The concentration of the cosurfactant 1-butanol (BuOH) determined the polymer weight and size for a series of poly(styrene-co-methyl methacrylate)s (P(St-co-MMA)) synthesized by the free-radical (o/w) microemulsion technique. A factorial design established the levels of the experimental conditions for the polymerization i.e., concentration of the surfactant, sodium dodecyl sulfate (SDS); concentration of the cosurfactant, BuOH; temperature and ratio of the styrene (St) to methyl methacrylate (MMA). An increase in the weight-average molecular weight (M-w) and number-average molecular weight (M-n) was observed in the P(St-co-MMA) series with an increase in BuOH concentration from 1 to 5 wt %. These effects could arise from the micellar aggregation induced by interfacial BuOH. The unique micellar conditions could be exploited to synthesize copolymers of varying molecular weight and size. Additionally, the composition of the copolymers was virtually templates of the feed composition. (C) 2007 Wiley Periodicals, Inc.