Aromatic polyamides. V. A general synthesis of polyamides from aromatic diacids and aromatic diamines in sulfur trioxide

The reaction of terephthalic acid (TA) and para-phenylenediamine sulfate (PPD-S) in sulfur trioxide to form anisotropic, sulfonated poly(p-phenyleneterephthalamide) (SPT) dopes was reported in Part IV of this series. We have found now that the TA/PPD-S polymerization is only one example of a more general polyamide condensation reaction of aromatic diamines and aromatic diacids. Sulfonation of the aromatic diamine ring during TA/PPD-S polymerization in SO₃ was a major side reaction. Sulfonation was reduced or eliminated by aromatic diamine ring substitution with unreactive substituents, particularly chlorine and fluorine. Polymerization of 2,3,5,6-tetrafluoro-phenylenediamine with TA in SO₃ at 80°C (18% concentration) produced unsulfonated poly(tetrafluoro-para-phenyleneterephthalamide) (F-PPT) with an inherent viscosity of 2.2. The halogenated, all-para aromatic polymers formed highly anisotropic (liquid crystalline) dopes. Monomers that formed polymers in which the chain bond angle deviated from 180° (e.g., meta-oriented monomers) yielded only isotropic polymer solutions. The mechanism and rate of diamine–diacid reactivity in SO₃ was related to diamine basicity. Whereas the less basic aromatic diamines (as sulfates) polymerized with aromatic diacids in SO₃, the more basic aliphatic diamines (as sulfates) would not. Aliphatic, cycloaliphatic, and aryl-aliphatic diacids were degraded by or reacted with the solvent (SO₃). Thermogravimetric analyses of F-PPT and monosulfonated poly(chloro-para-phenyleneterephthalamide) at 20°C/min showed weight loss only above 380 and 370°C, respectively.