Effects of Molecular Weight of Reaction Media on Formation of Volatile Oligomers by Thermal Degradation of Polyisobutylene

Abstract

ABSTRACT:For the thermal degradation ofpolyisobutylene, the effects of physical factors such as pressure in the reaction vessel and volume and molecular weight of the molten polymer matrix constituting the reaction media on the formation of the volatile oligomers were examined by a kinetic approach with measurements of composition ratios for terminal monoolefins [(TTD)P, (TVD)p, (TTD),, and (TVD) , do not depend on pressure and initial mass of sample, but depend on the initial molecular weight of sample. These values decrease clearly as the initial molecular weight decreases. It could be deduced that the decrease in molecular weight of the matrix decreases concentration ratio of two types of terminal macroradicals ([Rp · JI[R, · ]). According to the proposed kinetics of a chain mechanism including diffusion-controlled termination, a marked decrement of the radical concentration ratio suggests that the rate of termination increases with decrease in molecular weight of the matrix and that resulting decrement of kinetic chain length depresses the formation of the primary terminal macroradical (RP ·) in the depropagation step.,KEY WORDS Although some physical factors such as volume and molecular weight of molten polymers as a reaction media affect polymer reactions in the melt, la there is little information regarding this problem. We examined this effects from detailed structural and kinetic analyses of the products in a series of studies on the thermal degradation of polyisobutylene. One of the most remarkable features of the thermal degradation of mainchain-scission type of polymers such as polyisobutylene, polyethylene, polypropylene, polystyrene, and poly-(methyl methacrylate) is decrease in volume and molecular weight of the molten polymer matrix constituting the reaction media during degradation. In the previous papers, 2 we showed that the main products in the volatile oligomers formed by thermal degradation of polyisobutylene are four types of terminal monoolefins, 2 a as shown below: a terminal vinylidene double bond (TVD) type and a terminal trisubstituted double bond (TTD) type of olefins with a tert-butyl end (t-Bu) formed from a primary (p) terminal macroradical (Rp · ), and TVD and TTD type of olefins with an isopropyl end (i-Pr) formed from a tertiary (t) terminal macroradical (R 1 • ) CH 3 CH 3 CH 3 Each homologue consists of oligomers from dimers to dodecamers. The formation of these olefins could be reasonably explained by the reaction model consisting of the intramolecular hydrogen abstraction (back-biting) of RP · and R 1 • and the subsequent p scission at the inner position of the main chain. 3 The reactivities for abstraction of interest hydrogens (CH 2 and CH 3 ) are governed not by the steric hindrance-depending mechanism 1 b, 3 but by the bond energy-depending mechanism. The latter mechanism predicts that back-biting occurs predominantly at CH 2 rather than at CH 3 in the same manner as intermolecular hydrogen abstractions 4 of both RP · and R 1 ·, in conformity with the order of bond disso...