The Scission of Polysulfide Cross-Links in Rubber Particles through Phase-Transfer Catalysis

Abstract

Hydroxide ions transported by onium ions from water into swollen rubber particles rapidly break polysulfide crosslinks with little or no main chain scission. The reaction is not truly catalytic because the catalyst is consumed during the reaction. Model studies with N-methyl-N,N,N-tri-n-alkylammonium chlorides and di-2-cyclohexen-1-yl disulfide show that catalyst decomposition involves a highly selective demethylation of the quaternary ammonium ion by its 2-cyclohexenylthiolate counterion. This step is inhibited when certain alkylating agents are added. Rate studies of the Hofmann reaction show that it is too slow to be competative under these conditions. Several onium salts have been examined in this process. In general, those having several large alkyl substituents perform best, consistent with the known partitioning/reaction rate behavior of such catalysts in simple, low viscosity solvents. However, with N-methyl-N,N,N-tri-n-alkylammonium chlorides, the rate decreases as the tri-n-alkyl substituents become very large (e.g., C18H37).