The research of Vogel and Marvel truly represents a pioneering effort in the synthesis of high performance polymers. In this landmark article, they extended the knowledge of the thermal stability of im-idazole derivatives into the realm of high molecular weight, fully aromatic polybenzimidazole (PBI) polymers. They demonstrated the ability to produce high molecular weight polymers having potentially useful properties. One key to achieving a high molecular weight was the use of the phenyl ester derivative of the aromatic dioic acids. This provided superior polymer to that produced by using the free acid or the methyl ester. The commercial PBI products of today still rely on that finding. Their exploratory studies were extended through funding by the Air Force Materials Laboratory and NASA, who addressed a specific set of aerospace and defense needs. This subsequently led to some of the earlier developments of a prepolymer that could be used a8 a high temperature adhesive, a carbon foam made possible by the high carbon yield of PBI, and the development of PBI fibers. Essential to the practical utility of Vogel and Marvel's polymers was that several PBI compositions formed fully soluble, high molecular weight polymer solutions. This allowed subsequent processing into useful forms avoiding the nonprocess-able, proverbial "brick-duet," encountered with some other high performance polymers. From a commercial viewpoint, the balance of processability and polymer performance led to the selection of poly[2,2'-(rn-phenylene)-5,5'-bibenzim-idazole] as the principal product of commerce. Monomers needed to produce this PBI composition were a major concern. Diaminobenzidine (now more commonly referred to as tetraaminobiphenyl) was not commercially available and the diphenyl iso-phthalate was a limited-supply specialty chemical. Hence, dedicated facilities for manufacturing tetra-aminobiphenyl (Hoechst AG) and PBI polymer and fiber (Celanese Corporation, now Hoechst Celanese) were constructed and began operation in 1982/1983. Barring a significant discovery to reduce mono-mer costs, PBI remains a specialty polymer targeted at high performance niches. Even so, its unique performance attributes (e.g., has a glass transition temperature of 435°C; does not bum in air, contribute fuel to flames or produce much smoke; forms a tough flexible char in high yield; is hydrophdic, having high moisture regain; forms very comfortable fabrics for garments; has good chemical resistance) permita PBI to be used in a cost-effective manner in particular situations. Fire blocking of aircraft seats was among the earliest large-volume uses for PBI fiber. Currently, fire service applications are the predominant commercial area. Vogel and Marvel, in the course of their early investigations, may not have envisioned the future when people who had used PBI products in these commercial applications would have acknowledged that "PBI saved my life." They 1123

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Aromatic heteropolymers were synthesized from aromatic tetraamines and dianhydrides; some of the polymers had a ladder‐type structure. The properties and thermal stability of the polymer were studied. The experimental conditions yielding the imidazolone structure were determined on model compounds. Polymers containing mixed aromatic units in the chain were prepared from pyromellitic anhydride or 3,3′,4,4′‐benzophenone tetracarboxylic dianhydride, and 3,3′‐diaminobenzidine, 3,3′,4,4′‐tetraaminodiphenyl ether, 1,4,5,8‐tetraaminonaphthalene, or 1,2,4,5‐tetraaminobenzene. The polycondensations were carried out in melt and in solution (in melted phenol, pyridine, dimethylacetamide, 116% polyphosphoric acid). The inherent viscosities of a number of polymers synthesized by melt condensation were in the range of approximately 0.2–0.7; the higher polymers were obtained by carrying out the polycondensations in solution in 116% polyphosphoric acid; in this case, the intrinsic viscosities ranged from 1.1 to 1.8. Most of the polymers do not melt at 350°C. and are soluble in the usual organic solvents, but the ladder‐type structure polymers were soluble only in concentrated sulfuric acid.