Syntheses of Organic Solvent-Soluble Polybenzimidazole Derivatives with a Guanidinoid Structure

Although polybenzimidazoles (PBIs) exhibit outstanding thermal stability, their aromatic nature and rigidity render them barely soluble in traditional solvents, when they possess a high molecular weight. Therefore, highly acidic and toxic solvents are required to dissolve the high-molecular-weight PBI for film casting, which restricts its applications. This paper presents an innovative synthetic approach for fabricating a novel PBI variant with high thermoresistance using a novel precursor that is soluble in common solvents. The monomer synthetization involved two steps: (1) selective protection of the 3-amino group of 3,4-diaminobenzoic acid and (2) reaction of benzoic acid with p-diaminobenzene to produce an aromatic diamine. Thereafter, poly(amide o-aminourea) precursors were produced through polyaddition of the resulting diamine with diisocyanate compounds, followed by deprotection. These precursors were soluble in conventional polar solvents, such as dimethylformamide, and were cast into films. Precursors with o-aminourea groups were annealed at 210 °C to convert them into poly(benzimidazole guanidinoid) derivatives. The pyrolysis temperature of the film, estimated through thermogravimetry, exceeded 400 °C, and its mechanical toughness was sufficiently high for use in high-performance plastics.