Polyimide is a polymer of imide monomers. The structure of imide is as shown below.
There are two general types of polyimides. One type, so called linear polyimides, are made by combining imides into long chains. Aromatic heterocyclic polyimides are the other usual kind, where R′ and R″ are two carbon atoms of an aromatic ring. Examples of polyimides include Kapton, Ultem and Torlon. Polyimides have been in mass production since 1955.