Molded Rigid Monolithic Porous Polymers:  An Inexpensive, Efficient, and Versatile Alternative to Beads for the Design of Materials for Numerous Applications

A simple molding process carried out within the confines of a closed mold has been used for the preparation of porous polymer monoliths. The polymerization is carried out using a mixture of monomers, porogenic solvent, and free-radical initiator under conditions that afford macroporous materials with through-pores or channels large enough to provide the high flow characteristics required for their applications. The versatility of the preparation technique is demonstrated by its use with both hydrophobic and hydrophilic monomers such as styrene, (chloromethyl)styrene, glycidyl methacrylate, alkyl methacrylates, 2-(acrylamido)-2-methyl-1-propanesulfonic acid, acrylic acid, and acrylamide. Techniques have been developed that allow fine control of the porous properties of the polymers. These, in turn, determine the hydrodynamic properties of the devices that contain the molded media. Since all of the mobile phase must flow through the monolith, the mass transport within the molded material is accelerated considerably by convection, and the monolithic devices perform well even at much higher flow rates than those used with packed columns. This is particularly important for the chromatographic separations of large molecules such as proteins, for which diffusion is a serious problem that significantly slows down the separation processes. Similarly, this convection provides faster transport of substrates to active sites with immobilized enzymes, and it increases significantly the apparent activity of the conjugate. The molded separation media have been used for the chromatographic separation of biological compounds using modes such as reversed-phase, hydrophobic interaction, and ion-exchange chromatography, synthetic polymers and oligomers in reversed-phase and precipitation−redissolution mode, for electrochromatography, enzyme immobilization, and molecular recognition, and in advanced detection systems. Grafting of pores with selected polymer chains has led to "smart" materials that quickly respond to external stimuli and change the permeability and/or polarity of the monolith.