Membered Ring Heterocyclic Polymers

Polyimides are obtained from amine and carboxyl reactants when the ratio of amine to acid functional groups is 1 2. If reactants with the reverse ratio of amine to acid functional groups are employed, polybenzimidazoles (PBI) are produced for instance, polymerization of 3,3 diaminobenzidine and diphenyl isophthalate yields poly[(5,5 -bi-liT-benzimidazole]-2,2 -diyl)-l,3-phenylene] (Lilia) (Eq. 2-216) [Buckley et al., 1988 Hergenrother, 1987  

Marvel, 1975 Ueda et al., 1985]. The reaction probably proceeds by a sequence of two nucleophilic reactions a nucleophilic substitution to form an amine-amide (Eq. 2-217) followed by cyclization via nucleophilic addition (Eq. 2-218). 

Considerable efforts have centered on carrying out the synthesis of polybenzimidazoles at more moderate temperatures. Polymerization of the isophthalic acid or its diphenyl ester have been successfully carried out in polyphosphoric acid or methanesulfonic acid-phosphorous pentoxide at 140-180°C, but the reaction is limited by the very low solubilities ( 5%) of the reactants in that solvent. The lower reaction temperature is a consequence of activation of the carboxyl reactant via phosphorylation. Lower reaction temperatures are also achieved in hot molten nonsolvents such as sulfolane and diphenyl sulfone, but the need to remove such solvents by a filtration or solvent extraction is a disadvantage. 

It should be noted that structure Lilia is an oversimplification for the polymer formed from 3,3-diaminobenzidene and diphenyl isophthalate. Dehydration after ring closure can occur toward either of the two nitrogens, and one would expect more or less random placements of the carbon-nitrogen double bonds. Thus, the PBI structure is a random copolymer of repeating units Lilia, LUIb, and LIIIc. 

A wide range of polymers based on other heterocyclic 5-membered rings have been investigated, including oxadiazole, pyrazole, triazole, tetrazole, and thiophene [Bass et al., 1987 Briffaud et al., 2001 Cassidy, 1980 Critchley et al., 1983 Gebben et al., 1988 Ueda and Sugita, 1988], 

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All polymers derived from three-membered heterocycles have structures which allow the formation of six-membered rings. Therefore, it is thermodynamically expected that these polymers will degrade, as long as active species which allow these degradations to occur, are present in the polymer chains. This explains why poly-oxiranes degrade to 1,4-dioxanes, polythiiranes to 1,4-dithianes and poly(N-alkyl-aziridines) to dialkylpiperazines. 

The backbone of a nucleic acid is a polymer of ribofuranoside rings (five-membered rings of the sugar ribose) linked by phosphate ester groups. Each ribose unit carries a heterocyclic base that provides part of the information needed to specify a particular amino acid in protein synthesis. Figure 23-21 shows the ribose-phosphate backbone of RNA. 

Synthesis of Heterocycles. The convenient and efficient synthesis of a variety of 2-amino-substituted l-aza-3-(aza, oxa, or thia) heterocycles has been described using tosyl chloride/NaOH (eq 55). Different substitution patterns are tolerated and five- or six- membered rings can be formed in moderate to good yields. The application of polymer-supported tosyl chloride is possible in this reaction it facilitates the work-up and renders the reaction conditions very suitable for parallel or automated synthesis. 

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