Abstract: 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.

Abstract: Aromatic polyimides with amine end groups are chain-extended (molecular weight increased) by reacting them with aromatic di- or tri- anhydrides. The reaction can either be at a temperature above the melting point of the reactants or in solvents for the reactants. The polyimides can be shaped and formed prior to chain-extending.

Abstract: Aromatic polyimides with Schiff base end groups are chain-extended (molecular weight increase) by heating the polyimides, preferably in the presence of Lewis acid salts as catalyst. Molecular weights are increased with little or no by-product formation. The 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 a Schiff base monoamine such as p-amino-benzylidene-aniline, p-amino-benzylidene-4-vinylaniline and p-aminobenzylidine-4-ethynylaniline.

Abstract: Aromatic polyimides with phenolic end groups are chain-extended (molecular weight increased) by reacting them with formaldehyde, compounds capable of generating formaldehyde under the reaction conditions or hexamethylene tetramine in the presence of a catalytic amount of lime. These polyimides can be shaped and formed prior to the chain-extending.The aromatic polyimides appropriate for chain-extending are formed by the reaction of aromatic dianhydrides, aromatic diamines and phenolic compounds such as aminophenol.

Abstract: 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.

Abstract: A process for preparing substantially completely cyclized aromatic polyimides, either monomeric, oligomeric or polymeric, is provided. The process involved the reaction of a mixture of at least one tetracarboxylic acid dianhydride and at least one aromatic diamine in at least one aromatic phenol, preferably cresols, in the presence of an organic azeotroping agent such as a cyclic hydrocarbon, preferably benzene, until substantially all of the water of reaction is eliminated. Other monomers, e.g., a monoamine or an .alpha.,.beta.-carboxylic acid monoanhydride can be added to the mixture so as to provide reactive end groups, preferably olefinically terminated, which will enable the polyimide to cure by coupling together by addition or condensation thereby increasing molecular weight with little or no by-product. After the water is removed, it is preferred that the azeotroping agent be removed and then the polyimide increased in molecular weight by heating in the aromatic phenol.