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 disulfidic cross-links, providing new information about the m olecular processes underlying the macrophysical phenomenon of reversion. Th e 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 degrees C to yield olefins and alpha, beta-unsaturated thioaldehydes. T he latter species are not stable enough to be observed directly, but instea d 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)d isulfide was observed to degrade according to a previously unknown mechanis m, involving a 1,4-hydride shift, to furnish a 1,3-dipolar intermediate. Th e reaction is catalyzed by bis (diethyldithiocarbamato)zinc(II), ZDEC, and the activation enthalpy Delta H double dagger of the reaction has been dete rmined to be similar to 71(7) kJ mol(-1). Several dienophiles and dipolarop hiles were applied to trap the alpha, beta-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-lin ks via hydrogen shifts and indicates, for the first time, in what way zinc complexes may cause cross-link degradation and induce reversion in rubber v ulcanizates.