A bis(diketiminate)bromosamarium(III) complex was successfully synthesized and, after the activation with butyl lithium, was shown to be active in methyl methacrylate (MMA) polymerization. The effects of temperature, polymerization time and catalyst concentration were studied. Activities of ca. 16,000 g of poly(methyl methacrylate) (PMMA) per mol of samarium and per hour were obtained under optimum conditions (0 degrees C and a MMA/catalyst molar ratio of 90/1), providing a polymer with a molar mass M-n > 23.0 kg mol(-1) and a molar mass distribution (M-w/ M-n) < 1.4. After 1 h of polymerization, conversions of MMA as high as 90% were observed. (c) 2006 Elsevier B.V. All rights reserved.3172149153