Homogeneous Zinc(II) Catalysis in Accelerated Vulcanization III. Degradation Modes of Mono- and Disulfidic Cross-Links

Abstract

Abstract This paper reports a Reaction-Stage-Modeling study dealing with degradation of mono- and disulfidic cross-links, providing new information about the molecular processes underlying the macrophysical phenomenon of reversion. The monosulfidic model cross-link (2,3-dimethyl-2-buten-1-yl)(2,3-dimethyl-1-buten-3-yl)sulfide was found to degrade at a relatively low temperature of 140 °C to yield olefins and α, β-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-characterized—1,3-dithiin. The disulfidic model cross-link bis(2,3-dimethyl-2-buten-1-yl)disulfide 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(diethyldithiocarbamato)zinc(II), ZDEC, and the activation enthalpy ΔH‡ of the reaction has been determined to be ∼ 71(7) kJ mol− . Several dienophiles and dipolarophiles were applied to trap the α, β-unsaturated thioaldehyde and 1,3-dipolar intermediate, 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.