THERMAL-DEGRADATION OF POLYMERS IN THE MELT .1. CHARACTERIZATION OF VOLATILE OLIGOMERS FORMED BY THERMAL-DEGRADATION OF POLYISOBUTYLENE

Chemical structures of a number of components of volatile oligomers in cluding (n + 2)-mers (n greater than or equal to 0) produced by therma l degradation of polyisobutylene were systematically determined by hig h-resolution capillary gas chromatogaphy/mass spectrometry (GC/MS). Th e total ion current (TIG) chromatogram consists of about 100 peaks ran ging from dimers (2-mers, n = 0) to dodecamers (12-mers, n = 10) and m ost of the main peaks are classified into four types of terminal monoo lefins: a trisubstituted olefin with a tert-butyl end, a vinylidene ol efin with a tert-butyl end, a trisubstituted olefin with an isopropyl end, and a vinylidene olefin with an isopropyl end. The formation of t hese monoolefins is reasonably interpreted by intramolecular hydrogen abstractions (back-biting) of primary and tertiary terminal macroradic als and subsequent beta scissions at the inner position of the main ch ain. In all the chromatograms of each (n + 2)-mer (n greater than or e qual to 1), the retention times of terminal trisubstituted types of mo noolefins were shorter than those of terminal vinylidene types of mono olefins, in contrast to the elution order of dimers. The relative inte nsities between the interesting peaks of each (n + 2)-mer (n greater t han or equal to 1) clearly represent that back-biting more predominant ly occurs at the methylene hydrogen rather than at the methyl group co nsistent with the difference in the bond dissociation energy of the H- C bond of interest, as opposed to the steric hindrance mechanism(5,7)) .