Controlled Molecular Weight Polyimides from Poly(amic acid) Salt Precursors

Controlled molecular weight, thermoplastic polyimides have been prepared via poly(amic acid) salt precursors. 2,2-Bis[4-(3,4-dicarboxyphenoxy)phenyl]propane dianhydride and m-phenylenediamine (the Ultem monomers), together with calculated amounts of phthalic anhydride were reacted in N-methylpyrrolidinone as well as in tetrahydrofuran to form poly(amic acid)s with controlled molecular weights. Poly(amic acid) salts were prepared in heterogeneous reactions of the poly(amic acid)s with quaternary ammonium bases or triethylamine dissolved in methanol or water, to yield soluble salts. The poly(amic acid) salts were then melt imidized in air at 250 or 300 °C for 30 min. Results suggest that the poly(amic acid) salt counterion controls the mechanism by which the salt imidizes, which in turn controls the properties of the final polyimide. The triethylammonium poly(amic acid) salts yielded linear, thermoplastic, molecular weight controlled polyimides upon melt imidization. The polyimides prepared from the poly(amic acid) salts containing the triethylammonium, tetraethylammonium, and tetrapropylammonium counterions showed dynamic weight loss profiles comparable to the polyimide produced directly from the control poly(amic acid).