Polybenzimidazole (PBI, short for poly[2,2’-(m-phenylen)-5,5’-bisbenzimidazole]) fiber is a synthetic fiber with a very high decomposition temperature. It does not exhibit a melting point, does not readily ignite, and has exceptional thermal and chemical stability. It was first discovered in 1961, by American polymer chemist Carl Shipp Marvel in the pursuit of new materials with superior stability, retention of stiffness, and toughness at elevated temperature. Due to its high stability, polybenzimidazole is used to fabricate high-performance protective apparel such as firefighter's gear, astronaut space suits, high temperature protective gloves, welders’ apparel and aircraft wall fabrics. Polybenzimidazole has been applied as a membrane in fuel cells.

Brinker and Robinson first reported aliphatic polybenzimidazoles in 1949. However the discovery of aromatic polybenzimidazole, which shows excellent physical and chemical properties, was generally credited to Carl Shipp Marvel in the 1950s. The Material Laboratory of Wright Patterson Air Force Base approached Marvel. They were looking for materials suitable for drogue parachutes which could tolerate short-time mechanical stress. However, the thermal resistance of all known filaments at that time was inadequate. The original search concentrated on aromatic condensation polymers but the amide linkage proved to be weak link for the aim of maximal thermal stability of the polymer, whereas Marvel's research focused on condensation polymers with aromatic and heteroaromatic repeating units. This progressively led to the discovery of polybenzimidazole.

Its development history can be summarized in the following list:

PBI are usually yellow to brown solid infusible up to 400 °C (752 °F) or higher. The solubility of PBI is controversial, because while most of the linear PBI are partly or entirely dissolved in strong protonic acids (for instance, sulfuric acid or methanesulfonic acid), contradictory observations of solubilities have been recorded among weaker acids like formic acid, and in non-acidic media, such as the aprotic amide-type solvents and dimethyl sulfoxide. For example, one type of PBI prepared in phosphoric acid was found by Iwakura et al. to be partially soluble in formic acid, but completely soluble in dimethyl sulfoxide and dimethylacetamide, whereas Varma and Veena reported the same polymer type to dissolve completely in formic acid, yet only partially in dimethyl sulfoxide or dimethylacetamide.

Imidazole derivatives are known to be stable compounds. Many of them are resistant to the most drastic treatments with acids and bases and not easily oxidized. The high decomposition temperature and high stability at over 400 °C suggests a polymer with benzimidazole as the repeating unit may also show high heat stability. Polybenzimidazole and its aromatic derivatives can withstand temperatures in excess of about 500 °C (932 °F) without softening and degrading. The polymer synthesized from isophthalic acid and 3,3'-Diaminobenzidine is not melted by exposure to a temperature of 770 °C (1,420 °F) and loses only 30% of its weight after exposure to high temperature up to 900 °C (1,650 °F) for several hours.

A property of a material needed to be considered before putting it into application is flammability, which demonstrates how easily one material can ignite and combust under the realistic operating conditions. This may affect its application in varied areas, such as in construction, plant design, and interior decoration. A number of quantitative assessments of flammability exist, such as limiting oxygen index (LOI), i.e., the minimum oxygen concentration at which a given sample can be induced to burn in a candle like configuration. These permit estimation of a 'ranking' comparison of flammability. Data shows that PBI is a highly flame resistant material compared to common polymers.

PBI's moisture regain is useful in protective clothing; this makes the clothing comfortable to wear, in sharp contrast to other synthetic polymers. The moisture regain ability of PBI (13%) compares favorably with cotton (16%).

The preparation of PBI(IV) can be achieved by condensation reaction of diphenyl isophthalate (I) and 3,3’,4,4’-tetraaminodiphenyl (II) (Figure 1). The spontaneous cyclization of the intermediately formed amino-amide (III) to PBI (IV) provided a much more stable amide linkage. This synthetic method was first used in the lab and later developed into a two step process. In a typical synthesis, starting materials were heated at 270 °C (518 °F) for 1.5 h to form the PBI prepolymer and later the prepolymer was heated at 360 °C (680 °F) for another 1 h to form the final commercial-grade product.