Process for preparing aromatic polyimides and some polyimides prepared thereby

A process for preparing substantially completely cyclized aromatic polyimides, either monomeric, oligomeric or polymeric, is provided. The process involved the reaction of a mixture of at least one tetracarboxylic acid dianhydride, at least one aromatic diamine and an .alpha.,.beta.-carboxylic acid monoanhydride in at least one aromatic phenol, preferably cresols, in the presence of an organic azeotroping agent such as a cyclic hydrocarbon, preferably benzene, until substantially all of the water of reaction is eliminated. The monoanhydride provides reactive end groups which will enable the polyimide to cure by coupling together by addition or condensation thereby increasing molecular weight with little or no by-product. After the water is removed, it is preferred that the azeotroping agent be removed and then the polyimide increased in molecular weight by heating in the aromatic phenol. The polymer can then be isolated by concentration or precipitation as a powder, or the polymer in solution can be used to prepare coatings on various substrates.