Half-sandwich samarium(III) diketiminate bromide was successfully synthesized and was shown to be active in methyl methacrylate (MMA) polymerization. The effects of temperature, polymerization time and catalyst concentration were studied. Activities of ca. 18 kg of polymethacrylate (PMMA) per mol of samarium, per hour were obtained under optimum conditions (0 degrees C and a MMA/catalyst molar ratio of 100/1), giving a polymer with a molar mass M-n > 24,000 g mol(-1) and a molar mass distribution (M-w/M-n) < 1.4. After 1 h of polymerization, conversions of MMA as high as 96% were observed. (c) 2006 Elsevier Ltd. All rights reserved.