Monodisperse, micrometer-sized low molar mass polystyrene particles by two-stage dispersion polymerization

Free-radical dispersion polymerization of styrene was carried out in ethanol and in ethanol-water mixtures in the absence and presence of carbon tetrabromide (CBr4) as a chain transfer agent. When CBr4 was present at the onset of the reaction, particles with a broad size distribution were obtained. If, however, the addition of CBr4 was delayed ca 1 h, so that the particle nucleation step was complete, then 1-2 wt% chain transfer agent, dissolved in monomer plus solvent, could be added to the reaction without a deleterious affect on particle formation. The particle size and size distribution was essentially identical to that obtained in the absence of CBr4. When more CBr4 was added, other problems arose. These problems appeared to be due to solubility of low molar mass polymer in the reaction medium. They could be overcome by running the reaction in ethanol-water mixtures (e.g. 5 wt% water) to decrease the solubility of oligo-styrene at 70 °C, the reaction temperature. In this way, monodisperse particles could be prepared in the presence of 3 wt% CBr4 based upon total styrene, consisting of polymers with Mn=7060, Mw/Mn=2.4.