Polybenzimidazole structures

A week link in the polybenzimidazole structure is the imino hydrogen. When this hydrogen is replaced by a phenyl group as in N-phenyl polybenzimidazole, a dramatic increase in high-temperature properties... 

Carollo A, Quartarone E, Tomasi C, Mustarelli P, Belotti F, Magistris A, Maestroni F, Parachini M, Garlaschelli L, Righetti PP (2006) Developments of new proton conducting membranes based on different polybenzimidazole structures for fuel cells applications. J Power Sources 160 175-180... 

A considerable number of non-cross-linked aromatic and heterocyclic polymers has been produced. These include polyaromatic ketones, aromatic and heterocyclic polyanhydrides, polythiazoles, polypyrazoles, polytriazoles, poly-quinoxalines, polyketoquinolines, polybenzimidazoles, polyhydantoins, and polyimides. Of these the last two have achieved some technical significance, and have already been considered in Chapters 21 and 18 respectively. The most important polyimides are obtained by reacting pyromellitic dianhydride with an aromatic diamine to give a product of general structure (Figure 29.17). 

Sulfonated poly(arylene ether)s have shown promise for durability in fuel cell systems, while poly-(styrene)- and poly(imide)-based systems serve as model systems for studying structure-relationship properties in PEMs because their questionable oxidative or hydrolytic stability limits their potential application in real fuel cell systems. Sulfonated high performance polymer backbones, such as poly(phe-nylquinoxaline), poly(phthalazinone ether ketone)s, polybenzimidazole, and other aromatic or heteroaromatic systems, have many of the advantages of poly-(imides) and poly(arylene ether sulfone)s and may offer another route to advanced PEMs. These high performance backbones would increase the hydrated Tg of PEMs while not being as hydrolytically sensitive as poly(imides). The synthetic schemes for these more exotic macromolecules are not as well-known, but the interest in novel PEMs will surely spur developments in this area. 

Many heterocyclic polymers have been produced in an attempt to develop high-temperature-resistant polymers for aerospace applications. Among these are the polybenzimidazoles (PBIs), which are prepared from aromatic tetramines and esters of dicarboxylic acids (structure 4.63). In standardized procedures, the reactants are heated to below 300°C forming soluble prepolymer, which is converted to the final insoluble polymer by further heating ... 

As aromatic compounds have been exhausted as building blocks for life science products, A-heterocyclic structures prevail nowadays. They are found in many natural products, such as chlorophyll hemoglobin and the vitamins biotin (H), folic acid, niacin (PP), pyridoxine HCl (Be), riboflavine (B2), and thiamine (Bi). In life sciences 9 of the top 10 proprietary drugs and 5 of the top 10 agrochemicals contain A-heterocycIic moieties (see Tables 11.4 and 11.7). Even modern pigments, such as diphenylpyrazolopyrazoles, quinacri-dones, and engineering plastics, such as polybenzimidazoles, polyimides, and triazine resins, exhibit an A-heterocydic structure. 

A number of thermally stable polymers have been synthesized, but in general the types of structures that impart thermal resistance also result in poor processing characteristics. Attempts to overcome this problem have largely been concentrated on the incorporation of flexible groups into the backbone or the attachment of stable pendent groups. Among the class of polymers claimed to be thermally stable only a few have achieved technological importance, some of which are polyamides, polyimides, polyquinoxalines, poly quinolines, and polybenzimidazoles. Of these, polyimides have been the most widely explored. 

The e. s. r. and conductivity of polymers such as the polyacene/qui-none radical polymers (PAQR polymers), polyacetylenes and polybenzimidazoles have been investigated (59, 60, 61). The term eka conjugated has been coined to decribe their properties which are very similar to those previously described. The e. s. r. signal is without structure, the activation energy which ranges from 0.2—2.0 ev is considerably... 

Abbreviations AD, asymmetric dihydroxylation BPY, 2,2 -bipyridine DMTACN, 1,4-dimethyl-l,4,7-triazacyclonane EBHP, ethylbenzene hydroperoxide ee, enantiomeric excess HAP, hydroxyapatite LDH, layered double hydroxide or hydrotalcite-type structure mCPBA, meta-chloroperbenzoic acid MTO, methyltrioxorhenium NMO, A-methylmorpholine-A-oxide OMS, octahedral molecular sieve Pc, phthalocyanine phen, 1,10-phenantroline PILC, pillared clay PBI, polybenzimidazole PI, polyimide Por, porphyrin PPNO, 4-phenylpyridine-A-oxide PS, polystyrene PVP, polyvinylpyridine SLPC, supported liquid-phase catalysis f-BuOOH, tertiary butylhydroperoxide TEMPO, 2,2,6,6-tetramethyl-l-piperdinyloxy TEOS, tetraethoxysilane TS-1, titanium silicalite 1 XPS, X-ray photoelectron spectroscopy. 

Figure 3 shows some polymer structures of other sulfonic-acid-based materials and Fig. 4 shows the conductivity of propanesulfonated polybenzimidazole, showing the strong decrease in conductivity of PFSA above 80 °C and the much more thermally stable PBI derivative. 

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. 

These plastics, also known as pyrrones, are experimental materials prepared from aromatic dianhydrides and aromatic tetraamines. The polymer syntheses provide soluble prepolymers that are converted to the pyrrone structures by thermal or chemical dehydration. The precursors can be used to cast films or coatings, or can be molded under very high pressures into filled or unfilled forms. The pyrrones combine some of the best properties of the polybenzimidazoles and polyimides. The pyrrone films are exceptionally radiation resistant and retain their strength properties after 10,000 megarads of 1-MeV electrons. 

The molecular size of polybenzimidazoles has in the pertinent literature been expressed in terms of inherent (j7i h) or intrinsic ([ ]) viscosities, determined on sulfuric acid solutions or, less frequently, on solutions in formic acid or aprotic solvents. The effect of structure on viscosity behavior appears to be less pronounced than that of the polymerization methods used and of the monomer sensitivity to the employed reaction conditions. In general, melt polymerizations by Marvel s method give products with higher molecular mass than obtained in solution condensations, which may partly be due to increased end group reactivity and interaction at the much higher reaction temperatures encountered in the former process (Cf. Table 1). Furthermore, monomers like bis(phenoxycar-bonyl)ferrocene, diphenyl tetrafluoroterephthalate, or l,7-bis(phenoxycarbonyl)car-... 

Electro-insulation materials. The retention of dielectric properties in a high-temperature environment, coupled with good corrosion resistance in contact with certain reactive chemicals, suggests excellent possibilities of polybenzimidazole use in electrical insulation and other dielectric applications at high operating temperatures and/or in aggressive chemical environments. Typical applications, hence, can be foimd in special cable and wire insulation, in the manufacture of circuit boards and radomes for supersonic aircraft, as battery and electrolytic cell separators, and as fuel cell frame structural materials. Some recent publications in the patent and technical report literature may serve to illustrate such applications. 

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