Process for preparing polyimide solution

A process for preparing a polyimide solution, which comprises the step of polymerizing ingredients consisting essentially of:

• (A) a tetracarboxylic acid ingredient consisting of, based on the tetracarboxylic acid ingredient,
• (a) approximately 80 to 100% by mole of 3,3',4,4'-biphenyltetracarboxylic acid or its functional derivative and
• (b) 0 to 20% by mole of at least one compound selected from the group consisting of 2,3,3',4'-biphenyltetracarboxylic acid, 2,2',3,3'-biphenyltetracarboxylic acid, 3,3',4,4'-benzophenonetetracarboxylic acid, bis(3,4-dicarboxyphenyl) methane, 2,2-bis-(3,4-dicarboxyphenyl) propane, bis(3,4-dicarboxyphenyl) ether, bis(3,4-dicarboxyphenyl)thioether, bis(3,4-dicarboxyphenyl) sulfone, 2,3,6,7-naphthalenetetracarboxylic acid, pyromellitic acid and functional derivatives thereof, and
• (B) an aromatic diamine ingredient containing at least approximately 50% by mole, based on the aromatic diamine ingredient, of 4,4'-diaminodiphenyl ether, at a temperature of from approximately 100.degree. to 300.degree. C. in phenol or a halogenated phenolic compound;
• the amounts of the tetracarboxylic acid ingredient and the aromatic diamine ingredient being approximately equimolar to each other;
• whereby the tetracarboxylic acid ingredient and the aromatic diamine ingredient are polymerized, while being accompanied by imidation, to produce a homogeneous polyimide solution containing a polyimide having a logarithmic viscosity of from 0.7 to 6, as measured at 50.degree. C., on a solution of 0.5 g polymer in 100 ml of p-chlorophenol.