The emulsif ier-f ree emulsion polymerization of styrene in the presence of 18-crown-6 has been studied. We found that 18-crown-6 can accelerate the polymerization reaction as a phase transfer catalyst. The latex particles obtained in this polymer ization have two types in size

and the latex particles are composed of two types of polymers in different size. Prom these results

a two phases particle nucleation mechanism has been proposed for this polymerization.