The Ni(II) complexes [Ni(eta(5)-C5H3R2)(X)(NHC)] 1a-f combined with MAO have been tested in methyl methacrylate (MMA) polymerization. We have found that the most active complex 1f, bearing the bulky 2,6-diisopropenylphenyl substituents in the NHC ligand, effected the MMA polymerization with TOF up to 200 h(-1). Predominantly syndiotactic, high molecular weight PMMAs were obtained suggesting an anionic, MAO-centered polymerization mechanism. (C) 2013 Elsevier B.V. All rights reserved.