This invention relates to a process for enhancing the molecular weight of polyimide resins by extraction of certain materials present in those resins. 
Polyimides, as a class, are well known industrial resins, which have found increasing use in the manufacture of articles that must be able to withstand high temperatures without degradation, for example, in aerospace applications such as missile nose cones and in the manufacture of printed circuit boards for electronic equipment, especially for computers. 
Polyimides are normally made from a tetracarboxylic acid dianhydride and a diamine by a process that requires two steps, namely, the formation of a polyamic acid and the subsequent cyclization of the polyamic acid to the polyimide.  Cyclization can be obtained either by heating, in which case it is usually referred to as "thermal conversion" or by chemical means (which still is followed by some heating), in which case it is referred to as "chemical conversion". 
An improved process for the manufacture of polyimides is described in my U.S. Pat.  No.  4,358,581, which process first produces a prepolymer of polyamic acid , and this material is further mixed with a monomer polymerizable with the prepolymer in an amount which would yield high molecular weight polyamic acid and with a conversion system.  The resulting solution is then converted into high molecular weight polyimide.  The above U.S. Pat.  No.  4,358,581 provides a very good and detailed explanation of that process and also discusses in its introductory part a number of U.S. patents generally pertinent to the art of polyimide manufacture. 
U.S. Pat.  No.  4,358,581 is hereby incorporated by reference into the present disclosure, and the process claimed in that patent will be generally referred to throughout this disclosure and claims as the "polyimide bipolymerization process". 
It is generally realized that the presence in the polyamic acid precursor or in the final polyimide of various impurities, including, e.g., residual solvents, is undesirable because such impurities adversely affect the physical and mechanical properties of the resulting polyimides, including their molecular weight, tensile strength, and dielectric properties.  Accordingly, extraction of impurities from either polyamic acids or polyimides has been proposed in the past.  Representative patents dealing with this problem are U.S. Pat.  No.  3,961,009 (to Yoda et al.)  and U.S. Pat.  No.  4,470,944 (to Asakura et al.),  both assigned to Toray Industries, Inc.
While such heretofore known extractions have produced improved polyimides, the degree of improvement obtained by those earlier processes still is considered quite modest, and further improvement is very desirable.