Homogeneous Zinc(II) Catalysis in Accelerated Vulcanization IV. The Mechanism of Cross-Link (De)Sulfuration

Abstract

Abstract This paper reports a detailed re-examination of the mechanism of zinc-mediated cross-link desulfuration by means of Reaction-Stage Modeling. Heating the model cross-link bis(2,3-dimethyl-2-buten- 1-yl)disulfide in the presence of catalytic amounts of bis(diethyl-dithiocarbamato)zinc(II), ZDEC, results in slow but definite formation of monosulfides displaying retention and isomerization of the double-bond position. The formation of monosulfides with retained double bonds implies that cross-link desulfuration occurs according to SNi reactions; whereas, the formation of isomerized monosulfides points to the operation of SNi′- and/or thione-like desulfuration reactions. Importantly, these observations constitute the first evidence for zinc accelerators acting as homogeneous catalysts in desulfuration. The activation energy for cross-link desulfuration of SNi reaction was established to be 86(16) kJ mol−1. In addition, ZDEC is reported to sulfurate model cross-links. The required sulfur atoms may originate either from desulfuration of other sulfides, or from elemental sulfur added to the mixture. Just as for cross-link desulfuration, zinc-mediated sulfuration was found to occur both with and without double-bond isomerization.