In this study, the emulsifier-free emulsion polymerization of methyl methacrylate (MMA) was initiated directly by a Cu2+/HSO3- redox system. Latex particles with negative charge due to the bonded anionic sulfite ion were successfully synthesized after 2 h of reaction at 40-60 degrees C. Scanning electron microscopy pictures showed a uniform particle size distribution, and the average size decreased from 223 to 165 nm wit increasing reaction temperature from 40 to 60 degrees C. The initiation step in the polymerization mechanism was proven to be a redox reaction, in which Cu2+ oxidized the bisulfite ion to produce an anionic sulfite radical and proton. The produced anionic sulfite radical then initiated the polymerization of MMA. Moreover, Cu2+ not only served as one component in the redox initiator system but also as a chain-transfer agent that terminated growing polymer chains to produce chains with unsaturated end groups [poly(methyl methacrylate) (PMMA)-CH = CH2]. For this system, about 17% PMMA-CH = CH2 was produced. The tacticities of the PMMA latex prepared at 40-60 degrees C were almost the same, about 62-64% syndiotactic, 33-35% heterotactic, and 3% isotactic. These PMMA latexes had almost the same glass-transition temperature, 125-127 degrees C, regardless of the reaction temperatures, and their weight-average molecular weights were in the range between 254,000 and 315,000. (c) 2008 Wiley Periodicals, Inc.