Kinetics of polymer degradation involving the splitting off of small molecules: Part 8. Thermal degradation of polyvinyl esters

Abstract

High conversion kinetic runs of the thermal degradation of polyvinyl esters have been treated using the theory developed previously. For polyvinyl acetate, polyvinyl propionate, polyvinyl butyrate and polyvinyl valerate, excellent agreement has been obtained between theory and experiment for the model of gradual zip propagation. In the case of polyvinyl caproate, better agreement has been reached for the model of immediate zip growth. The results obtained suggest that the pre-exponential factor and activation energy for random elimination of acetic acid from polyvinyl acetate increase with increasing molecular weight and the extrapolation to infinite chain length gives the values log( A s −1 ) = 14·0 and E a = 192 kJ mol −1. In all cases, the initiation step of the zip reaction is not autocatalytic. The pre-exponential factors and activation energies for random elimination of acids are arranged in a zig-zag fashion, but the rate constants of random elimination increase with increasing length of the acyl substituent. The ratio of the rate constants for zip propagation and random elimination exhibits a minimum at polyvinyl butyrate. The content of structural irregularities causing premature zip termination is rather high.