DEGRADATION AND STABILIZATION OF POLY(VINYL CHLORIDE) .5. REACTION-MECHANISM OF POLY(VINYL CHLORIDE) DEGRADATION

The degradation of poly(vinyl chloride) is a complex chain dehydrochlorination that consists of an initiation process to generate an active intermediate and a series of chain reactions that generates additional active intermediates with progressively increased numbers of double bonds. Each intermediate partitions between an intermediate with one more double bond and a stable conjugated polyene with the same number of double bonds. At low and moderate temperatures the thermal degradation of PVC in an inert atmosphere is a succession of molecular concerted reactions. The initiation process is a 1,2-elimination through a four center transition state requiring a synperiplanar conformation. It can be catalyzed by a molecule of hydrogen chloride that makes possible 1,2-elimination through a transition state of six centers with much lower activation enthalpy but also with very low activation entropy. There are two main chain reactions: the first is a 1,4-elimination from allylic chlorine atoms and methylenes cis- to a double bond through a transition state of six centers; the second is a 1,3-rearrangement of hydrogen atoms catalyzed by hydrogen chloride. The chain reaction is interrupted when a relatively stable trans-double bond is formed and no hydrogen chloride is present to catalyze trans-cis-isomerization or 1,3-rearrangement. Macro carbocations formed by heterolysis of carbon-halogen bonds in the presence of strong Lewis acids react much faster than does the original PVC in concerted elimination by 1,2-syn- or 1,4-cis-mechanisms promoting a so-called 'catastrophic', very fast degradation. Macro radicals formed by thermal homolysis, irradiation or reaction with promoters can also promote very fast hydrogen chloride elimination due to a special mechanism consisting of a 1,2-rearrangement of a chlorine atom followed by a concerted 1,3-elimination through a five center transition state.