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.
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