Evolution of aromatics in the thermal degradation of poly(vinyl chloride): A mechanistic study

Abstract Various aspects of the thermal decomposition, flammability and smoke evolution from PVC have been investigated in an attempt to account for the mechanism of evolution of aromatics from this polymer. The most relevant experimental observations for the understanding of the mechanism of thermal decomposition of PVC were obtained by the method of direct pyrolysis in the mass spectrometer. A number of facts that had partly escaped previous workers was revealed: that in each of the two degradation steps of PVC a family of aromatic hydrocarbons was formed with different structures; that the two families of aromatics are suppressed at considerably different rates by the addition of metal oxides. The selective suppression of aromatics is related to the fact that the two families of aromatics are evolved from linear and from crosslinked polyene chains, respectively. Another conclusion is that the intramolecular cyclization processes in linear and crosslinked polyenes are satisfactorily explained by the cyclization of polyene radicals, whereas mechanisms based solely on Diels-Alder processes meet with several difficulties. The close analogy found with the thermal degradation of polyacetylene supports these conclusions. Another model system is provided by PVC 2 and PVF 2 . These vinylidene polymers do not form alkyl-aromatics, only unsubstituted aromatics are evolved and the thermal degradation occurs in a single stage.

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