Abstract: In preparing polyimides by heating a solution of intramolecular tetracarboxylic acid dianhydrides and organic diisocyanates in an aprotic organic solvent in the presence of a polymerization accelerator, a catalytic amount of an alkali metal hydroxide is used as the polymerization accelerator. By this process, polyimides having high molecular weights are produced in a simple manner. The polyimides produced exhibit a high-temperature resistance.

Abstract: To produce mixed polyimides having structural units of the general formulae ##STR1## wherein n is an integer larger than 1 and R represents a divalent aromatic residue, a solution of an intramolecular tetracarboxylic acid dianhydride mixture including 50 to 99% by mol benzophenone-3,3',4,4'-tetracraboxylic acid dianhydride and 50 to 1% by mol pyromellitic acid dianhydride is heated in a polar aprotic solvent and is admixed with an altogether equimolar amount of 2,4- and/or 2,6-toluenediisocyanate and/or methylenebis(phenylisocyanate). The mixed polyimides according to the invention are readily soluble in organic solvents, are flame-retardant and exhibit a good thermostability.