The controlled radical polymerization of styrene in water medium, in the presence of polyoxyethylene nonyl phenyl ether, catalyzed and initiated by CuCl/BDE bis(N,N′‐dimethylaminoethyl)ether/R—X was studied. The results show that the molecular weight increased with conversion of the monomer. Using this controlled system, the block copolymer, poly(St‐b‐MMA), was successful synthesized in water medium. In reference to the system of CuCl/BDE/PhCH2Cl, the polymerization may also occur in the micelle to produce a superhigh molecular mass (Mn = 1,500,000) polymer with monodispersion (MWD, Mw/Mn = 1.03). The Cu(I) and Cu(II) partition ratio in two phases, which may affect the reversible deactivation and debase the catalyst efficiency, was detected. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 75: 802–807, 2000