CONTROLLED MOLECULAR-WEIGHT POLYIMIDES FROM POLY(AMIC ACID) SALT PRECURSORS

Controlled molecular weight, thermoplastic polyimides have been prepar ed via poly(amic acid) salt precursors. 2,2-Bis[4-(3,4-dicarboxyphenox y)phenyl]propane dianhydride and m-phenylenediamine (the Ultem monomer s), together with calculated amounts of phthalic anhydride were reacte d in N-methylpyrrolidinone as well as in tetrahydrofuran to form poly( amic acid)s with controlled molecular weights. Poly(amic acid) salts w ere prepared in heterogeneous reactions of the poly(amic acid)s with q uaternary ammonium bases or triethylamine dissolved in methanol or wat er, to yield soluble salts. The poly(amic acid) salts were then melt i midized in air at 250 or 300 degrees C for 30 min. Results suggest tha t 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 imidiz ation. The polyimides prepared from the poly(amic acid) salts containi ng the triethylammonium, tetraethylammonium, and tetrapropylammonium c ounterions showed dynamic weight loss profiles comparable to the polyi mide produced directly from the control poly(amic acid).