The semicontinuous polymerization of methyl methacrylate (MMA) in heterogeneous medium under monomer-starved conditions is reported here. The effect of monomer addition rate on kinetics, particle size, particle number, and PMMA average molar masses are reported. This process permits the synthesis of high-solid content latexes containing nano-sized particles (< 40 nm) with narrow particle size distributions [(D-w/D-n) < 1.1]. Moreover, the molar masses (M-n approximate to 0.3-1.2 x 10(6) g/mol) are much lower than those expected by chain transfer to monomer, which is the. typical termination mechanism in 0-1 emulsion and microemulsion reactions. Both particle size and average molar masses decrease as the rate of monomer addition is diminished. Possible explanations for this process are provided. (c) 2006 Wiley Periodicals, Inc.