Micron size Monodispersed polymethyl methacrylate (PMMA) and polystyrene (PSt) latices were synthesized by emulsifier free emulsion polymerization in single step. By suitable variation of the ionic strength

the monomer concentration

or the initiator concentration in the aqueous phase

the final particle sizes of PMMA and PSt ranged from 0 6μm to 1 8μm

and from 0 8μm to 2 2μm

respectively

with the distribution coefficient of particle size below 5%.(The mean diameter of latex particles increased with increasing the ionic strength and mononer concentration.On the contrary

it decreased as the initiator concentration increased for PMMA system.Therefore

a combination of low initiator

high ionic strength and hight monomer concentration is expected to result in larger size particles and narrower size distribution than ordinary emulsion polymerization.) The effects of the ionic strength and the monomer concentration in the emulsifier-free emulsion polymerization on the particle size are slight but significant for PMMA system versus PSt system under the same conditions.This is explained by the different nucleation mechanism of emulsifier-free emulsion polymerization of MMA and St.