The sequential block copolymerization of styrene (St) and butadiene (Bd) was carried out with an activated rare earth catalyst composed of catalyst neodymium tricarboxylate (Nd), cocatalyst Al(i-BU)(3) (Al), and chlorinating agent (Cl). The microstructure, composition, and morpholog of the copolymer were characterized by FTIR, H-1 NMR, C-13 NMR, and TEM. The results show that styrene-butadiene diblock copolymer with high cis-1,4 microstructure of butadiene units (similar to 97 mol %) was synthesized. The cis-selectivity for Bd units was almost independent on the content of styrene units in the copolymer ranging from 18.1 mol % to 29.8 mol %. The phase-separated morphology of polystyrene (PS) domains of about 40 nm tethered by the elastomeric polybutadiene (PB) segments is observed. The PS-b-cis-PB copolymer could be used as an effective compatilizer for noncompatilized binary PS/cis-PB blends. (c) 2007 Wiley Periodicals, Inc.