Mathematical models of the initial stage of the thermal degradation of poly(vinyl chloride)—IV. Influence of stereoisomery

Abstract

Two mathematical models for the initial stage of the thermal degradation of PVC initiated by decomposition of normal units and the internal chloroallylic fragments have been proposed. Solution of the systems of differential equations was carried out according to Euler's method with the computer. The theoretical curves for PVC dehydrochlorination at 200, 240 and 250 ° were calculated. They coincide with the experimental ones up to 30–40% conversion. The results give evidence of the validity of the equations which have previously been deduced for the initial stage of PVC degradation initiated by decomposition of normal units and unstable fragments such as the internal chloroallylic groups, the branching fragments with chlorine atom near tertiary carbon atom or sum of these fragments in all proportions. The mathematical model for the PVC degradation with the assumption that PVC dehydrochlorination is initiated by decomposition of normal units and the unstable conformer of the definite isotactic stereoisomer, has been proposed. It has been shown that in this case there is no agreement between the experimental kinetic curve for PVC dehydrochlorination at 200 ° and the theoretical ones. The conclusion has been made that at elevated temperatures the PVC thermal degradation is not the stereoselective reaction and that the internal chloroallylic fragments and branching groups with a chlorine atom near the tertiary carbon atom may be the major unstable fragments which have a significant influence on the early stage of PVC degradation.