Structure of Ion-exchange Resins
The most widely used type of resin is a copolymer of styrene and divinyl benzene produced in bead
form by suspension polymerization in an aqueous medium.
The proportion of divinyl benzene (DVB) is 2–20%, which results in a three-dimensional cross-linked
structure that is both rigid, porous and highly insoluble. The degree of cross-linking, expressed as the
weight percent of DVB, affects the rigidity of the structure and the size of the pores. A low degree of
cross-linking produces beads which swell appreciably when in contact with a polar solvent and have
large pores enabling ions to diffuse into the structure and exchange rapidly. Resins with a high degree
of cross-linking have smaller pores and are more rigid. Swelling is less, the exchange process is slower,
and large ions may not be able to diffuse into the interior of the bead at all. By control of the
polymerization process, the resin produced can be made gel-like (micro-reticular) having only pores of
sizes comparable to the dimensions of inorganic and small organic ions. Alternatively, a more open
structure can be produced (macro-reticular) in which the pores are tens of nanometres in diameter and
through which bulky organic ions can diffuse readily.
Cation or anion-exchanging properties are introduced into the resin by chemical modification after
polymerization. Cation-exchangers can be subdivided into strong-acid types containing —SO 3 H groups
and weak-acid types containing —COOH groups. The former are produced by reacting the resin with
chlorosulphonic acid which results in mainly para substitution of the benzene rings. The sulphonic acid
groups are dissociated over a wide pH range and these resins will exchange their protons for other
cations under both acid and alkaline conditions. The weak-acid type differs in that it is prepared by
direct polymerization of DVB and methacrylic acid, CH 3 C( CH 2 )COOH. Protons are exchanged for
other cations only above pH 5 as below this value the carboxylate groups are not dissociated.
Anion-exchangers comprise strong-base types incorporating quaternary ammonium groups (—N+R 3 )
and weak-base types incorporating primary, secondary or tertiary amines. They are prepared by
chloromethylating the resin followed by treatment with the appropriate amine.