Trialkylaluminums as cocatalysts for bis(phenoxy-imine)Zr complexes, bis [N-(3-tert-butylsalicylidene)-anilinato] zirconium dichloride (1) and bis [N- (3-tert-butylsalicylidene)-cyclohexy]aminato] zirconium dichloride (2), are investigated for ethylene polymerization without MAO or Ph3CB(C6F5)(4). The effects of alkylaluminums, Al/Zr molar ratio, polymerization temperature and time as well as the comonorner I-hexene on polymerization performance are examined. In appropriate range of Al/Zr molar ratio, the catalyst system bis(phenoxy-imine)Zr complex/trialkylaluminum shows high activity for ethylene polymerization, which approximates to bis(phenoxy-imine)Zr complex/MAO system. Their catalytic activities depend on not only the nature, but also the amount, of the trialkylaluminum. Both the molecular weights and the polydispersities (M-w/M-n) of PEs produced by 1 with Et3Al, He3Al, or Me3Al are higher than those with MAO. The nature, rather than the amount, of alkylaluminum detertmines the M. and the polydispersity of polymer in polymerization with the same complex. Unexpectedly, M, of the polyethylenes produced with complex 1/triethylaluminum increases with polymerization time. The catalyst system bis(phenoxy-imine)Zr complex/trialkylaluminum has low capacity of copolymerization. (c) 2005 Elsevier B.V. All rights reserved.