DEGRADATION AND STABILIZATION OF POLY(VINYL CHLORIDE) .4. MOLECULAR-ORBITAL CALCULATIONS OF ACTIVATION ENTHALPIES FOR DEHYDROCHLORINATION OF CHLOROALKANES AND CHLOROALKENES

Molecular orbital calculations at the MNDO level with AM1 or PM3 parametrization give reliable activation enthalpies for chloroalkane and chloroalkene dehydrochlorination. Dehydrochlorination of chloroalkanes and some allyl chlorides is a molecular 1,2-elimination through a four center transition state generated in a synperiplanar conformation. The transition state for chloroalkanes requires very strong polarization of the carbon-chlorine bond. When structure allows, allyl chlorides eliminate hydrogen chloride in a 1,4 process through a six center transition state generated from a cis-configuration of the double bond. This transition state requires much lower activation enthalpy and less polarization of the molecule than does the 1,2 process. Initiation of PVC degradation by 1,2-elimination from normal chain residues, or at the very beginning of degradation from structural irregularities such as tertiary chlorine atoms, takes place through a four center transition state. Allylic chlorine atoms formed in this way or pre-existing in small amounts as structural irregularities have a reactivity not much different from the secondary chlorine atoms of the main chain of the polymer if they adopt a trans-configuration of the double bond. On the other hand, if allylic chlorine atoms have a cis-configuration, they rapidly eliminate hydrogen chloride, forming conjugated polyenes through a transition state of six centers. This elimination constitutes the chain reactions in the PVC degradation process. The chain stops when a relatively stable trans-conformation of the allylic chloride is formed.