Pyromellitic dianhydride

Pyromellitic dianhydride, usually shortened to PMDA, is a small, flat molecule with a benzene ring at its center and a reactive group on each side. It's a white crystalline powder that reacts readily with water and with amines, which is exactly what makes it useful.

It's built from ten carbon atoms (the six-carbon ring plus four more), two hydrogen atoms left on the ring and six oxygen atoms. The oxygens sit in two five-sided anhydride rings, one on each side. Industrially it's made from durene, a benzene ring carrying four methyl groups. Durene vapor and air pass over a hot vanadium oxide catalyst, which oxidizes the four methyl groups into acid groups. Each neighboring pair then loses a water molecule and closes into an anhydride ring. It's purified by sublimation.

In polyimide, each anhydride ring opens and grabs the nitrogen end of an oxydianiline molecule. With a reactive group on each side, PMDA links to two neighbors and builds a long chain. Its rigid, flat shape is a big part of why polyimide is so stiff and heat-resistant. It must be kept dry, since moisture opens its rings before they can react.