The crosslink polymerization of styrene and methyl methacrylate in supercritical carbon dioxide

Governmental regulations restricting the release of chemicals to the environment are becoming increasingly more stringent. For this reason, there is not only a thrust toward new product development; but, there is also a drive to introduce more "environmentally friendly" processes for the manufacture of these new products. Conventional heterogeneous polymerizations (dispersion, emulsion, precipitation, and suspension) remain processes of interest due to the cost and environmental advantages of using water as the polymerization medium over a typical solvent as in homogeneous or solution polymerizations. More recently, studies have been conducted involving the use of supercritical fluids, supercritical carbon dioxide in particular, as solvents for polymerizations. The benign chemical composition of carbon dioxide in conjunction with its relatively low raw material cost heighten the interest in studying supercritical carbon dioxide as a potential processing fluid.; The present study investigates the feasibility of using supercritical carbon dioxide as a polymerization medium for producing crosslinked polymers. Precipitation polymerization experiments were conducted involving two monomers, styrene and methyl methacrylate, and two crosslink monomers, divinyl benzene and ethylene glycol dimethacrylate. A free radical initiator was employed. Variables of interest included: temperature, carbon dioxide density, monomer content, and crosslink monomer level. Product form and final polymer conversion characterized the effects of these variables.; Stable polymer powder and spheres were produced from these precipitation polymerizations involving only monomer, crosslinking monomer, and free radical initiator. This was very encouraging since not only was a benign solvent used as the polymerization medium, but there were also no impurities present in the final polymer. This represents the first successful precipitation polymerziation in carbon dioxide. A carbon dioxide soluble surfactant has been required to form stable polymers in other studies involving carbon dioxide as the polymerization medium.; The conclusion from this investigation is that supercritical carbon dioxide at select process conditions is comparable to conventional solvents, such as acetonitrile, in crosslink precipitation polymerizations. The temperature, carbon dioxide density, monomer to solvent ratio, and crosslink monomer level each influence the conversion and final product form.