Seeded preswelling emulsion polymerization was carried out by using monodispersed poly(4-vinylpyridine-co-butyl acrylate) [P(4VP-BA)] particles as the seed, and styrene and butyl acrylate as the second-stage monomers under different polymerization conditions, to obtain hemispherical polystyrene (PST)-rich-P4VP-rich microspheres. Prior to polymerization, toluene was added into the preswelling system together with the second-stage monomers. It was found that, with the increase of the amount of toluene, the particle morphology showed a tendency toward desirable hemispherical structure, and the colloidal stability of composite latex was improved. When the weight ratio of toluene/seed latex was increased up to 7.5/40 (g/g), the stable hemispherical latex could be obtained. However, when toluene was not added, the coagulum formed on the wall of the reactor during polymerization, and the composite particles with multiple surface domains (such as sandwich-like, popcorn-like) were formed. In addition, the final morphology of composite particles was influenced by the polarity of the seed crosslinker and the hydrophilicity of the second-stage initiator, which could affect the mobility of poly(styrene-co-butyl acrylate) [P(ST-BA)] chains. The morphology development during the polymerization was investigated in detail, and a schematic model was derived to depict the formation mechanism of hemispherical P(4VPBA)/P(ST-BA) composite microspheres. The results revealed that the mobility of the P(ST-BA) chains influenced the diffusion of the P(ST-BA) domains on the surface of the P(4VP-BA) matrix. When the mobility of the P(ST-BA) chains allowed small-size P(ST-BA) domains to coalesce into one larger domain, complete phase-separated morphology (hemisphere) could be achieved. (C) 2003 Wiley Periodicals, Inc.