Synthesis of conducting polymers

A neutral chloroaluminate ionic liquid, which contains ATCh- and Cl anions, is obtained by using an exactly equimolar amount of the organic chloride and the AICI3. The melts can also be buffered to neutrality using alkali halides. 

Although the use of these molten salts is hampered by their instability in air and water, and this instability may also be reflected in the resultant polymer films, it is important to note that much of this earlier work clearly identifies a number of the potential benefits of using ionic liquids for the synthesis of conducting polymers. 

Pickup and Osteryoung investigated the polymerization of pyrrole in both the AlCl3/[C4py][Cl] molten salt [48] and the more conductive AlCl3/[C2mim][Cl] [49]. Synthesis of poly(pyrrole) is only possible in neutral melts. In basic melts oxidation of Cr to CI2 occurs before the monomer oxidation and in acidic melts no films are produced due to the formation of a 1 1 AlC -pyrrole adduct, as determined by NMR spectroscopy [50]. 

The electrochemical behavior of poly(pyrrole) films prepared and cycled in an AICI3 [C2mim][Cl] melt was investigated in detail and improvements in reproducibility and the rate of oxidation and reduction of these films were observed compared to films prepared under similar conditions in acetonitrile [49]. This was postulated to be a result of an increase in the porosity of poly(pyrrole) films deposited from the melt compared to those from acetonitrile, although attempts to describe this porosity using porous electrode models were not totally conclusive. 

This appears to be contradictory to the smoother poly(pyrrole) films that are formed in air- and water-stable ionic liquids [46, 51]. 

Stable immobilisation of macromolecular biomolecules on conducting microsurfaces with complete retention of their biological recognition properties is a crucial problem for the commercial development of miniaturised biosensors. Various conducting polymers have been utilised for immobilisation of enzymes at an electrode surface including PPy [74-76, 103-106], polyindole [79], PANI [77, 107, 108], poly (N-methylpyrrole) [65] and copolymers of N-substituted pyrroles [34]. 

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The synthesis of conducting polymers can be divided into two broad areas, these being electrochemical and chemical (i.e., non-electrochemical). Whilst the latter may be considered to be outside the scope of this review, it is worth noting that many materials which are now routinely synthesised electrochemically were originally produced via non-electrochemical routes, and that whilst some may be synthesised by a variety of methods many, most notably polyacetylene, are still only accessible via chemical synthesis. In view of this it is useful to have an appreciation of the synthesis of these materials via routes which do not involve electrochemistry. 

Diphasic Biocatalytic System for the Synthesis of Conducting Polymers... 

Fig. 12.6 Cell for electrolytic synthesis of conducting polymer (roll-type).

Although the methods reviewed above have been most widely used for synthesis of conducting polymers, there are many other organic reactions capable of producing conjugated structures. One example is the Wittig condensation of bis-triphenyl-phosphonium compounds with dialdehydes in the presence of a strong base, typified by the synthesis of poly(p-phenylene-vinylene) 102). 

Useful electrochemical synthesis of conducting polymers requires a combination of conductivity with insolubility in the electrolyte, which causes the polymer to precipitate, hopefully directly on the electrode surface, as a coherent film which does not passivate the electrode and may be removable from its surface for further study. Electrochemical synthesis is generally limited to rather small amounts of polymer, although a method of producing large amounts of film by continuous stripping of a rotating electrode has been described 109). 

Virtually all of the real interest in electroinitiated synthesis of conducting polymers has focussed on the anodically active aromatic monomers, of which the most highly studied examples are pyrroles and thiophenes (Table 1). 

These polymerizations depend upon the ability to oxidize the monomer to a radical cation, whose further reactions lead to polymer. Since the oxidation potentials of the polymers are lower than those of the corresponding monomer, the polymer is simultaneously oxidized into a conducting state so that it is non-passivating. Some of the more important electrochemically-synthesised structures are discussed in more detail below and Chandler and Pletcher U4) have reviewed the electrochemical synthesis of conducting polymers. Detailed discussion in terms of thermodynamic parameters is impossible because the polymerizations are irreversible, so that E0 is undefined for the monomer-polymer equilibrium. 

Chemical preparation methods for the synthesis of conducting polymers have been widely used [35]. It has become clear that to fully exploit the potential of the conducting polymer, better-defined soluble materials with a clear correlation between structure and properties need to be prepared. In this section, monosubstituted alkylthiophenes will be discussed as an example of how, via the development of well-documented systematic methods, ordered polymer layers can be obtained with improved conductive properties. 

Further to their role as supporting electrolytes, the conductivity and electrochemical stability of ionic liquids clearly also allows them to be used as solvents for the electrochemical synthesis of conducting polymers, thereby impacting on the properties and performance of the polymers from the outset. Parameters such as the ionic liquid viscosity and conductivity, the high ionic concentration compared to conventional solvent/electrolyte systems, as well as the nature of the cation and... 

There are also a number of other variables to consider when planning the electrochemical synthesis of conducting polymers in ionic liquids. While most of these variables also exist for the synthesis of the polymers in molecular/solvent systems and have been investigated in detail, it is worth considering that the influence of any of these factors may be different when utilizing ionic liquids as the growth medium because of their distinctly different properties. These are discussed in more detail below. 

There is a plethora of different ionic liquids readily available, either commercially or through straightforward laboratory synthesis investigations into their use for the synthesis of conducting polymers has, so far, focused on a relatively small number (Figure 7.4). Thus, the avenues for future investigation in this area are vast. 

Fig. 7.4 The cations and anions utilized to date for the electrochemical synthesis of conducting polymers in ionic liquids, and their abbreviations.

Abstract Conductive polymers are established materials for development of chemical and biological sensors. Properties of these polymers are influenced by a number of different physical and chemical factors. Application of combinatorial and high-throughput techniques to development and optimization of chemo and biosensors is reviewed. Methods for addressable synthesis of conductive polymers and protocols for comprehensive description of chemosensitive properties are discussed. 

Synthesis of conductive polymers can be realized either by addition of oxidizing agents or by electrochemical oxidation at anodic potentials. To distinguish from electrochemical polymerization, polymerization by external oxidizer is often marked in literature as a chemical polymerization. Many types of conductive polymers formed by this way have a strong trend to adsorb on the surfaces... 

Experimental approaches to combinatorial synthesis of conductive polymers are summarized in the Fig. 13.2. Combinatorial polymerization driven by an introduced chemical oxidizer can be performed with a usual liquid handling robot or with a microfluidic system. Surprisingly, much more sophisticated techniques were usually used (Fig. 13.2). 

Fig. 13.2 Experimental approaches for combinatorial synthesis of conductive polymers. The figure includes a parts of illustration reprinted with permissions from Xiang and La Van39 (copyright 2006 The Institute of Electrical and Electronics Engineers, Inc.)...

The results demonstrate several approaches to combinatorial synthesis of conductive polymers and an example of automated high-throughput screening of their chemosensitive properties. Convenient format of combinatorial libraries in the form of small amounts of polymers (few Jg) deposited on silicon substrate with electrodes allows one not only simple characterization, but also simple storage and organization of banks of materials with electrical and optical access for further investigations. 

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