| 摘要 |
To produce mixed polyimides having structural units of the general formulae (see fig. I) and (see fig. II) 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.
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