Chemical Composition

Organic ion exchangers are made by polymerization of organic monomers into large molecules which are made insoluble by crosslinking with a polyfunctional monomer. The nature and the level (concentration) of the crosslinking influences the elasticity, the dimensional stability or strength of the copolymer particle, and the available space surrounding the ionogenic or catalytic site within the solvent-swelled gel phase.

Many monomers have been transformed into insoluble ion exchangers by various polyfunctional monomers, but an unabridged listing of these will not be given here. Instead, this discussion will consider only those monomer systems most used.

The most prominent insoluble copolymer matrices for constructing organic ion exchangers are those derived by the free radical copolymerization of styrene with divinylbenzene. Other matrices that have been used and are presently used to a much lesser extent are those made by the condensation polymerization of phenol (including the other hy-droxylated aromatic derivatives of phenol such as catechol, resorcinol, hydroquinone, etc.) with formaldehyde and the copolymer matrices made by the free radical polymerization of the acrylate and meth-acrylate monomers with divinylbenzene.

The functional groups that perform the catalytic work are attached to the preformed crosslinked polymeric matrix. A vast array of chemistry allows the attachment of many different functional groups for anchoring the catalytic agent. With the aromatic polymers, electrophilic substitution reactions provide the means of functional group attachment, and with the aliphatic acrylic and methacrylic resins, the car-boxyl group provides the means of functional group attachment by nucleophilic substitution reactions. A plethora of chemistry is available to build a heterogeneous catalyst upon the polymeric matrices employed to make ion exchangers.

Much of the chemistry for designing effective catalysts built upon crosslinked polymers, however, has not been pursued to a fruitful outcome. Many of the special heterogeneous catalysts have been built upon crosslinked polymers with poor mass transport in the solvent systems necessary for effective chemical transformations and, therefore, have had inferior performance to the corresponding homogeneous catalysts. This inferior performance resulting from poor mass transport has partially quenched the commercial development of what could be excellent heterogeneous catalysts when built upon the properly designed structures of the crosslinked polymers. As a result, most of the commercial effort to use ion exchangers as catalysts has been in two areas - acid-and base-catalysed reactions. Solid acids and solid bases are the two major ion exchangers employed in water demineralization and purification and have been most explored as catalysts.

The strong acid ion exchangers are sulfonated polymers of styrene crosslinked with divinylbenzene.

The solid bases are copolymers of styrene and/or vinylpyridine crosslinked with divinylbenzene and functionalized to give either a quaternary ammonium hydroxide group or a tertiary amine group. Solid bases are also prepared from copolymers of the acryl-ate and methacrylate monomers by crosslinking with divinylbenzene followed by attachment of the amino group to the polymer via an amide linkage.

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Solar Panel Basics

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