title: Pyromellitic dianhydride
text: # Pyromellitic dianhydride
  
  Pyromellitic dianhydride (PMDA) is an organic compound with the formula $C_6H_2(C_2O_3)_2.$ 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 (or related tetrasubstituted benzene derivatives). An idealized equation is:
  
      $C_6H_2(CH_3)_4$ + 6 $O_2$ → $C_6H_2(C_2O_3)_2$ + 6 $H_2O$
  
  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, $C_6H_2[(CO)_2NR]_2$ 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.
  
  