Micron-sized monodisperse crosslinked polystyrene (PS) beads have been prep
ared by a multistage emulsion polymerization using styrene monomer, divinyl
benzene crosslinking agent, and potassium persulfate initiator in the absen
ce of emulsifier. In the first stage of the reaction, the lower the reactio
n temperature, the larger the bead size obtained. In the later stages of th
e reaction, the particle size is increased with the initiator concentration
and monomer content. Particle nucleation of the preexisting polymer seed o
f 0.7-0.8 mu m in diameter is prepared at 60 degrees C, then the monodisper
se crosslinked PS beads > 2 mu m are synthesized up to the third stage of t
he reaction. As the particle size grows, the number of free radicals in the
growing particles increases, and the conversion of the next stage is conti
nuously increased. The reaction mechanism is suggested that the continuous
polymerization be conducted due to the diffusion of monomer into the preexi
sting particles to induce spherical beads in the later stages of the reacti
on. Otherwise, phase separation or the formation of protrusion by the captu
re of free radicals will be taking place. (C) 1999 John Wiley & Sons, Inc.