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 including (n + 2)-mers (n greater than or equal to 0) produced by thermal degradation of polyisobutylene were systematically determined by high-resolution capillary gas chromatogaphy/mass spectrometry (GC/MS). The total ion current (TIG) chromatogram consists of about 100 peaks ranging from dimers (2-mers, n = 0) to dodecamers (12-mers, n = 10) and most of the main peaks are classified into four types of terminal monoolefins: a trisubstituted olefin with a tert-butyl end, a vinylidene olefin with a tert-butyl end, a trisubstituted olefin with an isopropyl end, and a vinylidene olefin with an isopropyl end. The formation of these monoolefins is reasonably interpreted by intramolecular hydrogen abstractions (back-biting) of primary and tertiary terminal macroradicals and subsequent beta scissions at the inner position of the main chain. In all the chromatograms of each (n + 2)-mer (n greater than or equal to 1), the retention times of terminal trisubstituted types of monoolefins were shorter than those of terminal vinylidene types of monoolefins, in contrast to the elution order of dimers. The relative intensities between the interesting peaks of each (n + 2)-mer (n greater than or equal to 1) clearly represent that back-biting more predominantly occurs at the methylene hydrogen rather than at the methyl group consistent with the difference in the bond dissociation energy of the H-C bond of interest, as opposed to the steric hindrance mechanism(5,7)).