Polyacetylene conducting polymers/synthetic metals

Polyacetylene (PA), the simplest linear conjugated polymer, has been actively studied for two main reasons. First, the discovery of the direct synthesis method of PA films on the surface of a Ziegler-Natta catalyst solution [1]. Second, the discovery of a large increase in electronic conductivity, due to a synthetic metal by doping with small quantities of electron-attracting species such as iodine, AsFs, etc., or with an electron donor such as sodium. However, because of its high reactivity and poor solubility, it is difficult to obtain the experimental structural data of PA. 

In the ten years that have elapsed since the appearance of the Handbook of Conducting Polymers [1] in 1986, the effort dedicated to the research of synthetic metals has continued to grow. The group of polymers on which this research is focused nowadays is largely the same as then but the emphasis has shifted. Ten years ago the attention of the scientific community was very much on two polymers (i) polyacetylene, for which various synthetic routes leading to appreciably crystalline material had become available, and (ii) polypyrrole, which showed very attractive conduction properties although it was found to be highly disordered and hard to characterize,... 

Figure 10.43 Current-voltage characteristics of individual polyacetylene nanofibers. (Reprinted with permission from Synthetic Metals, Current-voltage characteristics of conducting polymers and carbon nanotubes by A. B. Kaiser and Y. W. Park, 152, 1-3. Copyright (2005)...

Since the initial discoveiy in 1977,2 that polyacetylene, (CH)x, now commonly known as the prototype conducting polymer, could be p- or n-doped, either chemically or electrochemically to the metallic state, the development of the field of conducting polymers has continued to accelerate at an unexpectedly rapid rate. This rapid growth rate has been stimulated not only by the field s fundamental synthetic novelty and importance to a cross-disciplinary section of investigators - chemists, electrochemists, experimental and theoretical physicists and electronic and electrical engineers - but to its actual and potential technological applications. 

The high electronic conductivity of the 7c-conjugated polymers (e.g. polypyrrole, polyacetylene) determined by the presence of delocalized electronic states, puts those polymers into the category of synthetic metals. The lower electronic conductivity of the redox and ion-exchange polymers, compared with the 7r-conjugated polymers, is due to the presence of localized electronic states. The redox polymers, which contain redox electroactive centres, conduct current by electron self-exchange... 

A polymeric material that possesses the electronic, magnetic, electrical, and optical properties of a metal while retaining the processibility and mechanical properties usually associated with a conventional organic polymer is called a conducting pol ymer. Since the first demonstration of metallic conductivity in AsF5-doped polyacetylene by MacDiarmid and co-workers in 1977 [1], conducting polymers have become the main part of a larger class of materials commonly known as synthetic metals. 

Since the 1970s polymer chemistry has enabled the development of so-called synthetic metals , i.e. polymers with conductivies up to 10 S cm , comparable to those of metals. There are still many technological hurdles in processing the materials into suitable forms for them to find widespread applications. The conductivity in these materials results from delocalized n orbitals along the backbone. Representative conductive polymers are polyacetylene (PA) or poly(3,4-ethylene dioxythiophene) (PEDOT), as sketched in Fig. 2.33. The role of dopant is vital in conferring quasi-metallic properties. Indeed, the original discovery of metallic conductivity in PA (in 1977) was serendipitous, involving the accidental addition of excess iodine dopant. 

Polyanitine was the first CP polymer, which was described in the mid-19th century by Henry Letheby [36]. Since then numerous intrinsically CP have been developed, among others polyacetylene, polythiophene, polypyrrole. CPs, also referred to as synthetic metals, have found applications in many fields. They are integrated for example in solar cells, rechargeable batteries and biomedical devices [37]. CPs are also very attractive for biosensors. In biosensors, CP can be used as excellent non-metallic electrodes. Numerous biosensors have been developed over the past 20 years with electrodes made of CP. The fabrication is fairly easy and flexible. This allows the biosensors to be single-use system avoiding any risk of contamination and adaptation of the biosensors to new targets can be rapidly made. They are mostly biocompatible, can easily be synthesized and can be modified for immobilization of bioelements [38]. These conductive polymers are referred to as intrinsic conductive polymers in comparison to extrinsic conductive polymers that are a polymer matrix in which some metal particles have been entrapped [39]. 

Conducting polymers have now been studied for nearly a decade since the report [1] of the achievement of a highly conducting "metallic state upon doping of polyacetylene, (CH)x, with acceptors such as iodine and AsFs and donors such as sodium. This report led to vigorous activity in exploration of the physics and chemistry of the phenomena associated with these systems. A review of the full extent of theoretical development as well as synthetic and experimental work is beyond the scope of a short article. Therefore, only a brief introduction to the physical concepts as applied to a quasi-one-dimensional system is given below, followed by application to a conducting polymer of current interest, polyaniline. 

The search for new organic metals and superconductors has attracted a great deal of attention in synthetic chemistry and material science since the discovery of high electrical conductivity in conjugated polymers such as polyacetylene [1], Lots of theoretical studies have been carried out in order to understand the mechanism of conductivity and superconductivity in the conjugated polymers and related... 

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