Polychloromethylstyrene (PCMS) micrometer-sized particles of narrow size distribution were prepared by the dispersion polymerization of chloromethylstyrene in a mixture of ethanol and dimethyl sulfoxide. Micrometer-sized PCMS/poly(butyl methacrylate) hemispherical composite particles of narrow size distribution were prepared by a single-step swelling process of the uniform PCMS template particles with emulsion droplets of butyl methacrylate (BMA) containing benzoyl peroxide, followed by the polymerization of BMA at 73 degrees C within the swollen template particles. The effects of various polymerization parameters, for example, BMA volume, initiator type and concentration, and toluene as the swelling solvent, on the properties (size and size distribution, morphology, polymerization yield, and composition) of the hemispherical composite particles were elucidated. (C) 2008 Wiley Periodicals, Inc.