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Abstract

A polyamic acid having a three-dimensional network molecular structure produced by a gel forming ring-opening polyaddition reaction in an organic solvent of the reaction components comprising: (A) an acid component consisting of at least one tetracarboxylic acid dianhydride selected from the group consisting of a tetracarboxy benzene dianhydride, a tetracarboxy dianhydride of a compound having 2 to 5 condensed benzene rings, and compounds represented by formula (III), and substituted compounds thereof: ##STR1## wherein R.sub.1 represents --O--, --CO--, --SO.sub.2 --, --SO--, an alkylene group, an alkylene bicarbonyloxy group, an alkylene bioxycarbonyl group, a phenylene group, a phenylene alkylene group, or a phenylene dialkylene group, n.sub.4 is 0 or 1, n.sub.5 is 0 or 1; and n.sub.6 is 1 or 2, provided that the sum of n.sub.5 and n.sub.6 is 2, and (2) 0 to 10 wt % of at least one acid component selected from the group consisting of a di- or tricarboxylic acid, a carboxylic acid anhydride and a substituted compound thereof; (B) at least one aromatic diamine; and (C) at least one polyamino compound selected from the group consisting of compounds represented by formula (I) or (II): ##STR2## wherein A.sub.1 represents ##STR3## A.sub.2 represents ##STR4## n.sub.1 is 0 or an integer from 1 to 4; n.sub.2 is 0 or an integer from 1 to 3; X represents an acid; q represents the base number of the acid; R represents --O--, --CH.sub.2 --, --CO-- or --SO.sub.2 --; and n.sub.3 is 0 or 1; a polyimide derived from the polyamic acid; and a process for preparing a film of the polyamic acid or polyimide. The polyimide film has superior heat resistance and mechanical characteristics.

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First Claim