A novel method has been developed to prepare amphiphilic core-shell polymer nanoparticles via polymerizations of styrene (St) initiated by hydrophilic a-methylstyrene (AMS)-containing prepolymers. AMS-containing prepolymers were first synthesized by soap-free emulsion copolymerization of AMS, butyl acrylate (BA), and acrylic acid (AA) and then were kept at 90 degrees C to generate free radicals on the AMS segments, which subsequently initiate the polymerization of St. Thus, well-defined, amphiphilic core-shell nanoparticles, ranging from 126 to 282 nm in diameter, were produced in the absence of surfactant. The St conversion, molecular weights, and size of products strongly depended on the AMS-containing prepolymer concentration and the AMS content of it. Transmission electron microscopic (TEM) images of the particles clearly show well-defined core-shell morphologies where PSt cores are coated with hydrophilic AMS-containing prepolymer shells. The amphiphilic core-shell nanoparticles can be produced in high concentrations (up to 25% solids content). This new method is scientifically and technologically significant because it provides a commercially viable route to a wide variety of novel amphiphilic core-shell nanoparticles. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012