Low-melting polyimide copolymer and method for preparing the same

A method for preparing a low-melting polyimide copolymer containing 30 to 90 mole % of the following imide repeating unit (I), containing 10 to 70 mole % of the following imide repeating unit (II), containing or not containing the following imide repeating unit (III), and having a logarithmic viscosity number of 0.1 to 5 dl/g when measured in concentrated sulfuric acid at a temperature of 30.degree..+-.0.01.degree. C. at a concentration of 0.5 g/dl, the aforesaid method being characterized by comprising the steps of feeding raw materials at one time or dividedly to a reactor;

• reacting diacid anhydrides of A.sup.1 mole of a compound represented by the following formula (IV) and A.sup.2 mole of a compound represented by the following formula (V) with diamines of B.sup.1 mole of a compound represented by the following formula (VI), B.sup.2 mole of a compound represented by the following formula (VII) and B.sup.3 mole of a compound represented by the following formula (VIII) in compliance with relations of formulae (IX), (X), (XI) and (XII) regarding A.sup.1, A.sup.2, B.sup.1, B.sup.2 and B.sup.3 in the presence of a solvent at a temperature of 0.degree. to 200.degree. C. under conditions where the acid anhydride represented by th formula (IV) which accounts for at least 30 mole % of the whole diacid anhydride is reacted with the diamine represented by the formula (VI) which accounts for at least 30 mole % of the whole diamine and under conditions where the diacid anhydride represented by the formula (IV) which accounts for at least 10 mole % of the whole diacid anhydride is reacted with the diamine represented by the formula (VII) which accounts for at least 10 mole % of the whole diamine, in order to form a polyimide precursor;
• and heating the precursor at 100.degree. to 400.degree. C. or adding a known imide formation accelerator thereto and converting the precursor into the imide at a relatively low temperature of 10.degree. to 100.degree. C.:
• ##STR27## wherein R.sup.1 is at least one of ##STR28## R.sup.2 is at least one of ##STR29## R.sup.3 is a carbon cyclic aromatic group having 6 to 50 carbon atoms except ##STR30## R.sup.4 is at least one of ##STR31## and R.sup.5 is a carbon cyclic aromatic group having 6 to 50 carbon atoms, but when R.sup.4 is ##STR32## R.sup.5 is neither ##STR33## and when R.sup.5 is ##STR34## R.sup.4 is not ##STR35##