DISPERSION POLYMERIZATION IN SUPERCRITICAL CO2 WITH SILOXANE-BASED MACROMONOMER - 2 - THE PARTICLE FORMATION REGIME

Dispersion polymerization of methyl methacrylate in supercritical CO2 is studied in situ by turbidimetry at 65 degrees C from 2000 to 5000 p sia for various concentrations of a poly( dimethylsiloxane) monomethac rylate (PDMS-mMA) macromonomer stabilizer. The average particle size, particle number density, and overall surface area are reported vs time during particle formation. Coagulative nucleation and controlled coag ulation regions have been identified. They are governed by the amount of stabilizer available relative to the total surface area of the disp ersion. Near the end of the controlled coagulation region, which can l ast tens of minutes, the particle number density approaches the final value. The time in this region is longer than predicted by the model p roposed by Paine (Macromolecules 1990, 23, 3190) due to incomplete inc orporation of stabilizer, solubility limitations of polymerized stabil izer in the continuous phase, and plasticization of the particles by C O2, which increase particle coagulation. Threshold values of pressure and stabilizer concentration are required to achieve a solvent quality and surface coverage sufficient to prevent uncontrolled coagulation d uring particle formation.