Reaction mechanism of poly(vinyl chloride) degradation. Molecular orbital calculations

Semiempirical Molecular Orbital Calculations (MNDO AMl) support kinetic results concerning the molecular mechanism of thermal degradation of PVC and show that under special conditions radical and ionic mechanisms are also possible. The degradation of poly(vinyl chloride) is a complex chain dehydrochlorination that consists of an initiation process to generate an active intermediate followed by chain reactions that generate 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 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. 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 because of 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.