vulcanization of BMS polymer-brominated poly(isobutylene-co-p-methylstyrene) Metal oxide cure of brominated poly(isobutylene-co-p- methylstyrene), BIMS polymer, is unique to the halogenated elastomers. In this polymer, metal oxide cure proceeds via electrophilic aromatic substitution chemistry, not nucle- ophilic substitution chemistry. The reaction is catalyzed by the in situ formation of the Lewis acid catalyst zinc hydroxy- bromide(figure 14). This is thought to form from the reac- tion of advantitious water or by the action of stearic acid. The zinc hydroxybromide then catalyzes an electrophilic aromatic substitution reaction forming'diphenylmethyl' crosslinks MBTS and sulfur are sometimes used in BIMS com pounds. The MBTS decomposes to MBT by the action of zinc oxide and sulfur. Since MBT is a mono-functional nucleophile, it will react with the benzyl bromide sites form-