By using in situ UV—Vis spectroscopic technique for ethylene polymerization under atmospheric pressure

three catalvtic systems with different iron or cobalt 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[1-(2

4

6-trimethylphenylimino)ethyl]pyridine iron (Ⅱ) dichloride (b) and 2

6-bis[1-(2

6-diisopropylphenylimino)ethyl] pyridine cobalt dichloride (c)．The results showed that the activation of MAO to procatalyst is fast；active absorptions at 560 nm

630 nm and 580 nm were observed

respectively．The rule of growth for active absorptions in polymerization and their disappearance when the catalyst was poisoned by anhydrous batches ethanol

were investigated. Catalytic active species and mechanism were discussed．