ELECTROPHILIC AROMATIC-SUBSTITUTION IN THE CURING OF BROMINATED POLY(ISOBUTYLENE-CO-4-METHYLSTYRENE) - A MECHANISTIC MODEL STUDY WITH ZINC SALTS

The crosslinking chemistry of a new elastomer based on brominated poly (isobutylene-co-4-methylstyrene) has been investigated using model compounds. In order to mimic the conditions that prevail within the highly aliphatic rubber, the study was carried out in mineral oil using catalysts that are compatible with such low polarity media. Electrophilic aromatic substitution reactions occur in a system consisting of p-isopropylbenzyl bromide and p-isopropyl toluene in the presence of zinc oxide and zinc stearate. The reaction proceeds after a significant induction period while no induction period is seen for a similar reaction with zinc bromide as the catalyst. The in situ formation of reactive species containing Zn-Br bonds appears to be an important step in the overall process. The stoichiometric ratio of zinc salt to benzylic bromide is important. High ratios lead to the accumulation of benzylic stearate or benzylic alcohol in the system which retards the alkylation reaction. The model study emphasizes the importance of stoichiometry of reagents in this reaction and provides insight into the crosslinking mechanism.