The polymerization of styrene with a series of bispyrazolylimine dinickel (II) complexes of bis-2-(C3HN2(R-1)(2)-3,5)(C(R-2) = N(C6H3(CH3)(2)-2,6)Ni2Br4 (complex 1: R-1 = CH3, R-2 = Ph; complex 2: R-1 = CH3, R-2 = 2,4,6-trimethylphenyl; complex 3: R-1 = R-2 = Ph; complex 4: R1 = Ph, R-2 = 2,4,6-trimethylphenyl) in the presence of methylaluminoxane (MAO) was studied. The influences of polymerization parameters such as polymerization temperature, Al/Ni molar ratio, reaction time, and catalyst concentration on catalytic activity and molecular weight of the polystyrene were investigated in detail. The influence of the bulkiness of the substituents on polymerization activity was also studied. All of the four catalytic systems exhibited high activity (up to 10.50 x 10(5) gPS/(mol Ni h)) for styrene polymerization and provide polystyrene with moderate to low molecular weights (M-w = 4.76 x 10(4)-0.71 x 10(4) g/mol) and narrower molecular weight distributions about 2. The obtained polystyrene was characterized by means of FTIR, H-1-NMR, and C-13-NMR techniques. The results indicated that the polystyrene was atactic polymer. The analysis of the end groups of polystyrene indicated that styrene polymerization with bispyrazolylimine dinickel complexes/MAO catalytic systems proceeded through a coordination mechanism. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 122: 545-550, 2011