characteristically ipns swell but do not dissolve in solvent nor flow when heated ipns using polystyrene polystyrene as models for ion exchange resin matrices were first introduced in the 1960s see for example j r millar journal of chemical society pg 1311 1960 pg 1789 1962 and pg 218 1963 ipns have also been described in c h sperling u201c interpenetrating polymer networks and related materials u201d plenum press ny 1981 and in u201c sulfonic acid resins with interpenetrating polymer networks u201d d klempner and k c rrisch ed advances in interpenetrating polymer networks volume ii technomic publishing co inc pg 157 176 lancaster basel 1990 ion exchange resins including ipns have been commercially available for many years examples of such resins along with techniques for their preparation are provided in u s pat nos 4 419 245 4 564 644 u s pat no 4 582 859 5 231 115 5 834 524 6 251 996 6 924 317 us 2002 0042450 and us 2008 0237133 commercial examples of ipn gel type ion exchange resins are described in ion exchange at the millennium u2014 proceedings of iex 2000 ed j a greg imperial college press london 2000 the subject copolymer may be subsequently functionalized to form a chelating or ion exchange resin including cationic and anionic by reaction with a functionalizing agent representative examples of functionalizing agents useful for creating ion exchange resins of the.