A well-defined, amphiphilic poly(styrene-co-acrylic acid) copolymer was synthesized in a single step by nitroxide-mediated controlled free-radical copolymerization of styrene and acrylic acid, without protection of the acid groups: M-n = 6500 g mol(-1). M-w/M-n = 1.5 and a composition of F-AA = 0.70 +/- 0.03 in acrylic acid. In addition to the good control over molar mass and molar mass distribution. the copolymer exhibited a narrow composition distribution with a slight gradient. Such copolymer was an efficient stabilizer for the emulsion polymerizations of styrene and of mixtures of methyl methacrylate and n-butyl acrylate, until 45 wt% solids. A low amount (typically 3-4 wt% based on the monomer(s)) was needed for a good stabilization. This is approximately a decade lower than the required amount of random. amphiphilic copolymers prepared via conventional free-radical polymerization. The performances were, however. below those of analogous diblock copolymers, but the great advantage is the very easy synthetic procedure. (c) 2006 Elsevier Ltd. All rights reserved.