Correlation between tacticity and thermal stability in comparison with other defect structures in PVC

Abstract

Fractions from six PVC samples, which have been extensively studied in respect of defect structures content by the IUPAC Working Party on PVC Defects and Stability, were obtained by extracting with acetone. The whole polymers, as well as their acetone soluble and insoluble fractions, were characterized by determining the osmotic molecular weight and the tacticity. Then they were thermally degraded up to conversion of 0.3% in powder state at 180°. The polyene distribution in the degraded polymers was studied by u.v.-vis spectroscopy. The molecular weights of the soluble fractions were lower and the isotactic contents higher than those of both the insoluble fraction and the whole polymer. The variation of degradation rate with both the content of acetone soluble material and the overall isotactic content appears to be linear, suggesting that the influences are closely related. On the other hand, two of the samples deviate from the linear behaviour, probably because of their higher content of defect structures as a result of severe polymerization conditions used for these samples. The u.v.-vis spectra of the equally degraded polymers show that insoluble and soluble fractions are very different in polyene distribution, that of the soluble fraction exhibiting an enhanced concentration of polyenes of 7–9 double bonds (λ between 393 and 437 nm). The soluble fractions behave in much the same way as those from polymers of different tacticities, as studied earlier, so that the above results confirm the influence of some isotactic conformations on PVC degradation.