Chain-extending polyimides with aromatic polyisocyanates

Aromatic polyimides with anhydride end groups are chain-extended (molecular weight increased) by reacting them with aromatic polyisocyanates such as tolylene-2,4-diisocyanate. These polyimides can be shaped and formed prior to chain-extending. Thermally stable, polyimide foams are formed by the reaction. The aromatic polyimides appropriate for chain-extending are formed by the reaction of aromatic diamines with a molar excess of aromatic dianhydrides.