Effect of molecular weight on formation of non-volatile oligomers by thermal degradation of polyisobutylene and its kinetic analysis

Abstract

The formation of functional groups of non-volatile oligomers by the thermal degradation of polyisobutylene is characterized by a kinetic approach including intermolecular hydrogen abstraction of primary (p) and tertiary (t) terminal macroradicals (R p • and R t •) and volatile small radicals (S •), followed by β-scission. By assuming in the kinetic analysis that the reaction occurs competitively under a steady state regarding the on-chain macroradicals, various composition ratios for the functional groups can be represented in terms of the rates of respective hydrogen abstraction. The ratio between the tert-butyl endgroup (t-Bu) and the isopropyl endgroup (i-Pr), which corresponds to that between the abstraction rates of R p • and R t • , is expressed by the product of the rate constant ratio and the integrated macroradical concentration ratio (R p •]/[R t •]). The observed value of the ratio [ t-Bu] [ i-Pr] decreases with reaction time. This is induced by a decrease in the molecular weight ( M) of the reaction medium. The molecular weight dependence ( M a ) of [ t-Bu] [ i-Pr] is expressed by that of R p bull;/[R t •]. The value of exponent a was determined to be about 1.2 and 0.7 at 300 and 320°C, respectively, from the analysis of data at various reaction times. These values are roughly consistent with average values (1.0 and 0.9) of the same power-law exponent for the volatile oligomers. These results support the hypothesis that the concentrations of the respective radicals decrease in different ways.