Dispersion copolymerization of vinyl monomers in supercritical carbon dioxide

The thermally initiated free-radical dispersion copolymerization of methyl methacrylate (MMA) and N,N-dimethylacrylamide (DMA) in supercritical carbon dioxide has been studied in the presence of polysiloxane surfactants. The formation of a stable latex seems to be related to the presence of at least one reactive end group in the surfactant macromolecule, thus suggesting that anchoring of the stabilizer to the surface of the growing polymer particles occurs through chemical grafting. In the case of a molar ratio of DMA to MMA lower than 0.40, microspherical copolymer particles with yields ranging from 81 to 97% were collected from batch polymerizations carried out at 65degreesC and 23-28 MPa using AIBN as an initiator and PDMS-mMA as a surfactant. The effect of conversion on the cumulative composition of the copolymer was investigated at a 0.20 mol/mol DMA/MMA ratio, and this composition, determined through NMR analyses, was found to be roughly constant during the entire process and, at the adopted initial feed composition, to remain close to the initial monomer feed mixture. The effectiveness of the surfactant stabilization toward the growing polymer particles was found to be dependent both on the concentration of the surfactant and on the amount of the hydrophilic monomer (DMA) with respect to the hydrophobic one (MMA). The experimental results indicate that, by a proper selection of the polymerization recipe, it is possible to prepare microspherical copolymer particles with tunable swelling behavior in aqueous media.