Polystyrene-based monoliths, preparation

Three main types of polymer-based monoliths are polymethacrylate-based monoliths where methacrylate forms the major component of the monomers for polymerization, polyacrylamide-based monoliths where cross-linked polyacrylamide is synthesized directly within the capillary, and polystyrene-based monoliths that are usually prepared from styrene and 4-(chloromethyl) styrene as monomers and divinylbenzene (DVB) as the cross-linker. 

Hydrophilic polystyrene-based continuous beds bearing a hydrophilic surface on a hydrophobic polymeric support were prepared by a two-step modification of polychloromethylstyrene monolith first with ethylenediamine followed either by a reaction with y-gluconolactone or with chloroacetic acid [57,122]. Activation could also be performed by grafting 4-vinyl-2,2-dimethylazactone onto the monolith porous surface [123]. 

Instead of packed columns, monolithic (continuous bed), analytical, or capillary columns in the form of a rod with flow-through pores offer high porosity and improved permeability. Silica-based monolithic columns are generally prepared by gelation of a silica sol to a continuous sol-gel network, onto which a Cjg or another stationary phase is subsequently chemically bonded. Such columns provide comparable efficiency and sample capacity as conventional columns packed with 5-pm particle materials, but have three to five times lower flow resistance, thereby allowing higher flow rates and fast HPLC analyses. Rigid polyacrylamide, polyacrylate, polymethacrylate, or polystyrene monolithic columns are prepared by in sim polymerization. 

The monoliths are prepared both from organic (mainly based not only on polystyrene, acrylamides, acrylates, or methacrylates but also on imprinted polymers) and inorganic materials (based on sUica, zirconia, titania, or aluminium oxide) as continuous... 

The second concept for the generation of monolithic polymers is based on diblock copolymers which were prepared by Hillmyer and coworkers [27]. These copolymers contain oriented nanoscopic cylinders of the degradable polymer polylactide (PLA) which were embedded in polystyrene. The latter served as an inert thermoplastic matrix, while PLA could be selectively removed under well-defined conditions using sodium hydroxide in aqueous methanol. The resulting mesoporous monolithic polystyrene contains nanochannels with defined pore size. The major drawback of this material free of any cross-linker is associated with reduced mechanical and chemical stability. 

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