No abstract

Autocatalysis during the thermal dehydrochlorination of poly(vinyl chloride) (PVC) is shown to be a free-radical process that converts the ordinary monomer units of the polymer into chloroallylic structures that have low thermal stabilities. In the first stage of dehydrochlorination, conjugated polyene sequences are created by a nonfree-radical route. They react with HCl to give cation monoradicals and/or excited cation diradicals. One or both of these species, or other radicals formed from them, can then abstract methylene hydrogen in order to produce new radicals that are also carbon-centered. These are converted by chlorine-atom β scission into the chloroallylic segments, which start the growth of new polyenes in the usual (nonradical) way. At 180 °C in solid PVC, autocatalysis was inhibited by free-radical scavengers (a hindered phenol, triphenylmethane, and metallic mercury) but greatly enhanced by an increased concentration of HCl when all-trans-β-carotene, a model for PVC polyene sequences, was introduced simultaneously. When they were subjected to autocatalytic conditions, other model compounds gave products that apparently resulted from the abstraction of hydrogen by free-radical intermediates.

This article describes, in narrative style, the research of the author and his associates, performed over a period of 30 years, that led to the identification and quantification of the anomalous structures in poly(vinyl chloride) (PVC) and to detailed descriptions of their mechanisms of formation. Also examined here are the implications of this work for the thermal stability of PVC, for the overall chemical mechanism of vinyl chloride polymerization, and for other relevant aspects of the chemistry and technology of vinyl chloride homopolymers and copolymers. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 2451–2467, 2005