New unsymmetric alpha-diimine ligands based on phenanthrenequinone were synthesized by using TiCl4 and 1,4-diazabicyclo[2.2.2]octane. The coordination reactions of these ligands with (DME)NiBr2 afforded the corresponding dibromide nickel alpha-diimine catalysts, which were subjected to ethylene polymerization with MAO. The unsymmetrical structure variation of the pentacyclic diimine ligand revealed that more encumbered structure led to higher activity of the catalysts. The polyethylenes produced by these catalysts at the same temperature showed similar branching frequency while those branch distributions were markedly changed as the steric bulkiness varied, resulting in the significant change of morphology from completely amorphous to semicrystalline with meting points above 100 degrees C. (C) 2014 Elsevier B.V. All rights reserved.