On the mechanism of the thermal degradation of polyvinyl acetate

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Abstract
The kinetics of deacetylation and benzene formation, and the polymer crosslinking and absorption spectra of PVAc during thermal degradation (in vacuo at 200–280°C with or without removal of volatile products) were studied. The effects of the molecular weight of PVAc samples and the addition of various substances such as acids, metal chlorides, amines, phenols, triphenylmethane, organic salts of Ca, and dialkyltin on the thermal decomposition of PVAc were investigated.Deacetylation kinetics of the polymer with continuous removal of volatile products can be described by the first order reaction equation with the rate constant ket = 9.6 × 1015 e−51400/RT sec−1. The PVAc degradation rate does not depend on the molecular weight of the polymer. During the thermal degradation of PVAc, polyenes having up to six conjugated double bonds form. A scheme is proposed for the thermal degradation of PVAc by the molecular mechanism.In the presence of volatile degradation products the kinetic curves of PVAc deacetylation have a distinct autocatalytic character. This is due to the accelerating effect of acetic acid. Degradation of PVAc is also accelerated by other acids. There is a certain correlation between the degradation rate and dissociation constants of acids. This indicates that acids accelerate the thermal degradation of PVAc by the ionic mechanism.A rapid crosslinking of the polymer takes place in the process of PVAc thermal decomposition. This evidently results from Diels‐Alder reactions between polyenes. The inhibitors of the free radical reactions, organic salts, have no effect on deacetylation rate of PVAc.