HOMOGENEOUS ZINC(II) CATALYSIS IN ACCELERATED VULCANIZATION III. DEGRADATION MODES OF MONO- AND DISULFIDIC CROSS-LINKS

This paper reports a Reaction-Stage-Modeling study dealing with degradation of mono- and disultidic cross-links, providing new information about the molecular processes underlying the macrophysicat phenomenon of reversion. The monosultidic model cross-link (2,3-dimethy]-2-buten-l-y])(2,3-dimethyl-l-hiiten-3-yl)sulfide was found to degrade at a relatively low temperature of 140 °C to yield olefins and a, beta-unsaturated thioaldehydes. The latter species are not stable enough to be observed directly, but instead dimerize via an auto-Diels-Alder reaction to give a-fully-characterized1,3-dithiin. Thedisulfidic model cross-link bis(2,3-dimethyl-2-buten-1 -yl)disutfide was observed to degrade according to a previously unknown mechanism, involving a 1,4-hydride shift, to furnish a 1,3-dipolar intermediate. The reaction is catalyzed by bis(diethy]dilhiocarbamalo)zinc(ll), ZDEC, and the activation enthalpy deltaH+ of [he reaction has been determined to be ~ 71 (7) kJ mol^-1 Several dienophiles and dipolarophiles were applied to trap the a. beta-unsaturated thioaldehyde and 1,3-dipolar intermediat.e, but neither was successful. Overall, this study has furnished direct evidence for degradation of mono- and disulfidic cross-links via hydrogen shifts and indicates, for the first time, in what way zinc complexes may cause cross-link degradation and induce reversion in rubber vulcanizates.