Polyamides and polyimides from anhydrides

Polyimides, first reported in 1908 [103], have been prepared from dianhydrides and diamines or by self-condensation of an AB monomer such as 4-aminophthalic anhydride. 

Aliphatic diamines react with a diester-diacid derived from a dianhydride to form an ammonium salt which can be polymerized, usually in the melt, to a polyamic acid. Further heat converts the polyamic acid to the polyimide 

Aliphatic polyimides are in general fusible and soluble in many solvents, and they can often be made in one step. Aromatic polyimides, however, are usually made in two steps. 

The reaction between an aromatic diamine and an aromatic dianhydride such as pyromellitic dianhydride is fast and exothermic. It is usually carried out in a solvent for the polyamic acid, which is, of course, a polyamide. Both meta- and para-diamide units are formed 

In the case of pyromellitic dianhydride, the rate of polyamidation is limited only by the rate of solution of the dianhydride [104]. 

---