The combination of 1,4-bis(2,6-diisopropylphenyl)-acenaphthenediimine-dichloronickel (II) (1) and methylaluminoxane (MAO) has been shown as being highly active in ethylene polymerization under high pressure and temperature. Herein we describe the effects of ethylene pressure and reaction temperature on polymer properties and reaction performance. The polyethylenes synthesized with the system 1/MAO are highly branched, with 105 to 277 branches per 1000 backbone carbon atoms, depending on the reaction conditions. The branching index increases with the rise of temperature or with the decrease of ethylene pressure. These branches go from methyl to hexyl, or even farther, and present a pattern in which 1,4; 1,5 and 1,6 methyl groups appear mainly and isolated methyl groups are not present. These branches are generated by a chain-walking system. The polyethylenes produced with these systems have a molecular weight (Mw) between 44,000 and 105,000 Daltons and polydispersions from 2,0 to 4,0, depending on reaction conditions. The polymer molecular weight tends to decrease with the increasing temperature of polymerization.