In situ UV-VIS spectroscopic system for ethylene polymerization under atmospheric pressure was designed and built, and was used to mechanistic studies for late-transition metal catalysts. Four catalytic systems with four different iron, cobalt and nickel complexes as catalyst precursor were studied. The corresponding complexes are 2,6-bis(1-(2,6-dimethylphenylimino)ethyl)pyridine iron(II) dichloride (a), 2,6-bis[l-(2,4,6-trimethylphenylimino)ethyl]pyridine iron(II) dichloride (b), 2,6-bis[1-(2,6-diisopropylphenylimino)ethyl]pyridine cobalt dichloride (c) and 1,2-bis(2,6-diisopropylphenyl)ethane nickel(II) dibromide (d). The experimental results showed that the step of activation of MAO to procatalyst is very fast; active absorptions at 560, 640, 580 and 545 run were observed, respectively, and were probably assigned to catalytically active species which were coordinatively cationic alkyl complexes. For active absorptions, the increasing regularity in polymerization initiation and the decreasing regularity during the catalyst being poisoned by anhydrous batches ethanol, were investigated. Catalytically active species and catalytic mechanism were discussed. (C) 2002 Elsevier Science B.V. All rights reserved.