Synthesis of Poly(α-methylstyrene-b-isobutylene) Copolymers by Living Cationic Sequential Block Copolymerization: Investigation on the Crossover from Living Poly(α-methylstyrene) Chain End to Isobutylene

The synthesis of poly(α-methylstyrene- b -isobutylene) (P(αMeSt- b -IB)) copolymers via sequential monomer addition was studied using the adduct of the olefinic αMeSt dimer and HCl (DiαMeSt·HCl) as initiator in the presence of 2,6-di- tert -butylpyridine (DTBP) in methylcyclohexane (MeChx)/methyl chloride (MeCl) (60/40 v/v) at −80 °C. With BCl 3 employed as co-initiator in the first step of the synthesis, well-defined PαMeSt with controlled molecular weight and narrow molecular weight distribution ( M̄ w / M̄ n = 1.05 ∼ 1.20) was obtained. Chain end decomposition was not detected for up to 30 min under monomer-starved condition. Model reactions using DiαMeSt·HCl indicated that, upon addition of IB, quantitative crossover takes place followed by immediate termination and the selective formation of PαMeSt-IB 1 -Cl. Therefore, the addition of TiCl 4 was necessary to polymerize IB. The obtained P(αMeSt- b -IB) diblock copolymer was found to be contaminated by a small amount of homo P(αMeSt), apparently formed via intramolecular alkylation. It has been shown that the blocking efficiency ( B eff ) was a linear function of [IB], but this appeared to be very different for DiαMeSt-IB 1 -Cl and PαMeSt-IB 1 -Cl. This difference was attributed to selective solvation of PαMeSt-IB 1 -Cl by the solvent, which results in local [IB], which is about one-tenth of that in the bulk.