Aqueous polymerizations of ethylene catalyzed by a series of neutral nickel(II) complexes coordinated with salicylaldimines have been performed in the presence of surfactants, and the lower branched high molecular weight polyethylene was obtained. The results showed that the catalytic activity of the catalysts and the properties of the polyethylene were influenced by the electronic and steric effects of the substituents attached on the benzene ring in the complexes. With increasing the electron withdrawing substituents in the phenoxy ring, the catalytic activity was increased, and the molecular weight of polyethylene decreased. Meanwhile, both of the catalytic activities and the molecular weights of polyethylene were increased with increasing the steric effect of the substituents in aniline. The molecular weights of the polyethylene obtained in an aqueous system were in the range of 10⁴～10⁵. The DSC analysis of the polyethylene indicated that the degrees of crystallinity and the melting points were 50%～70% and 115～137 ℃, respec-tively. The GPC results displayed that the molecular weight distribution of the polyethylenes cocatalyzed by bis(1,5-cyclooctadiene)nickello)[Ni(COD)₂] presented bimodal to unimodal with the electronegativity of the groups attached on the phoenxy circle being increased. The variation of the molecular weight and polydispersity of the polyethylenes were further demonstrated by the dynamic rheological researches.

Key words: aqueous catalytic polymerization, ethylene, salicylaldimine, neutral nickel(II) complex