Stereoselective substitution on PVC using phase transfer catalysts

The nucleophilic substitution with sodium benzenethiolate in aqueous suspension in the presence of a phase-transfer catalyst has been studied for two samples of PVC of different tacticities. The kinetics shows two well-defined periods: one period is very fast and involves low conversions that are greater as the isotactic content of PVC increases, while another steady period is slow and its slope seems to depend on physical features, e.g., molecular weight and crystallinity, which would affect the accessibility of nucleophile. The evolution of unreacted syndio, iso, and heterotactic triad contents with degree of substitution has been followed by ¹³C-NMR spectroscopy. Since the content of syndiotactic triads does remain unreacted at the end of the reaction and that of isotactic and heterotactic triad decreases, but at a different rate, the reaction is proved to proceed by a stereospecifically controlled mechanism. The influence of degree of substitution on both the thermal degradation rate and the evolution of UV–Visible spectra of equally degraded samples has been also studied; the highest stability in terms of degradation rate and content of short polyenes is obtained when a specific fraction of isotactic triads has reacted. From the correlation between the results of this work and those previously obtained in solution, some enlightening conclusions about the reaction mechanism are drawn.