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 param etrization give reliable activation enthalpies for chloroalkane and ch loroalkene dehydrochlorination. Dehydrochlorination of chloroalkanes a nd 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 el iminate hydrogen chloride in a 1,4 process through a six center transi tion state generated from a cis-configuration of the double bond. This transition state requires much lower activation enthalpy and less pol arization 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 t ertiary chlorine atoms, takes place through a four center transition s tate. Allylic chlorine atoms formed in this way or pre-existing in sma ll amounts as structural irregularities have a reactivity not much dif ferent from the secondary chlorine atoms of the main chain of the poly mer if they adopt a trans-configuration of the double bond. On the oth er hand, if allylic chlorine atoms have a cis-configuration, they rapi dly eliminate hydrogen chloride, forming conjugated polyenes through a transition state of six centers. This elimination constitutes the cha in reactions in the PVC degradation process. The chain stops when a re latively stable trans-conformation of the allylic chloride is formed.