EFFECT OF MOLECULAR-WEIGHT ON FORMATION OF NONVOLATILE OLIGOMERS BY THERMAL-DEGRADATION OF POLYISOBUTYLENE AND ITS KINETIC-ANALYSIS

The formation of functional groups of non-volatile oligomers by the th ermal degradation of polyisobutylene is characterized by a kinetic app roach including intermolecular hydrogen abstraction of primary (p) and tertiary (t) terminal macroradicals (R(p)(.) and R(t)(.)) and volatil e small radicals (S-.), followed by beta-scission. By assuming in the kinetic analysis that the reaction occurs competitively under a steady state regarding the on-chain macroradicals, various composition ratio s for the functional groups can be represented in terms of the rates o f respective hydrogen abstraction. The ratio between the tert-butyl en dgroup (t-Bu) and the isopropyl endgroup (i-Pr), which corresponds to that between the abstraction rates of R(p)(.) and R(t)(.), is expresse d by the product of the rate constant ratio and the integrated macrora dical 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 th at of [R(p)(.)]/[R(t)(.)]. The value of exponent a was determined to b e about 1.2 and 0.7 at 300 and 320 degrees C, respectively, from the a nalysis of data at various reaction times. These values are roughly co nsistent with average values (1.0 and 0.9) of the same power-law expon ent for the volatile oligomers. These results support the hypothesis t hat the concentrations of the respective radicals decrease in differen t ways. Copyright (C) 1996 Elsevier Science Ltd.