Synthesis of well-defined macroporous polymer monoliths by sol-gel polymerization in supercritical CO2

The synthesis of continuous macroporous polymers (i.e., "polymer monoliths") is currently a subject of great interest for a variety of applications. These materials may have certain advantages over more traditional macroporous polymer beads, mainly because of the absence of interstitial voids in the "packed" state. Typically, a mold is filled with a polymerization mixture containing a cross-linking monomer, functional comonomers, initiator, and a porogenic diluent. This mixture is then polymerized to form a continuous porous monolith that conforms to the shape of the mold. One drawback of the method is that large volumes of organic solvents are required (typically similar to1:1 solvent to monomer), and these solvents can be hard to remove from the polymer matrix at the end of the reaction. Also, the pore structure of the polymer can be remarkably sensitive to very small changes in the composition of the porogenic solvent mixture. Recently, we have developed methods for the synthesis of highly cross-linked polymer monoliths using supercritical carbon dioxide as the porogenic solvent (Cooper, A. I.; Holmes, A. B. Adv. Mater. 1999, 11, 1270). In this paper, we describe how it is possible to achieve fine control over average pore sizes and pore size distributions, both by variations in the density of the supercritical solvent and also via reverse micellar imprinting.