This article reports a new methodology for preparing highly stereoregular styrene (ST)/1,3-butadiene (BD) block copolymers, composed of syndiotactic polystyrene (syn-PS) segments chemically bonded with cis-polybutadiene (cis-PB) segments, through a stereospecific sequential block copolymerization of ST with BD in the presence of a C5Me5TiMe3/B(C6F5)(3)/Al(oct)(3) catalyst. The first polymerization step, conducted in toluene at -25 degreesC, was attributed to the syndiospecific living polymerization of ST. The second step, conducted at -40 degreesC, was attributed to the cis-specific living polymerization of BD. The livingness of the whole polymerization system was confirmed through a linear increase in the weight-average molecular weights of the copolymers versus the polymer yields in both steps, whereas the molar mass distributions remained constant. The profound cross reactivity of the styrenic-end-group active species with BD toward ST led to the production of syn-PS-b-cis-PB copolymers with extremely high block efficiencies. Because of the presence of crystallizable syn-PS segments, this copolymer exhibited high melting temperatures (up to 270 degreesC), which were remarkably different from those of the corresponding anionic ST-BD copolymers, for which no melting temperatures were observed. Scanning electron microscopy pictures of a binary syn-PS/cis-PB blend with or without the addition of the syn-PS-b-cis-PB copolymers proved that it could be used as an effective compatibilizer for noncompatibilized syn-PS/cis-PB binary blends. (C) 2005 Wiley Periodicals, Inc.