.1 ± 1.7 ºC (∆𝐻𝑐𝑐= 7.0 ± 0.7 J·g-1), Tm =
137.4 ± 0.8 ºC (∆𝐻𝑚 = 35.1 ± 1.2 J·g-1).
4.4.2.2. Attempted Synthesis of Poly(L-Lactide-co-OPD) with Increased
OPD Initial Feed
L-lactide and OPD were introduced in a degassed Schlenk vessel under nitrogen
atmosphere. The Schlenk vessel was then sealed under nitrogen atmosphere and
immersed into a silicone oil bath preheated at 110 °C. Once the monomers were
completely melted, tin (II) octanoate was added to the Schlenk vessel under nitrogen
atmosphere. After 7 hours of reaction, the polymerisation was thermally quenched.
The polymeric mixture was purified by dissolution - reprecipitation using chloroform
and cold methanol (0 - 1 ºC; 1:10 vol:vol) as solvent and non-solvent, respectively.
The polymer was recovered by filtration
Compound 5: L-lactide (1.0395 g, 7.212 mmol), OPD (505.6 mg, 3.950 mmol), tin
(II) octanoate (5 mg, 12 µmol). Yellow wax. Yield: 50.0 %. 1H NMR (CDCl3, δ ppm):
5.17 (q, H, CHPLLA), 5.10 (q, H, CHL-lactide), 4.43 (m, 2H, -CH2OOPD), 4.36 (m, H, -
CH(CH3)OHL-LA), 2.79 (m, 2H, CH2C=OOPD), 2.68 (m, 2H, C=OCH2OPD), 1.59 (d, 3H,
CH3L-LA), 1.50 (d, 3H, CH3L-lactide). ATR-FTIR: ʋ max = 2996 (w, -CH2 stretching,
asymmetric), 2932 (w, -CH2 stretching, symmetric), 1752 (s, -C=O carbonyl
stretching) with a shoulder from 1730 to 1700 cm-1, 1456 (-CH3 bending), 1355 (-CH-
deformation, asymmetric), 1094 (-C-O- stretching), 1053 (-OH bending), 934 (s, -CO-
O- deformation of L-lactide).
Chapter 4: Functionalization of Poly(l-lactide) with 2-Oxepane-1,5-Dione 167
Compound 6: L-lactide (1.0068 g, 6.986 mmol), OPD (653.5 mg, 5.105 mmol), tin
(II) octanoate (5 mg, 12 µmol). Yellow wax. Yield: 48.4 %. 1H NMR (CDCl3, δ ppm):
5.19 (q, H, CHPLLA), 5.09 (q, H, CHL-lactide), 4.45 (m, 2H, -CH2OOPD), 4.37 (m, H, -
CH(CH3)OHL-LA), 2.81 (m, 2H, CH2C=OOPD), 2.687 (m, 2H, C=OCH2OPD), 1.59 (d,
3H, CH3L-LA), 1.49 (d, 3H, CH3L-lactide). ATR-FTIR: ʋ max = 2996 (w, -CH2 stretching,
asymmetric), 2932 (w, -CH2 stretching, symmetric), 1751 (s, -C=O carbonyl
stretching) with a shoulder from 1730 to 1700 cm-1, 1456 (-CH3 bending), 1355 (-CH-
deformation, asymmetric), 1094 (-C-O- stretching), 1052 (-OH bending), 934 (s, -CO-
O- deformation of L-lactide).
4.4.2.3. Attempted Synthesis of Poly(L-Lactide-co-OPD) at Higher
Temperatures
L-Lactide and OPD were introduced in a degassed Schlenk vessel under nitrogen
atmosphere. The Schlenk vessel was then sealed under nitrogen atmosphere and
immersed into a silicone oil bath preheated at the relevant temperature. Once the
monomers were completely melted, tin (II) octanoate was added to the Schlenk vessel
under nitrogen atmosphere. After 7 h of reaction, the polymerisation was thermally
quenched. The polymeric mixture was purified by dissolution - reprecipitation using
two different solvent systems: chloroform and cold methanol (0 - 1 ºC; 1:10 vol:vol)
as solvent and non-solvent, respectively and chloroform and cold hexane (0 - 1 ºC;
1:10 vol:vol) as solvent and non-solvent, respectively. The polymer was recovered by
filtration as yellow waxes.
Compound 7: L-lactide (2.0850 g, 14.47 mmol), OPD (402.6 mg, 3.145 mmol), tin
(II) octanoate (5 mg, 12 µmol). Purification yield: 67 %. ATR-FTIR: ʋ max = 2997
(w, -CH2 stretching, asymmetric), 2948 (w, -CH2 stretching, symmetric), 1754 (s, -
C=O carbonyl stretching) with a shoulder from 1730 to 1666 cm-1, 1455 (-CH3
bending), 1383 and 1358 (-CH- deformation, symmetric and asymmetric), 1181, 1129
and 1086 (-C-O- stretching), 1043 (-OH bending), 934 (-CO-O- deformation of L-
lactide), 871 (w, -C-C- stretching).
Compound 8: L-lactide (2.0392 g, 14.15 mmol), OPD (412.5 mg, 3.223 mmol), tin
(II) octanoate (5 mg, 12 µmol). Purification yield: 74 %. ATR-FTIR: ʋ max = 2997
(w, -CH2 stretching, asymmetric), 2948 (w, -CH2 stretching, symmetric), 1749 (s, -
C=O carbonyl stretching) with a shoulder from 1730 to 1666 cm-1, 1454 (-CH3
168 Chapter 4: Functionalization of Poly(l-lactide) with 2-Oxepane-1,5-Dione
bending), 1383 and 1359 (-CH- deformation, symmetric and asymmetric), 1181, 1129
and 1085 (-C-O- stretching), 1043 (-OH bending), 871 (w, -C-C- stretching).
4.4.2.4. Organocatalysed Copolymerisation of L-Lactide and OPD in
Solution
All the following experiments of this section were performed under an argon
atmosphere (Ultra High Purity) using a glovebox (Labconco). All the glassware was
oven dried at 60 °C, before being introduced into the glovebox. DCM, dried over a
purification system, was degassed 5 times and stored over molecular sieves (4 Å) under
argon atmosphere. L-lactide and OPD were both dissolved in DCM and dried over
molecular sieves (4 Å) at least 24 h prior to the reaction.
4.4.2.4.1. Homopolymerisation of L-lactide
In a typical experiment, L-lactide (298.6 mg, 2.072 mmol) was added to a glass vial.
Benzyl alcohol (2.575 mg, 23.81 µmol) and DBU (20.45 mg, 134.3 mmol) were added
to the monomer solution. Samples were collected at different times of the reaction to
evaluate the monomer conversion. The polymerisation was quenched with glacial
acetic acid after 7 minutes 30 before being concentrated under reduced pressure. The
polymeric mixture was purified by dissolution / reprecipitation using chloroform and
cold hexane (0 - 1 ºC) as the solvent and non-solvent respectively (1:10, vol:vol). The
purified product was recovered by filtration and dried under vacuum. 1H NMR (CDCl3,
δ ppm): 7.33 (m, 6H, CHBnO), 5.16 (q, H, CH), 1.57 (d, 3H, CH3). ATR-FTIR: ʋ max
= 2996 (w, -CH2 stretching, asymmetric), 2948 (w, -CH2 stretching, symmetric), 1749
(s, -C=O carbonyl stretching), 1454 (-CH3 bending), 1383 and 1359 (-CH-
deformation, symmetric and asymmetric), 1182, 1129 and 1084 (-C-O- stretching),
1044 (-OH bending), 871 (w, -C-C- stretching). GPC (Refractive Index detector): 𝑀𝑛
(Ð) = 12,000 ± 0 g·mol-1 (1.29 ± 0.01).
4.4.2.4.2. Batch Polymerisations of L-lactide and OPD
In a typical experiment, volumes of L-lactide and OPD solutions were added to a glass
vial. Benzyl alcohol and DBU were both dissolved in 2 mL of DCM and an aliquot of
each solution was added to the monomer solution (the volumes and masses are
reported in Table 4.8). The reaction was performed at room temperature and protected
from the light. Samples were collected at different times of the reaction to evaluate the
Chapter 4: Functionalization of Poly(l-lactide) with 2-Oxepane-1,5-Dione 169
monomer conversions. The polymerisation was quenched with glacial acetic acid
before being concentrated under reduced pressure. The polymeric mixture was purified
by dissolution/reprecipitation using chloroform and cold hexane (0 - 1 ºC) as the
solvent and non-solvent respectively (1:10, vol:vol). A crude fraction was collected
for conversion calculation. The purified product was recovered by filtration and dried
under vacuum. A control polymerisation of L-lactide alone was carried out using the
same procedure.
Table 4.8. Conditions of batch copolymerisations in DCM at room temperature using
DBU and benzyl alcohol as catalyst and initiator, respectively.
Entry L-lactide OPD Benzyl alcohol DBU
mg μmol mg μmol mg μmol mg μmol
13 99.50 690.30 - - 1.04 9.65 7.84 51.50
14 99.50 690.30 9.97 77.85 1.06 9.83 7.85 51.56
15 99.50 690.30 20.04 156.66 1.06 9.83 7.85 51.56
16 99.50 690.30 54.75 427.73 1.04 9.65 7.84 51.50
17 99.50 690.30 76.65 598.83 1.04 9.65 7.84 51.50
Compound 13: white powder. Purification yield: 21.6 %. 1H NMR (CDCl3, δ ppm):
7.33 (m, 6H, CHBnO), 5.17 (q, H, CHL-LA), 1.58 (d, 3H, CH3L-LA). ATR-FTIR: ʋ max =
2996 (w, -CH2 stretching, asymmetric), 2948 (w, -CH2 stretching, symmetric), 1751
(s, -C=O carbonyl stretching), 1454 (-CH3 bending), 1383 and 1359 (-CH-
deformation, symmetric and asymmetric), 1182, 1130 and 1082 (-C-O- stretching),
1044 (-OH bending), 871 (w, -C-C- stretching). GPC (Refractive Index detector): 𝑀𝑛
(Ð) = 9,400 ± 0 g·mol-1 (1.31 ± 0).
Compound 14: white powder. Purification yield: 27.2 %. 1H NMR (CDCl3, δ ppm,
subscripts L-LA and OPD denote each repeating units): 7.34 (m, 6H, CHBnO), 5.17 (q,
H, CHL-LA), 4.42 (t, 2H, -CH2OOPD), 4.35 (m, H, -CH(CH3)OHL-LA), 2.80 (m, 2H,
CH2C=OOPD), 2.67 (m, 2H, C=OCH2OPD), 1.58 (d, 3H, CH3L-LA). ATR-FTIR: ʋ max =
2995 (w, -CH2 stretching, asymmetric), 2948 (w, -CH2 stretching, symmetric), 1749
(s, -C=O carbonyl stretching), 1453 (-CH3 bending), 1382 and 1359 (-CH-
170 Chapter 4: Functionalization of Poly(l-lactide) with 2-Oxepane-1,5-Dione
deformation, symmetric and asymmetric), 1182, 1129 and 1082 (-C-O- stretching),
1044 (-OH bending), 870 (w, -C-C- stretching). GPC (Refractive Index detector): 𝑀𝑛
(Ð) =10,400 ± 0 g·mol-1 (1.28 ± 0).
Compound 15: white powder. Purification yield: 24.4 %. 1H NMR (CDCl3, δ ppm,
subscripts L-LA and OPD denote each repeating units): 7.34 (m, 6H, CHBnO), 5.17 (q,
H, CHL-LA), 4.42 (t, 2H, -CH2OOPD), 4.35 (m, H, -CH(CH3)OHL-LA), 2.80 (m, 2H,
CH2C=OOPD), 2.67 (m, 2H, C=OCH2OPD), 1.58 (d, 3H, CH3L-LA). ATR-FTIR: ʋ max =
2996 (w, -CH2 stretching, asymmetric), 2948 (w, -CH2 stretching, symmetric), 1749
(s, -C=O carbonyl stretching), 1453 (-CH3 bending), 1383 and 1359 (-CH-
deformation, symmetric and asymmetric), 1181, 1129 and 1082 (-C-O- stretching),
1044 (-OH bending), 870 (w, -C-C- stretching). GPC (Refractive Index detector): 𝑀𝑛
(Ð) = 5,800 ± 0 g·mol-1 (1.22 ± 0).
Compound 16: white powder. Purification yield: 9.94 %. 1H NMR (CDCl3, δ ppm,
subscripts L-LA and OPD denote each monomers): 5.01 (q, H, CHL-LA), 4.42 (t, 2H, -
CH2OOPD), 2.80 (m, 2H, CH2C=OOPD), 2.67 (m, 2H, C=OCH2OPD), 1.70 (d, 3H, CH3L-
LA). ATR-FTIR: ʋ max = 3304 (b, w, -COOH stretching), 2984 (w, -CH2 stretching,
asymmetric), 2948 (w, -CH2 stretching, symmetric), 1722 (s, -C=O carbonyl
stretching), 1704 (s, -C=O carbonyl stretching of OPD), 1454 (-CH3 bending), 1374
and 1346 (-CH- deformation, symmetric and asymmetric), 1123 and 1082 (-C-O-
stretching), 1042 (-OH bending), 870 (w, -C-C- stretching).
Compound 17: white wax. Purification yield: 6.31 %. 1H NMR (CDCl3, δ ppm,
subscripts L-LA and OPD denote each monomers): 5.01 (q, H, CHL-LA), 4.42 (t, 2H, -
CH2OOPD), 2.80 (m, 2H, CH2C=OOPD), 2.67 (m, 2H, C=OCH2OPD), 1.70 (d, 3H, CH3L-
LA). ATR-FTIR: ʋ max = 2996 (w, -CH2 stretching, asymmetric), 2948 (w, -CH2
stretching, symmetric), 1749 (s, -C=O carbonyl stretching), 1453 (-CH3 bending),
1383 and 1359 (-CH- deformation, symmetric and asymmetric), 1181, 1129 and 1082
(-C-O- stretching), 1044 (-OH bending), 940 (-CO-O- deformation of L-lactide), 867
(w, -C-C- stretching).
4.4.2.5. Investigation on the Role of the OPD Ketone Moity on the
Polymerisation
L-Lactide (202.0 mg, 1.402 mmol) and TOSUO (60.00 mg, 348.8 µmol) and were
dissolved in 1 and 1.2 mL DCM respectively, dried over molecular sieves, and mixed
Chapter 4: Functionalization of Poly(l-lactide) with 2-Oxepane-1,5-Dione 171
together. Benzyl alcohol (2.268 mg, 20.97 μmol) and DBU (14.5 mg, 95.27 µmol)
were both added to the monomers solution. After 10 minutes, a fraction was collected
for conversion calculation by 1H NMR spectroscopy. The polymerisation was
quenched with glacial acetic acid before being concentrated under reduced pressure.
The polymeric mixture was purified by dissolution/reprecipitation using chloroform
and cold hexane (0 - 1 ºC) as the solvent and non-solvent respectively (1:10, vol:vol).
1H NMR (CDCl3, δ ppm, subscript L-LA denotes the PLLA repeating units, subscript
TOSUO denotes the TOSUO monomer): 7.34 (m, 6H, CHBnO), 5.17 (q, H, CHL-LA),
4.29 (t, 2H, C=OOCH2TOSUO), 3.99 (s, 4H, C-O-CH2CH2OTOSUO), 2.70 (t, 2H,
CH2C=OOCH2TOSUO), 2.01 (t, 2H, C-OCH2CH2O-CH2CH2TOSUO), 1.90 (t, 2H, C-
OOCH2CH2C-OTOSUO), 1.58 (d, 3H, CH3L-LA).
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Chapter 5: Photodegradation of Functionalized Poly(l-Lactide) with 2-Oxepane-1,5-Dione 175
Chapter 5: Photodegradation of
Functionalized Poly(L-Lactide)
with 2-Oxepane-1,5-Dione
5.1 BACKGROUND
In the preceding chapter (Chapter 4), modification of poly(L-lactide) was achieved
through bulk copolymerisation of L-lactide and 2-oxepane-1,5-dione using tin (II)
octanoate as the catalyst. However, only low levels of incorporated OPD were
achievable because of a competition between the ketone moiety in OPD and the ester
groups of the monomers for the coordination process with the tin catalyst. In line with
these findings, the synthesis of copolymers containing higher concentrations of
chromophores by employing a modified OPD monomer is explored in this chapter.
This modified OPD monomer, 1,4,8-trioxaspiro[4.6]-9-undecanone (TOSUO)
features an ethylene ketal protecting group (Figure 5.1).1 The first reported synthesis
of OPD was achieved via a Baeyer-Villiger oxidation of 1,4-cyclohexane
monoethylene acetal with m-chloroperoxybenzoic acid (mCPBA) to afford TOSUO.
Subsequent deprotection of the ketone moiety with triphenylcarbenium
tetrafluoroborate furnished OPD.2
Figure 5.1. Structure of 1,4,8-trioxaspiro[4.6]-9-undecanone.
TOSUO has been used by others to generate a platform of polyesters bearing additional
functional groups. For instance, TOSUO was successfully polymerized with ε-
caprolactone (ε-CL) to yield random and block copolymers in solution. In this case,
176 Chapter 5: Photodegradation of Functionalized Poly(l-Lactide) with 2-Oxepane-1,5-Dione
aluminium isopropoxide was employed as the initiator. Close agreement was found
between the initial TOSUO feed and the concentration of repeating units with values
up to 90 mol%.3-5 Tin (II) octanoate, as a catalyst, also proved to efficiently
copolymerize ε-CL and TOSUO when combined with poly(ethylene glycol) or benzyl
alcohol as initiators.6-9 Prime et al.10 first reported the synthesis of poly(L-lactide-co-
TOSUO) in the bulk using tin (II) octanoate and butanol as the catalyst and initiator
respectively. A level of incorporated TOSUO of 3.8 mol% was achieved for a 7.4
mol% initial feed. Babasola and coworkers synthesized low molecular weight
copolymers of poly(TOSUO-co-ε-caprolactone) and D,L-lactide in the bulk at 110 ºC,
using tin (II) octanoate. They used both octan-1-ol and methoxy poly(ethylene glycol)
(350 Da) as initiators to tune the viscosity. They obtained copolymers featuring
number average molecular weights ranging from 2,200 to 2,900 Da.11
In this chapter, L-lactide was copolymerized with TOSUO in the bulk at 110 °C, via
initiation with benzyl alcohol and tin (II) octanoate to afford copolymers with various
compositions (Scheme 5.1.a). The ketones were subsequently deprotected to afford
poly(L-lactide-co-OPD) (Scheme 5.1.b). An ageing study was undertaken for two
PLLA-co-OPD to assess the influence of the ketone moieties and their concentration
on the photodegradation of the copolymers.
Chapter 5: Photodegradation of Functionalized Poly(l-Lactide) with 2-Oxepane-1,5-Dione 177
Scheme 5.1. Two step synthesis of poly(L-lactide-co-OPD): Conditions and reagents:
a. tin (II) octanoate, in the bulk, 110 ºC; b. triphenylcarbenium tetrafluoroborate
(TPFB), DCM, room temperature, 2 hours.
5.2 RESULTS AND DISCUSSION
1,4,8-Trioxaspiro[4.6]-9-undecanone (TOSUO) was first synthesized and
characterized. Following the synthesis of TOSUO, the monomer was copolymerized
with L-lactide in the bulk at 110 ºC, using tin (II) octanoate and benzyl alcohol as the
catalyst and initiator respectively.
5.2.1 Synthesis of 1,4,8-Trioxaspiro[4.6]-9-Undecanone
1,4,8-Trioxaspiro[4.6]-9-undecanone was synthesized via a Baeyer-Villiger oxidation
of 1,4-cyclohexane monoethylene acetal with m-chloroperoxybenzoic acid (mCPBA)
according to reported procedures (Scheme 5.2).1, 11 TOSUO and the mCPBA by-
product, 3-chlorobenzoic acid, were separated via column chromatography
(hexane/ethyl acetate 65/35), with TOSUO obtained as an oil in 40% yield. TOSUO
178 Chapter 5: Photodegradation of Functionalized Poly(l-Lactide) with 2-Oxepane-1,5-Dione
remained as an oil (no solid obtained as reported in the literature) even after prolonged
drying under vacuum.
Scheme 5.2. Baeyer-Villiger oxidation of 1,4-cyclohexane monoethylene acetal by
mCPBA to afford TOSUO and the mCPBA by-product, 3-chlorobenzoic acid.
1H NMR spectroscopy confirmed the presence of the ethylene ketal protons at 3.99
ppm, the methylene protons α to the ethylene ketal groups at 2.01 and 1.90 ppm, as
well as the methylene protons adjacent to the ester group at 4.29 and 2.70 ppm (Figure
5.2).1 The obtained 13C NMR spectrum of TOSUO was also consistent with data
reported in the literature (Figure 5.3)12. The measured melting point was in the range
44 – 47 ºC. The Attenuated Total Reflectance Fourier Transform Infrared (ATR-FTIR)
spectra revealed a strong ester stretching band at 1724 cm-1 supporting the formation
of the ester group.
Once prepared, the TOSUO monomer was subsequently used to modify the backbone
of PLLA. Previous report stated the thermal instability of TOSUO for temperatures
above 120 ºC, leading to the deprotection of the ketone acetal groups to afford 2-
oxepane-1,5-dione.11 Based on both the literature and the findings in Chapter 3, no
reactive extrusion was performed between PLLA and TOSUO. Instead, the TOSUO
monomer prepared here was subsequently copolymerised with L-lactide in the bulk.
Chapter 5: Photodegradation of Functionalized Poly(l-Lactide) with 2-Oxepane-1,5-Dione 179
Figure 5.2. 1H NMR spectrum of TOSUO, measured in CDCl3.
Figure 5.3. 13C NMR spectrum of TOSUO, measured in CDCl3.
180 Chapter 5: Photodegradation of Functionalized Poly(l-Lactide) with 2-Oxepane-1,5-Dione
5.2.2 Synthesis of Poly(L-Lactide-co-TOSUO)
5.2.2.1. Ring-Opening Polymerisation of L-Lactide and TOSUO
Poly(L-lactide-co-TOSUO) was synthesized by a transition-metal catalysed ring-
opening polymerisation of L-lactide and TOSUO using tin (II) octanoate and benzyl
alcohol as the co-initiator (Scheme 5.3). A typical polymerisation involved the
introduction of both monomers and benzyl alcohol in a flame-dried Schlenk vessel
under an inert atmosphere in a glovebox, followed by the melting of the monomers
and subsequent addition of tin (II) octanoate. The initial molar fraction of TOSUO
ranged from 5 to 25 mol%. This range was selected because higher OPD incorporation
within the copolymer would lead to insolubility of the compounds.13, 14 Moreover, this
range is suitable enough to check whether the incorporated OPD has any effect on the
photooxidation process.
Scheme 5.3. ROP of L-lactide and TOSUO in the bulk at 110 °C to afford poly(L-
lactide-co-TOSUO) using tin (II) octanoate and benzyl alcohol as the catalyst and the
initiator respectively.
Efforts were made to remove traces of moisture from the reactants and atmosphere
used during polymerisation. This involved drying the monomers under vacuum prior
to each polymerisation, and distilling benzyl alcohol over calcium hydride and storing
it under an inert atmosphere. The polymerisations were carried out under an inert
Chapter 5: Photodegradation of Functionalized Poly(l-Lactide) with 2-Oxepane-1,5-Dione 181
atmosphere in the bulk at 110 °C to ensure the complete melting of both monomers
and yet limit the risk of thermal degradation via backbiting transesterifications as well
as epimerization.13, 14 Deprotection of TOSUO was also reported to occur at 120 ºC
after twenty four hours, revealing the ketone pendent moieties.11 Benzyl alcohol was
used as an initiator in order to control the architecture of the synthesized copolymers
to deliver a known end-group. The high boiling point of benzyl alcohol allowed its use
at 110 °C. The reactions were performed until no stirring of the polymeric mixture was
possible anymore due to an increase in viscosity. At this stage they were thermally
quenched. The polymers were purified by reprecipitation using tetrahydrofuran and
cyclohexane as solvent and non-solvent respectively to remove any unreacted
monomer and tin (II) octanoate residues. In comparison to Soxhlet extraction, the
dissolution / reprecipitation technique is efficient at removing higher quantities of
residual tin.15
5.2.2.1.1. Chemical Structures of Synthesized Poly(L-Lactide-co-TOSUO)
The incorporation of TOSUO onto PLLA was confirmed by 1H NMR spectroscopy
analysis. The quadruplet at 5.16 ppm and the doublet at 1.57 ppm were respectively
assigned to the methine and methyl protons of the L-lactide repeating unit (Figure 5.4).
The doublet at 1.50 ppm and the multiplet at 4.35 ppm correspond to the methyl and
methine of the -CH(CH3)-OH end group respectively.16 The TOSUO repeating unit
was confirmed by the presence of ethylene ketal protons at 3.92 ppm, the methylene
protons adjacent to the ethylene ketal groups at 1.98 ppm and the methylene protons
adjacent to the ester moiety at 2.45 and 4.23 ppm.1 The multiplet at 1.98 ppm supports
successful ring-opening of TOSUO as reported in the literature.1, 12 In comparison, this
multiplet resonates at 1.90 ppm in the monomer. The multiplet at 7.35 ppm shows the
presence of benzyl alcohol end-groups, with a small multiplet observed next to the
methine protons of the L-lactide repeat unit (at 5.23 – 5. 19 ppm) being attributed to
stereosequence combinations including D-lactide units.17, 18 This suggests that
epimerization occurred during polymerisation, which can be caused by both inter- and
intramolecular transesterifications that are favoured by prolonged reactions as well as
the use of tin (II) octanoate as the catalyst.15
182 Chapter 5: Photodegradation of Functionalized Poly(l-Lactide) with 2-Oxepane-1,5-Dione
Figure 5.4. Representative 1H NMR spectrum of poly(L-lactide-co-TOSUO),
measured in CDCl3.
The concentration of TOSUO repeat units was determined, after several purification
steps to ensure complete removal of any residual unreacted TOSUO, by calculations
based on the integrations of the methylene and alkyl protons of PLLA and PTOSUO
respectively. The composition of each copolymer is summarized in Table 5.1.
Although the polymerisations were thermally quenched after stirring was ceased (due
to high conversion of monomers), the reaction times varied from 19 to 44 hours. This
suggests some instability in the process which could arise from impurities that
potentially acted as initiating species.
Chapter 5: Photodegradation of Functionalized Poly(l-Lactide) with 2-Oxepane-1,5-Dione 183
Table 5.1. Copolymerisation of L-lactide and TOSUO in the bulk at 110 °C catalysed
by tin (II) octanoate.
a Calculated from integration ratios in 1H NMR spectra of purified copolymers.
The chemical structure of the product was further analysed by ATR-FTIR
spectroscopy. The ATR-FTIR spectra of the different poly(L-lactide-co-TOSUO)s are
shown in Figure 5.5. All the absorption bands matched the characteristic bands of a
neat PLLA: including a -C=O stretching band at 1755 cm-1, -C-O- stretching bands at
1182 and 1087 cm-1, and a -CH3 bending at 1455 cm-1.19, 20 Closer observation of the
carbonyl region (1850 – 1650 cm-1) indicates two shoulders around 1775 and 1715 cm-
1 (Figure 5.6). The shoulder at 1775 cm-1 was assigned to a butyrolactone carbonyl
group arising from the transesterifications between an ester group and a hydroxyl
group;21, 22 while that at 1715 cm-1 represented the ketone stretching band, suggesting
that deprotection of the ketone occurred to a small degree during the polymerisation
(favoured by relatively high temperatures and long reaction times, as discussed in
Section 5.2.1). The 1H NMR spectra did not reveal any resonance at 2.9 and 2.7 ppm,
assigned to ketone pendent moieties.2, 23 The concentration of such groups could be
below the detection limit of NMR spectroscopy. The two maxima observed around
1755 cm-1 were due to noise resulting from the resolution (4 cm-1) used to run the
spectra and the normalization process.
Entry Initial feed ratio
(mol%)
(LLA / TOSUO)
Reaction time Copolymer ratio
(mol%) a
(PLLA / PTOSUO)
18 100 / 4.4 19 h 100 / 4.8
19 100 / 9.6 43 h 100 / 4.8
20 100 / 15 44 h 100 / 8.3
21 100 / 25 20 h 100 / 12.7
184 Chapter 5: Photodegradation of Functionalized Poly(l-Lactide) with 2-Oxepane-1,5-Dione
Figure 5.5. ATR-FTIR spectra of different poly(L-lactide-co-TOSUO)s (average of 9
spectra per film after baseline correction and normalization with the -CH3 bending
band at 1455 cm-1).
Chapter 5: Photodegradation of Functionalized Poly(l-Lactide) with 2-Oxepane-1,5-Dione 185
Figure 5.6. Enlarged view of the carbonyl region (1850 – 1650 cm-1) in the ATR-
FTIR spectra of the different poly(L-lactide-co-TOSUO)s (the two maxima observed
around 1755 cm-1 were due to noise resulting from the resolution (4 cm-1) used to run
the spectra and the normalization process).
5.2.2.1.2. Molecular Weights of Synthesized Poly(L-lactide-co-TOSUO)
The molecular weights and polydispersities of the copolymers were evaluated by GPC
using chloroform as eluent (Figure 5.7).
186 Chapter 5: Photodegradation of Functionalized Poly(l-Lactide) with 2-Oxepane-1,5-Dione
Figure 5.7. GPC traces of purified poly(L-lactide-co-TOSUO) measured in
chloroform (the traces were baseline-corrected and normalized).
Table 5.2. Comparison of the theoretical 𝑀𝑛 and the measured values for the
different poly(L-lactide-co-TOSUO) copolymers.
Copolymer Theoretical
𝑀𝑛 a
𝑀𝑛
(Da)b
𝑀𝑤
(Da)b
Ð b 𝑀𝑛
(g·mol-1)c
18 16,250 5,600 ± 200 7,300 ± 100 1.32 ± 0.05 2,600
20 18,300 6,100 ± 500 8,900 ± 100 1.47 ± 0.10 2,220
21 13,630 4,700 ± 900 6,000 ± 1,300 1.28 ± 0.06 2,820
a Theoretical value of 𝑀𝑛 in the case of full conversion of both monomers; b Measured
by GPC in chloroform; c Calculated from integration ratios of benzyl alcohol end
groups at 7.35 ppm with both methine protons of PLLA at 5.16 ppm and the methylene
protons adjacent to the ethylene ketal groups at 1.98 ppm, in 1H NMR spectroscopy.
Chapter 5: Photodegradation of Functionalized Poly(l-Lactide) with 2-Oxepane-1,5-Dione 187
The initial ratios of monomer to initiator were calculated to reach a 𝑀𝑛 that was greater
than the entanglement molecular weight of PLLA (8-10 kg·mol-1), assuming living
polymerisation conditions were achieved.24 The 𝑀𝑛 values measured by GPC were
significantly lower than the expected values. However, these values were relative to
polystyrene. Compound 19 featured the lowest values measured by GPC with a 𝑀𝑛 of
800 ± 0 Da, a 𝑀𝑤 of 1,200 ± 0 Da and a polydispersity of 1.52 ± 0.01. However,
compound 19 was analysed by GPC six months after being synthesized. It was stored
in the dark in the meantime. These low values suggested some degradation upon
storage for several months. Regarding the other compounds, the lower molecular
weights obtained suggest the potential presence of other initiators such as solvent
impurities or residual moisture that could react with tin (II) octanoate. The
polymerisations follow a coordination-insertion mechanism in which tin (II) octanoate
reacts with benzyl alcohol or residual protic impurities to afford tin (II) alkoxide as the
actual initiator (Scheme 5.4).25 The polymerisation then proceeds via the cleavage of
the acyl-oxygen bond of the monomer, followed by its subsequent insertion into the
chain.26, 27 Adding benzyl alcohol to the initial reacting mixture is aimed to control the
resulting molecular weight from the monomer / initiator molar ratio.25 However, other
impurities may have initiated tin (II) octanoate, as revealed by the GPC measurements.
Scheme 5.4. Conversion of tin (II) octanoate into tin (II) alkoxide via reaction with
alcohol or residual protic impurities.
Moreover, the results were obtained following a calibration with narrow polystyrene
standards, which can also explain the observed differences. The molecular weights
were determined by 1H NMR spectroscopy as well. The experimental 𝑀𝑛 was deduced
from the integration ratios of the multiplet at 7.35 ppm assigned to benzyl end-group
protons with the methine proton of PLLA at 5.16 ppm and the methylene protons of
PTOSUO at 1.98 ppm. Similar to the values measured by GPC, the calculated 𝑀𝑛 were
lower than the theoretical values. However, only the protons from the benzyl end-
group could be observed in the 1H NMR spectra.
Sn(Oct)2 + ROH Oct-Sn-OR + OctH
Oct-Sn-OR + ROH Sn(OR)2 OctH+
188 Chapter 5: Photodegradation of Functionalized Poly(l-Lactide) with 2-Oxepane-1,5-Dione
5.2.2.1.3. Thermal Properties of Synthesized Poly(L-lactide-co-TOSUO)
The thermal properties of the copolymers were analysed by DSC on the second heating
run after erasing the thermal history of the copolymers. All copolymers were
semicrystalline, exhibiting a glass transition (Tg), an exothermic peak assigned to cold
crystallization (Tcc) and an endothermic peak corresponding to melting (Tm) (Figure
5.8). The unique glass transition indicates the amorphous phases of both PLLA and
PTOSUO were miscible and suggests the random distribution of TOSUO segments
along the PLLA chains. The values of Tg randomly varied with the concentration of
TOSUO segments (Table 5.3). For random copolymers, the Tg can be predicted based
on the Fox relationship:
1
𝑇𝑔=
𝑤1
𝑇𝑔1+
𝑤2
𝑇𝑔2
Where 𝑤1, 𝑤2 represent the weight fraction and 𝑇𝑔1, 𝑇𝑔2 are the glass transitions of
PLLA and POPD respectively.28 PTOSUO was reported to exhibit a Tg ranging from
-35 to -14 ºC (for number average molecular weights ranging from 2,600 to 12,400
g·mol-1 respectively).4 Therefore lower values of Tg for the copolymers compared to
neat PLLA were expected. The endothermic peak featured a shoulder, referred to as
Tm1 in Table 5.3.
Tian and coworkers synthesized random copolymers of -caprolactone and TOSUO
and reported a single glass transition suggesting the homogeneity of amorphous phases
and their randomness while block copolymers were characterized by two glass
transitions.3, 4 An increase in the glass transition of poly(ε-caprolactone-co-TOSUO)
was observed with the TOSUO content (-60 to -40 ºC with a TOSUO content of 0 to
1 mol% respectively).3 On the contrary, the melting temperatures decreased until a
TOSUO content of 15 mol%, above which the copolymers became amorphous.3 The
double melting behaviour of PLLA was explained by the melting-recrystallization
model, with the melting of original crystals and the melting of crystals formed during
the heating cycle of the DSC measurement.29
Chapter 5: Photodegradation of Functionalized Poly(l-Lactide) with 2-Oxepane-1,5-Dione 189
Figure 5.8. DSC thermograms from the second heating cycle of purified poly(L-
lactide-co-TOSUO).
Table 5.3. Thermal properties of the different poly(L-lactide-co-TOSUO) (three
measurements were performed and the values were averaged).
Entry Tg (°C) Tcc (°C) ∆𝐻𝑐𝑐
(J.g-1)
Tm1 (°C) Tm2 (°C) ∆𝐻𝑚(J.g-
1)
𝜒𝑐 (%)
18 49.5 ±
0.9
88.8 ±
0.4
14.3 ±
4.7
- 148.2 ±
1.7
49.1 ±
5.2
67.6 ±
9.8
19 35.3 ±
2.0
100 ±
2.0
1.1 ±
0.4
- 113.9 ±
1.3
1.6 ± 0.8 2.5 ±
1.4
20 43.7 ±
1.7
99.8 ±
2.1
27.9 ±
3.1
125.5 ±
1.4
139.9 ±
0.7
40.9 ±
0.9
73.4 ±
4.3
21 40.1 ±
0.2
88.8 ±
0.3
15.2 ±
0.9
128.1 ± 0 143.2 ±
0
43.9 ± 0 42.0
190 Chapter 5: Photodegradation of Functionalized Poly(l-Lactide) with 2-Oxepane-1,5-Dione
5.2.3 Synthesis of Poly(L-Lactide-co-2-Oxepane-1,5-Dione)
5.2.3.1. Deprotection of the Ketone Acetal Groups
As shown above, poly(L-lactide-co-TOSUO) was successfully synthesized with
various levels of TOSUO incorporated into the structure. Copolymers 18 and 20, with
4.8 and 8 mol% incorporated TOSUO and featuring relatively the same molecular
weight, were selected to be deprotected for subsequent artificial ageing. Deprotection
of the ethylene ketal groups afforded the corresponding ketone-containing
copolymers. Deprotection of acetal groups is usually performed under acidic
conditions. However, such conditions tend to hydrolyse the ester linkages of the L-
lactide repeating units which leads to reduction of the molecular weight.3
Triphenylcarbenium tetrafluoroborate (TPFB), on the other hand, efficiently
deprotected the ethylene ketal groups without altering the polymer backbone (Scheme
5.5).3, 10 The mechanism involves hydride transfer from the ethylene acetal to
triphenylcarbenium tetrafluoroborate, resulting in the formation of an oxonium ion that
is subsequently hydrolysed during aqueous work-up (Scheme 5.6).30, 31
In a typical deprotection reaction, poly(L-lactide-co-TOSUO) and TPFB were
dissolved in DCM and stirred at room temperature for 2 hours (Scheme 5.7).
Following the re-precipitation of the polymeric mixture from cold methanol, the
precipitate was isolated by filtration to give a white powder.
Scheme 5.5. Chemical structure of triphenylcarbenium tetrafluoroborate (TPFB).
Chapter 5: Photodegradation of Functionalized Poly(l-Lactide) with 2-Oxepane-1,5-Dione 191
Scheme 5.6. Mechanism of the deprotection using TPFB involving a hydride
abstraction from the ethylene acetal that affords an oxonium ion that is subsequently
quenched by aqueous work-up.30, 31
Scheme 5.7. Deprotection of the ketone acetal groups of poly(L-lactide-co-TOSUO)
using TPFB in DCM at room temperature to afford poly(L-lactide-co-OPD).
Conditions and reagents: a. poly(L-lactide-co-OPD), TPFB (1.5 equivalents of
ethylene ketal groups), DCM, 2 hours, room temperature, 80 - 85 %.
5.2.3.2. Chemical Structures of Copolymers After Deprotection
The conversion to ketone groups after deprotection was monitored using 1H NMR
spectroscopy. A representative spectrum of the copolymer before and after the
deprotection step is depicted in Figure 5.9. The acetal protons at 3.99 ppm in the
192 Chapter 5: Photodegradation of Functionalized Poly(l-Lactide) with 2-Oxepane-1,5-Dione
poly(L-lactide-co-TOSUO) spectrum were not visible in the poly(L-lactide-co-OPD)
spectrum. Moreover, the multiplets at 1.99 and 2.47 ppm (referred to as e and d
respectively in the starting material) shifted to 2.80 and 2.66 ppm respectively in the
product, thus supporting the completion of the deprotection reaction. The ratio a:b
between the resonances of the protons of benzyl chain-ends and of the methine protons
of the PLLA segments slightly decreased after the deprotection step. This either
suggests an increase in molecular weight or cleavage of benzyl groups.
Figure 5.9. Comparison of the 1H NMR spectra of copolymer before (referred to as
A) and after deprotection of the ketones in DCM at room temperature (referred to as
B).
Chapter 5: Photodegradation of Functionalized Poly(l-Lactide) with 2-Oxepane-1,5-Dione 193
ATR-FTIR spectroscopy confirmed the recovery of the ketone. The carbonyl region
(1850 – 1650 cm-1) of the ATR-FTIR spectra of the copolymer before and after the
deprotection of the ketone acetal is shown in Figure 5.10. The spectra shows two
distinct bands with maximum at 1745 and 1755 cm-1 respectively. However, these two
bands result from noise due to the resolution (4 cm-1) and the normalization process.
The ester band of poly(L-lactide-co-OPD) shows additional shoulders compared to
poly(L-lactide-co-TOSUO). The shoulder from 1730 to 1700 cm-1 corresponds to the
ketone stretching band of OPD repeating units.32 The shoulder around 1770 cm-1 could
be due to the carbonyl stretching of a butyrolactone-type product resulting from
intramolecular transesterification during the deprotection step. The shoulder observed
after the deprotection step remained small, suggesting a low extent of
transesterification. Such unwanted intramolecular transesterification was reported
before for crosslinked star-poly(OPD-co--CL) with terminal hydroxyl groups
attacking an internal ester function during ketone reduction step.21, 33
Figure 5.10. ATR-FTIR spectra of poly(LLA-co-TOSUO) and poly(LLA-co-OPD)
with an enlarged view of the carbonyl region (1850 – 1650 cm-1) revealing a
194 Chapter 5: Photodegradation of Functionalized Poly(l-Lactide) with 2-Oxepane-1,5-Dione
shoulder at 1715 cm-1 corresponding to the C=O stretching band of the ketone moity
of OPD, a shoulder at 1775 cm-1 corresponding to the carbonyl stretching of a
lactone-type product resulting from transesterification during the deprotection step.
5.2.3.3. Molecular Weights of Poly(L-Lactide-co-OPD)
The extent of degradation during the deprotection step was further investigated by
analysing the molecular weights of the different poly(L-lactide-co-OPD) polymers by
GPC in chloroform with a refractive index detector. The deprotection step did not
significantly alter the molecular weights as shown by the negligible difference between
the GPC traces (Figure 5.11).
Figure 5.11. Comparison of the GPC traces of the copolymer before and after
deprotection of the ketones, measured in chloroform (the traces were baseline-
corrected and normalized).
Chapter 5: Photodegradation of Functionalized Poly(l-Lactide) with 2-Oxepane-1,5-Dione 195
Table 5.4. Molecular weights of copolymers before and after deprotection, as
measured by GPC in chloroform.
𝑀𝑛
(g·mol-1)
𝑀𝑤
(g·mol-1)
Đ
18 Protected 5,600 ± 800 6,500 ± 1,200 1.28 ± 0.06
18’ Deprotected 5,400 ± 400 6,710± 100 1.32 ± 0.01
20 Protected 6,100 ± 500 8,900 ± 100 1.47 ± 0.10
20’ Deprotected 5,500 ± 100 7,500 ± 100 1.37 ± 0.03
5.2.3.4. Thermal Properties of Poly(L-Lactide-co-OPD)
Thermal properties of the copolymers were analysed by DSC using the second heating
run after erasing the thermal history of the samples. Similar to the protected
copolymers (refer to Section 5.2.2.1.3.), the deprotected copolymers were
semicrystalline, featuring a glass transition, an exothermic peak assigned to cold
crystallization (Tcc), and an endothermic peak corresponding to melting (Tm). The
variations in glass transition were within the measurement error, while the other
thermal transitions appeared randomly with no specific trend (Table 5.5).
Table 5.5. Thermal transitions of poly(L-lactide-co-OPD) after the deprotection of
the ketone acetal groups (three measurements were performed and values were
averaged).
Entry Tg
(°C)
Tcc
(°C)
∆𝐻𝑐𝑐
(J·g-1)
Tm1
(°C)
Tm2
(°C)
∆𝐻𝑚
(J·g-1)
𝜒𝑐
(%)
18 49.5 ±
0.9
88.8 ±
0.4
14.3 ±
4.7
- 148.2 ±
1.7
49.1 ±
5.2
67.6 ±
9.8
18’ 49.6 ±
5.7
- - - 149.4 ±
0.4
69.1 ±
6.5
73.7 ±
7.0
20 43.7 ±
1.7
99.8 ±
2.1
27.9 ±
3.1
125.5 ±
1.4
139.9 ±
0.7
40.9 ±
0.9
73.4 ±
4.3
20’ 44.3 ±
1.5
94.9 ±
1.0
10.8 ±
2.8
131.0 ±
1.7
142.2 ±
0.9
39.7 ±
2.6
53.9 ±
4.9
196 Chapter 5: Photodegradation of Functionalized Poly(l-Lactide) with 2-Oxepane-1,5-Dione
Previous studies have shown that removal of the ethylene ketal protecting groups of
poly(ε-caprolactone-co-TOSUO) random copolymers induced a marked increase in
glass transition, melting temperature and melting enthalpies (-53 to -43 ºC; 46 to 64
ºC and 55.5 to 87.5 J·g-1, respectively).3, 4 The variations of glass transitions observed
in the literature could be explained by the Fox-Flory relationship. For instance, the
glass transition of PTOSUO was determined to be in the range -35 to -14 ºC (depending
on the number average molecular weight) whereas POPD displayed a Tg of 37 ºC.2, 4
A series of poly(L-lactide-co-TOSUO) were synthesized with various initial feeds of
TOSUO. Several low molecular weight copolymers were obtained with a maximum
incorporation level of 8 mol%. The low molecular weights resulted from remaining
initiator-derived impurities in the TOSUO monomer, as well as the difficulty to handle
tin (II) octanoate due to its high viscosity. A deprotection step afforded poly(L-lactide-
co-OPD) as confirmed by spectroscopic techniques.
5.2.4 Photodegradation of Poly(L-lactide-co-OPD)
Following the synthesis of copolymers of L-lactide and OPD, the prodegradant effect
of the ketone moieties incorporated into the polymer backbone was investigated with
respect to the photodegradation rate. Copolymers were artificially aged using a QUV
accelerated weathering tester (Q-lab, Ohio) under UV-A light at 50 °C for 240 hours.
As discussed in Section 5.2.2.3., the molecular weights of the synthesized copolymers
range between 2,500 and 6,000 g·mol-1. The obtained molecular weights were thus
below the entanglement molecular weight of PLLA, reported to be 8,000 – 10,000
g·mol-1.34 Attempts to obtain films via a solvent-casting method resulted in recovering
the polymer as a powder and films could not be produced. Therefore, the copolymers
were aged as powders between quartz plates, which were then mounted onto
aluminium holders in the QUV. Two poly(L-lactide-co-OPD) copolymers were
artificially aged to assess the prodegradant role played by the ketone moieties on the
evolution of the chemical structure and molecular weight with UV-A irradiation. The
copolymers featured different concentrations of OPD segments, 5.2 and 8 mol%, to
study the effect of this concentration on the photodegradation rate.
As reviewed in Chapter 1, PLLA features a carbonyl group in its backbone. Such
group absorbs UV irradiation at 280 nm via the n-π* transition with the corresponding
extinction coefficient ɛ at that wavelength less than 100 L·mol-1·cm-1.35 The low value
Chapter 5: Photodegradation of Functionalized Poly(l-Lactide) with 2-Oxepane-1,5-Dione 197
of ɛ induces a relative stability towards UV light. Thus, the aim herein was to
investigate the extent by which incorporated ketone moieties accelerate the
photooxidation rate Two PLLA-co-OPD copolymers featuring various concentrations
of OPD segments (5.2 and 8 mol%) were artificially aged under UV-A light at 50 ºC.
Samples were collected every two days to monitor the photodegradation by visual
observations, to assess the evolution of molecular weight and investigate any chemical
changes. In terms of visual observations, the copolymers were white powders before
irradiation and remained the same throughout the ageing process.
5.2.4.1. Changes of Molecular Weight
The evolution of molecular weight and polydispersity of irradiated poly(L-lactide-co-
OPD) with both 5.2 mol% (referred to as 18’) and 8 mol% (referred to as 18’)
incorporated OPD was assessed by GPC using tetrahydrofuran (THF) as the eluent.
The GPC traces of the copolymer 18’ as a function of irradiation time are shown in
Figure 5.12.
198 Chapter 5: Photodegradation of Functionalized Poly(l-Lactide) with 2-Oxepane-1,5-Dione
Figure 5.12. Evolution of the GPC traces of poly(L-lactide-co-OPD) (5.2 mol% OPD
segments) as a function of irradiation time, measured in THF (the traces were
baseline-corrected and normalized).
Before UV exposure, the copolymer was characterized by a unimodal trace. After two
days of irradiation, a shoulder appeared towards higher molecular weight. This
shoulder continuously increased with increasing irradiation time. The SEC traces of
poly(L-lactide-co-OPD) with 8 mol% incorporated OPD exhibited similar changes
(Figure 5.13). A shoulder appeared after two days irradiation with the intensity
increasing until four days irradiation, after which it remained constant.
Chapter 5: Photodegradation of Functionalized Poly(l-Lactide) with 2-Oxepane-1,5-Dione 199
Figure 5.13. Evolution of the GPC traces of poly(L-lactide-co-OPD) (8 mol% OPD
segments) as a function of irradiation time, measured in THF (the traces were
baseline-corrected and normalized).
In terms of molecular weights, the weight average molecular weights increased more
than the number average for both copolymers (Table 5.6). Broadening of the GPC
traces was also observed and shown by polydispersity values which increased for both
copolymers throughout the ageing process.
These observations suggest poly(L-lactide-co-OPD) undergoes changes at early stages
of the degradation process, with the broadening of the distributions after only two days
of irradiation in the QUV. The stable values of number average molecular weight
combined with the increase in weight average molecular weight and polydispersity
suggest crosslinking as the dominant pathway in the photooxidation of poly(L-lactide-
co-OPD).36 Under UV-visible light irradiation and air, polymers can undergo various
photodegradation reactions, from photooxidation to photolysis. Absorption of UV-
200 Chapter 5: Photodegradation of Functionalized Poly(l-Lactide) with 2-Oxepane-1,5-Dione
visible light by chromophores, either within the polymer structure (e.g. ketones) or as
defects, generates radicals that induce the formation of macroradicals by hydrogen
abstraction. Those macroradicals then react with oxygen to yield peroxide radicals.
Depending on the type of polymers, those peroxide radicals can propagate the
photooxidation mechanism and result in photolysis with chain cleavages and
formation of various photodegradation products (including hydroperoxides,
anhydrides, esters).37, 38 On the other hand, the macroradicals can recombined and
results in crosslinking. An increase in the weight average molecular weight and
broadening of the molar mass are characteristics of crosslinking.36, 39 As discussed in
Chapter 1, poly(L-lactide) did not undergo drastic modification when artificially aged
in the QUV under UV-A light at 50 ºC. Studies have shown that when exposed to UV-
A light in a Sepap 12.24 at 60 ºC up to 670 hours, PLLA photooxidation solely results
into chain scission and no crosslinking.19, 20 The occurrence of crosslinking revealed
by GPC could be attributed to the OPD segments in the copolymer.
Table 5.6. Evolution of the number and weight averaged molecular weights,
polydispersity and the chain scission of poly(L-lactide-co-OPD) as a function of
irradiation time in the QUV.
Incorporated
OPD
Irradiation (Days) 𝑀𝑛 (g·mol-1) 𝑀𝑤
(g·mol-1) Đ
5.2 mol% 0 5,100 ± 900 6,100 ± 1,000 1.26 ± 0.04
2 5,200 ± 300 7,000 ± 100 1.35 ± 0.07
4 5,500 ± 250 7,100 ± 100 1.30 ± 0.05
6 5,500 ± 100 7,300 ± 100 1.34 ± 0.01
8 5,500 ± 400 7,100 ± 0 1.31 ± 0.09
8 mol% 0 5,500 ± 100 7,500 ± 100 1.37 ± 0.03
2 5,800 ± 0 7,500 ± 0 1.29 ± 0.01
4 5,800 ± 200 7,900 ± 400 1.36 ± 0.01
6 5,500 ± 100 7,600 ± 100 1.38 ± 0.02
8 6,000 ± 300 7,200 ± 800 1.38 ± 0.01
5.2.4.2. Thermal Properties and Crystallinity
The variation in molecular weight of the copolymers throughout ageing were expected
to impact their thermal properties. Aged samples were characterized by DSC using a
Chapter 5: Photodegradation of Functionalized Poly(l-Lactide) with 2-Oxepane-1,5-Dione 201
heat / cool / heat cycle. When cooling down from the melt at 10 ºC·min-1, copolymers
with 5.2 (18’) and 8 mol% (20’) OPD exhibit different behaviours (thermograms not
shown here). The thermograms of poly(L-lactide-co-OPD) 18’ show an exothermic
peak assigned to crystallization before and after ageing, whereas no such transition is
observed for the copolymer 20’. This difference could be explained by different
crystallization rates between the two copolymers, with 20’ showing a slower rate of
crystallization than 18’.
After a second heating run to erase the thermal history of the samples, copolymer 18’,
before and after ageing, featured a glass transition (Tg) and an endothermic peak
corresponding to its melting temperature (Tm) (Figure 5.14). The initial glass
transition, 45.7 ± 1.9 ºC, was below the temperature used during the ageing process in
the QUV (50 °C). This suggests the polymer is in a rubbery state and therefore
facilitates oxygen diffusion within the polymer matrix during the ageing process.
Overall, the glass transitions were weak and hardly visible (Figure 5.15). The values
did not significantly vary during the ageing process with differences within the error
of the measurements. This lack of variation was expected based on GPC measurements
which showed no changes in 𝑀𝑛 with ageing since Tg is linked to 𝑀𝑛
according to the
Fox-Flory relationship:
𝑇𝑔 = 𝑇𝑔,∞ −𝑘
𝑀𝑛
Where k is the Flory-Fox constant, 𝑇𝑔,∞ the glass transition of polylactide having an
infinite molecular weight (reported value of 55 °C for PLA40) and 𝑀𝑛 is the number
average molecular weight.41 The 𝑀𝑛 values did not decrease with irradiation time,
therefore the glass transitions remained constant as well. Similar to Tg, there was no
noticeable shift towards lower temperatures for the melting peak. The melting
temperature also depends on the molecular weight and follows the relationship:
𝑇𝑚 = 𝑇𝑚,∞ −𝐴
𝑀𝑛
With A a constant, and 𝑇𝑚,∞ the melting temperature of PLA having an infinite
molecular weight.42
202 Chapter 5: Photodegradation of Functionalized Poly(l-Lactide) with 2-Oxepane-1,5-Dione
Figure 5.14. DSC thermograms from the second heating cycle of poly(L-lactide-co-
OPD) with 5.2 mol% OPD with increasing irradiation time in the QUV.
Chapter 5: Photodegradation of Functionalized Poly(l-Lactide) with 2-Oxepane-1,5-Dione 203
Figure 5.15. Enlarged view of the glass transitions in the DSC thermograms from the
second heating cycle of poly(L-lactide-co-OPD) with 5.2 mol% OPD with increasing
irradiation time in the QUV.
On the other hand, copolymer 20’ exhibits a glass transition (Tg), an exothermic peak
assigned to cold crystallization (Tcc), and a double endothermic peak corresponding to
melting temperatures (Tm) before and after ageing (Figure 5.16). The difference
between the thermograms of 18’ and 20’ could arise from different crystalline states,
with 18’ being more crystalline than 20’.43 As for 18’, the initial glass transition was
below the temperature used during the ageing in the QUV (50 °C). Overall, the glass
transitions did not significantly vary during the ageing process, with differences within
the error of the measurements.
The temperature of cold crystallization increased from 95.6 ± 0.7 °C, before
irradiation, to 100.9 ± 1.1 °C after ten irradiation days, as well as their associated
crystallization enthalpies (12.7 ± 0.8 to 24.8 ± 0.9 J·g-1). Such an increase in Tcc was
previously observed during the photooxidative degradation of PLA under UV-A light,
204 Chapter 5: Photodegradation of Functionalized Poly(l-Lactide) with 2-Oxepane-1,5-Dione
from 115 to 118 ºC after 400 hours in a Sepap 12.24 device. The formation of
additional nuclei of crystallization was suggested to induce that increase.20
Concerning the melting behaviour, 20’ exhibits a double melting peak that did not shift
to lower temperatures with irradiation days. This double behaviour was previously
reported for polylactide and was attributed to several possible reasons. It could be
caused by a melt-recrystallization process involving the melting of original crystals,
recrystallization and subsequent melting of recrystallized crystals.44, 45 Melting of
crystals featuring different lamellar thicknesses or melting of the crystalline phases α
and α’ are also reported explanations.46
Figure 5.16. DSC thermograms from the second heating cycle of poly(L-lactide-co-
OPD) with 8 mol% OPD with irradiation days in the QUV.
As shown by GPC measurements, an increase in molecular weight, which was likely
due to crosslinking occurred during the irradiation process. The formation of
crosslinked networks induced changes in the thermal properties of the resulting
Chapter 5: Photodegradation of Functionalized Poly(l-Lactide) with 2-Oxepane-1,5-Dione 205
polymer. Contrary to the results observed here, glass transition usually increases when
crosslinking occurs due to a variation of conformational entropy.47 For instance, films
of crosslinked PLLA with 3 wt% of triallyl isocyanurate, a crosslinking agent, were
characterized by an increase in Tg with irradiation time (electron beam irradiation at
room temperature). A correlation was observed between the crosslinking density and
glass transition, with higher Tg for higher crosslinking densities.48 The appearance of
a cold crystallization peak for aged PLLA was reported by Gardette and coworkers
who suggested that additional crystallization nuclei were formed during the
photodegradation.20 Concerning the melting temperature, the combined increase in gel
fraction and decrease in melting temperature was previously reported in the literature
for crosslinked polylactide.49, 50 The crystallization was inhibited by the crosslinked
network that restrained the mobility of the macromolecular chains, resulting in a lower
degree of crystallization and a lower melting enthalpy.49, 50
Table 5.7. Thermal properties of aged poly(L-lactide-co-OPD) copolymers
determined by DSC for samples before irradiation and after UV irradiation for 2-10
days (two measurements were performed and values were averaged).
UV
irradiation
(days)
Tg
(°C)
Tcc
(°C)
∆𝐻𝑐𝑐
(J.g-
1)
Tm1
(°C)
Tm2
(°C)
∆𝐻𝑚
(J.g-1)
𝜒𝑐
(%)
18
’
0 45.7 ±
1.9
- - 149.2
± 0.2
- 73.4 ±
3.0
2 43.4 ±
3.7
- - 147.3
± 1.1
- 63.7 ±
7.9
4 44.1 ±
1.6
- - 147.7
± 1.5
- 63.8 ±
2.1
6 50.1 ±
3.6
- - 148.6
± 0.2
- 57.3 ±
2.1
8 - - -
10 - - -
20
’
0 45.2 ±
1.0
95.6
± 0.7
12.7
± 0.8
130.1
± 1.3
141.6 ±
0.3
40.2 ±
3.0
56.5 ±
4.1
206 Chapter 5: Photodegradation of Functionalized Poly(l-Lactide) with 2-Oxepane-1,5-Dione
2 41.7 ±
1.9
93.3
± 2.0
15.5
± 3.0
127.2
± 0.8
139.3 ±
0.1
42.2 ±
2.9
61.6 ±
6.3
4 43.0 ±
1.3
96.3
± 1.8
22.1
± 6.0
127.5
± 1.1
139.3 ±
0.1
38.8 ±
0.1
69.2 ±
1.5
6 45.5 ±
0.1
99.4
± 0.3
26.5
± 1.4
128.7
± 0
139.3 ±
0.1
38.4 ±
0.1
69.2 ±
1.6
8 44.9 ±
0.9
98.2±
0.1
12.7
±0.8
130.1
± 1.8
141.6 ±
0.3
40.2 ±
3.0
56.5 ± 0
10 42.9 ±
3.6
100.9
± 1.1
24.8
± 0.9
125.0
± 4.7
138.6 ±
2.1
37.4 ±
2.1
66.3 ±
1.9
5.2.4.3. Chemical Structure of the Films
The chemical structure of the irradiated samples of poly(L-lactide-co-OPD) were
further analysed by ATR-FTIR spectroscopy. Nine spectra were collected for each
sample after every two days in the QUV, baseline-corrected, averaged and normalized
to the band at 1454 cm-1 (–CH3 bending) to eradicate any effect from differences in
contact with the ATR crystal and depth of penetration of the IR beam.
The averaged spectra of poly(L-lactide-co-OPD) (5.2 mol% OPD), before and after ten
days of UV irradiation in the QUV, are shown in Figure 5.17. No particular trends
were observed as highlighted by the carbonyl region (Figure 5.18). The shoulder in
the range 1720 - 1710 cm-1 seems to undergo little change before and after eight
irradiation days, and disappears after ten days in the QUV. This suggests a
disappearance of the ketone moieties within the copolymer. However, no absorbance
band of potential products resulting from these chemical changes could be detected by
ATR-FTIR spectroscopy for this copolymer. In comparison, the evolution of the
averaged spectra of aged poly(L-lactide-co-OPD) (8 mol% OPD), before and after UV
exposure, are illustrated in Figure 5.19. The carbonyl band changed throughout the
ageing process, with variation in absorbance (Figure 5.20). Any trend was hardly
observed between 1700 and 1730 cm-1, assigned to the OPD ketone. However, a
shoulder appears throughout the ageing process between 1830 and 1860 cm-1, with a
maximum at 1845 cm-1. This shoulder increased with increasing irradiation time. Such
a band has been previously attributed to anhydrides as the major photodegradation
products for irradiated PLLA.19, 20
Chapter 5: Photodegradation of Functionalized Poly(l-Lactide) with 2-Oxepane-1,5-Dione 207
Figure 5.17. ATR-FTIR average spectra of poly(L-lactide-co-OPD) (5.2 mol% OPD)
before and after ten days of UV irradiation (average of 9 spectra after baseline
correction and normalization to the -CH3 bending band at 1454 cm-1).
208 Chapter 5: Photodegradation of Functionalized Poly(l-Lactide) with 2-Oxepane-1,5-Dione
Figure 5.18. Enlarged view of the carbonyl band in the ATR-FTIR spectra of poly(L-
lactide-co-OPD) (5.2 mol% OPD): anhydride region 1900 - 1810 cm-1 and ketone
region 1740 - 1690 cm-1.
Chapter 5: Photodegradation of Functionalized Poly(l-Lactide) with 2-Oxepane-1,5-Dione 209
Figure 5.19. ATR-FTIR average spectra of poly(L-lactide-co-OPD) (8 mol% OPD)
powder before and after irradiation (average of 9 spectra after baseline correction and
normalization with the -CH3 bending band at 1454 cm-1).
210 Chapter 5: Photodegradation of Functionalized Poly(l-Lactide) with 2-Oxepane-1,5-Dione
Figure 5.20. Enlarged view of the carbonyl band in the ATR-FTIR spectra of poly(L-
lactide-co-OPD) (8 mol% OPD): anhydride region 1900 - 1810 cm-1 and ketone
region 1740 - 1690 cm-1.
5.2.5 Mechanism of Photodegradation
The photooxidation of two poly(L-lactide-co-OPD) was assessed under accelerated
artificial conditions. Both copolymers were characterized by lower values of molecular
weights than the entanglement molecular weight, preventing from obtaining them as
films. The copolymers were then aged as white powders. No visual sign of degradation
(change of colour) was observed during the ten irradiation days. However, GPC
analysis revealed changes at early stages of the UV irradiation process, with the
apparition of a shoulder towards the high molecular weights. This shoulder increased
with irradiation days for both copolymers. Although number average molecular
weights remained stable, both weight average molecular weights and polydispersities
increased throughout ageing. These results suggest little or no chain scission occurred,
while crosslinking seemed to predominantly happen and increase with irradiation days.
Along with the GPC results, ATR-FTIR spectroscopy showed the disappearance of the
Chapter 5: Photodegradation of Functionalized Poly(l-Lactide) with 2-Oxepane-1,5-Dione 211
1730 – 1700 cm-1 shoulder, assigned to ketone moieties, for the copolymer with 5.2
mol% incorporated OPD, while the apparition of a shoulder at 1845 cm-1 for the
copolymer with 8 mol% incorporated OPD.
As reviewed in Chapters 1 and 2, ketone moieties undergo chain scissions via Norrish
type I and II processes. As reviewed in Chapter 2, the absence of a hydrogen on the
carbon γ to the ketone in the OPD monomer reduced the likelihood of a contribution
of a Norrish type II mechanism. However, incorporated OPD could potentially
undergo both pathways as shown in Scheme 5.8. If following the Norrish type I, ring-
opened OPD would undergo α-cleavage of the ketone to give two free radical
intermediates, including an acyl radical. Acyl radicals can eliminate carbon monoxide
to give alkyl radicals. On the other hand, the Norrish type II would involve an
intramolecular hydrogen abstraction from the carbon γ to the carbonyl group. This
abstraction is followed by the cleavage of the α-β C-C bond resulting in the formation
of an enol and a terminal C=C double bond. The enol subsequently tautomerizes to the
more stable ketone.53 Regarding poly(L-lactide), the photooxidation under UV-A light
resulted in chain scissions, reducing the molecular weight, and anhydrides as major
degradation products, as confirmed by the presence of a band at 1845 cm-1 in the IR
spectrum.21, 22
In the work described here, the ketone of ring-opened OPD was expected to undergo
photocleavage via Norrish type reactions, generating radicals. Such radicals could
recombine due to cage effect, resulting in the crosslinking events demonstrated by
GPC. For the copolymer with 8 mol% incorporated OPD, some radicals potentially
attacked the PLLA segments to initiate the photooxidation mechanism by abstracting
hydrogen and forming anhydrides as degradation products, as revealed by ATR-FTIR
spectroscopy.
212 Chapter 5: Photodegradation of Functionalized Poly(l-Lactide) with 2-Oxepane-1,5-Dione
Scheme 5.8. Norrish type I and II cleavages of the ketone of ring-opened OPD.
5.3 SUMMARY
OPD was investigated as a photoprodegradant to enhance the photodegradation rate of
random copolymers of poly(L-lactide-co-OPD). Following the findings presented in
Chapter 4, a modified OPD with ketal protecting groups was selected and
copolymerised with L-lactide. Synthesis of random copolymers of L-lactide and
modified OPD was performed in the bulk at 110 ºC, using tin (II) octanoate and benzyl
Chapter 5: Photodegradation of Functionalized Poly(l-Lactide) with 2-Oxepane-1,5-Dione 213
alcohol as catalyst and initiator respectively. Copolymers with TOSUO incorporation
up to 12.7 mol% were obtained. Additional initiating species resulted in lowering of
the expected molecular weights, with values lower than the entanglement molecular
weight obtained. A subsequent deprotection step of the TOSUO units enabled the
recovery of the ketone moieties without significantly lowering the molecular weights.
Subsequently, two poly(L-lactide-co-OPD), featuring similar molecular weight and
two concentrations of incorporated OPD (5.2 and 8 mol%), were artificially aged under
UV-A light in the QUV. GPC analysis revealed crosslinking events occurred at early
stages of the photooxidation process, and increased with irradiation time. ATR-FTIR
spectroscopy analysis revealed the apparition of anhydrides as degradation products.
The photooxidation was proposed to be initiated by the cleavage of the ketone moieties
via Norrish type reactions, generating radicals. Such radicals could either recombine
leading to crosslinking, or initiate the photooxidation pathway of the poly(L-lactide)
segments.
5.4 EXPERIMENTAL
5.4.1 Material
1,4-Cyclohexanedione monoethyleneketal (97 %), 3-chloroperbenzoic acid (≤ 77 %),
tin (II) octanoate (95 %), and triphenylcarbenium tetrafluoroborate were purchased
from Sigma-Aldrich and used as received. L-lactide ((3s)-cis-3.6-dimethyl-1,4-
dioxane-2,5-dione) (98 %) was purchased from Sigma-Aldrich, recrystallized twice
from toluene and dried under vacuum prior to use. Hexane, ethyl acetate, and
dichloromethane were all AR grade, purchased from ChemSupply and used as
received.
5.4.2 Methods
5.4.2.1. Synthesis of 1,4,8-Trioxaspiro[4.6]-9-Undecanone
214 Chapter 5: Photodegradation of Functionalized Poly(l-Lactide) with 2-Oxepane-1,5-Dione
m-Chloroperbenzoic acid (4.44 g, 25.7 mmol, 2.0 equiv.) was dissolved in 110 mL
DCM and dried over anhydrous Na2SO4. The solution was filtered into a 250 mL flask.
1,4-Cyclohexanedione monoethyleneketal (2 g, 12.8 mmol) was slowly added to the
mCPBA solution under stirring. The solution was refluxed at 40 °C for 24 h before
allowing to cool to ambient temperature. The solution was washed with brine. The
organic layer was collected, dried over anhydrous Na2SO4 and concentrated under
reduced pressure. The product was purified by column chromatography using hexane
/ ethyl acetate (65 / 35). TOSUO was recovered as an oil in 39 - 40 % yield. Mp. 44-
45 °C (Lit., 49-51 °C1). 1H NMR (CDCl3, 600 MHz), δ ppm = 4.28 (t, 2H, C=OOCH2),
3.98 (s, 4H, C-O-CH2CH2O), 2.69 (t, 2H, CH2C=OOCH2), 2.00 (t, 2H, C-OCH2CH2O-
CH2CH2), 1.89 (t, 2H, C-OOCH2CH2C-O).13C NMR (CDCl3, 600 MHz), δ ppm =
175.4 (C=O), 107.9 (C-OCH2CH2O-), 64.7 (C-OCH2CH2O-), 64.3 (C=OOCH2), 39.1
(C=OOCH2CH2), 32.7 (C-OCH2CH2O-CH2CH2), 28.8 (CH2C=OOCH2). ATR-FTIR:
ʋ max = 2990 (w, -CH2 stretching), 1724 (s, -C=O carbonyl stretching of the lactone),
1454 (-CH3 bending), 1121 and 1094 (s, -C-O- stretching).
5.4.2.2. Synthesis of Poly(L-lactide-co-TOSUO) in the bulk
All glassware was oven dried overnight at 80 °C. L-lactide, TOSUO and benzyl alcohol
were introduced into a Schlenk vessel under inert atmosphere in a glovebox. Tin (II)
octanoate was added to the Schlenk vessel under a nitrogen atmosphere (Table 5.8).
The Schlenk vessel was then sealed under an inert atmosphere and immersed into a
preheated silicone oil bath at 110 °C. The polymerisations lasted from one to two days
O
O
O
O
Chapter 5: Photodegradation of Functionalized Poly(l-Lactide) with 2-Oxepane-1,5-Dione 215
depending on the conditions. The polymerisations were thermally quenched when the
stirring was no longer working because of an increased viscosity. The polymeric
mixture was dissolved in THF. A crude fraction was kept for the determination of the
monomer conversion. The rest was reprecipitated into cold cyclohexane (10 fold
excess). The purification step was repeated until the unreacted monomer was
completely removed.
Table 5.8. Conditions of the ROP of L-lactide and TOSUO in the bulk at 110 °C.
Compound 18: White polymer. Purification yield: 55.3 %. 1H NMR (CDCl3, δ ppm,
subscripts L-LA and TOSUO denote each repeating units): 7.33 (m, 6H, CHBnO), 5.17
(q, H, CHL-LA), 4.36 (m, H, -CHCH3-OHL-LA-end group), 4.22 (t, 2H, CH2OTOSUO), 3.93
(s, 4H, OCH2CH2TOSUO), 2.46 (t, 2H, OC=OCH2TOSUO), 1.99 (m, 4H, CH2C-
OCH2CH2O-CH2TOSUO), 1.58 (d, 3H, CH3L-LA). 13C NMR (CDCl3, δ ppm): 170.0 (-
C=OL-LA, -C=OTOSUO), 128.6 (-C=CBnO), 69.4 (-C-CH3L-LA), 67.1 (-CO-CH2-CH2-
OCTOSUO), 57.5 (C=OOCH2CH2TOSUO), 27.3 (OC=OCH2CH2TOSUO), 17.0 (-C-CH3L-LA).
ATR-FTIR: ʋ max = 2996 (w, -CH2 stretching, asymmetric), 2948 (w, -CH2 stretching,
symmetric), 1749 (s, -C=O carbonyl stretching), 1455 (-CH3 bending), 1383 and 1359
(-CH- deformation, symmetric and asymmetric), 1180 and 1084 (-C-O- stretching),
1043 (-OH bending), 871 (w, -C-C- stretching). GPC (Refractive Index detector): 𝑀𝑛
(Ð) = 5,600 ± 200 g·mol-1 (1.52 ± 0.01). DSC (second heating cycle): Tg = 49.5 ± 0.9
ºC, Tcc = 88.8 ± 0.4 ºC (∆𝐻𝑐𝑐 = 14.3 ± 4.7 J·g-1), Tm = 148.2 ± 1.7 ºC (∆𝐻𝑚 = 49.1 ±
5.2 J·g-1).
Compound 19: White polymer. Purification yield: 71.3 %. 1H NMR (CDCl3, δ ppm,
subscripts L-LA and TOSUO denote each repeating units): 7.33 (m, 6H, CHBnO), 5.16
(q, H, CHL-LA), 4.36 (m, H, -CHCH3-OHL-LA-end group), 4.23 (t, 2H, CH2OTOSUO), 3.93
code Initial TOSUO
content
L-lactide TOSUO Benzyl
alcohol
mol% g mmol mg mmol μL μmol
18 4.42 1.484 10.30 78.30 0.4552 10 96.17
19 9.60 0.9947 6.901 113.9 0.6622 10 96.17
20 15.3 1.489 10.33 271.5 1.579 10 96.17
21 24.4 1.007 6.987 293.3 1.705 10 96.17
216 Chapter 5: Photodegradation of Functionalized Poly(l-Lactide) with 2-Oxepane-1,5-Dione
(s, 4H, OCH2CH2TOSUO), 2.46 (t, 2H, OC=OCH2TOSUO), 1.97 (m, 4H, CH2C-
OCH2CH2O-CH2TOSUO), 1.59 (d, 3H, CH3L-LA). 13C NMR (CDCl3, δ ppm): 170.0 (-
C=OL-LA, -C=OTOSUO), 128.6 (-C=CBnO), 69.4 (-C-CH3L-LA), 67.1 (-CO-CH2-CH2-
OCTOSUO), 57.5 (C=OOCH2CH2TOSUO), 27.3 (OC=OCH2CH2TOSUO), 17.0 (-C-CH3L-LA).
ATR-FTIR: ʋ max = 2996 (w, -CH2 stretching, asymmetric), 2948 (w, -CH2 stretching,
symmetric), 1747 (s, -C=O carbonyl stretching), 1454 (-CH3 bending), 1383 and 1359
(-CH- deformation, symmetric and asymmetric), 1182 and 1082 (-C-O- stretching),
1042 (-OH bending), 871 (w, -C-C- stretching). GPC (Refractive Index detector): 𝑀𝑛
(Ð) = 800 ± 0 g·mol-1 (1.52 ± 0.01). DSC (second heating cycle): Tg = 35.3 ± 2.0 ºC,
Tcc = 100.0 ± 2.0 ºC (∆𝐻𝑐𝑐 = 1.1 ± 0.4 J·g-1), Tm = 113.9 ± 1.3 ºC (∆𝐻𝑚 = 1.6 ± 0.8
J·g-1).
Compound 20: White polymer. Purification yield: 81.6 %. 1H NMR (CDCl3, δ ppm,
subscripts L-LA and TOSUO denote each repeating units): 7.33 (m, 6H, CHBnO), 5.17
(q, H, CHL-LA), 4.35 (m, H, -CHCH3-OHL-LA-end group), 4.23 (t, 2H, CH2OTOSUO), 3.93
(s, 4H, OCH2CH2TOSUO), 2.45 (t, 2H, OC=OCH2TOSUO), 1.99 (m, 4H, CH2C-
OCH2CH2O-CH2TOSUO), 1.58 (d, 3H, CH3L-LA). 13C NMR (CDCl3, δ ppm): 170.0 (-
C=OL-LA, -C=OTOSUO), 128.6 (-C=CBnO), 69.4 (-C-CH3L-LA), 67.1 (-CO-CH2-CH2-
OCTOSUO), 57.5 (C=OOCH2CH2TOSUO), 27.3 (OC=OCH2CH2TOSUO), 17.0 (-C-CH3L-LA).
ATR-FTIR: ʋ max = 2996 (w, -CH2 stretching, asymmetric), 2948 (w, -CH2 stretching,
symmetric), 1755 (s, -C=O carbonyl stretching), 1454 (-CH3 bending), 1383 and 1359
(-CH- deformation, symmetric and asymmetric), 1181 and 1086 (-C-O- stretching),
1042 (-OH bending), 871 (w, -C-C- stretching). GPC (Refractive Index detector): 𝑀𝑛
(Ð) = 6,100 ± 500 g·mol-1 (1.47 ± 0.10). DSC (second heating cycle): Tg = 43.7 ± 1.7
ºC, Tcc = 99.8 ± 2.1 ºC (∆𝐻𝑐𝑐 = 27.9 ± 3.1 J·g-1), Tm1 = 125.5 ± 1.4 ºC, Tm2 = 139.9 ±
0.7 ºC (∆𝐻𝑚 = 40.9 ± 0.9 J·g-1).
Compound 21: White polymer. Purification yield: 87.9 %. 1H NMR (CDCl3, δ ppm,
subscripts L-LA and TOSUO denote each repeating units): 7.33 (m, 6H, CHBnO), 5.16
(q, H, CHL-LA), 4.28 (t, 2H, CH2OTOSUO), 3.94 (s, 4H, OCH2CH2TOSUO), 2.45 (t, 2H,
OC=OCH2TOSUO), 2.00 (m, 4H, CH2C-OCH2CH2O-CH2TOSUO), 1.58 (d, 3H, CH3L-LA).
13C NMR (CDCl3, δ ppm): 169.5 (-C=OL-LA, -C=OTOSUO), 128.2 (-C=CBnO), 67.2 (-C-
CH3L-LA), 65.0 (-CO-CH2-CH2-OCTOSUO), 63.4 (C=OOCH2CH2TOSUO), 35.8
(C=OOCH2CH2TOSUO), 26.9 (OC=OCH2CH2TOSUO), 16.6 (-C-CH3L-LA). ATR-FTIR: ʋ
max = 2994 (w, -CH2 stretching, asymmetric), 2948 (w, -CH2 stretching, symmetric),
Chapter 5: Photodegradation of Functionalized Poly(l-Lactide) with 2-Oxepane-1,5-Dione 217
1747 (s, -C=O carbonyl stretching), 1454 (-CH3 bending), 1384 and 1359 (-CH-
deformation, symmetric and asymmetric), 1182 and 1085 (-C-O- stretching), 1043 (-
OH bending), 871 (w, -C-C- stretching). GPC (Refractive Index detector): 𝑀𝑛 (Ð) =
4,700 ± 900 g·mol-1 (1.28 ± 0.06). DSC (second heating cycle): Tg = 40.1 ± 0.2 ºC, Tcc
= 88.8 ± 0.3 ºC (∆𝐻𝑐𝑐 = 15.2 ± 0.9 J·g-1), Tm1 = 128.1 ± 0 ºC, Tm2 = 143.2 ± 0 ºC (∆𝐻𝑚
= 43.9 ± 0 J·g-1).
5.4.2.3. Synthesis of Poly(L-lactide-co-OPD)
In a typical deprotection reaction, poly(L-lactide-co-TOSUO) (531.6 mg, 186.1 μmol,
TOSUO 6.701 μmol) was dissolved in 1 mL DCM. Trityl fluoroborate (2 equivalents,
4.4 mg, 13 μmol) was dissolved in 0.3 mL DCM and added to the copolymer solution.
The yellow solution was stirred for 2 hours at room temperature. The solution was then
reprecipitated in cold methanol (10 fold excess) as a white powder.
Compound 18’: White polymer. Yield: 84.6 %. 1H NMR (CDCl3, δ ppm, subscripts
L-LA and OPD denote each repeating units): 7.33 (m, 6H, CHBnO), 5.16 (q, H, CHL-
LA), 4.35 (t, 2H, -CH2OOPD), 2.80 (m, 2H, CH2C=OOPD), 2.66 (m, 2H, C=OCH2OPD),
1.58 (d, 3H, CH3L-LA). ATR-FTIR: ʋ max = 2996 (w, -CH2 stretching, asymmetric),
2948 (w, -CH2 stretching, symmetric), 1748 (s, -C=O carbonyl stretching) with a
shoulder from 1730 to 1700 cm-1, 1454 (-CH3 bending), 1383 and 1359 (-CH-
deformation, symmetric and asymmetric), 1180 and 1084 (-C-O- stretching), 1043 (-
OH bending), 871 (w, -C-C- stretching). GPC (Refractive Index detector): 𝑀𝑛 (Ð) =
5,400 ± 400 g·mol-1 (1.32 ± 0.01). DSC (second heating cycle): Tg = 49.6 ± 5.7 ºC, Tm
= 149.4 ± 0.4 ºC (∆𝐻𝑚 = 69.1 ± 6.5 J·g-1).
Compound 20’: White polymer. Yield: 84.6 %. 1H NMR (CDCl3, δ ppm, subscripts
L-LA and OPD denote each repeating units): 7.33 (m, 6H, CHBnO), 5.16 (q, H, CHL-
LA), 4.35 (t, 2H, -CH2OOPD), 2.80 (m, 2H, CH2C=OOPD), 2.66 (m, 2H, C=OCH2OPD),
1.58 (d, 3H, CH3L-LA). ATR-FTIR: ʋ max = 2996 (w, -CH2 stretching, asymmetric),
2948 (w, -CH2 stretching, symmetric), 1749 (s, -C=O carbonyl stretching) with a
shoulder from 1730 to 1700 cm-1, 1453 (-CH3 bending), 1382 and 1359 (-CH-
218 Chapter 5: Photodegradation of Functionalized Poly(l-Lactide) with 2-Oxepane-1,5-Dione
deformation, symmetric and asymmetric), 1181 and 1084 (-C-O- stretching), 1043 (-
OH bending), 870 (w, -C-C- stretching). GPC (Refractive Index detector): 𝑀𝑛 (Ð) =
5,500 ± 100 g·mol-1 (1.37 ± 0.03). DSC (second heating cycle): Tg = 44.3 ± 1.5 ºC, Tcc
= 94.9 ± 1.0 ºC (∆𝐻𝑐𝑐 = 10.8 ± 2.8 J·g-1), Tm1 = 131.0 ± 1.7 ºC, Tm2 = 142.2 ± 0.9 ºC
(∆𝐻𝑚 = 39.7 ± 2.6 J·g-1).
5.4.2.4. Accelerated Photo Ageing
The copolymer powders were placed between quartz plates and mounted onto 35 mm
aluminium slide holders and exposed to UV-A 340 lamps at an irradiance of 0.68
W/m2 at 340 nm in a QUV accelerated weathering tester (Q-lab, Ohio). Water was
present in a tray at the bottom of the QUV chamber in order to maintain maximum and
consistent levels of humidity for the degradation study. The QUV was operated at a
black panel temperature of 50 °C and cycles lasted for 24 h. The irradiance sensors
were calibrated every 500 hours.
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222 Chapter 6: Conclusions and Future Research Directions
Chapter 6: Conclusions and Future Research
Directions
6.1 CONCLUSIONS
As a wider strategy towards controlling the degradation of biodegradable polymers for
limiting and reducing pollution, the main aim of this PhD research project was to tune
the photodegradability of a biodegradable polyester by employing a photosensitizing
molecule, 2-oxepane-1,5-dione (OPD), as an additive mixed with commercial polymer
and as a monomer to be copolymerized..
Films of blended commercial grade poly(L-lactide) and OPD synthesised here were
produced via a solvent-casting process with an initial OPD concentration ranging from
0 to 10 wt%. The films were artificially aged in a QUV accelerated weathering device
using conditions relevant to outdoor natural conditions; UV-A light at 50 °C. OPD as
an additive led to accelerated photodegradation of the polymeric blends, as revealed
by drastic decreases in molecular weights as a result of chain scissions. The number of
chain scissions increased with the initial OPD concentration, with the
photodegradation proposed to be initiated by the cleavage of OPD via a Norrish type
I mechanism, resulting in the formation of initiating radicals that further attacked the
macromolecular chains.
Following these results, the photosensitizing effect of OPD was investigated when it
was incorporated into the PLLA backbone as a copolymer. In-melt modification
studies were carried out initially to try to prepare PLLA - OPD copolymers using
reactive extrusion of commercial grade PLLA and OPD. The incorporation was
expected to occur due to transesterification reactions, catalysed by tin (II) octanoate.
However, the purified extrudates were only composed of PLLA, demonstrating the
absence of transesterification with OPD, independent of the initial feeds, the residence
time or the transesterification catalyst. Subsequent thermal analysis of OPD revealed
that thermal degradation of the compound occurred at temperatures close to 160°C,
which is expected to have limited the success of the in-melt modification. In order to
avoid thermal degradation of OPD, direct copolymerisation of OPD with L-lactide
Chapter 6: Conclusions and Future Research Directions 223
monomer was investigated. Various conditions were trialled, and included
polymerisation in the bulk at 110 °C with tin (II) octanoate as the catalyst, and in
solution at room temperature with DBU and benzyl alcohol as the catalyst and initiator,
respectively. The tin-catalysed ring-opening polymerisations of L-lactide and OPD
resulted in only very low amounts of incorporated OPD within the PLLA structure. It
is proposed that the ketone moiety of OPD hindered the polymerisation of both
monomers by reacting with the tin (II) octanoate catalyst to form a complex. Regarding
the other catalytic system investigated (DBU and benzyl alcohol), it was found that the
nucleophilicity of DBU was high enough to efficiently polymerize L-lactide, while not
being nucleophilic enough to polymerize the lactone-type monomer, OPD.
In order to increase the amount of OPD incorporated into the PLLA backbone, further
work focussed on copolymerising L-lactide with a functionalised OPD monomer,
TOSUO, which contained ketone acetal groups to protect the ketone. L-lactide-
TOSUO copolymers were successfully synthesized, with TOSUO incorporation
ranging from 4.8 to 12.7 mol%. Subsequent deprotection of the ketone acetals using
triphenylcarbenium tetrafluoroborate afforded several poly(L-lactide-co-OPD)
copolymers. Photodegradation studies were carried out on deprotected copolymers in
the QUV using UV-A light at 50 °C and showed that cross-linking of the copolymers
occurred, which is in contrast to the scission mechanism found when OPD was used
as a non-polymerized additive.
It has been found here that the method of incorporation of 2-oxepane-1,5-dione, OPD,
into poly(L-lactide) has a significant effect on the photodegradation mechanism. When
used as an additive physically blended with poly(L-lactide) up to 10 wt%, OPD
drastically decreased the molecular weight via chain scissions, resulting in a
accelerated embrittlement of the blend films during irradiation compared to neat
PLLA. However, when incorporated into poly(L-lactide) as a copolymer at 5.2 and 8
mol%, UV irradiation resulted in crosslinking, as revealed by GPC measurements.
Based on its structure and the photochemistry of related compounds, OPD was
expected to undergo cleavage through a Norrish type I mechanism, to form carbon-
centred radicals that could attack the PLLA macromolecular chains, leading to chain
scissions. This was found to be the case where OPD was used as a blend, but the
contrary result found for poly(L-lactide-co-OPD) copolymers suggests that these
radicals, or their radical by-products recombined between copolymer molecules,
224 Chapter 6: Conclusions and Future Research Directions
resulting in crosslinking. In both cases, OPD was found to accelerate the rate of
photodegradation of PLLA, with the differing mechanisms of degradation found for
blends and copolymers providing scope for tuning the photodegradability of PLLA
polymers via alteration of the method of OPD incorporation.
6.2 FUTURE RESEARCH DIRECTIONS
Following the findings on the photosensitizing potential of 2-oxepane-1,5-dione when
physically mixed with commercial grade PLLA, or used as a copolymer, films of
similar formulations could be aged under outdoor natural conditions to confirm the
photodegradation mechanisms found using accelerated laboratory studies. In addition,
monitoring the mechanical properties during the ageing study would provide a
quantitative assessment of property change until embrittlement to guide their potential
use as degradable packaging films.
The biodegradation rate of poly(L-lactide) is impacted by the temperature, the relative
humidity and the type of microorganisms present in the environment. However, the
influence of OPD incorporation is unknown. Comparative studies could be performed
between PLLA and PLLA - OPD copolymers and blends placed in compost (where
the conditions are controlled) and in natural environments so that the influence of OPD
on the biodegradation of PLLA can be assessed.
Further work regarding the photodegradation of poly(L-lactide-co-OPD) copolymers
could focus on varying the concentration of incorporated OPD and artificially ageing
the copolymers to determine what the effect of a wider concentration range of OPD is
on crosslinking and chain scission mechanisms of degradation.
Appendices 225
Appendices
CHAPTER 2
1H NMR spectrum of OPD
DSC thermograms of OPD
226 Appendices
DSC thermograms of the transparent and opaque sections of the PLLA-OPD 10
wt% film
ATR-FTIR average spectra of PLLA –OPD 4 wt% film before and after one
and ten irradiation days (average of 9 spectra after baseline correction and
normalization with the -CH3 bending band at 1455 cm-1).
Appendices 227
ATR-FTIR average spectra of PLLA –OPD 6 wt% (top) and PLLA - OPD 8
wt% (bottom) films before and after one and ten irradiation days (average of 9
spectra after baseline correction and normalization with the -CH3 bending band
at 1455 cm-1).
228 Appendices
UV-visible spectra from the PLLA – OPD 2 wt% (top) and PLLA – OPD 4 wt%
(bottom) films as a function of irradiation time in the QUV.
Appendices 229
UV-visible spectra from the PLLA – OPD 6 wt% (top) and 8 wt% OPD
(bottom) films as a function of irradiation time in the QUV.
230 Appendices
CHAPTER 3
DSC thermograms of polyOPD
DSC thermograms from the cooling cycle of purified extrudate of PLLA – tin
(II) octanoate formulation without OPD
Appendices 231
DSC thermograms from the cooling cycle of purified extrudates of PLLA - OPD
(top: 5 wt% OPD; bottom: 10 wt% OPD).
232 Appendices
DSC thermograms from the cooling cycle of purified extrudates of PLLA –
OPD 15 wt%
Appendices 233
DSC thermograms from the cooling cycle of double-purified extrudates after 20
(top) and 40 minutes (bottom) residence times.
234 Appendices
DSC thermograms from the cooling cycle of double-purified extrudates after 60
(top) and 80 minutes (bottom) residence times.
Appendices 235
CHAPTER 4
DSC thermograms of compound 5 (top) and compound 6 (bottom).
236 Appendices
DSC thermograms of red crystals.
GPC traces of the homopolymerisation of L-lactide using DBU and benzyl
alcohol measured in chloroform (the traces were baseline-corrected and
normalized).
Appendices 237
CHAPTER 5
DSC thermogram of TOSUO