Chain-extending nitrile end-capped polyimides

Aromatic polyimides with nitrile end groups are chain-extended (molecular weight increased) by polymerization using Lewis acid salts as catalyst. Molecular weights are increased with little or no by-product formation. These polyimides can be shaped and formed prior to the polymerization. The aromatic polyimides appropriate for chain-extending are formed by the reaction of aromatic dianhydrides, aromatic diamines and nitrile compounds such as aminobenzyl cyanide and aminobenzonitrile.