Thermal dehydrochlorination and stabilization of poly(vinyl chloride) in solution: Part V. Influence of structural defects in the polymer

Polymer Degradation and Stability 28 (1990) 173-184

Thermal Dehydrochlorination and Stabilization of Poly(vinyl chloride) in Solution: Part V. Influence of Structural Defects in the Polymer N. Bensemra, Tran Van Hoang & A. Guyot CNRS-Laboratoire des Materiaux Organiques, BP 24, F.69390, Lyon-Vernaison, France (Received 12 May 1989; accepted 29 May 1989)

ABSTRACT The thermal degradation of various samples of poly( vinyl chloride) ( P VC ) in trichlorobenzene ( T C B ) solution at 187°C has been studied in the presence of a 1/1 mixture of zinc and calcium stearates. Two series of P V C samples have been studied, both prepared at 55°C using dicetylperoxydicarbonate as initiator. The first series was prepared in TCB solution and the second in suspension. In each series the conversion of the monomer was varied. HCl evolution and stabilizer consumption were measured. The dehydrochlorination ( D HC ) rate was studied as a function of the amounts of allyllic chlorine, tertiary chlorine and oxygenated structures initially present in the samples. All these structures are active, especially the oxygenated ones. although quantitative correlations were not clearly obtained.

INTRODUCTION Following studies of model compounds 1 it has been generally estimated that the thermal dehydrochlorination process should be initiated by structural defects, such as allyllic chlorine associated with internal unsaturation, tertiary chlorines associated with branches or at certain chain ends. Attempts to get quantitative correlation have been made in various laboratories 2-4 and in a I U P A C cooperative working party. 5 However, reliable measurements of the a m o u n t s o f these structural defects have been obtained from N M R studies only recently 6 - 8 so that the number of samples 173 Polymer Degradation and Stability 0141-3910/90/$0350 © 1990 Elsevier Science Publishers Ltd, England. Printed in Great Britain

174

N. Bensemra, Tran Van Hoang, A. Guyot

which are fully characterized is very limited. On the other hand, the major influence o f oxygenated structures, such as ketoallyl structures, has also been pointed out recently 9 as well as their possible catalytic action/°'11 In a recent study, 12 two series o f samples were prepared in solution and in suspension using the same initiator and the same temperature, the polymerization being terminated at various conversions. These samples were also well characterized by 1H and 13C N M R spectroscopy before and after reduction to polyethylene. The purpose of the present paper is to present D H C studies of these samples in the presence of metal soap (Ca, Zn) stabilizers. A further characterization of the carbonyl structures in these samples was carried out by F T I R and the correlation of all the structural defects with the D H C process is discussed.

EXPERIMENTAL Two series o f PVC were used. Samples SUS2 to SUS5 were prepared in suspension at 55°C using dicetylperoxydicarbonate as initiator. The same temperature and initiator were used for the solution polymerization of samples SOL9, SOL11 and SOL12. Some data about the polymerization and molecular weights of the samples are reported in Table 1. The data obtained in a previous study 12 of the N M R analysis of these samples, are reported in Table 2. Thermal treatment at 187°C and measurements of HC1 evolution and stabilizer consumption from coulometric titration of the chloride ions were previously described. 13,14 F T I R analysis of the polymers before and after TABLE I Synthesis of the Polyvinylchloride Samples at 55°C using Dicetylperoxydicarbonate as Initiator Sample Polymerization Vinyl Initiator Duration Conversion Mn Mw/ process chloride (weight %) (h) (%) x 10 -3 Mn (mole/ litre )

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117 114 103 94

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thermal treatment in the presence of zinc and calcium stearates were carried out on films cast from distilled tetrahydrofuran. In the present experiments, 2 wt % each of zinc and calcium stearates was used.

RESULTS A N D DISCUSSION The experimental data for HC1 evolution and stabilizer consumption during the thermal treatment at 187°C are shown in Figs 1 and 2 for the samples prepared in solution (Figs la and 2a) and in suspension (Figs lb and 2b). As expected, the polymers prepared in solution are far less stable than those prepared in suspension. From the data reported in Table 2, correlations were attempted between the initial DHC rate (sum of HCI evolved plus stabilizer consumption) and either the amount of allylic chlorine or the tertiary chlorine atoms (sum of the number of branches except the chloromethyl branches), or their sum. The corresponding data are shown in Figs 3, 4 and 5, respectively. Obviously there are no clear correlations, but only some trends. For suspension polymers both the number of defects and the DHC rate tend to increase with the conversion, with the exception of sample SUS5 whose thermal stability is smaller than expected. For solution polymers, sample SOL12 is much less stable than the two other samples, possibly because it was obtained after a longer time and in the presence of a larger amount of initiator. As shown in Fig. 6, an attempt to correlate the D H C rate with the molecular weight of the polymer was not successful, although there is a trend towards a better stability for higher molecular weights (comparison of SUS and SOL samples); however, there are definite exceptions (such as SUS2 and SOL12). 20

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In addition to possible 'labile chlorine atoms' from internal double bonds or branch points, other weak structures may arise from oxygenated structures. One of these is the carbonate group associated with chain ends from the initiator (cetyl groups). As shown in Table 2, the SOL samples do contain larger amounts of such groups (about four times as many as the SUS samples). In the infrared spectra (Figs 7 and 8) the carbonate bands are expected to appear near 1747cm -1. Obviously, and specially for SOL samples, there are many other oxygenated structures. The band at 1722cm-1 has already been assigned to aliphatic aldehyde and ketone groups; 9 the authors have suggested that transfer reactions to formaldehyde

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are responsible for the formation of these structures; the presence of formaldehyde is expected to arise from the reaction of oxygen with vinyl chloride monomer. The two other bands at 1690 and 1626cm- 1 (SOL9) or 1670 and 1632 cm-1 (SOL12) might be associated with ketoallyl structures in the cis or trans configurations. 15 Such structures (or at least the cis configuration a6) have been claimed to be highly active in initiating the DHC process. 9'17 Other defects can be suspected to be present on examination of the infrared spectra in the high wave number region 2800-3700cm-1. As shown in Figs 9 and 10, in both suspension and solution samples, but chiefly for the more unstable SOL12 sample, there are bands in the region 38003500 cm-~ which may be associated to OH and OOH structures. ~8 Rather large differences are observed between the samples; for instance, these bands are not present in the spectrum of the more stable SOL11 sample.

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Important changes in the spectra result from the D H C process; as shown in Figs 11 and 12 for the SOL12 sample, the bands associated with the aldehyde, ketone and carbonate groups disappear very quickly, thus demonstrating that the corresponding structures are concerned in the D H C mechanism. The changes shown in Fig. 12 are more difficult to interpret: the shift of the maximum from 3260 to 3313 cm-1 may be due to the thermal decomposition of the hydroperoxide structure to give OH groups and carbonyl groups are possibly responsible for the growing band around 1720 c m - 1 in Fig. 11. But the grafting of the ester group from the stabilizer 14

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must also be considered. Quantitative interpretation of these changes is not possible, unfortunately, owing to lack of knowledge of the appropriate extinction coefficients. A final point to be considered is the various tacticities of the samples: there are practically no differences for the suspension samples, but significant variations are observed in the A1426/A1435 ratio between SOL12, SOL9 and SOL11: these values are, respectively, 1-00, 1"07 and 1"13. The syndiotacticity of the polymer is decreasing with conversion due to the increasing inhomogeneity of the polymerization medium; it has been shown recently that even in the suspension process, the polymer produced in the initial stages, where the contribution of the polymerization in homogeneous conditions is significant, is a little more syndiotactic than the polymer produced at higher conversion.~ 9 It has been also speculated that the special conformation of the end of isotatic sequences might be responsible for the initiation of the DHC. 2°

N. Bensemra, Tran Van Hoang, A. Guyot

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CONCLUSION An effort has been made to correlate the initial rate of thermal D H C in PVC with structural defects for two series of well characterized samples. Quantitative correlations are extremely difficult, due to the variety of structures involved in the process: allyllic and tertiary chlorine, possible conformations o f normal units and various kinds o f oxygenated structures. The last of these seems to be very important, as shown by the very low stability of sample SOL12 which obviously contains more of these

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structures, possibly formed by the presence of small amounts of oxygen during the polymerization.

ACKNOWLEDGEMENTS The authors are indebted to Dr Krantz of A T O C H E M , who kindly prepared the samples for us.

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