Miniemulsion of styrene and butyl acrylate with sodium dodecyl sulfate (SDS) as surfactant and hexadecane (HDE) as cosurfactant was prepared under ultrasonification,then the miniemulsion copolymerization was carried out with a redox initiator ((NH4)2S2O8/NaHSO3) at lower temperature.The effects of temperature and concentrations of surfactant,cosurfactant,and initiator on the polymerization rate and conversion were determined.Under the same conditions,the polymerization rate of miniemulsion was lower than that of conventional emulsion and the initiation period was much longer.Polymerization rate increased with the increase of recreation temperature and the concentration of initiator,and surfactant.In a certain range,the polymerization rate increased with increasing HDE concentration.However,beyond that range,it decreased with increasing HDE concentration.By using computer treatment of the experimental data,a mathematical model was established for miniemulsion kinetics,then studied the kinetics characters in detail.By comparing with the results of conventional emulsion,a"monomer droplet nucleation"mechanism of miniemulsion was proposed.