PROFLUORESCENT NITROXIDES AS PROBES FOR · Spectrofluorimetry of UV-aged polypropylene doped with...

THE SYNTHESIS AND APPLICATION OF NOVEL

PROFLUORESCENT NITROXIDES AS PROBES FOR

POLYMER DEGRADATION

Submitted by

James Peter BLINCO

Bachelor of Applied Science (Honours, Chemistry)

Submitted to the School of Physical and Chemical Sciences, Queensland University

of Technology, in partial fulfilment of the requirements of the degree of

Doctor of Philosophy

April 2008

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ABSTRACT

This PhD project has expanded the knowledge in the area of profluorescent

nitroxides with regard to the synthesis and characterisations of novel profluorescent

nitroxide probes as well as physical characterisation of the probe molecules in

various polymer/physical environments.

The synthesis of the first example of an azaphenalene-based fused aromatic nitroxide

TMAO, [1,1,3,3-tetramethyl-2,3-dihydro-2-azaphenalen-2-yloxyl, was described.

This novel nitroxide possesses some of the structural rigidity of the isoindoline class

of nitroxides, as well as some properties akin to TEMPO nitroxides. Additionally, the

integral aromatic ring imparts fluorescence that is switched on by radical scavenging

reactions of the nitroxide, which makes it a sensitive probe for polymer degradation.

In addition to the parent TMAO, 5 other azaphenalene derivatives were successfully

synthesised.

This new class of nitroxide was expected to have interesting redox properties when

the structure was investigated by high-level ab initio molecular orbitals theory. This

was expected to have implications with biological relevance as the calculated redox

potentials for the azaphenalene ring class would make them potent antioxidant

compounds. The redox potentials of 25 cyclic nitroxides from four different

structural classes (pyrroline, piperidine, isoindoline and azaphenalene) were

determined by cyclic voltammetry in acetonitrile. It was shown that potentials

related to the one electron processes of the nitroxide were influenced by the type of

ring system, ring substituents or groups surrounding the moiety. Favourable

comparisons were found between theoretical and experimental potentials for

pyrroline, piperidine and isoindoline ring classes. Substitution of these ring classes,

were correctly calculated to have a small yet predictable effect on the potentials. The

redox potentials of the azaphenalene ring class were underestimated by the

calculations in all cases by at least a factor of two. This is believed to be due to

another process influencing the redox potentials of the azaphenalene ring class which

is not taken into account by the theoretical model.

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It was also possible to demonstrate the use of both azaphenalene and isoindoline

nitroxides as additives for monitoring radical mediated damage that occurs in

polypropylene as well as in more commercially relevant polyester resins. Polymer

sample doped with nitroxide were exposed to both thermo-and photo-oxidative

conditions with all nitroxides showing a protective effect. It was found that

isoindoline nitroxides were able to indicate radical formation in polypropylene aged

at elevated temperatures via fluorescence build-up. The azaphenalene nitroxide

TMAO showed no such build-up of fluorescence. This was believed to be due to the

more labile bond between the nitroxide and macromolecule and the protection may

occur through a classical Denisov cycle, as is expected for commercially available

HAS units.

Finally, A new profluorescent dinitroxide, BTMIOA (9,10-bis(1,1,3,3-

tetramethylisoindolin-2-yloxyl-5-yl)anthracene), was synthesised and shown to be a

powerful probe for detecting changes during the initial stages of thermo-oxidative

degradation of polypropylene. This probe, which contains a 9,10-diphenylanthracene

core linked to two nitroxides, possesses strongly suppressed fluorescence due to

quenching by the two nitroxide groups. This molecule also showed the greatest

protective effect on thermo-oxidativly aged polypropylene. Most importantly,

BTMIOA was found to be a valuable tool for imaging and mapping free-radical

generation in polypropylene using fluorescence microscopy.

KEYWORDS

Azaphenalene, Cyclic Voltammetry, Fluorescence, Fluorophore, Free Radical,

Hydroxylamine, Isoindoline, Nitroxide, Oxidation, Oxoammonium, Paramagnetic,

Polymer Degradation, Polypropylene, Photo-oxidation, Profluorescent, Thermo-

oxidation, Redox.

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PUBLICATIONS ARISING FROM THIS PROJECT

Journal Articles J. P. Blinco, J.C. McMurtrie and S. E. Bottle (2007) “The First Example of an Azaphenalene Profluorescent Nitroxide.” Eur. J. Org.Chem. 28(3): 4638-4641 S. E. Bottle, J. P. Blinco, G. A. George, A. S. Micallef and T. Wade (2007) “Profluorescent Nitroxide Compounds” PCT Int. Appl. WO 2007124543 H. Sato, V. Kathirvelu, A. Fielding, J. Blinco, A. Micallef, S. Bottle, S. Eaton and G. Eaton (2007) “Impact of Molecular Size on Electron Spin Relaxation Rates of Nitroxyl Radicals in Glassy Solvents between 100 and 300 K” Mol. Phys. 105 (15-16): 2137-2151 A. S. Micallef, S. E. Bottle, J. P. Blinco and G. A. George (2008) “Monitoring Free Radical Reactions in Degrading Polymers with a Profluorescent Nitroxide.” ACS Symposium Series, 978 (Polymer Durability and Radiation Effects): 59-69. K. E. Fairfull-Smith, J. P. Blinco, D. J. Keddie, G. A. George and S. E. Bottle (2008) “A Novel Profluorescent Dinitroxide for Imaging Polypropylene Degradation” Macromolecules 41(5): 1577-1580 H. Sato, S. E. Bottle, J. P. Blinco, A. S. Micallef, G. R. Eaton and S. S. Eaton (2008) “Electron Spin-Lattice Relaxation of Nitroxyl Radicals in Temperature Ranges that Spans Glassy Solutions to Low-Viscosity Liquid” J. Magn. Reson. 191(1): 66-77 J. P. Blinco, J. L. Hodgson, B. J. Morrow, J. R. Walker, G. D. Will, M. L. Coote and S. E. Bottle (2008) “Experimental and Theoretical Studies of the Redox Potentials of Cyclic Nitroxides” J. Org. Chem. 73(17): 6763-6771 J. P. Blinco, D. J. Keddie, T. Wade, P. J. Barker, G. A. George and S. E. Bottle (2008) “Profluorescent Nitroxides: Sensors and Stabilisers of Radical-Mediated Oxidative Damage” Polym. Degrad. Stab. 93(9): 1613-1618

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Lectures

James P. Blinco. “Profluorescent Nitroxides” RACI QLD Polymer Group Student Symposium, Brisbane, Australia, September 2007. James P. Blinco, Graeme A. George and Steven E. Bottle. “Profluorescent Nitroxides: Novel Probes of Polypropylene Degradation” 233rd American Chemical Society National Meeting and Symposium, Chicago, USA, March 2007. James P. Blinco. “Novel Profluorescent Nitroxides Probes”, ARC Centre of Excellence in Free Radical Chemistry and Biotechnology Winter Carnival, Canberra, Australia, June 2006. James P. Blinco, Aaron S. Micallef, Graeme A. George & Steven E. Bottle. “Development and Application of Novel Profluorescent Nitroxides for Monitoring Polypropylene Degradation”, 28th Australasian Polymer Symposium, Rotorua, New Zealand, February 2006.

Poster Presentations James P. Blinco and Steven E. Bottle. “The First Example of an Azaphenalene Profluorescent Nitroxide” RACI Organic & Physical Chemistry Conference, Adelaide, Australia, February 2007. James P. Blinco, Aaron S. Micallef, Graeme A. George, Ezio Rizzardo, San H. Thang & Steven E. Bottle. “Development of a Novel Profluorescent Nitroxide for Monitoring Polymer Degradation” RACI CONNECT 2005, Sydney, Australia, June 2005. (Divisional Prize)

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TABLE OF CONTENTS

ABSTRACT ii

KEYWORDS iii

PUBLICATIONS ARISING FROM THIS PROJECT iv

TABLE OF CONTENTS vi

LIST OF FIGURES xi

LIST OF SCHEMES xvii

LIST OF TABLES xix

ABBREVIATIONS xx

DECLARATION xxiii

NOTES TO THE READER xxiv

ACKNOWLEDGEMENTS xxv

CHAPTER 1

1. Introduction 2

1.1 Degradation and Stability of polyolefins 2

1.2 Monitoring polyolefin degradation 6

1.3 Polyolefin Stabilisers 9

1.3.1 Primary antioxidants 10

1.3.2 Preventative or secondary antioxidants 13

1.4 Nitroxides 14

1.4.1 General 14

1.4.2 Nitroxide quenching of excited states 17

1.4.3 Profluorescent Nitroxides 18

1.4.4 Applications of Profluorescent nitroxides 19

1.4.5 Isoindoline Nitroxides 22

1.4.6 Profluorescent isoindoline nitroxides 24

1.5 Research problem investigated 29

1.6 Project outline and objectives 30

1.7 Relationship of the research papers 31

1.8 References 33

vii

CHAPTER 2

2. The First Example of an Azaphenalene Profluorescent Nitroxide 37

ABSTRACT 39

INTRODUCTION 40

RESULTS AND DISCUSSION 41

CONCLUSIONS 46

Acknowledgements 46

EXPERIMENTAL SECTION 46

REFERENCES 47

ELECTRONIC SUPPORTING INFORMATION 48

Experimental 49

Crystallography 53

Electronic Paramagnetic Resonance Spectroscopy 59

CHAPTER 3

3. Experimental and theoretical studies of the redox potentials of cyclic

nitroxides 60

ABSTRACT 62

INTRODUCTION 63


Synthetic Results 65

Cyclic Voltammetry 68

Theoretical Calculations 73

CONCLUSIONS 77

Acknowledgements 78

EXPERIMENTAL SECTION 78

REFERENCES 85


CHAPTER 4

4. Profluorescent Nitroxides: Sensors and Stabilizers of Radical-

Mediated Oxidative Damage 91

ABSTRACT 93

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INTRODUCTION 94

EXPERIMENTAL 95


Thermo-oxidative degradation of nitroxide-doped polypropylene 97

Photo-oxidative degradation of nitroxide-doped polypropylene 102

Thermo-oxidative degradation of nitroxide-doped polyesters 104

CONCLUSIONS 108

Acknowledgements 108

REFERENCES 108

CHAPTER 5

5. A Novel Profluorescent Dinitroxide for Imaging Polypropylene

Degradation 110

ABSTRACT 112

MANUSCRIPT TEXT 113

CONCLUSIONS 120

Acknowledgements 120

REFERENCES 120


CHAPTER 6

6. Conclusions 129

APPENDIX A

A. Further investigations into the synthesis and application of

profluorescent nitroxides 132

A.1 Photo-physical measurements of profluorescent nitroxides and their

diamagnetic derivative 133

A.1.1 Introduction 133

A1.2 Results and Discussion 134

A1.3 Experimental 136

A1.4 References 137

A.2 Halogenation of the azaphenalene nitroxide 137

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A.2.1 Introduction 137

A2.2 Results and Discussion 138

A2.3 Experimental 141

A2.4 References 144

A.3 Synthesis and EPR Studies of 1,1,3,3-tetraphenylisoindolin-2-

-yloxyl 145

A.3.1 Impact of molecular size on electron spin relaxation rates of

nitroxyl radicals in glassy solvents between 100 and 300 K 146

ABSTRACT 146

KEYWORDS 147

INTRODUCTION 148

METHODS 150


Spin lattice relaxation, 1/T1, for nitroxyl radicals 156 Temperature Dependence 156

Comparison of the dependence of the Raman coefficients, libration,

and tumbling correlation times on molecular volume 159

Estimation of effective volumes 161

Spin-lattice relaxation rates scaled by V-γT 165

Characterization of the additional relaxation process 165

Comparison of molecule-dependent scaling parameters 166

Comparison of 1/T1 for nitroxyls and other organic radicals 167 Impact of methyl rotation 171

Impact of other motions on spin echo dephasing 172

CONCLUSIONS 174 Acknowledgements 175

References 175

A3.2 Electron spin–lattice relaxation of nitroxyl radicals in

temperature ranges that span glassy solutions to low-viscosity

liquids 178

ABSTRACT 178

KEYWORDS 179

INTRODUCTION 179

EXPERIMENTAL METHODS 183

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Nitroxyl tumbling correlation times 186

Temperature dependence of 1/T1 190

Isotope effects on relaxation 192

Characterization of processes that depend on molecular tumbling 195

Spectral density function 197

Simulation of temperature dependence of 1/T1 198 Raman process 199

Local mode 200

Tumbling-dependent processes 201

Comparison of CA,g with theory 202

SUMMARY AND CONCLUSIONS 204 Acknowledgements 204

REFERENCES 204

xi

LIST OF FIGURES

CHAPTER 1

Figure 1.1. Schematic diagram of a typical CL apparatus 7

Figure 1.2. Typical CL curve of the oxidation of polypropylene at 150 °C 8

Figure 1.3. FTIR spectral comparison 8

Figure 1.4. Commercially available examples of hindered phenol stabilizers 10

Figure 1.5. Commercial examples of HAS 12

Figure 1.6. Commercial examples of secondary antioxidants 13

Figure 1.7. General Structure of Stable Nitroxide 15

Figure 1.8. Profluorescent nitroxide listed within the literature 21

Figure 1.9. Di-tert-alkyl nitroxides 23

Figure 1.10. Degradation of 44 doped polypropylene simultaneously

monitored by fluorescence, chemiluminescence and FTIR-ATR128 26

Figure 1.11. Comparison of non-degraded and degraded polypropylene

doped with 44 27

Figure 1.12. Fluorescence confocal microscopy image of HeLa cells doped

with 55135 29

Figure 1.13. Flow diagram of the relationship between project aims, published

manuscripts and chapters within the thesis 32

CHAPTER 2

Figure 1. Fluorescence spectra of 5 and 6 excited at 290 nm in cyclohexane 42

Figure 2. Fluorescence spectra of 7 and 8 excited at 290 nm in cyclohexane 42

Figure 3. Spectrofluorimetry of UV-aged polypropylene doped with 5,

compared with that of undoped polypropylene monitored at

340 nm; excitation at 290 nm 43

Figure 4. The molecular structure of one of five molecules in the

symmetric unit of TMAO, 5 [N(1), O(1), C(17)–C(32)] 44

Figure 5. The molecular structure of 7. The molecule has twofold

symmetry [special positions occupied by N(1) and O(1) with the

axis passing between C(8) and C(81)] (1: y + 1, x – 1, –z + 1) 44

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Figure S1. ORTEP representation of one of the five molecules in the

asymmetric unit. Ellipsoids are drawn at the 30% probability

level 56

Figure S2. ORTEP representation of 7 (ellipsoids drawn at the 30%

probability level). The molecule has 2-fold symmetry with special

positions occupied by N(1) and O(1) with the axis passing between

C(8) and C(81) (1 y + 1, x − 1, −z + 1) 57

Figure S3. EPR Spectrum of 5 58

Figure S4. Hyperfine splitting of 5 at different oxygen concentrations 58

CHAPTER 3

Figure 1. Cyclic voltammograms of the nitroxide/N-oxo ammonium

couple of 2a and 4a 68

Figure 2. B3LYP/6-311+G(3df,2p) Orbital Diagrams showing the Singly

Occupied Molecular Orbital (SOMO) of the parent nitroxide

radicals and the highest occupied molecular orbital (HOMO) of

the oxidised species 71

Figure 3. Oxidation potentials of mono-substituted isoindoline nitroxides

plotted with respect to the Hammett constant of the functional

group (σp) 72

Figure 4. Cyclic voltammograms of the nitroxide/N-oxo ammonium

couple of 5a, 4a and 5b 72

Figure 5. Cyclic voltammograms of the nitroxide/hydroxylamine couple

of 2a and 4a 73

Figure S1. 1H NMR (300 MHz, CDCl3) of 2-benzyl-1,1,3,3-tetramethyl-

6-nitro-2,3-dihydro-2-azaphenalene (3`b) 88


6-nitro-2,3-dihydro-2-azaphenalene (3`b) – Expansion of

aromatic region 88

Figure S3. 13C NMR (300 MHz, CDCl3) of 2-benzyl-1,1,3,3-tetramethyl-

6-nitro-2,3-dihydro-2-azaphenalene (3`b) 89


6,7-dinitro-2,3-dihydro-2-azaphenalene (3`c) 89

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6,7-dinitro-2,3-dihydro-2-azaphenalene (3`c) – Expansion of

aromatic region 90

Figure S6: 13C NMR (300 MHz, CDCl3) of 2-benzyl-1,1,3,3-tetramethyl-

6,7-dinitro-2,3-dihydro-2-azaphenalene (3`c) 90

CHAPTER 4

Figure 1. Nitroxides investigated in this study 96

Figure 2. Molar concentration of 3 nitroxides (TMAO, TMDBIO and

MeCSTMIO) in polypropylene 98

Figure 3. Chemiluminescence of polypropylene doped with; Blank,

TMAO, TMDBIO and MeCSTMIO, aged in O2 at 150 °C 99

Figure 4. Fluorescence intensity of polypropylene doped with;

MeCSTMIO, TMAO or TMDBIO, aged in O2 at 150 °C 99

Figure 5. Chemiluminescence of polypropylene doped with; TMAO and

TMDBIO aged in O2 at 135 °C 100

Figure 6. Spectrofluorimetery of polypropylene doped with TMAO, aged

in O2 at 120 °C for; 0 mins, 180 mins , 180 mins then BPO

initiator for 15 mins and comparison to Methoxyamine adduct of

TMAO 101

Figure 7. Fluorescence of polypropylene doped with; MeCSTMIO,

TMAO or TMDBIO, UV-aged in air at 35 °C 102

Figure 8. Oxidation indicies as determined by FTIR-ATR of polypropylene

doped with; MeCSTMIO, TMAO, TMDBIO and a Blank,

UV-aged in air at 35 °C 103

Figure 9. Fluorescence of polyesters aged at 95 °C in air; polyester A

undoped, polyester B undoped, polyester A MeCSTMIO-doped

and polyester B MeCSTMIO-doped 105

Figure 10. FTIR oxidation of polyesters aged at 95 °C in air; polyester A

undoped and polyester B undoped 106

Figure 11. Percentage mass loss of polyesters aged at 95 °C in air;

polyester A undoped and polyester B undoped 107

xiv

CHAPTER 5

Figure 1. Fluorescence and chemiluminescence emission of BTMIOA 6

doped polypropylene aged under O2 at 150oC (260 nm excitation).

Note the induction period for chemiluminescence from undoped

polypropylene is essentially zero over this timescale 117

Figure 2. Polypropylene plaque images excited between 254-290 nm using

short-range UV lamp (a) blank (b) doped with BTMIOA 6 at

0.17 mM (c) doped with TMDBIO-Me at 1.1 mM and (d) doped

with BTMIOA-Me 7 at 0.17 mM 118

Figure 3. Fluorescent images of a BTMIOA 6 doped, 2.5 cm x 2.5 cm

polypropylene plaque spiked with benzoyl peroxide initiator and

heated at 100oC in O2 for (a) 0 hr (b) 1 hr (c) 8 hr (d) 12 hr (e)

18 hr and (f) 24 hr. (a′)-(f′) are three-dimensional representations

of fluorescence for images (a)-(f) 119

APPENDIX A

Figure A.1.1. Compounds examined in this study 134

Figure A.1.2. Fluorescence decay rates of compounds A1-A6 135

Figure A.3.1. Structures and molecular weight of nitroxyls and other

organic radicals examined in this study 150

Figure A.3.2. Temperature dependence of 1/T1 for nitroxyls in (a) sucrose

octaacetate or (b) sorbitol; DTBN, tempone, CTPO, tempol,

TEIO and TPHIO. 158

Figure A.3.3. X-band CW EPR spectra of DTBN in sucrose octaacetate at

several temperatures, showing the changes in Azz 159

Figure A.3.4. X-band CW EPR spectra of DTBN, tempone, TMIO, TEIO and

TPHIO in decalin at 233 K and fit lines calculated using the NLSL

program34 160

Figure A,3.5. Comparison of the impact of molecular size on tumbling,

libration, and the Raman coefficient. (*) (< α2 >/< α2tempone >)–1

in sucrose octaacetate, τ/τtempone in decalin and heavy mineral oil

as a function of (C”Ram)-0.5. 161

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Figure A.3.6. X-band spin-lattice relaxation rates, 1/T1, for nitroxyls in (a)

sucrose octaacetate and, (b) sorbitol plotted as a function of

(Veff/Vtempone)- γ T; DTBN, tempone, tempol, CTPO, TEIO and

TPHIO 164

Figure A.3.7. (a) Contribution to spin-lattice relaxation rates from the additional

process, 1/T1ADD plotted as a function of log(T) and (b) 1/T1ADD

plotted as a function of log(Veff/Vtempon)- γ T, for nitroxyls; DTBN,

tempone, CTPO, TEIO and TPHIO in sucrose octaacetate and

DTBN, tempone and CTPO in sorbitol 166

Figure A.3.8. Spin-lattice relaxation rates, 1/T1, for organic radicals; tempone,

TPHIO, DPPH, galvinoxyl, and BDPA in sucrose octaacetate at

X-band, and for tempone, TPHIO, galvinoxyl, and BDPA in

sucrose octaacetate at Q-band. The solid lines are fit lines for the

combined impact of the Raman process and the additional

process. 168

Figure A.3.9. (a) Correlation between contributions to the relaxation rates from

the Raman and additional process at X-band for nitroxyls

(tempone, TEIO, TPHIO), galvinoxyl, DPPH, BDPA in sucrose

octaacetate and literature values for trityl-CD3 radical in 1:1 water

glycerol.29 (b) Relaxation rates for the same samples replotted with

both the x and y axes scaled by the same factor, CRA 169

Figure A.3.10. Temperature dependence of spin echo dephasing rates, 1/Tm, of

DTBN, tempone, CTPO, TMIO, tBuPyrr, TEIO and TPHIO and

galvinoxyl in sucrose octaacetate 172

Figure A.3.11. Spin echo dephasing rates for nitroxyls in sucrose octaacetate

as a function of (Veff/Vtempone)- γT using the values of Veff/Vtempone

shown in table 3 with (a) γ = 3.5 as observed for libration and (b) γ =

0.89 as observed for the Raman process; DTBN, tempone, CTPO,

TMIO, and TPHIO. 173

Figure A.4.1. Structures of the nitroxyls studied 182

Figure A.4.2. Tumbling correlation times, τ, of tempo or tempone in several

solvents as a function of temperature: tempo in 3-methylpentane,

tempone in 1-propanol, tempone in decalin, tempone-d16 in 1:1

water:glycerol, tempone in glycerol. 188

Figure A.4.3. Tumbling correlation times for tempone, tempone-d16, tempol,

tempol-d17, 15N-tempol-d17, CTPO and CProxyl in 1:1

water:glycerol mixtures as a function of temperature. 190

Figure A.4.4. Temperature dependence of 1/T1 for tempone in decalin, sucrose

octaacetate, 1:1 water:glycerol, sorbitol, and tempo in 3-

methylpentane. Values of 1/T1 for trityl-CD3 in water:glycerol

(1:1) are included for comparison. [43] 191

Figure A.4.5. 1/T1 at X-band for tempone in decalin, OTP, or sucrose

octaacetate 192

Figure A.4.6. Temperature dependence of 1/T1 at X-band for tempone, tempone

-d16, and 15N-tempone-d16 in 1:1 water:glycerol, glycerol or

decalin, showing isotope effects. 194

Figure A.4.7. Temperature dependence of isotope effects on 1/T1 for isotopically-

substituted tempone in glycerol and 1:1 water:glycerol. The (□) and

(■) points are the H/D ratio in water glycerol and glycerol,

respectively. The ( ) and ( ) points are the 14N/15N ratio in

water:glycerol and glycerol, respectively. 194

Figure A.4.8. Temperature dependence of (a) 1/T1 for 14N tempone in glycerol

and water:glycerol and the tumbling-dependent contribution to

relaxation [1/T1 (water:glycerol) − 1/T1 (glycerol)] and (b) the

tumbling-dependent contribution to relaxation [1/T1(water:glycerol)

− 1/T1 (glycerol)] for 14N tempone, 15N tempone, tempone-d16,

tempol, tempol-d17, 15N-tempol-d17, CTPO and CProxyl. 196

Figure A.4.9. Dependence of 1/T1 in water:glycerol and [1/T1 (water:glycerol)

–1/T1 (glycerol)] on tumbling correlation time between 228 and

288 K for tempone, tempone-d16 and 15N-tempone. 197

Figure A.4.10. Simulation of the temperature dependence of 1/T1 for (a)

tempone in glycerol, (b) tempone in water:glycerol, (c) tempone

in decalin, and (d) tempo in 3-methylpentane. 199

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xvii

LIST OF SCHEMES

CHAPTER 1

Scheme 1.1. Mechanism of action of hindered phenol stabilisers 11

Scheme 1.2. Regeneration Cycle of HAS Additive 12

Scheme 1.3. Action mechanism of phosphites and thioethers 14

Scheme 1.4. Resonance Structures of Nitroxide 14

Scheme 1.5. Disproportionation of a Nitroxide to a Hydroxylamine and a

Nitrone42 15

Scheme 1.6. Disproportionation of a nitroxide to an amine and a nitrone 16

Scheme 1.7. Radical Trapping Ability of Nitroxide 16

Scheme 1.8. Comparison of α-cleavage stability of 38 to 39 24

CHAPTER 2

Scheme 1. Reagents and conditions: (i) BnNH2, AcOH, reflux, >95%;

(ii) MeMgI, xylenes, reflux, 15.5%; (iii) 50 psi H2, 10% Pd/C,

AcOH, 85%; (iv) H2O2, Na2WO4·2H2O, NaHCO3/MeOH, 83%;

(v) FeSO4·7H2O, DMSO, H2O2, >95% 41

CHAPTER 3

Scheme 1. SOD mechanisms of cyclic nitroxides 63

Scheme 2. Nitroxides investigated in cyclic voltammetry and theoretical

studies 64

Scheme 3. Synthesis of azaphenalene derivatives 3b-e 65

Scheme 4. Synthesis of isoindoline derivatives 4e and 4f 66

Scheme 5. Synthesis of isoindoline derivatives 4d and 4i 67

CHAPTER 5

Scheme 1. Synthesis of BTMIOA 6 and BTMIOA-Me 7 115

xviii

APPENDIX A

Scheme A.2.1. General synthetic route for 5-bromo-1,1,3,3

tetramethylisoindoline-2-yloxyl4 138

Scheme A.1.2. Attempted bromination of A11 using isoindoline

conditions 138

Scheme A.2.3. Optimised brominating conditions for A11 139

Scheme A.2.4. Attempted bromination of A13 139

Scheme A.2.5. Attempted bromination of A15 and A17 140

Scheme A.2.6. Attempted halogenation of A15 140

Scheme A.2.7. General synthetic route for 5-iodo-1,1,3,3-

tetramethylisoindoline-2-yloxyl (A21) via diazotation 141

Scheme A.2.8. Synthesis of A23 via diazotation 141

Scheme A.3.1. Synthetic pathway for 1,1,3,3-tetraphenylisoindolin-2-

yloxyl (TPHIO). (1) PhMgBr, toluene, reflux; (2)

MCPBA, DCM 150

xix

LIST OF TABLES

CHAPTER 3

Table 1. Experimental redox potentials (V) of the studied nitroxidesa 70

Table 2. Comparison of experimental and theoretical oxidation potentials

of the studied nitroxides (V) 76

APPENDIX A

Table A.1.1 Photophysical data of compounds A1-A6 134

Table A.2.1. Experimental conditions for bromination attempts 142

Table A.3.1. Raman coefficients C”Ram in sucrose octaacetate and sorbitola 157

Table A.3.2. Dependence of tumbling correlation times and libration on

molecular size 162

Table A.3.3. Effective molecular volumesa 163

Table A.3.4. Comparison of relaxation rates for organic radicals with

different isotropic g-values 170

Table A.4.1. Parameters used for the simulations and theoretical values of CA,g 187

xx

ABBREVIATIONS

AcOH Acetic Acid

AIBN 2,2'-Azodiisobutyronitrile

Ar aryl

ATR attenuated total reflectance

au arbitrary units

BnNH2 benzylamine

BPO benzoyl peroxide

br broad

BTMIOA 9,10-bis(1,1,3,3-tetramethylisoindolin-2-yloxyl-5-yl)anthracene

calc. calculated

CL chemiluminescence

CV cyclic voltammetry

d doublet

dd doublet of doublets

dec. decomposed

DCM dichloromethane

DMF N,N-dimethylformamide

DMSO dimethylsulfoxide

DODB dioxane dibromide

DTBN di-tert-butylnitroxide

EI electron impact

EPR electron paramagnetic resonance

equiv equivalent(s)

ESI electrospray ionisation

Et ethyl

Et2O diethyl ether

EtOAc ethyl acetate

FT fourier transform

GC gas chromatography

h hour(s)

xxi

HAS hindered amine stabilisers

HOMO highest occupied molecular orbital

HPLC high performance liquid chromatography

HRMS high resolution mass spectrometry

IR infrared

ISC inter system crossing

m multiplet

MAD mean absolute deviation

Me methyl

MeCSTMIO 5-[2-(4-methoxycarbonyl-phenyl)-ethenyl]1,1,3,3

tetramethylisoindoline-2-yloxyl

MeMgI methylmagnesium iodide

mCPBA m-chloroperbenzoic acid

min minute(s)

mp melting point

MS mass spectrometry

NBS n-bromosuccinamide

NIS n-iodosuccinamide

NMR nuclear magnetic resonance

Pd/C palladium on charcoal

Ph phenyl

PP polypropylene

ppm parts per million

PMT photo-multiplier tube

PMMA poly(methyl methacrylate)

PROXYL 2,2,5,5-tetramethyl-1-pyrrolidinyloxyl

ROS reactive oxygen species

RT room temperature

s singlet

SOD superoxide dismutase

SOMO singly occupied molecular orbital

t triplet

TBAF tetrabutylammonium tetrafluoroborate

xxii

TEMPO 2,2,6,6-tetramethyl-1-piperidinyloxyl

THF tetrahydrofuran

TGA thermogravimetric analysis

TLC thin layer chromatography

TMAO 1,1,3,3-tetramethyl-2,3-dihydro-2-azaphenalen-2-yloxyl

TMBIO 1,1,3,3-tetramethylbenzo[f]isoindolin-2-yloxyl

TMDBIO 1,1,3,3 tetramethyldibenzo[e,g]isoindolin-2-yloxyl

TMIO 1,1,3,3-tetramethylisoindolin-2-yloxyl

TPhIO 1,1,3,3-tetraphenylisoindolin-2-yloxyl

TLC thin layer chromatography

UV ultraviolet

xxiii

DECLARATION

The work contained in this thesis has not been previously submitted to meet the

requirements for an award at this or any other higher education institution. To the

best of my knowledge and belief, this thesis contains no material previously

published or written by another person except where due reference is made.

James Blinco

April 2008

xxiv

NOTE TO THE READER

Due to the layout required for a thesis by published papers, the numbering scheme of

some compounds, figures, etc. may lead to confusion. Should a compound be

referred to in a later chapter, such as conclusions, it will be numbered as the chapter

number first, followed by a compound number. For example, compound 1.10 refers

to compound 10 in chapter 1. Some inconsistencies in the formatting of chapters is

to be expected as each chapter has been formatted to meet the specific requirement of

the journal in which the paper has been published. Some diagrams have been moved

and resized within the chapters to make it easier for the reader.

xxv

ACKNOWLEDGEMENTS

I would like to start by thanking my principal supervisor, Associate Professor Steven

Bottle for the support he has shown throughout my project. He always made time to

discuss problems, offer advice and encouragement or suggest ideas while still giving

me plenty of freedom to drive the project in the direction I wanted.

Thanks must also go to my associate supervisors, Professor Graeme George and

Doctor Aaron Micallef. Both were always available to offer input into many aspects

of my project. Their wisdom and friendship was much appreciated.

I would also like to show my gratitude to the remainder of the Bottle research group,

both past and present. Their support, advice and camaraderie helped make my time

in E411 both bearable and enjoyable.

I would like to acknowledge QUT, ARC Centre of Excellence in Free Radical

Chemistry and Biotechnology and the Royal Australian Chemical Institute for

financial support throughout my project, which as well as a scholarship also gave me

several opportunities to travel and present my work, for which I am grateful.

I must also thank a number of the chemistry postgraduate students and research staff

that helped make my time at QUT memorable. The laughter and friendship

throughout the years will not be forgotten.

Finally, I would like to thank my family, in particular my soon to be wife, Nicole.

While I may not have always shown it, I have really appreciated all the love and

support you have shown me over this journey. Thank-you.

1

CHAPTER 1

Introduction

Chapter 1: Introduction

2

1. INTRODUCTION

1.1 Degradation and stability of polyolefins

Based on annual production volume alone, polyolefins are by far the most important

class of polymer, industrially. Of the approximately 200 000 000 metric tons of

polymer produced annually, over half is various forms of polyolefins.1

The degradation of polyolefins has been studied extensively because of their

importance as both engineering and commodity polymers. Polymer degradation by

definition is “physical and chemical changes to a polymer accompanied by small

structural changes, which lead to significant and irreversible deterioration of the

quality of material.”2

Perhaps surprisingly, the intrinsic resistance of common polyolefins, such as

polyethylene and polypropylene, to oxidative degradation is extremely low. In the

absence of stabilisers, exposed polypropylene will very rapidly undergo degradation

reactions and quickly lose all mechanical effectiveness.3 It is now well established

that in the presence of oxygen, when a polymer is exposed to adverse environmental

conditions, reactive intermediates begin to form within the matrix. These, in time,

lead to a deterioration of mechanical properties of the polymer. The rate at which this

loss of properties occurs depends on the structural qualities of the polymer, the

environment in which the polymer is applied and the loadings of stabiliser. With

regard to structural features of the polymer, variables that affect the rate of

degradation include: catalyst residues from the synthesis of the polymer,4-8 the

amount of branching9-11 and the degree of unsaturation within the polymer

backbone.9,12,13

In terms of environmental factors, by far the most important parameters for

polyolefin degradation are oxygen, temperature and exposure to UV light. The fact

that UV light is a major factor could appear surprising as most polyolefins do not

contain a chromophore and, therefore, are non UV-light-absorbing substances.

However, studies have shown that the presence of trace chromophores within the


3

polyolefin, arising from processing (i.e. catalyst residues, etc.) or absorbed from the

environment (i.e. atmospheric polynuclear aromatic hydrocarbons, etc.) can lead to

photo-degradation.2 Environmental variables such as wind and rainfall also have an

effect on the rate of degradation.14,15

Although most polyolefins possess relatively simple molecular structures, the

chemical mechanism underpinning their degradation can be far more complex. In

terms of the chemical reactions involved, this degradation can be viewed as an

interdependent cascade of radical reactions in which oxidation results in chain

scission and the subsequent formation of further radical species to propagate damage.

This degradation is an auto-oxidative process and is proposed to occur through the

following steps:

I + RH IH + R Initiation(1)

Although not completely understood, initiation of oxidation of polyolefins is

generally thought to arise via a homolytic bond dissociation resulting from one or

more of the environmental factors shown above.

+ RHR-O-O Propagation(2)

+ O2 R-O-O(1) R

R-O-O-H + R

Once the alkyl radical, R., has been formed, it can undergo a reaction with molecular

oxygen to form peroxy-radicals at diffusion-controlled rates. This is followed by

hydrogen abstraction from the polymer to reform an alkyl radical. The ease of

hydrogen abstraction from the polymer becomes rate-determining in the propagation

of the radical species. That is, polymers with lower carbon-hydrogen bond strength

form more stable alkyl radical species and consequently oxidise faster. Both of the

propagation steps can be cycled numerous times.2


4

Chain Branching(5)

(4)

H2O+

R R

OOH

R R

O

+ OH

R R

OOH

R R

OO

R R

O+2 x

Chain branching occurs when reaction products (hydroperoxides) formed during the

previous steps decompose leading to an increase in radical concentration and an

auto-acceleration of degradation. Unimolecular decomposition of hydroperoxides

(equation 4) has a very high activation energy making this mechanism much less

likely at ambient conditions. More likely is a bi-molecular or “in-chain” reaction

(equation 5) in which adjacent peroxides decompose, yielding two radicals and water

at a much lower activation energy.2

Chain Scission(7)

(6)

+

RCH

O

+

R

OO

RCH

O

R +

R R

O

R

ROH

While there are various pathways by which chain scission can occur, the most

common is a unimolecular β-scission of oxygen-centred radicals. This process results

in a new radical site, which continues the oxidation process, and adds a point of

unsaturation on the polymer chain, prone to further oxidation. The main observed

effect of chain scission is a decrease in molecular weight, leading to changes in the

polymers properties, both mechanical and aesthetic.


5

Hydrogen Abstraction(9)

(8)

+

+R R

O

R+ RH R R

OH

OH + RH RH2O

Hydrogen abstraction allows chain transfer of the radical species both inter- and

intra-molecularly. The intra-molecular chain transfer is approximately 3 times more

likely than an intermolecular transfer.16 When an intra-molecular transfer occurs, the

radical migrates along the polymer chain giving rise to various types of new radicals

on the polymer backbone.

2 x R Cross-Linking(11)

+ R-O-O R-O-O-H(10) R

R-R

The likelihood of a polymer chain cross-linking depends on steric constraints at the

radical site. In a highly branched polymer, such as polypropylene, radical sites are

usually formed at tertiary carbons and are relatively hindered, leading to lower levels

of cross-linking. In polyolefins with less branching, such as polyethylene, cross-

linking is more common and is often observed as an initial increase in molecular

weight and viscosity as degradation occurs.

Chain Termination(13)

(12)

+

R

R

O

R-O-HR R

O

R

2 x R

R

O

R

OR

R

R

2 xR

CH

O

+ O2

In an oxygenated environment, the only pathway that needs to be considered for

chain termination is a bi-molecular process between two peroxy-radicals.ref The

resulting products from this reaction depend on the structure of the peroxy radicals

involved. If the radicals are hindered and tertiary, the product formed is usually a


6

dialkyl peroxide (equation 12), while secondary radicals tend to yield a ketone, an

alcohol and O2 via the Russell mechanism (equation 13).17 While termination

reactions do have the overall effect of stabilising a polymer as radicals are removed,

the subsequent products formed are much more prone to further oxidation than the

initial polyolefin.

1.2 Monitoring polyolefin degradation

Monitoring polyolefin degradation is very important due to the commercial relevance

of these polymers, as well as their acute sensitivity to oxidation. There is a rich

literature on polyolefin degradation, which examines this process from many

different points of view, including: aesthetically (crazing or yellowing of the

polymer), mechanically (elongation at break, etc.), morphologically (investigations

of surface micro-fractures) and chemically (oxidation index/carbonyl build-up,

change in molecular weight, etc.).18 Ultimately, the goal of much of this work is to

link these observations to the mechanism underpinning the degradation and thereby

use these changes as a predictor of the service lifetime of the polymer.

Investigations into the degradation of polyolefins are more complex than other

polymer systems as there is an “induction period” during which very little oxidation

or changes in properties are observed. This induction period is followed by a rapid

increase in oxidation and subsequent loss of the majority of mechanical properties. It

has been shown that the service lifetime of a polyolefin does not extend far beyond

this “induction period”. The challenge for research and development in this area

therefore has been to monitor the earliest stages of oxidative damage. Prior to this

work the best, current analytical techniques for detecting oxidative damage during

the “induction period” were measuring the oxygen uptake by the degrading

polymer19 and chemiluminescence (CL).

CL simply refers to the emission of light from a chemical reaction via an excited

state intermediate. The technique has a long history, being first used to monitor

polymer degradation by Ashby in 1961.20 It is believed that this light emission

comes about due to an excited state triplet carbonyl returning to the ground state. It


7

has been proposed that these triplet carbonyls can be formed from the decomposition

of hydroperoxides21,22 or from the bimolecular termination of peroxy radicals via the

Russell mechanism.21,23 The quantum yield of chemiluminescence is very weak for

the oxidation of polyolefins (ca. ΦCL=10-8–10-15).21 Even so, chemiluminescence

from the oxidation of polyolefins can be detected by sensitive instrumental

apparatus.24

Figure 1.1: Schematic diagram of a typical CL apparatus

A typical CL apparatus5,25 is shown in Figure 1.1. The sample chamber is protected

from external light sources and is both temperature and atmosphere controlled. The

detection unit of a CL instrument usually consists of a photo-multiplier tube which is

connected to a pulse height discriminator and counter. The photons of light emitted

from the polymer are detected by the PMT and give rise to a discrete pulse of

electrons. The discriminator converts these pulses to a digital signal, which is

recorded by the counter. Measurement of CL is usually plotted as photon count as a

function of temperature or time (an example is shown in Figure 1.2).


8

0

100000

200000

300000

400000

500000

0 50000 100000Time (s)

I (C

ount

s/s)

Figure 1.2: Typical CL curve of the oxidation of polypropylene at 150 °C

In more advanced stages of degradation, other instrumental monitoring techniques

can be employed for monitoring the degradation of polyolefins, the most common

being infra-red (IR) spectroscopy.3,26-28 This technique allows the detection of

chemical changes within the polymer by the observation of vibrational-band changes

within the polymer’s IR spectrum. The most pronounced change is the appearance

of carbonyl (and to a smaller extent hydroxyl) bands which are only present after

oxidative damage to the polymer. An example of the appearance of these bands in

polypropylene is shown in Figure 1.3.

Figure 1.3: Comparison of FTIR spectra of non-degraded and (blue) thermally

degraded polypropylene (red)

70090011001300150017001900Wavenumbers (cm-1)

PPOxidised PP


9

Other spectroscopic techniques that have been applied to monitoring polymer

degradation include: Electron Paramagnetic Resonance (EPR) which is used to

monitor the formation of radical species within the polymer matrix, Nuclear

Magnetic Resonance (NMR) which can be used to analyse structural changes to the

polymer and UV-Visible spectroscopy which can be used to detect the formation of

chromophoric groups within the polymer.29

Although much information can be derived from these analytical techniques, none of

them are without limitations. Most of these techniques involve the destruction or

some modification of the polymer being monitored. This transformation occurs

either in the sampling, through having to remove a small portion of polymer for the

analysis; the sample preparation, where the sample has to be dissolved, pressed, cut,

etc. in order to allow analysis; or in the analysis itself, in which the technique

determines changes within the polymer sample by ultimately destroying the sample.

Clearly, the development of a technique that allows non-destructive, real-time

monitoring of polymers in situ would be of benefit for analysing polymer

degradation, especially in regards to time-course measurements. This research

project involved the use of novel probe additives and aimed to allow such monitoring

with the additional benefit of heightened stabilisation of the polymer matrix by the

probe.

1.3 Polyolefin stabilisers

As mentioned in Section 1.2, in the presence of oxygen, polyolefins will readily

degrade via the various pathways shown. For this reason, it is essential for

polyolefins to have stabilisers incorporated into their bulk, to retard degradation and

increase service lifetime.30 With regard to the protection of polyolefins against

oxidative damage, the most important class of stabilisers are antioxidants. These

antioxidants are often used in association with UV screeners/absorbers or metal

chelator/deactivators. Antioxidants are usually incorporated into the polymer at low

levels (generally 0.5 %w/w or less) and can be subdivided into two important


10

classes: chain-breaking (primary) antioxidants and preventative (secondary)

antioxidants.

1.3.1 Primary antioxidants

Primary antioxidants are radical scavengers that look to interrupt the propagation

step of the degradation cycle. They achieve this by removing carbon-centred alkyl

radicals, as well as oxygen-centred acyl and peroxy radicals. The most prominent

classes of primary antioxidants are the hindered phenol and hindered amine

stabilisers.

HO (CH2)2 C

O

O CH2

4Irganox 1010

HO (CH2)2 C

O

O C18H37

Irganox 1076

HO

HN

O

(CH2)6

OH

HN

O

Irganox 1098

HO

OH

OHIrganox 1330

Figure 1.4: Commercially available examples of hindered phenol stabilisers

Examples of commercially available hindered phenol stabilisers are shown in Figure

1.4. Pospisil31-37 has extensively investigated the mechanism behind the efficacy of

hindered phenols and how the phenol additives are consumed (shown in Scheme

1.1). Initially the degradation radical (either carbon- or oxygen-centred) is able to

abstract hydrogen from the phenol group to form a much more stable phenoxy-

radical. This phenoxy-radical will not undergo further hydrogen abstraction from the

polymer, but free radical character delocalizes into the aromatic ring where it allows


11

reaction with another radical to form an inert quinoidal species. Phenol groups are

also integral groups of UV screeners and metal deactivator/chelator additives.2

HO O

O

R R

RO

O

R

RHO

R

P PH

RO

1

2 3 Scheme 1.1: Mechanism of action of hindered phenol stabilisers

Hindered amine stabilisers (HAS) are commonly used in various polymeric materials

and were originally thought to only be active as photo-stabilisers (commonly referred

to as hindered amine light stabilisers or HALS).38 It has recently been shown that

these compounds also have a remarkable ability to retard the thermo-oxidative

degradation of polymers.3 Commercially available HAS units (Figure 1.5) usually

consist of two or more piperidene units joined through UV-stable groups. HAS can

be single, monomeric compounds (such as Tinuvin 123) or are available as larger

oligomeric molecules (such as Chimassorb 944). Larger HAS have reduced

volatility and, hence, are less susceptible to leaching from the polymer matrix.

Most of the stability that HAS impart on polyolefins is due to the oxidation products

of the stabilisers. The mechanism by which HAS stabilise polymers involves their

interaction with hydroperoxides formed in the degradation of the polymer. These

peroxides oxidise the piperidine or similar amine moiety (4) to the corresponding

nitroxide (5). The nitroxide generated rapidly traps reactive, carbon-centered


12

N

H

N

H

N

H

N

H

HNN N

NNN

NHC8H17

N (CH2)6 H(CH2)6

n

Chimassorb 944

NC8H17O OCO(CH2)8COO N OC8H17

Tinuvin 123

N OO C2H4 O C

O

C2H4

O

nTinuvin 622

NH O NH

Tinuvin 770

C

O

(CH2)8 C

O

O

Figure 1.5: Commercial examples of HAS

NR

(HAS)

N O

RROO

ROOR

N O R

ROO

ROOH

N O H

(Nitroxide Radical)

(Alkyl Radical)(Peroxy Radical)

(Alkylperoxide)

(Alkoxyamine)

(Peroxy Radical)

(Hydroperoxide)

(Hydroxylamine)(Olefin)

(Oxidation)

4 5

6 7

R'

Scheme 1.2: Regeneration cycle of HAS additive


13

radicals at rates competitive with the rates these radicals can react with oxygen (as

shown in Scheme 1.2). This interrupts the radical propagation/degradation cycle and

hence slows the speed at which degradation can proceed.

As shown in Scheme 1.5, HAS owe their efficacy to the generation of nitroxide

radicals. Nitroxides do not initiate damage; instead, they rapidly scavenge the

radicals present in the degrading polymer matrix. The properties of the stable

nitroxide radicals involved are unique and impart much of the protecting abilities of

HAS additives. There has been much research on the nature of the nitroxides

involved with a view to understanding and improving their properties. As nitroxide

additives are the main focus of this project they will be discussed in greater detail in

Section 1.4.

1.3.2 Preventative or secondary antioxidants

O PO

O

O

OP O

Irgafos 126P

Irgafos 168

C12H25 O

O

S O

O

C12H25

Irganox PS 800

C18H37 O

O

S O

O

C18H37

Irganox PS 802

Figure 1.6: Commercial examples of secondary antioxidants

Preventative or secondary antioxidants work by decomposing hydroperoxides non-

radically; therefore, not allowing chain-branching to occur. These compounds are

generally most effective when used in tandem with a primary antioxidant.39 The two

most important classes of secondary antioxidants are thioethers and phosphites.

Commercial examples of both classes are shown in Figure 1.6. The stabilising effect


14

of secondary antioxidants is based on an oxidation mechanism and is shown in

Scheme 1.3.

(RO)3 PROOH

(RO)3 P Ophosphite phosphate

+ ROH

RROOH

S R

thioether sulfoxide

+ ROHS R R

O

S R

sulfonate

+ ROHR

O

O

ROOH

Scheme 1.3: Action mechanism of phosphites and thioethers

1.4 Nitroxides

1.4.1 General

Nitroxides, also known as aminoxyl or nitroxyl radicals, are free radical compounds

that have a general formula of R(NO.)R’. Nitroxides have been the subject of

extensive research over the last 40 years following their initial development in the

1960’s. In comparison to other common free radicals, nitroxides are stable,

kinetically persistent systems that can have lifetimes easily long enough to measure

EPR spectra. In certain examples they can even be isolated and stored as an inert

compound. The great stability of the nitroxide arises principally from the strong

stabilization energy (ca. 32 kcal/mol) of the unpaired electron shared between the

two hetero atoms as shown in Scheme 1.4.40

N OR

RN O

R

R

Scheme 1.4: Resonance structures of a nitroxide

Equally, the orbital overlap of a non-bonding lone pair (on the nitrogen) interacting

with the unpaired electron generates a three electron, two-centre π bond.


15

Additionally, the formation of a dimer between two nitroxide molecules is

energetically unfavourable, as a weak R2N-O-O-NR2 bond would be generated. It

has been found that the most stable nitroxide compounds are those that lack any

hydrogen atoms α to the nitroxyl moiety, with a general formula as shown in Figure

1.7.

N OR

R R

R

R

R

R = CR'3

Figure 1.7: General structure of a stable nitroxide

When α-hydrogens are present, it has been shown that nitroxides can undergo

disproportionation reactions, which, as shown in Scheme 1.5, results in the formation

of nitrone and hydroxylamine species.41,42 Similarly, substituents with increased

conjugation at the α position have also been shown to be less stable, despite adding

thermodynamic stability, as they generate new disproportionation pathways leading

to amine and nitrone species as shown in Scheme 1.6.42

N OR

H2CH

NOR

CH2H N OH

R

H2CH

N OR

H2C

8

9 10

+

Scheme 1.5: Disproportionation of a nitroxide to a hydroxylamine and a nitrone42

The steric bulk around the radical group can also increase stability within the

molecule as it does not allow the radical to participate as easily in bi-molecular

reactions with itself.40


16

N OR

N OR

N OR

OH

NR

N OR NH

R

O

NR O

11 12 13

Scheme 1.6: Disproportionation of a nitroxide to an amine and a nitrone

Although nitroxides are stable towards other nitroxides, in the presence of other

radical moieties (such as carbon-, sulphur-43-46 or phosphorus-47 centred radicals)

they react readily at close to diffusion-controlled speeds (ca. 107-109 M-1s-1)45,47-52 to

form even electron, diamagnetic compounds. Unlike the weak oxygen-oxygen bond,

the trapping of these radicals imparts the newly formed alkoxyamine with more

stability than the resonance structure shown in Scheme 1.4.

N OR

RN O

R

RR'R' R' = C, P, N

Scheme 1.7: Radical trapping ability of a nitroxide

While nitroxides do undergo a variety of chemistry with reactive oxygen species,

they do not form stable adducts with oxygen-centered radicals.

Another nitroxide characteristic is a positive magnetic susceptibility due to the spin

of their unpaired electron (S = ½). This property, known as paramagnetism, is used

by many applications that employ nitroxides as spin labels or probes, the most

common tool for analysing these systems being EPR spectroscopy.


17

1.4.2 Nitroxide quenching of excited states

As well as being useful EPR probes due to their paramagnetism, nitroxides have also

been shown to be efficient quenchers of excited singlet,53-59 triplet,53,55,60-62 and

excimeric57 states. The exact mechanisms by which this quenching takes place are

not fully understood and there is debate in the literature regarding the contributions

of a number of different proposed pathways to the overall effect. Although a number

of mechanisms have been investigated including energy56,58,63-65 and electron/charge

transfer,56,58,63,64 the most widely supported mechanisms for this quenching are

electron-exchange-induced intersystem crossing (ISC)54-56,58-66 and/or vibrational

quenching/internal conversion.55,56,60-64

In the case of many fluorophores, there is often some overlapping of the lower

vibrational levels of the first excited singlet state and the upper vibrational levels of

the first excited triplet state. Thus, it is energetically possible for the triplet state to

be populated from the excited singlet state. Such a process, however, involves a

change in the spin state of the electron, which, classically, is spin forbidden.

When a paramagnetic nitroxide interacts with a fluorophore, a doublet state is

established and this system is preserved when electrons are promoted to the first

excited singlet state. By virtue of its unpaired electron, the nitroxide acts to enhance

the extent of spin-orbit coupling, causing a relaxation between the singlet and triplet

states such that ISC becomes an ‘allowed’ transition. This is because the resulting

formation of the triplet state of the fluorophore has taken place by electron exchange

with the nitroxide such that that the overall doublet spin-state of the system is

preserved. Similarly, the same spin-orbit coupling ‘allows’ ISC from the triplet to

the singlet ground state.

One consequence of this excited-state quenching by the nitroxide is that if a

fluorophore “encounters” a nitroxide, its ability to fluoresce is suppressed or

quenched. The rate of intermolecular fluorescence quenching has been found to have

a high sensitivity to the proximity of the fluorophore to the nitroxide, with the

greatest effect being observed when the nitroxide and fluorophore are within 0-25 Å


18

of each other.58 This encounter quenching has been used successfully as an

analytical tool, predominantly within biology. The technique (referred to as parallax

analysis) has been used to probe the depth of fluorescent markers within lipid

membranes. This technique involves incorporating nitroxide-tagged phospholipids

into the matrix and observing the induced quenching.67-70

1.4.3 Profluorescent nitroxides

The ability of nitroxides to quench fluorescence is not necessarily just an

intermolecular process and, in fact, intramolecular quenching is extremely

efficient.63-65 Many of the intermolecular quenching mechanisms rely on an

interaction (such as encounter or collision) between an excited molecule and a

radical. By tethering a fluorophore via a spacer group to a radical moiety, the

fluorophore is considered to be in a constant “encounter complex” with the radical,

thus yielding a molecule with little or no fluorescence. When the radical is removed

(by radical trapping or change in redox state) the molecule becomes diamagnetic and

fluorescence is no longer quenched. These molecules may be described as

“profluorescent” as they possess natural suppressed fluorescence which is returned

upon conversion of the radical moiety to a diamagnetic species.

Such profluorescent systems were first proposed by Stryer,71 but it was not until a

series of elegant papers by Blough et.al.63,64,72,73 that the potential of these tethered,

optically-switching molecules as potent probes of radical and redox states was

realised. Since this time profluorescent probes have been applied to a number of

interesting studies in fields as far separated as biological and materials science under

many different names (fluorescence yield switches, prefluorescence probes or dual

chromophore nitroxides). A list of profluorescent nitroxides found in the literature is

shown in Figure 1.8 (the list does not include isoindoline-based profluorescent

nitroxides which will be discussed further in Section 1.4.6).


19

1.4.4 Applications of profluorescent nitroxides

Gerlock et al.72 was first to utilise profluorescent nitroxides as probes by applying

the naphthyl carboxylate 14 for the detection of 2-cyanoisopropyl radicals, noting

that with increasing fluorescence, radical signal (as measured by EPR) decreased

proportionally. Using this same probe, Moad et al.74,75 were able to extend the

application of this new method to the measurement of primary radical concentrations

generated by pulsed-laser photolysis. The method was capable of measuring

transient radical concentrations of less than 10-7 M and demonstrated that the

concentration of radicals generated in any pulse was only dependent upon the

concentration of initiator species, and independent of the concentration of probe.

Yang and Guo76,77 achieved comparable detection limits when applying 14, as well

as anthracene derivative 19, to the indirect detection of hydroxyl radicals via trapping

of methyl radicals generated from the quantitative reaction of hydroxyl radicals with

dimethyl sulfoxide (DMSO). Pou et. al.78 also used this method of detecting

hydroxyl radicals, utilizing the fluorescamine derivative 29. Li and Blough79,80

extended the analytical application of this method by using fluorescamine probes 25

and 28 for the detection of hydroxyl radicals within cellular systems. They

demonstrated that the efficacy of anticancer drugs based on quinoidal structures is

due, in part, to their ability to generate hydroxyl radicals in vivo.81,82

Coenjarts et al.83 demonstrated that thin poly(methyl methacrylate) (PMMA) films

doped with compounds 23 and 24, and a radical initiator, could be used to spatially

map photo-induced radicals. Spatial resolution of 10μm was achieved using this

technique. Scaiano and et.al.84 also applied 23 to the analysis of radical reactions in

PMMA films doped with AIBN, the assessment of the antioxidant capacity via

hydrogen transfer from phenols85 and the detection of carbon-centred radicals

generated during enzymatic processes.86

Hrdlovic et al.87 and Danko et al.88 doped anthracene derivatives 20 and 22 into thin

polyolefin films, but neither reported using these compounds as profluorescent

probes. Danko found that as the films were photo-oxidised, the probe’s


20

chromophore signal rapidly disappeared while monitored by UV-Visible

spectroscopy; suggesting that the probe was undergoing photo-chemically induced

reactions. No comment was made on fluorescence generated as the films degraded.

Lozinski et al.89 were the first to utilize the dansyl fluorophore 21, first proposed by

Stryer,71 and applied it to the analysis of vitamin C within fruit juice. Bilski et al.90

found that this fluorophore may act to quench singlet oxygen and that probes with

the dansyl fluorophore photo-sensitised singlet oxygen upon UV excitation,

highlighting the potential problems caused by UV irradiation in the presence of this

probe. Even so, Hideg et al.91 successfully applied this probe for determining that

photo-inhibition of photosynthesis by radiation resulted in the generation of singlet

oxygen in broad bean leaves.

Examination of the previously mentioned profluorescent nitroxides indicates that

most have been synthesised via a combination of commercially available

fluorophores and nitroxides. The linkages generally used to tether the moieties

together are potentially labile ester, amine, amide or sulphonamide linkages.

Problems may arise with these probes when they are exposed to harsh conditions

such as extreme heat or pH: conditions that could lead to cleavage of the fluorophore

from the nitroxide and result in artifacts during analysis.

Kieber and Blough,92,93 were able to circumvent this problem by using a simpler

nitroxide as a trap molecule, which was then derivatised further to form the

profluorescent nitroxide and adducts. They employed an amino nitroxide to trap

photo-chemically generated carbon-centred radicals in aqueous solutions. The

resulting mixture of nitroxide and adducts was reacted with fluorescamine. HPLC

with fluorescence detection allowed the observation of the diamagnetic adducts to a

limit of ca. 0.5 nM, two orders of magnitude more sensitive than common EPR

techniques.


21

COOR COORCOOR

COORHOOC

CO2R CO2R

N

S N

O

O

R

H

COORROOC

N

O

O

RN CO2R

OH

N

OO

CO2R

N

Ph

O

R

HO 2C

HO

S CO2R

O

OONH

NO

N

RO2N

NPh

O

R

HO2C

HO

N

Ph

O

HO 2C

HO

R

N

CN

R

O O

CONR

N

N

S N

O

O

R

H

N

S N

O

O R

N EtEt

OO NH

R

O

R

N O

N O

N O

14 15 16 17

18 19 20 21

22 23 24 25

26 27

28 29 30 31

32

33 34

35

R=

R=

R=

N

NO

R=

Figure 1.8: Profluorescent nitroxides described in the literature


22

Kieber et al.94 and Johnson et al.95 went on to develop this method further to include

dual fluorescence/mass spectrometry detection, allowing the trapped species to not

only be detected, but to elucidate structural information as well. Flicker and

Green96,97 employed a similar technique for the detection of reactive radical species

within cigarette smoke. In this case, the amino nitroxide was stabilised on a solid

support that the gases were passed over. The nitroxide and adducts were washed off,

derivatised with naphthalenedicarboxyaldehyde and submitted to HPLC analysis for

separation and detection of the fluorescent adducts. Attaching the fluorophore, after

trapping by the nitroxide, does eliminate some chance of cleavage. However, this

technique still has the drawback of having to subject the trapped species to further

chemical reactions with adducts that are not necessarily stable.

Targeting profluorescent nitroxides synthesised with a robust carbon-carbon

framework linking nitroxide and fluorophore limits the chance of cleavage or

fragmentation. Within the literature there are very few examples of profluorescent

nitroxides that employ such linkages.98-100 Our research group has found great

success in employing fused carbon linkages between a number of fluorophores and

the more chemically robust isoindoline class of nitroxides in particular.

1.4.5 Isoindoline nitroxides

Most literature pertaining to nitroxides has focused on the non-cyclic di-tert-

butylnitroxide (DTBN, 36), or derivatives of either the five-membered pyrrolidine

system 2,2,5,5-tetramethylpyrrolidin-1-yloxyl (PROXYL, 37), or the six-membered

piperidine system 2,2,5,5-tetramethylpiperdin-1-yloxyl (TEMPO, 38). This could be

partially due to the fact that a number of these aliphatic nitroxides are commercially

available. Within this group, research has focused on the fused aromatic, isoindoline

nitroxide: 1,1,3,3-tetramethylisoindoline-2-yloxyl (TMIO, 39).


23

N ON ON ON

O

36 37 38 39 Figure 1.9: Di-tert-alkyl nitroxides

The first literature report of the isoindoline nitroxide class was a series of papers by

Rozantsev and et.al.,101-103 which described a number of tetraethylisoindoline

analogues. Rassat and et.al.104,105 expanded upon this work to include several more

novel tetraethyl derivatives, as well as in depth EPR studies of these compounds.

The first tetramethylisoindoline analogue was published by Griffiths et al.106 in 1982.

This was followed by a number of publications107-110 by the same authors utilizing

TMIO as a radical trap to capture intermediates during polymerization in a number of

systems. Since this time, numerous other isoindoline derivatives have been

synthesized,111-114 including a number via an alternate Diels-Alder synthesis.115,116

These isoindoline derivatives have been utilized for applications including EPR

probes117,118 and nitroxide-mediated polymerization agents.119-121

Isoindoline nitroxides have been shown to have many advantages over their

commercial, aliphatic counter-parts. When investigated by Busfield et. al.,122 TMIO

displayed superior thermal and chemical stability under a variety of conditions. This

robust nature is generally thought to arise due to the fused, planar system forming a

much more rigid 6/5-membered bicyclic system. Another example of this robustness

is shown in the photodegradation study of Bottle et. al..123 Where TMIO and

TEMPO were compared, it was found that TMIO was more stable towards α-

cleavage and that recombination to reform TMIO, once cleavage had occurred, was

also favoured (as shown in Scheme 1.8). Micallef et. al.124 showed that even when

TMIO is subjected to flash vacuum pyrolysis, the integrity of the isoindoline ring

structure is maintained, with the major decomposition product being a demethylated

nitrone.


24

N Ohν N

O

N O hν NO

NO

N O+

39

38

40

41

42

Scheme 1.8: Comparison of α-cleavage stability of 38 and 39

Isoindoline nitroxide derivatives have also been shown to have superior EPR

linewidths over piperidine analogues,125 as well as being more efficient radical

traps,52 making them desirable synthetic targets as spin traps/probes.

The addition of the aromatic ring within the isoindoline nitroxide is also an

advantage. Having a UV chromophore incorporated into the ring structure aids

identification of products during synthesis as well as analytical identification of

trapped intermediates. The aromatic ring also allows additional sites for further

functionalisation (such as halogenation112 and nitration126), which have led to the

development of a number isoindoline-based profluorescent nitroxides as well as

water soluble nitroxides with applications as antioxidants in biological systems.

1.4.6 Profluorescent isoindoline nitroxides

As isoindoline nitroxides have been shown to be more robust than other classes of

nitroxide, profluorescent isoindoline nitroxides clearly represent an attractive goal as

probe molecules.


25

NO

N

PhO

HOHOOC

43

The first example of a profluorescent isoindoline probe was 43, synthesised by

Reid.127 This compound was based on a fluorescamine fluorophore, akin to those

used by Pou, et. al.78 and Herbelin et. al.64 Reid noted significant quenching of

fluorescence (ca. 30-fold) but found limited use for this probe as trapped adducts

were unstable and broke down into numerous products when isolation was

attempted.

N O N O

44 45 Reid also investigated the synthesis of the phenanthrene-based nitroxide 44. The

advantage of this system was the close proximity of nitroxide to fluorophore as well

as the robust fused ring system that was expected to be less reactive than the

fluorescamine backbone. Fluorescence spectroscopy showed almost double the

observed suppression of fluorescence of 44 compared to 43. An alternate synthesis

of 44 was carried out utilizing paldadium-catalysed coupling, by Hideg et. al.,98 who

also produced the initial synthesis for the naphthalene-based nitroxide 45, which was

a target of this project.

Studies of 44 as a probe for free radical formation during degradation of unstabilised

polypropylene were undertaken by Micallef et al..128 These studies showed the

potential of these compounds as radical sensors. Degradation of plaques, doped with

44, was monitored, in parallel, by fluorescence spectroscopy, infra-red spectroscopy

and chemiluminescence. As shown in Figure 1.10, the profluorescent probe


26

technique was able to detect radical formation from time zero, an area where other

established techniques gave little insight or even suggested no degradation was

occurring. It was also found that the “induction period” observed with other

monitoring techniques was extended when 44 was incorporated into the polymer.

This supports the hypothesis that the profluorescent nitroxide mode of action is

similar to that of HAS additives, when incorporated into a polymer. The detection of

radicals by this new method provided the first conclusive evidence that degradation

reactions occur from the onset of degrading conditions.

The fluorescence generated from the probe could also be imaged using photography.

It was found that by exciting the plaques with UV light, enough fluorescence could

be observed in degraded plaques to be evident in a digital image (example shown in

Figure 1.11). Such detection proved very sensitive and could be used to compare

plaques that had been degraded for differing lengths of time, as well as to compare

different areas on the surface of the same plaque. This indicated, for the first time,

the potential for profluorescent probes to provide spatial information for degradation

processes.

Figure 1.10: Degradation of polypropylene doped with 44 simultaneously

monitored by fluorescence spectroscopy, chemiluminescence and FTIR-ATR128

hallaThis figure is not available online. Please consult the hardcopy thesis available from the QUT Library


27

Figure 1.11: Comparison of non-degraded (top) and degraded (bottom)

polypropylene doped with 44

N O

O

O

N O

O

O

O

O46 47

Keddie and et.al.129 showed that the synthesis of profluorescent nitroxides with

carbon linkages was possible through palladium-catalysed coupling. The stilbene

analogues 46 and 47 were produced via Heck coupling of a styrene derivative with

either mono- or di-brominated TMIO, respectively.

NO

NO

48 49

Keddie also showed that it was possible to use the Sonagashira coupling technique to

link naphthalene and phenanthrene moieties to an alkyne-TMIO derivative yielding

48 and 49, respectively.130,131 Fairfull-Smith132,133 used the same methodology to


28

produce the bis-phenylethynylanthracene derivatives 50 and 51, while adopting

Suzuki coupling to produce diphenylanthracene derivatives 52 and 53.

NO

NO

NO

NO

NO

NO

50

51 53

52

Compounds 44-53 all exhibited potent inhibition of fluorescence, usually between

100 and 300 fold, and have a wide variety of wavelengths at which they can be

excited, and the subsequent fluorescence measured. This variety of excitation and

emission wavelengths is important when choosing a probe for a matrix with specific

light absorption characteristics, such as polymers. The testing of some of the afore-

mentioned compounds as probes for radical-mediated damage within a polymer

matrix forms part of this thesis.

N O

HN

O

N

N

O

O

HO

HO

54

As mentioned in Section 1.4.3 there are not only applications for profluorescent

nitroxides in materials science but many biological applications as well. Isoindoline

nitroxides have also been investigated in a biological context. Barhate and et.al.134

were able to attach an isoindoline nitroxide to a fluorescent nucleoside yielding 54,

followed by further reactions to form the subsequent ribonucleside fragment. It was

proposed that such a molecule would be a redox-active centre and could be

incorporated into nucleic acids to study their structure and dynamics via fluorescence

and EPR spectroscopy.


29

O

NO

OOC

O OH O

N

OOC

N N

O55 56

Keddie130 has also investigated strongly-fluorescent, xanthene-based dyes with

isoindoline nitroxides attached. He was successfully able to synthesise the green-

fluorescing fluoroscein adduct 55 and the red-fluorescing rhodamine adduct 56.

These probes have been applied by Morrow135 as cellular redox probes, as measured

by flow cytometery. Morrow was also able to show, by confocal microscopy

(example shown in Figure 1.12.), that 55 and its fluorescent adduct both localised in

the cellular membrane.135

Figure 1.12: Fluorescence confocal microscopy image of HeLa cells doped with

55135

hallaThis figure is not available online. Please consult the hardcopy thesis available from the QUT Library


30

1.5.1 Investigated research problem

Although most polymers are resistant to degradation, they are not immutable. Many

polymeric materials have substantial environmental residence times, but only limited

useful application lifetimes due to degradation, mechanical failure and surface

deterioration. As the service lifetimes of polymers vary depending on environmental

factors, it is important to have techniques in place to monitor polymer degradation.

While there are many different instrumental monitoring techniques available, almost

all suffer the limitation that they involve the destruction or severe modification of the

polymer being monitored.

Recently, we were able to demonstrate a novel degradation-monitoring method that

employs a profluorescent nitroxide as a probe for radical-mediated, thermo-oxidative

damage of polypropylene. This method detects radical species in the induction

period of the polyolefins’ degradation, where even the most sensitive alternative

monitoring techniques detect little or no change in the polymer.

This project looks to expand upon this area of research with two distinct foci; the

synthesis and characterisation of novel profluorescent nitroxide probes as well as

physical characterisation of the probe molecules in various polymer/physical

environments.

1.5.2 Project outline and objectives

The overall aim of this project was to investigate the relatively new area of

profluorescent nitroxides as probes for polymer degradation. Synthetic targets for

this project were designed with the following criteria being considered:

• Robust framework linking fluorophore and nitroxide

• Sensitivity between quenched nitroxide and fluorescent adduct

• Ability for the profluorescent nitroxides to be further functionalised to

enhance their use as probes (i.e. increased solubility, photo-physical

characteristics, etc.)


31

Based on this, the project looked to investigate profluorescent nitroxides that

incorporated a naphthalene moiety as the fluorophore. This investigation was not

limited to just the isoindoline nitroxides, usually focused on within the Bottle

research group, but also the novel fused piperidine class of azaphenalene nitroxides.

57

N O

This new azaphenalene nitroxide was suggested to be a hybrid of isoindoline and

piperidine nitroxides with theoretical calculations136 predicting interesting redox

properties. An electrochemical study was undertaken to validate the theoretical

calculations as well as being a method for comparing subtle differences in the cyclic

nitroxide ring classes.

Physical testing of the probes was undertaken with the incorporation of the

naphthalene probes as well as other profluorescent nitroxides synthesised within the

Bottle group into polypropylene (as a model polymer for polyolefins). The ability of

these profluorescent nitroxide probes to monitor radical-mediated damage (both

thermo- and photo-initiated) was investigated, and this profluorescence technique

compared with classical instrumental methods. The ability to use the probes to

visually image radical degradation was also explored.

1.5.3 Relationship of the research papers

The following flow diagram, Figure 1.1, illustrates the relationship between the aims

of this PhD project and the outcomes reported in published manuscripts arising from

this candidature.


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Chapter 1 – Introduction

A review of the literature pertaining to polyolefin degradation, nitroxides and

profluorescent nitroxides.

Chapter 2 – The First Example of an Azaphenalene Profluorescent Nitroxide

Synthesis of naphthalene-based nitroxides. Fluorescent properties of these compounds and preliminary investigations as polymer

degradation probes.

Chapter 3 – Experimental and Theoretical Studies of the Redox Potentials of Cyclic Nitroxides

Functionalisation of azaphenalene

nitroxides. Electrochemical comparison of nitroxide ring

classes.

Chapter 4 – Profluorescent Nitroxides: Sensors and Stabilizers of Radical-Mediated

Oxidative Damage

Investigations of profluorescent nitroxides for monitoring photo- and thermo-oxidative damage

in polymers.

Chapter 5 – A Novel Profluorescent Dinitroxide for Imaging Polypropylene

Degradation

Synthesis of a third-generation profluorescent nitroxide. Preliminary investigations of this

nitroxide as a radical degradation and imaging probe.

Figure 1.13 Flow diagram showing the relationship between project aims, published manuscripts and chapters within the thesis. Other unpublished studies are presented in Appendix A.


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1.6 References

(1) Mulhaupt, R. Macromol. Chem. Phys. 2003, 204, 289. (2) Vasile, C. In Handbook of Polyolefins; 2nd ed.; Vasile, C., Ed.; Marcel

Dekker Inc.: New York, 2000. (3) Gensler, R.; Plummer, C. J. G.; Kausch, H. H.; Kramer, E.; Pauquet, J. R.;

Zweifel, H. Polym. Degrad. Stab. 2000, 67, 195. (4) Gijsman, P.; Hennekens, J.; Vincent, J. Polym. Degrad. Stab. 1993, 39, 271. (5) Celina, M.; George, G. A. Polym. Degrad. Stab. 1995, 50, 89. (6) Celina, M.; George, G. A. Polym. Degrad. Stab. 1993, 40, 323. (7) Knight, J. B.; Calvart, P. D.; Billingham, N. C. Polymer 1985, 26, 1713. (8) Allen, N. S.; Fatinikum, K. O.; Henman, T. J. Eur. Polym. J. 1983, 19, 551. (9) Iring, M.; Földes, E.; Barabás, K.; Kelen, T.; Tüdós, F.;

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