The polymerizations of styrene and 4-methylstyrene (4MS) with a half-metallocene type catalytic system composed of (trimethyl)pentamethylcyclopen-tadienyltitanium (Cp*TiMe3), trioctylaluminum (Aloct(3)), and tris(pentafluorophenyl)borane [B(C6F5)(3)] were investigated at -25 degreesC. The addition of AlOct(3) as a third component of the catalytic system is effective both to promote the syndiospecific polymerization and to inhibit the nonstereospecific polymerization at the low-temperature region. The use of AloCt(3) was also effective to eliminate the chain transfer reaction to alkylaluminum. The number-average molecular weights (M-n's) of poly(4MS) or polystyrene increased proportionally with increasing monomer conversion. The molecular weight distribution (MWD) of polymer stayed narrow [M-w/M-n = similar to 1.1 for poly(4MS) and M-w/M-n = similar to 1.5 for polystyrene]. It was thus concluded that the polymerizations of the styrenic monomers with Cp*TiMe3/B(C6F5)(3)/AlOCt(3) catalytic system proceeded under living fashion at -25 degreesC. The living random copolymerization behaviors of styrene and 4MS were also confirmed. The C-13 NMR analysis clarified that each of the homopolymers and random copolymers obtained in this work had highly syndiotactic structure.