Correlation between the Tacticity and Content of LabileNondefect Structures in Poly(vinyl chloride)

GERARDO MARTINEZ, JOSE LUIS MILLAN

Instituto de Ciencia y Tecnologıa de Polımeros, Consejo Superior de Investigaciones Cientıficas, Juan de la Cierva 3,28006 Madrid, Spain

Received 19 June 2002; accepted 14 August 2002Published online 00 Month 2002 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/pola.10474

ABSTRACT: The content of mm isotactic triads taking the GTTG� conformation forsamples of poly(vinyl chloride) (PVC) fractions of different tacticities was measuredthrough a substitution reaction with sodium benzenethiolate. This quantity changedlinearly with the ratio of rmmmrx to mmmmrx (x � m or r), as accurately determinedby 13C NMR spectroscopy. In a comparison of this correlation and that obtainedbetween the thermal degradation stability and overall isotactic content, as studiedpreviously, some novel evidence for the GTTG� conformation of a few mm triads,termini of isotactic sequences no shorter than one heptad as specific labile sites in PVC,was obtained. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 3944–3949,2002Keywords: poly(vinyl chloride) (PVC); modification; degradation; tacticity; labilenondefect structures; local chain configuration

INTRODUCTION

The nature of the thermal labile structures inpoly(vinyl chloride) (PVC), which are responsiblefor the initiation of the degradation of this impor-tant material, has been very controversial formany decades.1 Most authors have proposed someabnormal structures such as allylic chlorine andtertiary chlorine, which might arise incidentallyduring the polymerization process, as the uniquelabile sites in PVC. Some attempts to characterizethem and to state the possible mechanisms oftheir appearance have been made.1–3 Overall, thetotal content has been proposed to be not higherthan 6/1000 monomeric units.1

A rather great number of courses of action de-signed in our laboratory led to the conclusion thatat least a considerable number of the labile struc-

tures are normal structures, namely, the GTTG�

conformation, which may occur only in the mmisotactic triad termini of isotactic sequences of atleast one heptad in length. Those studies havebeen extensively reported.4–6 They are based onprogress in two fields, the characterization of thestereochemical microstructure and the stereospe-cific nucleophilic substitution reactions on PVC.

The configuration and conformation of PVC insolution have been studied extensively with 1Hand 13C NMR.7–14 However, the 13C NMR spectrahave shown some advantage over 1H NMR spec-tra for determining triad and tetrad concentra-tions. Besides, it is known that the distribution ofthe rotational conformations depends on the con-figuration, temperature, and nature of the sol-vent,15–18 and the overlapping of the resonancecan be removed with the appropriate solvent.Usually, spectroscopic investigations of PVC areperformed in chlorinated benzenes, that is, o-di-chlorobenzene and 1,2,4-trichlorobenzene, at tem-peratures near the thermal decomposition of thepolymer.8,9,13–19 However, Elgert et al.20 found it

Correspondence to: G. Martinez (E-mail: gmart@ictp.csic.es)Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 40, 3944–3949 (2002)© 2002 Wiley Periodicals, Inc.

3944

possible to observe all possible configurationalpentad sequences at a lower temperature withdioxane (DI) as a solvent. To elucidate the mech-anism of the stereoselective nucleophilic substitu-tion in PVC, we succeeded in determining accu-rately the content of mmmr and rmmr pentads,21

using highly resolved 13C NMR spectra, as foundby Elgert et al. Furthermore, such resolutionmakes it possible to follow, at least to some ex-tent, the changes in rmmmrx and mmmmrx (x� m or r) heptad contents,21 which led us toconfirm that the specific substitution in PVC pro-ceeds through the isotactic GTTG� triad confor-mation4–6 as long as there are mmr tetrads at theend of the isotactic sequences no shorter than oneheptad.4,21 This mechanism is such that the con-formational equilibrium between the two likelyconformations of mmr, that is, GTGTTT %GTTG�TT, should be displaced toward the rightside by the use of solvents showing specific inter-actions with the GTTG�TT conformation.22 Be-cause that conformational equilibrium stronglylies over the left side, the effect of the solvent andthe equilibrium depart from that corresponding tothe amount of each conformation and, as a resultof polymerization, will be nil at low temperatures.Therefore, the total conversion of substitution attemperatures lower than �15 °C could be identi-fied as the real content of mmr under the GTTG�

conformation for any PVC sample.23

Through a comparison of the evolution of thisquantity and that of rmmmrx and mmmmrx (x� m or r) for PVC samples with different ste-reospecific microstructures as attempted in thiswork, our proposal that the GTTG� isotactic triadconformation is certainly a labile site will be con-firmed and the knowledge of the thermal degra-dation mechanism of PVC will take a great stepforward.

EXPERIMENTAL

Materials

The PVC samples were prepared at 90, 75, 60(samples A, B, and C, respectively), 40, 0, and�50 °C by a bulk polymerization process with2,2�-azobisisobutyronitrile as an initiator. Themethod used was the usual procedure in our lab-oratory. Full details have been published else-where.24 The polymers obtained at 40, 0, and �50°C were extracted with DI stirring for 24 h in athermostatic vessel under nitrogen. After separa-

tion of the insoluble fractions by centrifugation,the soluble fractions were precipitated with meth-anol and then washed and dried at 40 °C (samplesD, E, and F respectively). The average molecularweight was determined at 34 °C with solutions incyclohexanone (CH) with a Knauer membraneosmometer. Tetrahydrofuran (THF) was distilledunder nitrogen with aluminum lithium hydridefor the removal of peroxides immediately beforeuse. CH was purified by fractional distillation. DIwas refluxed over sodium and distilled from alu-minum lithium hydride. Sodium benzenethiolate(NaBT) was prepared by the reaction of sodium(Merck) with thiophenol (Merck) as already de-scribed.4

Substitution Reaction with NaBT

The substitution reactions were carried out in CHat �30 °C in an atmosphere of purified nitrogen.The full experimental details have been publishedelsewhere.4 At suitable intervals, the sampleswere precipitated in a large excess of methanol,purified from THF into methanol, washed, andfinally dried in vacuo at 40 °C. The degree ofsubstitution was determined by UV spectroscopyfrom the intensity of the benzenethiolate band at256 nm.4

13C NMR Spectroscopy

The tacticity of the polymers were measured with13C NMR decoupled spectra obtained at 80 °C ona Varian Unity instrument operating at 125 MHzwith DI-d8 as a solvent. The spectral width was2500 MHz, and a pulse repetition rate of 3 s and16,000 data points were used. The relative peakintensities were measured from the integratedpeaks area, which was calculated with an elec-tronic integrator.

RESULTS AND DISCUSSION

The starting PVC samples were prepared by abulk polymerization process at various tempera-tures so that different tacticity contents and, ac-cording to a previous work,24 different solubilitycharacteristics were obtained. To dispose of sam-ples capable of being used under the 13C NMRexperimental conditions, particularly the solubil-ity in DI, we carried out an extraction with DI forthe samples synthesized at temperatures lowerthan 60 °C. The extracts from the samples pre-

LABILE NONDEFECT STRUCTURES IN PVC 3945

pared at 40, 0, and �50 °C are those samplesdenoted D, E, and F respectively. A comprehen-sive characterization of all the samples was ac-complished, and the results are collected in Table1. As can be observed, the isotactic contents of thestudied samples were significantly lower as thetemperature of polymerization decreased, asshown by the IR absorbance ratio A1428cm

�1/A1434cm

�1, which is known to be a relative mea-sure of the syndiotactic content in PVC.24

From the latter results, it follows that the sam-ples were adequate for the aim of this study, andso we extended the characterization to analyzethe tacticity content with 13C NMR spectroscopy.Figure 1 shows some spectra (methynic region) ofthe samples. As can be seen, the resolution of thespectra allowed a quite accurate determination ofthe contents of the three indicated sets of pen-tads, which were centered by one syndio, hetero,or isotactic triad. From the overall surface areasof each type of pentad, the contents of syndio,hetero, and isotactic triads could be calculatedeasily. The results are collected in Table 2. Theisotactic triad content decreased in the same wayas shown in the IR measurements. However, Ta-ble 2 includes the persistence ratio (�; as definedby Reinmoller and Fox25 as the ratio between thenormalized intensity of isotactic dyads and theconditional probability of a syndiotactic place-ment on an isotactic chain end). It is apparentthat samples A–C exhibited Bernoullian behavior(� � 1), whereas at a temperature of polymeriza-tion lower than 40 °C, the samples departed grad-ually but rather slightly from Bernoullian behav-ior, indicating a small change in the steric rear-rangement.

Furthermore, the resolution of the spectramade it possible to follow the changes in rmmmrxand mmmmrx (x � m or r) heptad contents as theisotactic content decreased (Fig. 2). The mmmmrx(x � m or r) heptad content was markedly lower

as the isotactic triad content decreased. Thismeans that, as expected, the content of isotacticsequences equal to or longer than one heptad waslower as the overall isotactic content decreased.Consequently, the contents of the mmr tetrad andits fraction under the GTTG�TT conformation de-creased in a parallel way.

Figure 1. Evolution of 13C NMR spectra (methyniccarbon region) of PVC samples with different tacticitycontents: (a) sample A, (b) sample C, (c) sample E, and(d) sample F.

Table 1. Characteristics of the PVC Samples

Sample Mn � 10�3 A1428/A1434a

A 21.5 1.11B 31.2 1.13C 39.3 1.16D 43.0 1.20E 54.3 1.32F 32.8 1.47

a Syndiotacticity/isotacticity ratio from IR spectra.24

3946 MARTINEZ AND MILLAN

As already mentioned, it is widely acceptednowadays that the thermal instability of PVC isdue to the presence of some labile structures,which may be either defect structures arisingfrom a number of events in the polymerizationprocess3 or nondefect isotactic conformations thatwe have proposed to be the GTTG� isotactic triadconformation.4,21 The question arises as to whichof these types of labile structure is the more sta-bility-determining site, even though the straightrelation between PVC stability and the chemi-cally controlled isotactic content, as found in ourwork,26 gives clear support for the latter nonde-fect structure to be unstable.27

Because the content of this structure could bedetermined through nucleophilic substitutionwith NaBT at �30 °C for PVC samples with dis-tinct isotacticities23 and we were able to deter-mine accurately the rmmmrx/mmmmrx ratio, weendeavored to state the correlation between bothquantities for the PVC samples of Table 1. Theobtained values of nondefect structures (h) andthe rmmmrx/mmmmrx ratio are both listed inTable 3. Surprisingly, as shown in Figure 3, twowell-discriminated slopes can be observed. Theyrefer to the samples with polymerization temper-atures of 90 to 40 °C and 40 to �50 °C.

The change in slope agrees with the slightchange in stereochemical composition, as indi-

Figure 2. Evolution of 13C NMR spectra of PVC samples with different tacticitycontents in the methynic carbon region (mm triads): (a) sample A, (b) sample C, (c)sample E, and (d) sample F.

Table 2. Triad Probabilities of the PVC Samples

Sample Prra Prm � Pmr

a Pmma �b

A 29.2 49.8 21.0 1.00B 30.3 49.6 20.1 1.00C 30.4 49.9 19.7 0.99D 31.9 50.2 17.9 0.98E 35.3 49.5 15.2 0.97F 38.7 48.3 13.0 0.96

a Probability of syndio (Prr), hetero (Prm � Pmr), and iso-tactic (Pmm) triads.

b Persistence ratio.25

Table 3. Labile Nondefect Structures in the PVCSamples and Prmmmrx/Pmmmmrx Ratio

Sample h(%)a Prmmmrx/Pmmmmrx

A 0.78 1.08B 0.62 1.18C 0.55 1.20D 0.45 1.25E 0.34 1.39F 0.29 1.47

a Content of labile nondefect structures.23

LABILE NONDEFECT STRUCTURES IN PVC 3947

cated by the � values (Table 2). Therefore, themost plausible explanation of the results of Fig-ure 3 lies in the fact that at polymerization tem-peratures lower than 40 °C, the probability of theformation of mmr and GTTG� conformation de-creases and the probability of syndiotactic rr tri-ads increases simultaneously. In addition, as in-dicated by the slight decrease in the probability ofthe mr&rm heterotactic triad, the aforemen-tioned changes should be accompanied by someshortening of the isotactic sequence length. Be-sides, the last assertion is in agreement with ourprevious results on the correlation between thestability of PVC and the tacticity; the degradationrate increased drastically from a specific value ofisotacticity, which corresponded to a temperatureof polymerization of 40 °C.27

An explanation of the lability of the GTTG�

conformation of the mm triad in mmr sequencesis based on the fact that it exhibits high free-volume excess and relatively good rotational mo-bility with respect to the remaining conforma-tions in PVC. In addition, unlike the other con-formations, it is capable of rotating withoutcoupling with the adjacent units. These circum-stances may be observed through the comparisonin Figure 4 of the conformations GTGTGTTT andGTGTTG�TT at mmmr sequences.28

CONCLUSIONS

The linear correlation between the nondefectstructures and rmmmrx/mmmmrx ratio, coupledwith the correlation between tacticity and stabil-ity, accounts for the nondefect GTTG� isotactictriads being the specific labile sites of the initia-tion of thermal degradation of PVC. Therefore,these results should lead to a better understand-ing of the thermal degradation mechanism ofPVC on a molecular level.

REFERENCES AND NOTES

1. Guyot, A. Pure Appl Chem 1985, 57, 833.2. Hjertberg, T.; Sorvik, E. M. Polymer 1983, 24, 685.3. Starnes, W. H. Dev Polym Degrad 1981, 3, 135 and

references therein.4. Guarrotxena, N.; Martınez, G.; Millan, J. J Polym

Sci Part A: Polym Chem 1996, 34, 2387 and refer-ences therein.

5. Guarrotxena, N.; Martınez, G.; Millan, J. Polymer1997, 38, 1857.

6. Guarrotxena, N.; Martınez, G.; Millan, J. ActaPolym 1999, 50, 180.

7. Heatley, F.; Bovey, F. A. Macromolecules 1969, 2,241.

8. Carman, C. J. Macromolecules 1973, 6, 725.Figure 3. Correlation between the Prmmmrx/Pmmmmrx

(x � m or r) ratio and nondefect unstable structures (h).

Figure 4. GTGTGTTT and GTGTTG�TT conforma-tions at mmmr sequences in PVC.

3948 MARTINEZ AND MILLAN

9. Pham, Q. T.; Millan, J.; Madruga, E. L. MakromolChem 1974, 175, 945.

10. Abdel-Alim, A. H. J Appl Polym Sci 1975, 19, 1227and references therein.

11. Ando, I.; Nishioka, A.; Asakura, T. MakromolChem 1975, 176, 411.

12. Starnes, W. H.; Schilling, F. C.; Abbas, K. B.; Cais,R. E.; Bovey, F. A. Macromolecules 1979, 12, 556.

13. Liu, N. I.; Tong, S. N.; Koenig, J. L. J Appl PolymSci 1980, 25, 2205.

14. King, J.; Bower, D. I.; Maddams, W. F.; Pyszora, H.Macromol Chem 1983, 184, 879.

15. Mirau, P. A.; Bovey, F. A. Macromolecules 1986,19, 210.

16. Crowther, M. W.; Szeverenyi, N. M.; Levy, G. C.Macromolecules 1986, 19, 1333.

17. Ando, I.; Kato, Y.; Nishioka, A. Makromol Chem1976, 177, 2759.

18. Ando, I.; Kato, Y.; Kondo, M.; Nishioka, A. Makro-mol Chem 1977, 178, 803.

19. Nakayama, N.; Aoki, A.; Hayashi, T. Macromole-cules 1994, 27, 63.

20. Elgert, K. F.; Kosfeld, R.; Hull, W. E. Polym Bull1981, 4, 281.

21. Millan, J.; Martınez, G.; Jimeno, M. L. Eur PolymJ 1991, 27, 483.

22. Tiemblo, P.; Martınez, G.; Millan, J. J Polym SciPart A: Polym Chem 1995, 33, 1243.

23. Martınez, G.; Mijangos, C.; Millan, J. Polym Bull1985, 13, 151.

24. Martınez, G.; Mijangos, C.; Millan, J. J Appl PolymSci 1983, 28, 33.

25. Reinmoller, M.; Fox, T. G. Polym Prepr (Am ChemSoc Div Polym Chem) 1966, 7, 999.

26. Martınez, G.; Mijangos, C.; Millan, J. Eur Polym J1985, 21, 387.

27. Millan, J.; Martınez, G.; Gomez-Elvira, J. M.; Guar-rotxena, N.; Tiemblo, P. Polymer 1996, 37, 219.

28. Guarrotxena, N.; del Val, J. J.; Millan, J. PolymBull 2001, 47, 105.

LABILE NONDEFECT STRUCTURES IN PVC 3949