The reversible iodine transfer polymerization (RITP) of styrene in the absence of light was carried out using potassium persulfate (KPS) under argon atmosphere at 80 ℃ for 7 h and the properties of the polymers were compared to those obtained from the soap-free emulsion polymerization (SFEP). The variables were the concentration of KPS and iodine, molar ratios between KPS and iodine, reaction temperatures, and monomer contents. The results showed that the conversion retarded and reached from 99.5% to 82%, the particle size dramatically increased from 0.335 to 1.37 μm, the molecular weight decreased from 183,000 to 66,000 g/mol, and the polydispersity index (PDI) increased from 2.7 to 3.8 for the presence of 2 wt% of KPS and iodine content between 0 and 0.228 mmol, respectively, implying that the molecular weight was controlled and that the particle stability was reduced with iodine as usual. For the effects of the solid content on particle properties, the monomer conversion and the particle size increased, but the molecular weight and PDI decreased with the increased KPS content and [KPS]/[I2] ratio. The highest content was 30 wt% and this was less than that in the presence of sodium dodecyl sulfate (SDS) in the RITPemulsion and in the absence of SDS in the SFEP. Thus, the use of an emulsifier in the RITP or SFEP was one of the important factors for obtaining the maximum solid content needed for industrial applications.