The complex 1-(2-pyridyl)-2-azaethene-( N, N)-nickel dibromide and its derivatives can be used for the catalytic polymerization of ethylene after activation with methylaluminoxane (MAO). The polyethylenes made with these catalysts have a high degree of short-chain-branching with up to 240 branches per 1000 carbon atoms. Homogeneous ethylene polymerization reactions produce highly viscous oligomers with low molecular weights. Polymers with significantly higher molecular weights and lower amounts of branching are obtained when the ethylene polymerization reactions with these catalyst systems are carried out as heterogeneous reactions. The activities of these paramagnetic complexes are significantly lower than the activities of metallocene catalysts.