Dispersion polymerization in supercritical CO2 with a siloxane-based macromonomer:: 1.: The particle growth regime

Particle growth rates were analyzed for the dispersion polymerization of methyl methacrylate (MMA) in supercritical carbon dioxide at 65 degrees C stabilized with a poly(dimethyl siloxane)-methyl methacrylate (PDMS-mMA) macromonomer. Although pure CO2 is a mediocre solvent for PDMS even at 4000 psia, the monomer behaves as a cosolvent to prevent flocculation. As pressure is decreased, the dispersion flocculates sooner, as expected due to the reduced solvent quality of CO2. Final particle size is only mildly dependent on pressure as a result of the solvation from the high monomer concentration during the particle formation stage, however particle coagulation increases with decreasing pressure. There exists both a minimum pressure (similar to 3000 psia) and stabilizer concentration (similar to 2 wt % stabilizer/ monomer) below which particles are highly coagulated due to insufficient steric stabilization. Here polymerization rates are reduced due to diffusional restrictions. This threshold pressure and stabilizer concentration are required to change the mechanism from precipitation polymerization to dispersion polymerization, as indicated by product morphology, molecular weight, and molecular weight polydispersity. Final particle size and number density determined from the model of Paine {Macromolecules 1990, 23, 3109} agree with the measured values.