Di-, tri-, and tristar block copolymers consisting of rubbery poly(isobutylene) (PIB) and glassy poly(α-methylstyrene) (PαMeSt) sequences have been synthesized with 70 to > 95% efficiency. The syntheses involved initiation of α-methylstyrene polymerization by linear or three-arm star PIB's carrying 1,2, or 3 tertiary chlorine termini in conjunction with SnCl4coinitiator in the presence of 2,6-di-tert-butylpyridine (DtBP). Blocking efficiencies (Beff.) increased with the amount of DtBP and approached 100% under suitable conditions.