THERMAL-DEGRADATION OF POLYMERS IN THE MELT .2. KINETIC APPROACH TO THE FORMATION OF VOLATILE OLIGOMERS BY THERMAL-DEGRADATION OF POLYISOBUTYLENE

The thermal degradation of polyisobutylene is characterized by kinetic s consisting of four types of intramolecular hydrogen abstraction (bac k-biting) of primary (p) and tertiary (t) terminal macroradicals (R(p) (.) and R(t)(.)) and the successive beta scission at the inner positio n of the main chain. This reaction affords four types of terminal mono olefins in the volatile oligomers. Assuming the reaction occurs compet itively under a steady state regarding the on-chain macroradicals, the composition of the monoolefins are represented as the rate ratios of the respective back-biting processes. The rate ratio between the abstr actions of different types of hydrogens (CH2 and CH3) from the same ty pe of macroradicals is expressed only by the rate constant ratio. The (TTD)(p)/(TVD)(p) and (TTD)(t)/(TVD)(t) ratios remain constant during degradation, independently of the decreases in volume and molecular we ight of the reacting polymers and this tendency agrees fairly well wit h the kinetic expectation. This result suggests that back-biting depen ds only on the local motion of the reacting chain ends. The ratios bet ween the abstractions of the same type of hydrogen from different macr oradicals are expressed by the product of the rate constant ratio and the integrated macroradical concentration ratio ([R(p)(.)]/[R(t)(.)]). The observed values of (TTD)(p)/(TTD)(t) and (TVD)(p)/(TVD)(t) decrea se with reaction time. This decrease results from the decrease in macr oradical concentration.