The addition of soluble quaternaryammonium halides (QX) in catalytic amounts takes into solution CuX/pentamethyldiethylenetriamine (PMDETA) complex (X = Cl, Br) in methyl methacrylate (MMA). The soluble catalyst complex provided much better control of the polymerization of MMA at ambient temperature than did the insoluble catalyst formed in the absence of QX, with CuCl/PMDETA/Aliquat(R) 336 (AQCl) proving to be superior to the CuBr/PMDETA/Bu4NBr catalyst system. The effect was independent of the size of the quaternaryammonium ion. Also, the presence of Cl in the catalyst-QX combination either as CuCl or as QCl was enough to give much better control than that provided by a wholly Br-based system. Among the various initiators used, that is, ethyl 2-bromoisobutyrate (EBiB), methyl 2-bromopropionate (MBP), 1-phenylethyl bromide (PEBr), and p-toluenesulfonyl chloride (pTsCl), only EBiB gave a satisfactory result. With MBP and PEBr the initiation was slower than the propagation, whereas with pTsCl the initiation was very fast, so that instantaneous termination occurred. The living nature of the polymers was shown by block copolymer preparation. It has been suggested that some of the added halide ions entered into the coordination spheres of Cu(I) and Cu(II), leading to their improved solubility and stronger deactivation by the Cu(II) complex. (C) 2004 Wiley Periodicals, Inc.