Emulsifier-free cationic poly (styrene-methylmethacrylate) (P(St-MMA)) copolymer nanoparticles were synthesized under microwave irradiation in the presence of cationic radical initiator 2

2′-azaobisisobutyramidine dihydrochloride(AIBA)．Several factors affecting the polymerization were discussed．The polymer conversion was followed gravimetrically at different time intervals during the reaction. The particles were characterized using TEM

LLs．The latex stability in electrolyte-resistance was determined by the critical coagulation concentration of electrolyte．The results indicated that in acetone-water medium

the average particle hydrodynamic radius of P(St- MMA) decreased from 122.21 nm to 24.68 nm with the addition of acetone／water volume proportion in the range of 0～1.26:1

and the stability of latex increased

but the polymerization rate was lowered and the particle distribution broadened．With increasing the concentration of methylmethacrylate or AIBA

the particle size dropped and the polydispersity index enlarged．