Copper-encapsulated polystyrene nanocomposite particles were prepared through ex situ dispersion of Cu nanoparticles into monomer droplets and subsequent polymerization using water in supercritical carbon dioxide (water-in-sc-CO2) at 70 degrees C. First, colloidal dispersion of copper nanoparticles was synthesized by chemical reduction of copper chloride (CuCl2) using sodium borohydrate (NaBH4) as reducing agent. Colloidal dispersion of copper nanoparticles was added slowly during the polymerization of styrene using water-in-sc-CO2 medium at 70 degrees C and 20.68 MPa. Cu nanoparticle encapsulated polymer particles were characterized by UV, X-ray diffraction, thermograyimetric analysis, SEM, and TEM. Cu nanopartides were uniformly distributed inside the polymer matrix during the polymerization process. This work represents a simple way to prepare a variety of metal nanoparticles encapsulated polymer particles using waterin-sc-CO2 medium. The Cu/polystyrene nanocomposite particles exhibit antimicrobial activity against a number of bacteria. The current work represents a simple, cheap and universal way to prepare a variety of metal polymer nanocomposite materials. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 120: 1027-1033, 2011