Pyromellitic dianhydride


Pyromellitic dianhydride is an organic compound with the formula C6H22. It is the double carboxylic acid anhydride that is used in the preparation of polyimide polymers such as Kapton. It is a white, hygroscopic solid. It forms a hydrate.

Preparation

It is prepared by gas-phase oxidation of 1,2,4,5-tetramethylbenzene. An idealized equation is:
In the laboratory, it can be prepared by dehydration of pyromellitic acid using acetic anhydride.

Reactions

PMDA is an electron-acceptor, forming a variety of charge-transfer complexes. It reacts with amines to diimides, C6H22 which also have acceptor properties.

Applications

PMDA is used in PET bottle recycling as a chain extender. It increases the molecular weight of the polymer by linking-together alcohol and carboxylic acid groups formed by hydrolysis of the PET. This improves the rheological properties and overall quality of the recycled plastic.

Safety

Evidence suggests that PMDA causes occupational asthma.