Process for preparing polyimide resins

In the process for preparing polyimide resins comprising:

• (A) reacting one mole of a first diamine with about 2-10 moles of an acid dianhydride in an organic polar solvent at a temperature of from 140.degree.-220.degree. C. in the presence of a binary catalyst to form imide oligomers containing more than 80% by weight of an imide monomeric unit;
• wherein the acid dianhydride is selected from the group consisting of biphenyltetracarboxylic dianhydride;
• benzophenonetetracarboxylic dianhydride;
• 4,4'- 2,2,2-trifluoro-1-(trifluoromethyl)ethylidene!bis(1,2-benzenedicarbo xylic dianhydride);
• bis(dicarboxylphenyl)ether dianhydride and bicyclo(2,2,2)oct-7-ene-2,3,5,6-tetracarboxylic dianhydride, and
• the organic polar solvent is selected from the group consisting of N-methyl-2-pyrrolidone, N,N-dimethyl formamide N,N-dimethylacetamide, N-acetyl morpholine and tetramethyl urea;
• (B) reacting the imide oligomers with a second diamine so that the total amount of the first diamine and the second diamine is almost equivalent in moles to the acid dianhydride in the reaction solution at a temperature of 140.degree.-220.degree. C. to form a polyimide block polymer solution;
• wherein the first diamine is different from the second diamine, and the first and second diamines are selected from the group consisting of 1,4-benzenediamine;
• 6-methyl-1,3-benzenediamine;
• 4,4'-diamino-3,3'-dimethyl-1,1'-biphenyl;
• 4,4'-diamino-3,3'-dimethoxy-1,1-biphenyl;
• 4,4'-methylene bis(benzeneamine);
• 4,4'-oxybis(benzeneamine);
• 3,4'-oxybis(benzeneamine);
• 4,4'-thiobis(benzeneamine);
• 4,4'-sulfonyl(benzeneamine);
• 3,3'-sulfonyl(benzeneamine);
• 1-trifluoromethyl-2,2,2-trifluoroethylidene-4,4'-bis(benzeneamine);
• 2,2-bis 4-(4-aminophenoxy)phenyl!propane;
• bis 4-(3-aminophenoxy)phenyl!sulfone;
• 1,3-bis 1-(4-aminophenyl)-1-methylethylidene!benzene;
• 1,4-bis(4-aminophenoxy)benzene;
• 1,3-bis(4-aminophenoxy)benzene;
• 9,9-bis(4-aminophenyl)fluorene and diaminosiloxane;
• (C) preparing directly polyimide resins from the polyimide block polymer solution by heat evaporation, thereby removing both the solvent and the binary catalyst;
• the improvement wherein the binary catalyst comprises a first component which is a compound selected from the group consisting of .gamma.-valerolactone and crotonic acid and a second component which is a compound selected from the group consisting of pyridine and N-methyl morpholine, said first component being about 0.01-0.2 mole per mole of the acid dianhydride and said second component being about 1-2 moles per mole of the first component.