MECHANISM OF RADIATION DEGRADATION OF POLYISOBUTYLENE

GC, GC/MS, GPC, and Solution NMR spectra were used to study the gamma radiolysis of polyisobutylene (PIE) in vacuum to different total doses (0-900 kGy) and at different temperatures (77-423 K). NMR spectra sho w a large number of new resonances with relatively narrow line widths, and a variety of NMR techniques have been employed to determine and q uantify the structures associated with these new resonances. Chemical shift assignments were made by comparison with those for small molecul e model compounds and predictions based upon calculations according to several different schemes. Chain-end structures have been proposed th at account well for the majority of the new resonances, all being the result of an initial chain scission reaction initiated by the radiatio n. These assignments support some previous proposals for the mechanism of radiation degradation of polyisobutylene and exclude others. For e xample, NMR provides no evidence for the formation of ethyl chain ends and some of the main chain unsaturated structures previously proposed . NMR also indicate that at higher doses the chain end products formed during initial stages undergo secondary reactions. GC/MS data show th e formation of oligo oligomers during irradiation, which may be due to a backbitting mechanism. (C) 1996 John Wiley & Sons, Inc.