Selective Substitution Reactions on PVC. Lability of Some “Normal” Structures

Abstract

The nucleophilic substitution reaction on PVC with sodium thiophenate in cyclohexanone solution is studied. The reaction appears to be S_N2 substitution and it involves three different steps with decreasing reaction constants. By using ¹³C-NMR spectroscopy, the fast step is proposed to be due to the high reactivity of the central chlorine in isotactic triads and, to a lesser extent, in heterotactic triads. The study of the thermal degradation of the modified samples shows that during the fast step of substitution a stabilizing effect occurs, and that as soon as the first step is over, the polymer stability decreases markedly. These effects are accompanied by changes in poly-ene distribution as proved by UV-VIS absorption spectroscopy study on degraded samples. The results, as discussed on the basis of the possible conformations of triads in PVC, suggest that some chlorine atoms in both the GTTG isotactic and the TTTG heterotactic triads should be considered as labile structures in PVC. It follows from the results obtained that on the one hand the initiation of degradation may occur by normal structures, and on the other the build-up of polyenes is favored by the presence of …TTTT… sequences. Both features contributing to a better understanding of the degradation of PVC.