STRUCTURAL INVESTIGATIONS ON LEWIS-ACID MEDIATED SOLUBILIZATION OF POLY(P-PHENYLENEBENZOBISTHIAZOLE) IN AN APROTIC-SOLVENT

Investigations on aluminum chloride-mediated solubilization of poly (p -phenylenebenzo-bisthiazole), PBZT, in nitromethane revealed that, in general, dilute solutions of the polymer prepared in nitromethane with a large molar excess of the Lewis acid with respect to PBZT were rela tively stable and thus suitable for processing into films and coatings via regeneration with hydroxylic nonsolvents while polymer solutions with minimum stoichiometrically required (PBZT:AlCl3 molar ratio 1:4) or marginal molar excess of AlCl3 relative to PBZT tended to gel durin g handling. Structural studies on polymer-Lewis acid complex solutions were performed using H-1 and Al-27 NMR. H-1 NMR of the protonated str ucture of the model compound 2,6-diphenylbenzo [1,2-d:4,5-d'] bisthiaz ole was useful for confirming the H-1 NMR assignments of PBZT-AlCl3 co mplex solution in CD3NO2. Al-27 NMR was particularly useful in probing the different tetrahedrally coordinated environments of the Al nucleu s in the polymer solution, pointing to a complexation interaction betw een the polymer molecule and the Lewis acid and also between the solve nt and the Lewis acid. Direct evidence in the solid state for the pres ence of a reversible electron donor-acceptor complex (EDA complex) was obtained from FTIR spectra of complexed films, isolated and handled i n an inert atmosphere. The structural changes upon PBZT complexation c aused significant frequency shifts in both aromatic as well as heteror ing stretching modes relative to PBZT itself, and intensity variations indicative of changes in the resonance configuration between the 1,4- phenylene moiety and the aromatic heterocyclic ring were also observed upon complexation. A preliminary comparison between the pristine PBZT fibers and regenerated PBZT by dilute solution viscometry indicates t hat the process results, to some extent, in polymer degradation leadin g to a decrease in the molecular weight of the polymer. (C) 1993 John Wiley & Sons, Inc.