m-Polybenzimidazole (m-PBI), the condensation polymer of 3,3'-diaminobenzidine and isophthalic acid was evaluated using 1H- and 13C-NMR spectroscopy in a variety of deuterated polar aprotic solvents (DMSO-d6, DMF-d7, DMAC-d9 and NMP-d9). 1H-NMR spectra displayed characteristic repeat unit resonances and weaker resonances from aryl-substituted benzoate and 2-chloroaniline endgroups (the latter from the monomer impurity, 3-amino-3-chlorobenzidine).  Unexpectedly, aryl-substituted 2-aminoaniline endgroups from incompletely polymerized diaminobenzidine were not observed. Number average molecular weights (Mns), determined from endgroup analysis, qualitatively tracked Mws calculated from viscometry of dilute m-PBI/H2SO4 solutions. The 13C-NMR spectra (DMSO-d6), assigned from 1H-1H-COSY, 1H-13C-HMQC and 1H-13C-HMBC experiments, demonstrated coexistence of the 5- and 6-aryl benzimidazole tautomers in a 1.0:1.3 molar ratio. Under dilute conditions, carbons of the biphenylene aryl-aryl linkage and those para-oriented to them, exhibited sets of closely spaced resonances from the 5,5'-, 5,6'- and 6,6'-bibenzimidazole tautomers. The relative molar ratio of these bibenzimidazole tautomers, from integral areas of the deconvoluted resonances, was determined to be 1.0:2.5:1.6 (5,5'-/5,6'-/6,6'-) and matched DFT-calculations of the corresponding bibenzimidazole model compounds.  Variable temperature experiments coupled with spectral simulation of lifetime-broadened resonances proximal to the imidazole ring provided information relative to benzimidazole tautomerization kinetics.This work is supported as part of the Center for Electrocatalysis, Transport Phenomena, and Materials (CETM) for Innovative Energy Storage, an Energy Frontier Research Center (EFRC) funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0001055 and under DOE Contract # DE-AC02-05CH11231.

NMR evaluation of polybenzimidazole microstructure

248th National Meeting (2014)

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