A family of cobalt(II) complexes supported on tridentate dibenzimidazolyl ligands having a general formula: [N(CH3)(CH2)(2)(Bm-R)(2)]CoCl2 (where Bm=benzimidazolyl, R=H; -Me; -Bz), have been prepared by the condensation of o-phenylene diamine with methyliminodiacetic acid. The Co(II) complexes exhibited high activities for the polymerization of 1,3-butadiene, on activation with ethyl-aluminum sesquichloride (EASC), to yield predominantly cis-1,4 microstructure. The polymers are characterized by high molecular weight with polydispersity values between 2.35 and 3.37. The ligand modification shows remarkable influence on polymerization activity. The stereospecificity of the catalysts is consistent for a wide range of reaction conditions, except temperature. The electronic influence of ligand structure towards metal center is investigated by using cyclic voltammetric analysis and the generation of cationic active centers is identified via UV-vis spectroscopic analysis of the catalyst system. (c) 2009 Elsevier Ltd. All rights reserved.