Atom transfer radical polymerization of n-butyl methacrylate (BMA) was conducted in an aqueous dispersed system with different kinds of copper complexes. The partitioning behavior of the copper complexes, including CuCl/4,4'-di(5-nonyl)-2,2'-bipydine (dNbpy), CuCl2/dNbpy, CuCl/2,2'-bipydine (bpy), CuCl2/bpy, CuCl/bis(N,N'-dimethylaminoethyl)ether (bde), and CuCl2/bde between the monomer (BMA), and water was studied in detail with ultraviolet-visible spectroscopy. The results show that with a less hydrophobic ligand, such as bpy or bde, most of the Cu(I) or the Cu(II) complexes migrated from the BMA phase to the aqueous phase, the atom transfer equilibrium was destroyed, and the polymerization was nearly not controlled; it converted to classical emulsion polymerization. As to the very hydrophobic ligand dNbpy, although the partitioning study of the copper complexes indicated that not all the copper species were restricted to the organic phase, the linear correlation between the molecular weight and the monomer conversion and the narrow polydispersities confirmed that the polymerization was still quite well controlled. (C) 2003 Wiley Periodicals, Inc.