<正>The factors affecting particle size of reactive microgels formed during the self-emulsifying copolymerization of unsaturated polyester （UP）with butyl acrylate （BA）have been studied. The parameters discussed are: the proportion of the UP in the monomer mixture

the molecular weight and the carboxyl value of the UP

the phase ratio

the electrolyte concentration and the polar solvent additive. The seeding emulsion polymerization is discussed as well.It turned out that the particle size of the reactive microgels can be controlled in a definite range by changing the experimental conditions. However the particle size distribution becomes broader as the average diameter increases. It is suggested that the agglomeration of primary particles plays an important role during the growth of microgel particle.