Amphiphilic block copolymers of poly(acrylic acid-b-butyl acrylate) were prepared by reversible addition-fragmentation chain transfer polymerization in a one-pot reaction. These copolymers were characterized by NMR, static and dynamic light scattering, tensiometry, and size exclusion chromatography. The aggregation characteristics of the copolymers corresponded to those theoretically predicted for a star micelle. In a butyl acrylate and methyl methacrylate emulsion polymerization, low amounts of these copolymers could stabilize latices with solid contents up to 50%. (C) 2003 Wiley Periodicals, Inc.