Styrene (St) was polymerized in toluene solution by using bis(beta-ketoamino)nickel(II) complex as the catalyst precursor and methylaluminoxane (MAO) as the cocatalyst. The polymerization conditions, such as Al : Ni ratio, monomer concentration, reaction temperature, and polymerization time, were studied in detail. Both of the bis(beta-ketoamino)nickel(II)/MAO catalytic systems exhibited higher activity for polymerization of styrene, and polymerization gave moderate molecular weight of polystyrene with relatively narrow molecular weight distribution (M-w/M-n < 1.6). The obtained polymer was confirmed to be atactic polystyrene by analyzing the stereo-triad distributions mm, mr, and rr of aromatic carbon C-1 in NMR spectrum of the polymer. The mechanism of the polymerization was also discussed and a metal-carbon coordination mechanism was proposed. (c) 2007 Wiley Periodicals, Inc.