Polybenzimidazole

a polymer containing benzimidazole rings in the main macromolecular chain:

where R = H, C₆H₅, or an alkyl.

Polybenzimidazoles are produced mainly by solid-phase poly-cyclic condensation of aromatic tetraamines with diphenyl esters of aliphatic or aromatic dicarboxylic acids. Of current practical importance are polybenzimidazoles based on aromatic acids (mainly o,o′-diaminobenzidine and diphenyl isophthalate), called aromatic polybenzimidazoles:

Most polybenzimidazoles are colorless or slightly colored amorphous or crystalline compounds, depending on their structure, with molecular weights up to 40,000. They have high heat resistance: rapid decomposition of aromatic polybenzimidazoles in air begins at 450°-600°C, and of aliphatic polybenzimidazoles, at 300°-400°C. Polybenzimidazoles are also characterized by high chemical stability and, in particular, resistance to hydrolysis—for example, some polybenzimidazoles are unchanged upon boiling in 70-percent H₂ SO₄ and 25-percent NaOH for 10 hr.

Polybenzimidazoles made from aliphatic polymethylene dicarboxylic acids (for example, sebacic acid) are superior in strength to the aromatic polybenzimidazoles. The latter are soluble, for example, in dimethylacetamide.

Polybenzimidazoles are used to make adhesives, heat-resistant films and fibers, binders for glass-fiber-reinforced plastics used in the construction of airplanes and rockets, antifriction materials, and ablation heat-shielding coatings for spacecraft. Fabrics made from polybenzimidazoles have high resistance to heat and fire and are hydrophilic, comfortable to wear, and resistant to abrasion.

Polybenzimidazoles are produced in the USA. An adhesive produced under the name Imidite 850 and a binder for glass-fiber-reinforced plastics under the name Imidite 1850 are produced from polybenzimidazoles.