Cationic copolymer nanoparticles were prepared by emulsifier-free emulsion polymerization of styrene and n-butyl acrylate, using [2-(methacryoyloxy ethyl] trimethylammonium chloride as the cationic functional comonomer and 2,2'-azobis (2-methypropionamidine) as the cationic initiator. FTIR spectroscopy, 1H-NMR spectroscopy, and GPC were applied to characterize the chemical structure and molecular weight of the obtained copolymer. The size and size distribution of the nanoparticles were characterized through photon correlation spectroscopy. The interaction of nanoparticles with bovine serum albumin (BSA) was investigated by the means of transmission electron microscopy and fluorescence spectroscopy. It was found that the copolymer nanoparticles were monodisperse spheres with the diameter less than 90 nm and can complex well with BSA through electrostatic interaction. (c) 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012