Synthesis of PBIs

In a wide definition, PBI refers to a large family of aromatic heterocyclic polymers containing benzimidazole units. PBI with different structures can be synthesized from hundreds of combinations of tetraamines and diacids. In a specific way, PBI refers to the commercial product under the trademark Celazole , poly(2,2 -m-(phenylene)-5,5 -bibenzimidazole) (Fig. 4.1). In the context of PBI with different structures, this specific PBI is also named as meia-PBI because phenylene ring is meto-coordinated. As an amorphous thermoplastic polymer, the aromatic nuclei of PBI provide the polymer high thermal stabihty (glass transition temperature, Fg = 425-436 °C), excellent chemical resistance, retention of stiffness and toughness, but poor process-abihty [56-58]. Primarily used in textile fibers, the selection of poly(2,2 -m-(phenyl-ene)-5,5 -bibenzimidazole) as the commercial product was made on the basis of its 

ANHYDROUS PROTON-CONDUCTING POLYMERS FOR HIGH-TEMPERATURE PEMFCs 

More efforts have been made to modify the polymer stmctures for fuel cell uses. One motivation of these efforts is to improve the properties such as high molecular weight (MW) and good solubility and proeessability, which are of significance for mechanieal stability and functionalization processing of PBI membranes. Another motivation is to tailor the basicity of the polymers for improving the acid-base membranes. These modifications are accomplished in two ways either by syntheti-eaUy modifying the monomers prior to polymerization or by the postpolymerization substitution of the polymer at the reactive benzimidazole N-H sites. 

1 Heterogeneous Molten/Solid-State Synthesis In 1961, Vogel and Marvel [76] reported the first synthesis of PBI by heating an equimolar mixture of bis(o-diamine)s and phenyl esters of different dicarboxylic aeids. A two-stage synthetie procedure of poly(2,2 -m-(phenylene)-5,5 -bibenzimidazole) with tetraaminobiphenyl (TAB) and diphenyl isophthalate (DPIP) as monomers is shown in Rg. 4.2 [56]. 

The synthesized polymer is often characterized in terms of molecular weight by measurement of the inherent viscosity ([//] in dl/g) of a polymer solution in concentrated sulfurie acid. The inherent viscosity is related to the weight average 

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Polybenzimidazole (PBI) 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). The tetraaminobiphenyl required for the synthesis of PBI is obtained from 3,3 -dichloro-4,4 -diaminodi-phenyl (a dye intermediate) and ammonia. Many other tetraamines and dicarboxylic acids have been... 

Generally, three synthetic procedures are used for the synthesis of PBIs (1) solution polymerization, (2) thermally rearranged polymerization, and (3) catalytic polymerization. 

Figure 5.19 Synthesis of PBI/silica hybrid material. Reproduced with permission from Ref. [33],...

Recent progress in the synthesis of PBIs for using as high-temperature membrane electrolyte is updated. The polymer structure design with respect to the optimization of the properties of the polymer membranes and ultimately targeting improved fuel cell performance and durability is... 

Among a variety of PBI structures, only limited types of PBIs, which primarily included the commercially available PBI, poly[2,2 -(m-phenylene)-5,5 -bibenzimidazole] (i.e., m-PBI), sulfonated or phosphorylated m-PBI, as well as the poly(2,5-benzimidazole) (i.e., AB-PBI), have been explored for fuel cell applications [100]. A systematic synthesis of PBIs with different structures was initiated at RPI to study the effect of the PBI polymer molecular structure... 

PBI is being marketed as a replacement for asbestos and as a high temperature filtration fabric with exceUent textile apparel properties. The synthesis of whoUy aromatic polybenzimidazoles with improved thermal stabUities was reported in 1961 (12). The Non-MetaUic Materials and Manufacturing Technology Division of the U.S. Air Force Materials Laboratory, Wright-Patterson Air Force Base, awarded a contract to the Narmco Research and Development Division of the Whittaker Corp. for development of these materials into high temperature adhesives and laminates. 

A large number of PBIs have been investigated since their first synthesis in 1961 (35) the particular polymer used in many commercial and developmental appHcations is poly(2,2 -y -phenylene)-5,5 -diben2imida2ole). 

Poly(arylene thioether)s, 363-364 Poly(arylene thioether sulfone)s, 364 Poly(aryl sulfone) derivatives, 354 Poly(p-benzamide), synthesis of, 188-189 Polybenzimidazoles (PBIs), 265 ferrocene-containing, 315 synthesis of, 313... 

Polyazo dyes, 9 363—361 Polybenzamide, 15 109-110 Poly(p-benzamide) (PBA), 10 212 19 714 crystal lattice parameters of, 19 728t laboratory synthesis of, 19 719 Polybenzimidazole (PBI), asbestos substitute, 3 3141 Polybenzimidazole (PBI) fibers,... 

Polymers such as PBI have a weak link in them since a single covalent bond connects the phenyl rings in biphenyl. This weakness is overcome by the synthesis of ladder polymers, such as polyquinoxaline structures 4.64 and 4.65, and polydithione (structure 4.66), which have two covalent bonds throughout the chain. Thus, the integrity of the polymer is maintained even if one bond is broken ... 

Because PBI is expensive, other thermostable polymers were explored and tested as catalysts (246). A cross-linked version of a polyimide (PI) support with incorporated triazole rings (12b) gave better results than PBI for the epoxidation of cyclohexene. Moreover, it can be reused in the cyclohexene epoxidation at least 10 times without any loss of activity (247). Even less expensive, but thermooxidatively stable materials include polysiloxane-based resins, which have also been used for incorporation of Ti (see Section II,A). In this case, the synthesis comprises the polymerization of TEOS and an oligomeric dimethyl silanol with the addition of functional trialkoxysilanes such as trimethoxysilyl-2-ethylpyridine instead of Ti(OiPr)4 (248). Preliminary results show that the activity per Mo atom is higher than that of PBI-Mo. Furthermore, the degree of leaching of Mo is very low. Thus, it is expected that the polysiloxane-based systems may soon find wide application in oxidation chemistry. 

The synthesis of (32) and other analogues, described below, was carried out as shown in Scheme 4.1 by treating 2-nitroaniline with acrylonitrile followed by catalytic hydrogenation to a phenylenediamine. From this, the PBI nucleus was formed with carbethoxyacetimidate hydrochloride, followed by Dieckmann condensation of the resulting diester. Condensation of the ester with an appropriate amine afforded the target PBIs. 

Yu S, Zhang H, Xiao L, Choe EW, Benicewicz BC. Synthesis of poly (2,2 -(L4-phenylene) 5,5 -bibenzimidazole) (para-pbi) and phosphoric acid doped membrane for fuel cells. Fuel Cells 2009 9(4) 318-24. 

Poly(benzimidazoles) have high thermal stability, and there has been no lack of effort to increase the thermal stability even more through suitable choice of the initial monomers. If the dicarboxylic acids are replaced by tetracarboxylic acids or their anhydrides, and these are then converted with tetramines, then more or less perfect ladder polymers are formed. These ladder polymers all have a thermal stability about 100 K higher than PBI, and so can be used up to about 600° C. Poly(imidazopyrrolone) or pyrron, polypyrrolone, and poly(benzimidazobenzophenanthroline) or BBB may be specially mentioned in this respect. The synthesis of these difficultly soluble polymers must be mostly carried out in solvents such as polyphosphoric acid, zinc chloride, or eutectic mixtures of aluminum chloride and sodium chloride ... 

Concurrent with the development of PBI as a high temperature adhesive, work was also performed on polyimides (PI). The early synthesis of PI utilized the method of Edwards (7) and Endrey (8) where a soluble processable polyamic acid was formed from the stoichiometric reaction of an aromatic diamine with an aromatic tetracarboxylie acid dianhydride (eq. 2). The polyamic acid was subsequently cyclodehydrated to the PI. In adhesive work, this ring closure was accomplished thermally. 

In order to modify the state-of-the-art PBI stmcture and consequently its properties, significant effort has been focused on the synthesis of pyridine-based polybenzimidazoles. A systematic synthesis with different stractures was initiated to study the effect of the polymer molecular stmcture on the final film properties. The substitution of pyridine dicarboxylic acids (PDA) for the iso-/terephthalic acids is particularly interesting, because it increases the number of basic groups in the polymer backbones. The general stmcture of the series of pyridine-based polybenzimidazole (PPBI) homopolymers from 3,3 -4,4 -tetraaminobiphenyl (TAB) and 2,4-, 2,6-, 2,5- or 3,5-pyridine dicarboxylic acids using the PPA synthetic process is shown in Fig. 6. Monomer purity and accurate stoichiometry are cmcial to obtain high molecular weight polymers. 

Figure 5.5 Stepwise temperature control profile for the synthesis of high-molecular-weight 6F-PBI. Reproduced...

Figure 5.16 Synthesis of PBl in PPA and cross-linking of PBI by p-xylylene dichloride [34].

The general process used to synthesise aromatic polybenzimidazoles (PBIs) is presented in Section 4.3.3. More detailed information can be found in previously published books [87,88]. During 1960-1970 a number of publications, comparable to those on polyimides, reported the synthesis and properties of all aromatic and aryl-aliphatic polybenzimidazoles. Most of these polymers were prepared by the two-step process illustrated in Fig. 14 with the reaction of 1,3-benzenedicar-boxylic acid diphenyl ester 27 and [l,l -biphenyl]-3,3, 4,4 -tetramine 11 yielding ultimately PBI 29. All the applications - laminates and filament winding resins, adhesives, fibres and foams - used polymer 29, which was produced in semicommercial quantities by the Whittaker Corporation (Narmco Division) under the generic trade mark Imidite . Currently, forty years later, this polymer is manufactured by Hoechst-Celanese and its only commercial success is in the area of heat resistant fibres and fabrics. It is, however, worth noting the adhesive properties of this polymer and the reasons explaining the major obstacles to the development of PBIs as heat-resistant adhesives. 

Figure 15.11 (a) Antimicrobial activities (MIC) against bacterial strains E. coli, S. aureus, and S. mutans of Br-(PBI) -Br and Am-(PBI) -Am in relation to the molecular weight of the compounds. The MIC of PBI prepared by a polyaddition reaction is also shown, (h) Scheme of poly(3,4en-ionene) (PBI) synthesis by sequential monomer addition, (c) Log reduction of different bacterial strains in PBS after 1 and 10 minutes contact time with PBI at its MIC. 

In the early years, extensive work was carried out to synthetically modify PBI derivatives using different chemical structures of the monomers, i.e., different tetraamine and dicarboxylic acid derivatives, as comprehensively reviewed by Neuse et al. [2]. Since the mPBI membrane was successfully demonstrated as the membrane electrolyte in high temperature PEM fuel cells, much effort has been devoted to the synthesis of various novel PBI derivatives in order to improve the physicochemical properties of the membranes and their durability in fuel cells [9, 10]. This section summarizes the recent synthetic efforts on PBI structure analogues, primarily for applications in the high temperature PEM fuel cells. Table 7.1 lists the chemical structures of PBI variants with modified main chains. 

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