A previously proposed method of soap-free emulsion polymerization employing an amphoteric initiator, 2,2'-azobis[N-(2-carboxyethyl)-2-methylpropionamidine] (VA-057), was extended to synthesize micron-sized, monodisperse polystyrene (PSt) particles at low impurity levels. Initial pH values of solutions in which anionic PSt particles were formed were adjusted between neutral pH and 8.9 with a tiny amount of strong base such as NaOH to constitute a low-impurity polymerization system. Micron-sized polymer particles with a narrow size distribution were obtained in a pH range of 8.1-8.9. Different strong bases of KOH, Ca(OH)(2) and tetramethylammonium hydroxide were also added to examine electrostatic interaction between PSt particles formed in the low-impurity system, indicating that surface charge density of PSt particles is an important factor to determine size distributions of PSt particles formed. Polymerizations at high styrene concentrations and an appropriate VA-057 concentration succeeded in preparation of highly pure, monodisperse PSt particles with an average size of 1.86 mu m. (C) 2013 Elsevier Ltd. All rights reserved.