The initial heating velocity was surprisingly found to play a key role in the size distribution of the resulting polymer particles in the dispersion polymerization of methyl methacrylate. Monodisperse poly(methyl methacrylate) particles had to be prepared by the mild and gradual increase of the reaction temperature. A universal conclusion was drawn from a series of experiments: the more slowly the initial temperature rose, the more the dispersity of the poly(methy) methacrylate) particles improved with a monomer concentration of 5-20 wt %, a 2,2-azobisisobutyronitrile concentration of 0.5-3 wt %, and a polyvinylpyrrolidone K-30 ;concentration of 20-60 wt %. The optimal heating velocity was 10-15 degrees C/h. A heating ramp faster than 15 degrees C/h led to polydisperse polymer particles or a poor dispersity. These interesting phenomena might be explained as a delicate balance between the formation rate of radicals and the capture of oligomer chains in the continuous phase when a slow heating strategy was used. For comparison, the dispersion polymerization of styrene and glycidyl methacrylate in an alcoholic medium was also investigated. Slow heating was slightly disadvantageous for the dispersity of polystyrene and poly(glycidyl methacrylate). Presetting at the polymerization temperature was practical for the production Of monodisperse polystyrene and poly(glycidyl methacrylate). Furthermore, either a low reaction temperature or a long heating time caused the coagulation of poly(glycidyl methacrylate) or polydisperse particles. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 112: 917-925, 2009