Synthesis and Mechanism of PBI Phosphonate, Poly[2,2′-(-m-phenylene)-5,5′-Bibenzimidazole Phosphonate Ester], and its Polyphosphonic Acid Derivatives

Abstract

Polybenzimidazole diethyl phosphonates (PBIPEt) of high phosphorus content (as high as ∼13%) have been synthesized by reacting poly[2,2′-(m-phenylene)-5,5′-bibenzimidazole] (PBI) with diethyl phosphite and a peroxide, in N,N-dimethylacetamide solution, via a free radical mechanism. The phosphonate ester groups are linked directly to the aromatic backbone thus establishing a strong −(O═)P−Ph− bond. The highly phosphonylated products dissolve in lower alcohols and the corresponding polyphosphonic acids (PBIPOH) dissolve in aqueous sodium hydroxide (pH ∼ 7−8); membranes composed of either polymer are readily prepared. PBIPOH displayed high thermal stability as well as charge densities as high as ∼9 mequiv of H+/g. Model compounds benzimidazole and 2,2′-diphenyl-5,5′-bibenzimidazole have been phosphonylated under similar conditions in order to resolve phosphonylation mechanism, and some aspects of a rather complicated oxidative substitution. Benzoic acid is a byproduct of reactions involving benzoyl peroxide ...