Backbone structure benzimidazoles

Polymers containing heterocycles in the backbone include a variety of compounds, as the diversity of heterocyclic molecules is quite large. The polymers from this class may contain groups derived from furan, thiophene, pyrrole, isoindole, benzimidazole, benzothiazole, benzoxazole, quinoxaline, etc. Macromolecules with a ladder backbone containing, for example, a phenoxazine unit in their structure also are known. Amino thermosetting resins from melamine can be considered as polymers containing heterocycles in their structure. 

A variety of high-temperature-resistant PBIs were synthesized and reported in the Uterature. The most widely used PBI is poly[2,2 -(l,3-phenylene)-5,5-benzimidazole] (known as m-PBI) [22]. The other variations are poly[2,2 -( 1,4- phenylene)-5,5 -benzimidazole] (known as p-PBI) [13], poly(4,4 -diphenylether-5,5 -bibenzimidazole) (OPBI) [23], poly(2,5-benzimidazole) (AB-PBI) [24], pyridine-based PBI [25], sulfonated PBI [26], hyperbranched polybenzimidazole (HPBI) [27], naphthalene-based PBI [28], fluorinated PBI [29], PBI block copolymer [30], sulfonated PBI copolymer with sulfone or sulfonic acid groups in the backbone [31,32], PBFnano-composite [33], cross-linked PBI [34], and many others. Different approaches have been taken by the researchers to modify the structure and properties of the fluorinated PBI. A detailed summary of these works is discussed below. 

Table 14.1 Chemical structures of benzimidazole proton pump inhibitors (PPIs) showing the timoprazole backbone stmcture and structure of tenatoprazole, a new generation imidazopyridine PPI...

Synthetic routes to NHCs are very flexible, and a vast nimiber of structures are possible. NHCs based on imidazole and imidazolidine cores are the most common, although 1,2,3- and 1,2,4-triazole and oxazole, thiazole, and benzimidazole cores have also been used (among others). Several researchers, most notably Cavell and coworkers, have also explored the chemistry of so-called ring-expanded NHCs, where a six-, seven-, or eight-membered ring forms the core of the NHC [50]. By suitable modification of the synthetic route, it is possible to introduce a variety of substituents on the backbone of the core, or at the N-sub-stituents. This enormous flexibility has contributed to the rapid growth of this area of chemistry. 

The structure of four examples of these drugs is given in the illustration. It can be seen that they all share the same backbone, a 2-pyridyl methylsulfinyl benzimidazole with various substitutions on the pyridine or benzimidazole moieties, which are added to modify solution reactivity or to decrease toxicity. 

PBIs refer to a class of heterocyclic polymers consisting of benzimidazole linkages in polymer backbone. Poly(2,2 -m-phenylene-5,5 -bibenzimidazole) is almost the only commercial PBI (Celazole ). Its chemical structure is shown in Eigure 8.1. This polymer is often simply named as PBI or m-PBI (m refers to the meta-substitution positions of the phenylene moiety). 

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