Solid State NMR Characterization of Crosslinked Brominated Poly(Isobutylene-co-4-Methylstyrene)

The curing of brominated poly(isobutylene-co-4-methylstyrene) was studied using solid-state magic-angle-spinning NMR, a technique which is uniquely suited to investigate cured or crosslinked polymers. This brominated copolymer contains 4-bromomethylstyryl and 4-methylstyryl functional groups and was compression cured with pure ZnO. It was proposed that a precomplex is formed between zinc and the 4-bromomethylstyryl group, which generates a benzylic carbocationic intermediate. Crosslinking predominantly occurs by alkylation of the phenyl ring of another 4-bromomethylstyryl group by the benzylic intermediate. The ZnO brings the two bromides of these groups into proximity. The dependence of the cure chemistry on the concentration of 4-bromomethylstyryl functional groups was studied. Conversion of some 4-bromomethylstyryl groups to oxygenated species upon curing was also observed.