Solution polymerization of polybenzimidazole

Polybenzimidazoles (PBI) are an important class of heterocyclic polymers that exhibit high thermal and oxidative stabilities. The two dominant polymerization methods used for the synthesis of PBI are the melt/solid polymerization route and solution polymerization using polyphosphoric acid as the solvent. Both methods have been widely used to produce high-molecular weight PBI, but also highlight the obvious absence of a practical organic solution-based method of polymerization. This current work explores the synthesis of high-molecular weight meta-PBI in N,N-dimethyl acetamide (DMAc). Initially, model compound studies examined the reactivity of small molecules with various chemical functionalities that could be used to produce 2-phenyl-benzimidazole in high yield with minimal side reactions. 1H NMR and FTIR studies indicated that benzimidazoles could be efficiently synthesized in DMAc by reaction of an o-diamine and the bisulfite adduct of an aromatic aldehyde. Polymerizations were conducted at various polymer concentrations (2-26 wt % polymer) using difunctional monomers to optimize reaction conditions in DMAc which resulted in the preparation of high-molecular weight m-PBI (inherent viscosities up to 1.3 dL g−1). TGA and DSC confirmed that m-PBI produced via this route has comparable properties to that of commercial m-PBI. This method is advantageous in that it not only allows for high-polymer concentrations of m-PBI to be synthesized directly and efficiently, but can be applied to the synthesis of many PBI derivatives.