An ion-exchange resin is a polymer with groups that hold ions and can swap them for others. A cation-exchange resin is made by attaching sulfo groups, \mathrm{{-}SO_3H}, to a network polymer made from phenylethene (styrene) and a little diethenylbenzene (divinylbenzene).
When a solution containing sodium ions is passed through a column of the resin, the sodium ions are taken up by the sulfo groups, and an equal number of hydrogen ions are released in their place.
A solution of sodium chloride therefore comes out of the column as hydrochloric acid. The resin works only as long as it still has hydrogen ions to give.
An anion-exchange resin has groups such as \mathrm{{-}CH_2N^{+}(CH_3)_3OH^{-}}, which swap hydroxide ions for other anions.
When water is passed first through a cation-exchange resin and then through an anion-exchange resin, the cations are replaced by hydrogen ions and the anions by hydroxide ions. These combine to form water, so the water that comes out contains almost no ions.
This deionized water is used in laboratories and factories. Used resins can be regenerated by washing the cation-exchange resin with an acid and the anion-exchange resin with sodium hydroxide solution.
Sodium poly(propenoate) (sodium polyacrylate) is made into a lightly cross-linked network that can absorb a great deal of water.
When it meets water, the \mathrm{{-}COONa} groups ionize into \mathrm{{-}COO^{-}} and \mathrm{Na^{+}}. The negative charges on the chains repel each other and the network spreads out, while the many ions inside draw in more water.
Such a polymer can hold hundreds of times its own mass of water, and the water does not easily squeeze out. It is used in nappies and to keep soil moist.
Polyacetylene, made by polymerizing ethyne (acetylene), has a chain of alternating single and double bonds.
On its own it conducts electricity only slightly, but when a little iodine is added, the iodine takes electrons from the chain. The positive charges left behind can move along the chain of alternating bonds, so the polymer conducts electricity almost as well as a metal.
Hideki Shirakawa, Alan MacDiarmid and Alan Heeger shared the Nobel Prize in Chemistry in 2000 for this discovery. Conducting polymers are now used in capacitors and displays.
A photosensitive polymer changes when light falls on it. In one common kind, light makes the chains join to each other, so the parts that receive light no longer dissolve.
If light is shone through a mask with a pattern and the film is then washed, only the parts that received light remain, reproducing the pattern.
Such polymers are used for printing plates, for drawing the fine circuits of microchips and for dental fillings that are hardened by light.