Heteroaromatic polymers

Thiophene, pyrrole and their derivatives, in contrast to benzene, are easily oxidized electrochemically in common solvents and this has been a favourite route for their polymerization, because it allows in situ formation of thin films on electrode surfaces. Structure control in electrochemical polymerization is limited and the method is not well suited for preparing substantial amounts of polymer, so that there has been interest in chemical routes as an alternative. Most of the methods described above for synthesis of poly(p-phenylene) have been applied to synthesise polypyrrole and polythiophene, with varying success. 

The oxidative polymerization of pyrrole can also be induced in solvent-free systems and several papers have described synthesis via exposure of a matrix saturated with an Fe(III) salt to pyrrole vapour 90,91 . Mohammadi et al. 92 described synthesis on planar substrates by sequential exposure to FeCl3 and pyrrole and have also discussed a similar synthesis using H202 as the oxidant93). 

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Figure 13. (a) Substituted polypyrrole, (b) self-doped polypyrrole, (c) heteroaromatic polymer showing the monomer unit, (d) composite polypyrrole-polyelectrolyte, and (e) hybrid material. (Polyaniline macroion photo supplied by Gomez-Romero and M. Lira.)... 

Figure 6. Novel conjugated heteroaromatic polymers synthesized by redox elimination.

Scheme 3 Electrochemical reduction (n-doping) of n-conjugated heteroaromatic polymers. Effect of the imine group...

Kempe M, Fischer L, Mosbach K. Chiral separation using molecularly imprinted heteroaromatic polymers. J Mol Recognit 1993 6 25-29. 

In principle, it should be possible to prepare polyacetylene and its derivatives by coupling reactions of appropriately 1,2-substituted olefins. In practice this route does not appear to have been explored and step-reactions have mostly been applied to prepare aromatic and heteroaromatic polymers. Some of the more important syntheses ae reviewed below others have been reviewed by Feast29). 

Large third order susceptibilities have recently been observed for trans-polyacetylene, heteroaromatic polymers, and poly[p-phenylene vinylene] (4-11). Such nonlinear phenomena in electroactive polymers due to intense laser irradiation has been linked to the photogeneration of charged solitons on time scales of the order 10 s, and values of (3w-w+w+w)—4x 10 esu for... 

TABLE 1. Product conversions for selected electroluminescing polyaromatic and heteroaromatic polymer agents. 

The resistance of a material toward degradation at elevated temperatures is a feature increasingly in demand in advanced materials technologies in which operational temperature regimes are steadily raised for reasons of enhanced efficiency. It is this demand for even more increased heat resistance which, as pointed out before, has provided the most powerful impetus for heteroaromatic polymer development. The outstanding retention of useful properties in a high-temperature environment shown by the polyheteroaromatics primarily derives from the following factors ... 

Some of these reactions could provide a high yield, less severe route to similar heteroaromatic polymers. One such reaction is the... 

Kempe M., Fischer L. and Mosbach K. (1993) Chiral separation using molecularly imprinted heteroaromatic polymers, J. Mol. Recogn. 6, 25-29. Kempe M. and Mosbach K.(1994) Chiral recognition ofiV-fl// / a-protected amino acids and derivatives in non-covalently imprinted polymers, Int J. Peptide Protein Research, 44, 603-606. 

Figure 4. Relationship between force constants (Fee) and intemuclear carbon-carbon distance (toc. Our results, aromatic polymers heteroaromatic polymers, inter-ring x, and intra-iing . Polyacetylene o (Ref. 2S) Polycyclic aromatic hydrocarbons A (Ref. 26).

Finally, a slight "frequency dispersion" is observed in heteroaromatic polymers, and is not in compounds with phenyl rings, indicating a stronger 7C-electron delocalization in the former case. 

Most of the reported conjugated polymers with nonlinear optical or electrical activities are carbon-carbon conjugated systems, such as the extensively studied polyenes (e.g., polyacetylenes [15], polydiacetylenes [16,17], poly-/ -phenylenes) and heteroaromatic polymers (e.g., polypyrroles, polythiophenes, polycarbazoles, polyanilines) [18]. The synthetic methods, applications, and the structure-property relationships of these polymers have also been substantially investigated [19-21]. 

Fluoroalkylated heteroaromatic polymers can be prepared from 2,5-dibromo-3-perfluoropropylthiophene with NiCl2 [567]. The cross coupling reaction of dihaloheteroaromatic compounds and l,2-bis(tributylstannyl)ethylene catalyzed with palladium complexes leads in a single step to poly(thienylene vinylene) [353,568, 569]. The synthesis of structurally homogeneous PATs is described in Sect. 5.1.8. 

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