Condensation polymers polybenzimidazole

A number of plastics are condensation polymers and include polyesters and nylons that are not as highly oriented as the same materials but in fiber form. Other plastics have been developed that have outstanding heat stability, strength, and other properties that allow their wide use. These plastics include polycarbonates, polyimides, polybenzimidazoles, polysulfides, polyethers, polysulfones, and polyketones. 

Much attention has been paid to the synthesis of fluorine-containing condensation polymers because of their unique properties (43) and different classes of polymers including polyethers, polyesters, polycarbonates, polyamides, polyurethanes, polyimides, polybenzimidazoles, and epoxy prepolymers containing pendent or backbone-incorporated bis-trifluoromethyl groups have been developed. These polymers exhibit promise as film formers, gas separation membranes, seals, soluble polymers, coatings, adhesives, and in other high temperature applications (103,104). Such polymers show increased solubility, glass-transition temperature, flame resistance, thermal stability, oxidation and environmental stability, decreased color, crystallinity, dielectric constant, and water absorption. 

Poly-2-2 -(w-phenylene)-5,5 -bibenzimidazole, commonly called polybenzimidazole (PBI), was developed under the aegis of the U.S. Air Force Materials Laboratory in cooperation with the then-existing Celanese Corporation. The fiber went into commercial production in the United States in 1983. It is a condensation polymer obtained from the reaction of tetra-aminobiphenyl and diphenylisophthalate in a nitrogen atmosphere at temperatures that may reach 400°C in the final stages.29 The structure of a repeating unit is shown below. 

What are direct condensation polymers How are polybenzimidazoles, polybenzoxazoles, polybenzthiazoles, polyoxidiazoles, polybenzotriazoles, and polysulfodiazoles prepared Illustrate with chemical equations. 

Many polybenzimidazoles are prepared by direct condensation. They are colored polymers that mostly melt above 400 One such material is formed from 3,3 -diaminobenzidine and dipheyl isophthalate by heating the two together at 350-400 °C in an inert atmosphere ... 

Polybenzimidazoles are a class of thermally stable polymers, typically condensed from aromatic bis-o-diamines and dicarboxylates. The namesake polymer repeat unit, which contains a 1,3-dinitrogen heterocycle, is benzimidazole. As a monomer, benzimidazole, has a melting point of 170°C and a boiling point of >360°C. Benzimidazole s hydrocarbon analog, indene, has a melting point of -2°C and a boiling point of 183°C. With only a molecular weight difference of 2, the differences in thermal behavior between benzimidazole and indene foretells some of the unusual thermal characteristics to be found in benzimidazole polymers. 

The melting temperatures and thermal stabilities of several polybenzimidazoles are shown in TABLE I. For all aromatic polybenzimidazoies, melting points are over 600 C. Generally, the presence of other structural elements will reduce melting points and thermal stabilities. For example, TABLE I includes polybenzimidazoles condensed from either a tetraamine ether or a diacid ether. Strained polymer structures also reduce polybenzimidazole stability as evidenced by the low melting point of a polymer formed with a biphenyl-2,2 -dicarboxylate. Not surprisingly, the aliphatic, adipic acid forms a much less stable polybenzimidazole than the aromatic examples above it in the table. 

Aliphatic polybenzimidazoles were first synthesized by scientists in the 1950s. However, it was not until the 1960s that H. Vogel and C.S. Marvel first synthesized aromatic polybenzimidazoles and discovered that these unique, highly stable linear heterocyclic rigid-rod polymers exhibited exceptional thermal and oxidative stability [65]. Poly[2,2 -(m-phenylene)-5,5 -bibenzimidazole], PBI, was prepared by the condensation polymerization of... 

Where still more heat resistance is needed, we now have the polyimides, polybenzimidazole polyquinoxalines and other linear polymers in which the methylene content is reduced or eliminated. Conjugated unsaturation increases the thermal stability of these materials. Unfortunately, as the extent of condensation approaches that of the ladder polymer pyr-rones which might provide optimum heat resistance, the polymers become more intractable. 

Polybenzimidazole (PBl) is the most well-known commercial example of aromatic heterocycles used as high-temperature polymers. The synthesis of PBI is carried out as follows (see also Figure 1.36 of Plastics Fundamentals, Properties, and Testing). The tetraaminobiphenyl required for the synthesis of PBl is obtained from 3,3 -dichloro-4,4 -diaminodiphenyl (a dye intermediate) and ammonia. Many other tetraamines and dicarboxylic acids have been condensed to PBl polymeric systems. 

---