Cross-linked, monodisperse, micron-sized polystyrene particles by two-stage dispersion polymerization

It is well-known that cross-linking agents such as divinylbenzene (DVB) interfere with the dispersion polymerization of styrene in ethanol, a reaction carried out in the presence of polymeric stabilizers such as poly(vinylpyrrolidone). Small amounts of DVB broaden the particle size distribution and lead to irregular particles, whereas modest amounts lead to coagulation. We report that many of these problems can be avoided if one delays the addition of the cross-linking agent until after the end of the nucleation stage of the reaction (e.g., at less than 5% monomer conversion). In this way one can obtain monodisperse polystyrene particles containing 1 wt % ethylene glycol dimethacrylate (EGDNM) as a cross-linker. A second set of problems arises if the cross-linking agent is consumed too early in the reaction, presumably because early network formation limits the swellability of the growing particles. These problems can be overcome by feeding DVB into the reaction over 12 h or by adding EGDALA, in successive batches over 6 h. In this way, monodisperse particles with micrometer diameters, and a coefficient of variation of 2% or less, can be obtained containing up to 6 wt % cross-linker based on styrene.