COMPLEXATION-MEDIATED SOLUBILIZATION AND PROCESSING OF RIGID-CHAIN AND LADDER POLYMERS IN APROTIC ORGANIC-SOLVENTS

A reversible electron donor-acceptor complex formation between heterocyclic polymers and Lewis acids in aprotic organic solvents is used to solubilize otherwise insoluble thermally stable rigid-chain polymers, exemplified by benzimidazole-benzophenanthroline-type ladder (BBL) and semiladder (BBB) polymers, poly(p-phenylene-2,6-benzobisthiazole) (PBT), poly(p-phenylene-2,6-benzobisoxazole) (PBO), and related model compounds and polymers. It is shown that the soluble complexes arise from a Lewis acid-base reaction at specific heteroatom nonbonded electron pair donor sites (N, S, O) on the polymer chains rather than from a π-electron charge-transfer reaction. The resulting solutions of the rigid-chain macromolecules in organic solvents were investigated by spectroscopic and rheological measurements. In the case of polymers with rodlike structure in solution, such as BBL and PBT, liquid-crystalline solutions in aprotic organic solvents can be obtained at high concentrations. The isotropic and anisotropic solutions of the rigid-chain polymers can be processed into films, coatings, and fibers, taking advantage of the facile and complete regeneration of the pure polymers from their soluble complexes by precipitation in nonsolvents that are stronger Lewis bases than the polymers. Films and coatings so prepared were characterized and shown to be identical with the pristine polymers. Thus, complexation-mediated solubilization of rigid-chain heterocyclic polymers in organic solvents represents a novel and promising approach to solution processing of heretofore difficult to process high-temperature polymers. The results also have important implications for the understanding of intermolecular interactions in polymers, their effects on structure and properties, and origins of intractability of rigid-chain polymers. © 1990, American Chemical Society. All rights reserved.