Dispersion polymerization of styrene with n-dodecyl mercaptans (DDM) as the chain transfer agent was investigated. PS particles with various molecular weight, molecular weight distribution (MWD), and particle diameter were prepared by varying the concentration of DDM and also the addition time of DDM before and after the particle nucleation. The average particle diameter was increased, whereas polymerization rate, molecular weight, and MWD were decreased with increasing DDM concentrations from 0 to 10 wt %. The effect of addition of DDM before and after particle nucleation was studied at 0.4, 0.8, and 1.0 wt % DDM. The addition of DDM before particle nucleation produced PS particles of relatively large particle diameter and low molecular weight when compared with the addition of DDM after particle nucleation. This study shows that particle nucleation occurs in about 5-6 min, which corresponds to the 15-16% conversion, 372-378 nm in (D) over bar (n), and provides a facile way to control the particle size and interesting information about the particle formation using the delayed addition of DDM. (C) 2008 Wiley Periodicals, Inc.