THE DEGRADATION MECHANISM OF POLYISOBUTYLENE .2. CHARACTERIZATION OF THE PRODUCTS AND THE DEPENDENCE OF THEIR YIELDS ON SAMPLE THICKNESS PROVIDES DETAILED MECHANISTIC INFORMATION

The thermal degradation of polyisobutylene has been studied by pyrolysis gas chromatography, using a microthermocouple-controlled resistive filament. All samples were pyrolysed at 400 degrees C for 10 s. The overall pyrolysis mechanism, an array of both parallel and consecutive processes, has been elucidated by performing a sample size study. This involves the measurement of product yields as a function of sample thickness, for samples in the microgram range. The approach provides an indirect method of varying the residence time of primary products in the polymer melt, and thereby influencing the opportunities for secondary reactions. Results from this work have shown that monomer, dimers and trimers are produced in excess of those yields expected on the basis of primary reactions, whilst oligomers in the range of tetramer to octamer are produced in lower than expected yields. This leads to the conclusion that these higher oligomers are undergoing secondary reactions within the polymer melt to produce lower oligomers. Analysis of the corresponding bonus and deficit yields shows that monomer is the principal secondary degradation product and that oligomer depropagation is consequently a dominant secondary process. Previous work by the authors on the statistical analysis of the yields of decomposition products showed that the degradation of polyisobutylene proceeds essentially by parallel depropagation and random scission mechanisms. The present work has yielded further mechanistic information, and it may now be proposed that polyisobutylene thermally degrades by parallel depropagation with random scission, plus secondary depropagation of oligomers formed by random scission, with only small contributions from transfer processes. This mechanism is contrary to that put forward by Kiran and Gillham and Tsuchiya and Sumi, who proposed that, in addition to depropagation, intramolecular transfer with associated scission was the principal decomposition mechanism.