A modified emulsifier-free emulsion polymerization of butyl methacrylate (BMA) with ionic or/and nonionic comonomers was successfully used to prepare nanosized poly(butyl methacrylate) (PBMA) latices with high polymer contents. After seeding particles were generated in an initial emulsion system, consisting of a portion of BMA, water, ionic comonomer [sodium styrenesultonate (NaSS)] or nonionic comonomer [2-hydroxyethyl methacrylate (HEMA)] and potassium persulfate, most of the BMA monomer or the mixture of BMA and HEMA was added dropwise to the polymerizing emulsion over a period of 6-12 h. Stable latices with high PBMA contents up to 27% were obtained. It was found that the latex particle sizes (2R(h)) were largely reduced (34 nm) by the continuous addition of monomer(s) compared to those (107 nm) obtained by the batch polymerization method. The effect of comonomer concentration on the particle size, the number of PBMA particles/mL of latex (N-d), and the molar mass (M-w) of copolymer during the polymerization were discussed. The surface compositions of latex particles were analyzed by X-ray photoelectron spectroscopy, indicating that the surface of latex particles was significantly enriched in NaSS or/and HEMA. (C) 2004 Wiley Periodicals, Inc.