To clarify the growth mechanism of polystyrene (PSL) particles in the soap-free polymerization, characteristics of not only particles but also polymeric materials floating in the bulk were investigated oil the molecular scale by using atomic force microscope (AFM), where a cationic initiator V-50 is used to make the formed polymeric materials transfer oil the mica plate in sampling. Our main attention here is to know the reason why the particle size increases with increasing initiator concentration in the production of PSL particles. The following are found. (1) As far as the initiators and monomers remain in the bulk solution, the polymeric materials are born in the bulk continuously, because of the slow decomposition rate of initiators. (2) The growth of particles at the early stage of (t)r >= 0.75 h is considered to be attributable mainly to the particle swelling by absorbing monomers from the bulk. The rapid growth at the intermediate stage is due to the deposition of polymeric materials in the bulk oil the particle Surface and their simultaneous swelling by monomers in the bulk. (3) The reason why the particle size increases with increasing concentration of initiator is that the growth process is controlled by the deposition rate of polymeric materials in the bulk whose amount increases with the initiator concentration. (4) The particle size and the smoothness of particle surface depend oil the relative concentration of initiator, and monomers remained. (c) 2005 Elsevier Inc. All rights reserved.