The dispersion copolymerization of n-butyl methacrylate (BMA) in methanol-water media has been investigated. The reactions were carried out in the presence of a series of poly(ethylene oxide) (PEO) macromonomers (1) (C-1-EO(n)-(CH2)(m)-S), with a p-alkylstyrene (S) end group, with m = 1, 4, and 7 and n = 53 and 110. Nearly monodisperse PBMA latex particles from 90 to 500 nm in diameter are obtained. Factors which affect the particle size are the structure and molecular weight of the macromonomers, initial BMA, initiator, and macromonomer concentrations, temperature, and the composition of the solvent. The particle radius (R) follows the relationship R proportional to [BMA](0.82+/-0.06)(0)- [macromonomer](-0.54+/-0.03)(0)n(-0.30+/-0.02)[initiator](-0.10+/-0.01)(0). The power law exponents are close to those predicted from a coagulative nucleation model, particularly the multibin kinetics model for coalescence among unstabilized particles developed by Paine [Macromolecules 1990, 23, 3109]. R is also found to decrease with an increase in temperature and also with an increase in m (in 1). R, however, also increases-with an increasing volume fraction of water in the reaction medium. The mechanism for the dispersion copolymerization is discussed in detail.