Irregular structures in polyvinylchloride II. Unsaturated structures

In the present paper, considerations are made on the mechanism of the dehydrochlorination of PVC, mainly from information summarized from studies with unsaturated model compounds. A semi-empirical evaluation of the activation energy has been carried out for the monomolecular elimination of hydrogen chloride involving unsaturated structures in PVC. The semi-empirical method proposed by VEDENEYEV was used to study the C-Cl, C-H and C-C bonds in PVC. Activation energies for elimination of hydrogen chloride were calculated on the basis of the additivity of bond energies of the four-centre activated complex for various irregular structures in the polymer chain such as branching, head-to-head or tail-to-tail addition, and double bonds. From a number of possible structural abnormalities controlled by polymerization conditions only a few give a contribution to the initiation of the thermal dehydrochlorination but internal allyl type chlorine and branched structures having a tertiary chlorine appear to have significant effect as the initiators of the decomposition, while abnormal structures at the chain ends are rather stable. It should be pointed out that increasing delocalization or resonance stabilization in the case of unsaturated internal-irregular structures will lower the activation energy of elimination of hydrogen chloride in consequence of decreased thermal stability of PVC.