Structural and mechanistic aspects of the thermal degradation of poly(vinyl chloride)

A critical review of the title subject supports the following major conclusions. Thermal dehydrochlorination of poly(vinyl chloride) (PVC) begins with internal allylic chloride and tertiary chloride structural defects formed during polymerization. The tertiary chloride is associated with 2,4-dichloro-n-butyl, 1,3-di(2-chloroethyl), and chlorinated long branches. Mechanisms for the formation of all of the labile defects are well established. ‘Carbonylallyl’ structures and certain isotactic conformers of ordinary monomer units are unimportant as initiators of thermal dehydrochlorination. Both the initiation and the subsequent formation of conjugated polyene sequences occur via carbenium chloride ion pairs or by a closely related concerted four-center quasi-ionic route. Six-center concerted processes, pathways involving free radicals, and other mechanistic schemes suggested recently are not involved in polyene elongation. However, during thermal degradation, ordinary monomer units are converted into internal allylic chloride defects by a mechanism that may include the abstraction of hydrogen by triplet cation diradicals derived from polyene intermediates. Cyclization reactions seem likely to contribute to the termination of polyene growth. When PVC is thermolyzed in blends with other polymers, unusual kinetic phenomena are detected that remain to be fully explained.