A series of polystyrene (PS)/poly(vinyl acetate) (PVAc) crosslinked particles (240, 210, or 90 nm) with different concentrations of PS (75, 50, or 25 wt %) were prepared by soap-free emulsion polymerization. Based on the crosslinked polymer particles, three series of monodisperse core-shell particles with pH-sensitive poly[2(dimethylamino)ethyl methacrylate] (PDMAEMA) shells were synthesized by oxyanionic polymerization. During the oxyanionic polymerization, the acetate groups of PVAc were hydrolyzed, and the hydroxyl. groups that formed on the surfaces of the particles, acting as initiators, were transferred to -O-K+ by DMSO-K+ (where DMSO is dimethyl sulfoxide) at the same time; then, -O-K+ initiated the polymerization of 2-(dimethylamino)ethyl methacrylate. H-1 NMR and Fourier transform infrared studies confirmed the existence of PDMAEMA shells, and the contents of PDMAEMA were measured by elemental analysis. Because the PDMAEMA chain could be protonated at a low pH, these core-shell particles could adsorb negatively charged modified magnetite particles, and at higher pHs, the magnetite particles could be released again; this process was reversible. (C) 2004 Wiley Periodicals, Inc.