RuCl2(PPh3)(2)(triazol-5-ylidene) (4) induces, in the presence of primary or secondary amines and in conjunction of an organic bromide initiator (ethyl-2-bromo-isobutyrate), a controlled radical polymerization of methyl methacrylate (MMA). The optimal concentration of both the primary amine (CH3CH2CH2NH2) and the secondary amine (nBu(2)NH) is determined as a catalyst/amine ratio of 1/4. Polymers obtained, in the presence of both amines, have very narrow molecular weight distributions (MWD), 1.08 and 1.43 for the primary and secondary amine, respectively. The best initiation efficiency was obtained for the 4/initiator/nBu(2)NH (1/1/4) initiating system (93%). This system yields polymers with a very narrow MWD (1.24) and with a controlled M-n in direct proportion to the monomer conversion within a 24 h time period. NMR analysis reveals that the added amines interact with the Ru-complex 4 to generate a highly active catalyst for the polymerization of MMA. The controlled character of the polymerization is established via a monomer-addition experiment and 1 H NMR analysis. The microstructure of the synthesized polymers (syndiotactic) is in agreement with that of the free radical polymerization. (C) 2003 Elsevier Science B.V. All rights reserved.