Thermal degradation of polyisobutylene: effect of rotational motion around CC· bond on the β scission leading to monomer formation

Abstract

The formation of isobutylene monomer and volatile oligomers by the thermal degradation of polyisobutylene at 300 °C is simulated by using assigned values of the rate constants ( k td and k pd) of the depolymerization of primary and tertiary terminal macroradicals (R p· and R t·), based on a radical chain reaction model including diffusion-controlled termination reactions. With increasing reaction time, the composition of isobutylene monomer in the volatiles increases and that of the volatile oligomers decreases. This change in composition is well matched by the simulation using the value of k td larger than k td. This result is caused by a marked decrease in [R p·] and a gradual decrease in [R t·] with reaction time. The difference in the rate constants of the radicals R p· and R t· is estimated to be k td k pd = 50 − 100 and the ratio of rate constants corresponds to the values 18.8–21.7 kJ/mol of the difference of activation energy ΔE a. This difference is reasonably explained by assuming that the β scission depends on the energy barrier of rotation of the CC· bond of R p· and R t· whose value is estimated from an extended Hückel molecular orbital calculation on the molecular model of R p· and R t·.