WATCH IONS®

Watch Ion exchange resins are highly ionic, covalently cross-linked, insoluble polyelectrolytes supplied as beads. The beads have either a dense internal structure with no discrete pores (gel resins, also called microporous resins) or a porous multichannelled structure (macroporous or macroreticular resins). They are commonly prepared from styrene and various levels of the cross-linking agent divinyl benzene, wich controls the porosity of the particles. Porous beads can be made also by adding homopolystrene, wich is soluble in the monomer mixture, and leaching it out later with, toluene, for instance. The PS-DVB precursor beads are post-fractionalized to yield the finished resin. Acrylic based, ion exchange resins are also available.

These ionic polymers contain two types of ions, those wich are bound within the structure and the oppositely charged counter ions wich are free. The property of ion exchange is a consequence of Donnan exclusion - when the resin is immersed in a medium in wich it is insoluble, the counter ions are mobile and can be exchanged for other counter ions from the surrounding medium; ions of the same type of charge as the bound ions do not have free movement ion to and out of the polymer. Ion exchange resins have been classified based on the charge on the exchangeable counterion (cation exchanger or anion exchanger) and the ionic strenght of the bound ion (strong exchanger or weak exchanger). Thus, there are four primary types of ion exchange resins:

1. Strong cation exchange resins, containing sulfonic acid groups or the corresponding salts.

2. Weak cation exchange resins, containing carboxylic acid groups or the corresponding salts.

3. Strong anion exchange resins, containing quarternary ammonium groups. Of these, there are two types:

Type I resins contain trialkyl ammonium chloride or hydroxide and Type II resins contain dialkyl 2-hydroxyethyl ammoniumchloride or hydroxide.

4. Weak anion exchange resins, containing ammonium chloride or hydroxide.

Additional types of ion exchange resins include blends of cation and anion exchange resins, called mixed bed resins.

A resin wich contains both an anion and a cation as bound ions is said to be ampholytic. Some ion exchange resins are prepared witch chelating properties making them highly selective towards certain ions. In addition to their use in ion exchange, organic polymer supports, many of wich are based on PS-DVB resins, are being used as polymeric catalysts in the expanding research area involving heterogenization of homogenous catalysts and as polymeric supports and reagents in combinatorial chemistry.

The internal structure of the resin beads, i.e., whether microporous (gel-type) or macroporous, is important in the selection of an ion exhanger. Macroporous resins, with their high effective surface areas, facilitate the ion exchange process. Also they give access to the exchange sites for larger ions, can be used with almost any solvent, irrespective of wheter it is a good solvent for the uncrosslinked polymer, and take up the solvent with little or no charge in volume. They make more rigid beads, facilitating ease of removal from the reaction system. In the case of the microporous resins, since they have no discrete pores, solute ions diffuse through the particle to interact with exchange sites. Despite diffusional limitations on reaction rates, these resins offer certain advantages: they are less fragilie, requiring less care in handling, react faster in functionalization and applications reactions, and possess higher loading capapabilities.

In addition to being a function of bead morphology, the kinetics of the exchange depends on the particle size distribution of the resin. It is enhanced by a monodisperse resin, for example, see the Packed Bed resins; they permit faster elution and regeneration times with reduced back pressure.