A series of a-diimine Ni(II) and Pd(II) complexes containing bulky steric groups, {[(4-R-2,6-Ph2C6H2N=C)(2)Nap]MX2} (Nap: 1,8-naphthdiyl, MX2 =NiBr2, R=F (4a); R= CI (4b); R= Me (4c); R= n-Bu (4d) and MX2 = PdCl2, R=F (5a); R= CI, (5b); R= Me (5c); R= n-Bu (5d)), 1[4-F-2,6Ph(2)C(6)H(2)N=C(Me)](2)PdCl2}(5e), were synthesized and characterized. The crystal structures of ligands 3d, 3e and complexes 4a, 4b, 5a, 5b, 5e were determined by X-ray crystallography. These complexes, when activated with diethylaluminum chloride (DEAC), were used to catalyze polymerization of ethylene and methyl methacrylate (MMA) under mild conditions. The introduction of an electron-withdrawing group to the ligand framework improves the catalytic activity significantly. The Ni(II) catalysts 4a-d exhibited high catalytic activity in the polymerization of ethylene (up to 10(6) gPE/(mol Ni h bar)) and produced highly branched polyethylenes at high temperature. Interestingly, complex 4a could produce syndiotactic-rich PMMAs at low temperature (0 degrees C, up to 77.2% of rr triads). (C) 2014 Elsevier B.V. All rights reserved.