Electrical polyacetylenes

Copolymerizations of benzvalene with norhornene have been used to prepare block copolymers that are more stable and more soluble than the polybenzvalene (32). Upon conversion to (CH), some phase separation of nonconverted polynorhornene occurs. Other copolymerizations of acetylene with a variety of monomers and carrier polymers have been employed in the preparation of soluble polyacetylenes. Direct copolymeriza tion of acetylene with other monomers (33—39), and various techniques for grafting polyacetylene side chains onto solubilized carrier polymers (40—43), have been studied. In most cases, the resulting copolymers exhibit poorer electrical properties as solubiUty increases. 

Although polyacetylene has served as an excellent prototype for understanding the chemistry and physics of electrical conductivity in organic polymers, its instabiUty in both the neutral and doped forms precludes any useful appHcation. In contrast to poly acetylene, both polyaniline and polypyrrole are significantly more stable as electrical conductors. When addressing polymer stabiUty it is necessary to know the environmental conditions to which it will be exposed these conditions can vary quite widely. For example, many of the electrode appHcations require long-term chemical and electrochemical stabihty at room temperature while the polymer is immersed in electrolyte. Aerospace appHcations, on the other hand, can have quite severe stabiHty restrictions with testing carried out at elevated temperatures and humidities. 

Whilst the conductivity of these polymers is generally somewhat inferior to that of metals (for example, the electrical conductivity of polyacetylenes has reached more than 400 000 S/cm compared to values for copper of about 600 000 S/cm), when comparisons are made on the basis of equal mass the situation may be reversed. Unfortunately, most of the polymers also display other disadvantages such as improcessability, poor mechanical strength, poor stability under exposure to common environmental conditions, particularly at elevated temperatures, poor storage stability leading to a loss in conductivity and poor stability in the presence of electrolytes. In spite of the involvement of a number of important companies (e.g. Allied, BASF, IBM and Rohm and Haas) commercial development has been slow however, some uses have begun to emerge. It is therefore instructive to review briefly the potential for these materials. 

Fig. 1. (a) Comparison of normalised electrical conductivity of individual MWCNTs (Langer 96 [17], Ebbesen [18]) and bundles of MWCNTs (Langer 94 [19], Song [20]). (b) Temperature dependence of resistivity of different forms (ropes and mats) of SWCNTs [21], and chemically doped conducting polymers, PAc (FeClj-doped polyacetylene [22]) and PAni (camphor sulfonic acid-doped polyaniline [2. ]) [24]. 

Acetylene (ethyne), C2H2, can be polymerized, (a) Draw the Lewis structure for acetylene and draw a Lewis structure for the polymer that results when acetylene is polymerized. The polymer has formula (CH), where n is large, (b) Consider the polymers polyacetylene and polyethylene. The latter has the formula (CH2)W and is an insulating material (plastic wrap is made of polyethylene), whereas polyacetylene is a darkly colored material that can conduct electricity when properly treated. On the basis of your answer to part (a), suggest an explanation for the difference in the two polymers. 

Polyacetylene is not an electrical conductor. If it is doped with an impurity that either introduces electrons into the upper band or removes electrons from the lower band, it becomes a good conductor. 

The concept of electrochemical intercalation/insertion of guest ions into the host material is further used in connection with redox processes in electronically conductive polymers (polyacetylene, polypyrrole, etc., see below). The product of the electrochemical insertion reaction should also be an electrical conductor. The latter condition is sometimes by-passed, in systems where the non-conducting host material (e.g. fluorographite) is finely mixed with a conductive binder. All the mentioned host materials (graphite, oxides, sulphides, polymers, fluorographite) are studied as prospective cathodic materials for Li batteries. 

A non-electrochemical technique which has been employed to alter the physical characteristics of a number of polymers is that of stress orientation [26, 27], in which the material is stressed whilst being converted to the desired form. This has the effect of aligning the polymer chains and increasing the degree of order in the material, and is obviously most applicable to materials which can be produced via a precursor polymer. With Durham polyacetylene (Section 4.2.1) increases in length in excess of a factor of twenty have been achieved, with concomitant increases in order, as shown by X-ray diffraction and by measurements of the anisotropy of the electrical conductivity perpendicular and parallel to the stretch direction. 

All commodity polymers (that is those manufactured and sold in high volume) act as insulators because they have no free electrons to conduct electricity. Some low-volume polymers, such as polyacetylene, are conductive or semi-conductive, but their applications are specialized and their use limited. In this section, we shall concentrate on the properties of commodity polymers, because these materials represent the vast majority of polymers used in electrical applications. 

G A. Ozin, University of Toronto In view of the current interest in electrically conducting, undoped and doped polyacetylene thin films, do you envisage any possibility of initiating a controlled polymerization of a metal-metal triply bonded organometallie complex to produce a species of the form ... 

Shirakawa H, Louis EJ, MacDiarmid AG, Chiang CK, Heeger AJ (1977) Synthesis of electrically conducting organic polymers halogen derivatives of polyacetylene, (CH)X. J Chem Soc Chem Commun 578-580... 

Chiang CK, Fincher CR Jr, Park YW, Fleeger AJ, Shirakawa H, Louis EJ (1977) Electrical conductivity in doped polyacetylene. Phys Rev Lett 39 1098-1101, erratum (1978) Phys Rev Lett 40 1472... 

Electrically insulating films of polyacetylene, doped with iodine and sodium, became semiconductive (Shirakawa, MacDiarmid, Heeger, 1976). 

Nagels and Krikor143 studied the effect of y-irradiation on the electrical properties of fraws-polyacetylene. They reported a marked decrease of the conductivity and a slight increase of the thermopower after y-irradiation of 10 kGy (1 Mrad). Their study showed that no essential structural changes occur during irradiation. 

CK Chiang, CR Fincher, YW Park, AJ Fleeger, FI Shirakawa, EJ Louis, SC Gau, and AG MacDiarmid, Electrical conductivity in doped polyacetylene, Phys. Rev. Lett., 39 1098-1101, 1977. 

The first and most important event in the history of conducting polymers occurred in 1978 when it was announced that the electrical properties of polyacetylene could be dramatically changed by chemical treatment (Chiang et al, 1978). 

Note 2 The electric conductivity of a conjugated polymer is markedly increased by doping it with an electron donor or acceptor, as in the case of polyacetylene doped with iodine. 

An observant smdent has also heard of many new and old technologies that seemed powerful and promising, but have not found much application. The buckminster-fullerene and the related carbon tubules seem like marvelous materials with unmatched and fascinating properties, but they are not used in the marketplace. Neither are the electrically conducting polyacetylenes, which hold the promise of a moldable conductor that can be made at low temperature. Almost every professor of chemical engineering and chemistry has numerous research results that are not used in the marketplace today. A visit to their offices and discussions wifh fheir research sfaff may resulf in several suggestions that are worth further investigations. 

Five aspects of the preparation of solids can be distinguished (i) preparation of a series of compounds in order to investigate a specific property, as exemplified by a series of perovskite oxides to examine their electrical properties or by a series of spinel ferrites to screen their magnetic properties (ii) preparation of unknown members of a structurally related class of solids to extend (or extrapolate) structure-property relations, as exemplified by the synthesis of layered chalcogenides and their intercalates or derivatives of TTF-TCNQ to study their superconductivity (iii) synthesis of a new class of compounds (e.g. sialons, (Si, Al)3(0, N)4, or doped polyacetylenes), with novel structural properties (iv) preparation of known solids of prescribed specifications (crystallinity, shape, purity, etc.) as in the case of crystals of Si, III-V compounds and... 

Figure 6.48 (a) Effect of doping on the electrical conductivity (solid line) and thermopower (broken line) of polyacetylene. (Following Etemad et al, 1982.) (b) solitons in trans-polyacetylene (i) neutral, (ii) positive and (iii) negative solitons. Arrow marks the boundary between the two symmetric configurations. A, acceptor D, donor. (Following Subramanyam Naik, 1985.)... 

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