In general, an aromatic polyimide having a high molecular weight is highly heat-resistant. Accordingly, the aromatic polyimide is employed for various applications requiring heat-resistance.
On the other hand, the aromatic polyimide is usually low in solubility in an organic solvent. Further, the polyimide has an extremely high softening point or has no softening point. Therefore, the polyimide is not appropriate for molding, and it is difficult to use in combination with another resin for various applications.
For the reasons mentioned above, a solution of an aromatic polyamic acid in an organic solvent has been proposed and used in place of the aromatic polyimide for various applications. The aromatic polyamic acid is a precursor of the aromatic polyimide having a high molecular weight. However, the aromatic polyamic acid requires an imide ring closure to be converted into an aromatic polyimide. The ring closure reaction forms water, which should be removed. The process for removing water causes some troubles.
On the other hand, a terminal-modified imide oligomer which has a relatively small molecular weight and an unsaturated group as a terminal group or an unsaturated imide compound is excellently soluble in an organic solvent as well as thermosetting. Further, the imide oligomer or the imide compound has a relatively low softening point. Therefore, the imide oligomer or compound has been proposed to be used as an adhesive, while it is difficult to use the aromatic polyimide having a high molecular weight as an adhesive.
The above-mentioned known terminal-modified imide oligomer and the unsaturated imide compound are hard and fragile. Therefore, the mechanical property of the heat-cured product made from the oligomer or compound is inferior.
Recently, a thermosetting resin composition comprises a soluble aromatic polyimide made from benzophenone tetracarboxylic acid or its derivative and an imide oligomer made from bismaleimide or its derivative has been proposed in Japanese Patent Provisional Publications No. 62(1987) 30122 and No. 62(1987) 179558. The heat-cured product a improved in the mechanical property is accessible from therein composition.
But, the resin composition proposed in the Publications should be used together with phenol or its derivative as an organic solvent. The resin composition is low in solubility in a usual organic polar solvent. Further, the heat-resistance of the heat-cured product made from the composition is low.