Thermal degradation of polyisobutylene: effect of end initiation from terminal double bonds

Abstract

The effect of terminal double bonds on the thermal degradation of polyisobutylene was examined by using a structurally well-defined sample ( M n0 = 7.09 × 10 3) prepared by hydrogenation. The end initiation reactions from a terminal trisubstituted double bond (TTD) and a terminal vinylidene double bond (TVD) lead to the formation of a primary terminal macroradical (R p·) and a tertiary terminal macroradical (R t·), respectively. It is expected from kinetic analysis that the extent of occurrence of R p· is 2–3 times higher than that of R t· depending on the functionalities f TTD and f TVD and M n of the reacting polymer, in addition to the random initiation from the scission of skeletal CC(C) 2, and that the stationary ratio [R p·] [R t·] becomes slightly lower by hydrogenation. The effect of hydrogenation was found to be a decrease in the overall rate of degradation, owing to a marked decrease in the end initiation rate. However, the integrated concentration ratio [R p·] [R t] , which is estimated from the composition ratio of the interesting components, does not decrease, but increases. This result shows that this concentration ratio scarcely depends on the initiation reactions. A similar relationship between the ratio [R p·] [R t] and M n of the molten polymer matrix was found for the degradation of samples with and without TVD and TTD. This result supports the view that [R p·] decreases markedly with decreasing M n during the degradation, owing to a marked depression of the regeneration rate of R p· resulting from an increase in the rate of diffusion-controlled termination.