Structure polyaniline synthesis

The mechanism of polyaniline formation is an area of active research and controversy. The wide range of reaction conditions used in polyaniline synthesis and the resulting differences in the structure and characteristics of the polymers has probably contributed to the proposal of many different mechanisms. The majority of the proposed mechanisms begin with the oxidation of aniline to a cation radical (445). Two of these cation radicals couple to form /V-phenyl-p-phenylenediamine (443). The oxidation of the aniline monomers to form dimeric species is the slow step in the polymerization [271,285,286]. The subsequent steps of polymer growth are under discussion. 

D. Zhang and Y. Wang, Synthesis and applications of one-dimensional nano-structured polyaniline An overview. Mater. Sci. Eng. B, 134, 9-19 (2006). 

M. Malta, G. Louam, N. Errien, and R. M. Torresi, Nanofibers composite vanadium oxide/ polyaniline synthesis and characterization of an electroactive anisotropic structure, Electrochem. Commun., 5, 1011-1015 (2003). 

A. A. Karyakin, I. A. Maltsev, L. V. Lukachova, The influence of defects in polyaniline structure on its electroactivity optimization of self-doped polyaniline synthesis, /oarwit/ of Electroanalytical Chemistry 1996, 402, 217. 

Electroactive polyaniline films were synthesized by the catalysis of biUru-bin oxidase (BOD, a copper-containing oxidoreductase). The polymerization of aniline was carried out on the surface of a sohd matrix such as glass sUde, plastic plate, or platinum electrode to form homogeneous films [33]. The BOD was immobilized on the surface by physical absorption. The optimum pH was around 5.5. Some aniline derivatives such as p-aminophenol and p-phenylenediamine were good substrates for BOD. Structural analysis suggested the BOD synthesized polyanihne possessed partially 1,2-substititued structures. Cyclic voltammetric studies demonstrated that the PANl films were electrochemically reversible in redox properties, but differed from that of chemically or electrochemically synthesized PANl. The difference was attributed to the partial 1,2-substitution. Laccases are known to oxidize phenolic compounds in nature in the presence of oxygen and are capable in polyaniline synthesis in vitro [34-36]. 

Besides synthesis, current basic research on conducting polymers is concentrated on structural analysis. Structural parameters — e.g. regularity and homogeneity of chain structures, but also chain length — play an important role in our understanding of the properties of such materials. Research on electropolymerized polymers has concentrated on polypyrrole and polythiophene in particular and, more recently, on polyaniline as well, while of the chemically produced materials polyacetylene stih attracts greatest interest. Spectroscopic methods have proved particularly suitable for characterizing structural properties These comprise surface techniques such as XPS, AES or ATR, on the one hand, and the usual methods of structural analysis, such as NMR, ESR and X-ray diffraction techniques, on the other hand. 

A possible synthesis of poly(p-phenylene methine) from polybenzyl via redox elimination (A). The two limiting structures of polyaniline (B). 

Punniyamurthy, T. Iqbal, J. Polyaniline Supported Cobalt(II) Salen Catalyzed Synthesis of Pyrrolidine Containing a-Hydroxyamide Core Structures as Inhibitors for HIV Proteases, Tetrahedron Lett. 1997, 38, 4463. 

Polyanilines. Initial preparations of polyaniline (PANI) led to insoluble materials that were difficult to characterize. Use of model compounds and polymers (124,125) allowed for definitive structural analysis. Poly(/>-phenylene amineimine) (PPAI) was synthesized direcdy to demonstrate that PANI is purely para-linked (126). The synthesis was designed so as to allow linkage through the nitrogen atoms only (eq. 9). Comparison of the properties of PPAI and PANI showed PPAI to be an excellent model both structurally and electronically. 

Wnek 180> proposed that the structure of the oxidized insulating form of conventionally formed polyaniline is approximately a 50 % copolymer of diamine and diimine units, corresponding to the emeraldine structure and Hjertberg et al.180 obtained CPMAS NMR evidence for this conclusion. Some confirmation of the structure has also been obtained by chemical synthesis of the polymer182). However, Kitani et al.183) have suggested that the normal electrochemical synthesis leads to partially cross-linked polymers. 

An extensive review of the synthesis of rc-conjugated polymers is presented using a tutorial approach to provide an introduction to the field intended for the undergraduate student and the experienced chemist alike. The many synthetic methodologies that have been used for the synthesis of conjugated polymers are outlined for each class of polymers with a focus on research from the 1990s. The effect of structure on electrical properties is detailed. Specific systems reviewed include the polyacetylenes, polyanilines, polypyrroles, polythiophenes, poly(arylene vinylenes), and polyphenylenes. 

It should be mentioned that the defined interaction of dextran sulfate with amino functions is not only applied for the design of structures on the su-permolecular level but also on the molecular level. Thus, a preferred handed helical structure was induced into the polyaniline main chains by chemical polymerisation of achiral aniline in the presence of dextran sulfate as a molecular template. This affords a novel chemical route for the synthesis of chiral conducting polymers [158]. 

De, A. Basak, R Iqbal, J. Polyaniline Supported Cobalt Catalyzed One Pot Stereoselective Synthesis of the Structural Analogs of Aminopeptidase Inhibitor Bestatin, Tetrahedron Lett. 1997,38, 8383. 

Some conjugated polymers, such as polythiophene and polyaniline were synthesized already in the last century [8a,b], It is not surprising that, for example, polyaniline has played a major role in research directed toward synthetic metals because it possesses a relatively stable conducting state and it can be easily prepared by oxidation of aniline, even in laboratories without pronounced synthetic expertise (see section 2.6). It is often overlooked, however, that a representation of, for example, polypyrrole or polyaniline by the idealized structures 1 and 2 does not adequately describe reality, since various structural defects can occur (chart 1). Further, there is not just one polypyrrole, instead each sample made by electrochemical oxidation must be considered as a unique sample, the character of which depends intimately on the conditions of the experiment, such as the nature of the counterion or the current density applied (see section 2.5). Therefore, one would not at all argue against a practical synthesis, if the emphasis is on the active physical function and the commercial value of a material, even if this synthesis is quick and dirty . Care must be exercised, however, to reliably define the molecular structure before one proceeds to develop structure-property relationships and to define characteristic electronic features, such as effective conjugation length or polaron width. 

Scheme 1. (Left) Synthesis of Polyaniline by Electropolymerization in the Presence of an Optically Active Acid and (Right) Structure of Oxazoline-Containing Polythiophene 29...

The following chapters will describe how the design and assembly of various important polymers can be used to produce predetermined properties. (The synthesis and properties of polypyrroles, polyanilines, and polythiophenes are discussed in detail and differences between these systems emphasized.) A multitude of other CEP systems exist, and the interested reader is referred to the extensive literature now available. Furthermore, the synthesis of CEPs to produce different forms that enable integration of all the functions required for intelligent operation or that allow incorporation into a larger structure will be described. 

We have shown that the hexahydrous ferric chloride completely changes the microstructure of the blend. We observe a phase segregation creation of PVC nodules similar to the one formed by re-precipitation of PVC in the same conditions. A recent study [172] shows that in the case of polyaniline, the imine structure can be protonated with an acid HCl (by-product of the synthesis). The free water in the medium dissociates the acid, allowing the protonation of the chain, and explaining the increase in conductivity. This conductivity rise is about two decades when six water molecules are added for each molecule of FeClj, while the conductivity of the blend produced with FeCb, 6 H2O is lower than the one produced by anhydrous FeClj [127],... 

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