Bis(aminophosphines) were prepared by reaction of the corresponding diamines with chlorodiphenylphosphine. As phosphines are readily oxidized to pentavalent phosphorus forming P=O bonds, the analogous bis(aminophosphine oxide) were also synthesized. These bis(ammophosphines) and their products of air oxidation were employed to prepare the nickel complexes I, II, III and IV by reaction of dimethoxyethane nickel dibromide adduct and the bis(aminophosphorus) compounds in THF, at room temperature. The new Ni complexes activated by methylaluminoxane (MAO) were active as catalysts for ethylene polymerization. The influences of Al/Ni ratio and polymerization temperature on the catalyst activity, as well as on the polymer microstructure, were investigated. Highest yield was obtained at Al/Ni ratio = 2000. The more flexible aminophosphine complex 11 showed the highest catalytic activity. The temperature influence on polymerization reaction present a peculiar behavior: at low temperatures the oligomer production was higher and yield of polymer was lower. At temperatures bellow zero, octene and/or its isomers were formed predominantly. The polyethylenes obtained by polymerization with aminophosphine complexes I and II presented molecular weight even higher than 200,000 as well as broad molecular weight distribution. C-13 NMR analyses indicated that they were branched polymers with the melting temperature (T-m) in the range of 117-127 degrees C. The complexes containing the oxidized ligands (III and IV) produced more linear polyethylenes with T-m around 130 degrees C. (c) 2006 Published by Elsevier B.V.