Oxydianiline

Oxydianiline, or ODA, is two benzene rings joined by a single oxygen atom, with an amine group (a nitrogen carrying two hydrogens) at each far end. It's an off-white powder. Its full name, 4,4'-oxydianiline, records where those groups sit.

It's built from carbon in the two rings, hydrogen on the rings and amines, nitrogen in the two reactive ends, and one oxygen linking the rings. Industrially, two benzene rings carrying nitro groups are first joined through an oxygen bridge. The nitro groups are then reduced to amines with hydrogen over a catalyst. The product is purified carefully, because impurities would cut the polymer chains short.

In polyimide, each amine end attacks an anhydride ring on pyromellitic dianhydride, so ODA and PMDA alternate along the chain. The oxygen bridge acts as a small hinge. It lets the otherwise rigid chain flex a little, which is why Kapton film can be bent and folded instead of cracking like glass.