Well-defined polystyrene-b-polyisoprene-b-polystyrene (SIpS) triblock copolymers with different microstructures were synthesized by living anionic polymerization. The synthesis of star-branched polyisobutylene (PIB) was accomplished by the cationic polymerization in 2-cl-doro-2,4,4-trimethylpentane/titanium tetrachloride/SIpS triblock copolymer/2,6-di-tert-butylpyridine initiating system. The double bonds in SIpS triblock copolymer were activated as starting points for isobutylene polymerization. The formation of star-branched architecture was demonstrated by size-exclusion chromatography with quadruple detection: refractive index, multiangle laser tight scattering, viscometric, and ultraviolet detectors. SIpS triblock copolymer with high 3,4-PIp content is more reactive than that with high 1,4-PIp content in cationic initiating stage. The yields of star-branched PIB were remarkably dependent on the reaction time of TMP+ with SIpS. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 112: 3294-3299, 2009