An ionomer (/ˌaɪˈɑːnəmər/) (iono- + -mer) is a polymer composed of repeat units of both electrically neutral repeating units and ionized units covalently bonded to the polymer backbone as pendant group moieties. Usually no more than 15 mole percent are ionized. The ionized units are often carboxylic acid groups.

The classification of a polymer as an ionomer depends on the level of substitution of ionic groups as well as how the ionic groups are incorporated into the polymer structure. For example, polyelectrolytes also have ionic groups covalently bonded to the polymer backbone, but have a much higher ionic group molar substitution level (usually greater than 80%); ionenes are polymers where ionic groups are part of the actual polymer backbone. These two classes of ionic-group-containing polymers have vastly different morphological and physical properties and are therefore not considered ionomers.

Ionomers have unique physical properties including electrical conductivity and viscosity—increase in ionomer solution viscosity with increasing temperatures (see conducting polymer). Ionomers also have unique morphological properties as the non-polar polymer backbone is energetically incompatible with the polar ionic groups. As a result, the ionic groups in most ionomers will undergo microphase separation to form ionic-rich domains.

Commercial applications for ionomers include golf ball covers, semipermeable membranes, sealing tape and thermoplastic elastomers. Common examples of ionomers include polystyrene sulfonate, Nafion and Hycar.

Ionomer: A polymer composed of ionomer molecules.

Ionomer molecule: A macromolecule in which a small but
significant proportion of the constitutional units have ionizable
or ionic groups, or both.

Note: Some protein molecules may be classified as ionomer
molecules.

Usually ionomer synthesis consists of two steps – the introduction of acid groups to the polymer backbone and the neutralization of some of the acid groups by a metal cation. In very rare cases, the groups introduced are already neutralized by a metal cation. The first step (introduction of acid groups) can be done in two ways; a neutral non-ionic monomer can be copolymerized with a monomer that contains pendant acid groups or acid groups can be added to a non-ionic polymer through post-reaction modifications. For example, ethylene-methacrylic acid and sulfonated perfluorocarbon (Nafion) are synthesized through copolymerization while polystyrene sulfonate is synthesized through post-reaction modifications.