Polyacetylene

The first, rather extensive section focuses on polyacetylene. It presents a number of concepts which will not be reintroduced in later sections. The rationale is that the studies of polyaeetylene have been the most comprehensive of all and have preceded similar studies on other conducting polymers. The other two early conducting polymers, poly(para-phenylene) and polypyrrole, will be considered next. Subsequent sections deal with the new polymers, poly(para-phenylene 

Polythiophene has been the subject of many diffraction studies. Several soluble, film-forming derivatives have been synthesized, and several oligomers and substituted model compounds have been obtained as single crystals or deposited as ordered thin films. This broad field will be covered in a separate contribution to this Handbook, written by Samuelsen and MIrdalen (.see chapter 2). For this reason, the current chapter does not contain a section on polythiophenes. 

A dramatic change has come with the introduction of a versatile precursor route by Edwards, Feast and coworkers [18]. A dense polymer, called Durham polyaeetylene, is obtained from the high-temperature conversion of a film cast from a soluble precursor polymer. During the conversion, the film can be drawn and high structural orientation can be achieved [19]. 

Over the years, still different preparative methods have emerged, such as those based on a liquid-crystalline reaction medium, oriented by flow or magnetic field [20,21]. These developments are discussed in detail in various contributions to this Handbook that deal with polyacetylene. For an overview, see Tsukamoto [22] and Shirakawa [23]. 

The intractability of the early preparations of polyacetylene has severely hampered the establishment of clear-cut relationships between structure, morphology and (electrical) properties. An early example of an integrated approach to structure-property relations is a paper by Haberkom et al. [24], From a combination of x-ray data with NMR and IR investigations, these authors have found a relationship between the content of sp defects and crystallinity in polyacetylene prepared by the Shirakawa, Luttinger and other methods. Such defects are apparently expelled to the amorphous phase. The authors find a correlation with conductivity in both undoped and iodine-doped samples. 

Doping may impose further structural effects that depend on the size and nature of the dopant species. Although there may still not be complete accord on the crystal structure of lithium-doped polyacetylene, it appears that at low doping levels entropic factors are important in inserting a small nonaggregating ion, such as Li , into polyacetylene (PAc), with the dopant occupying sites with minimal strain or disruption of the host lattice. Iodine, on the other hand, in the form of IJ and IJ (and possibly higher polyiodides), produces structures in which anions cluster in columns or sheets to form intercalated layers between polymer chains.  

With the possible exception of lithium, which may interact chemically with organic chains, the small sizes of alkali metal n-dopants and high charge concentration compared with those of common p-dopants (IJ, FeCl4, AsF, etc.) favor the 

FIGURE 5.9. The potassium-doped polyacetylene channel in Fig. 5.8(a) viewed perpendicular to the channel direction for two assumed doping levels. If both ions shown are present, the lattice is described as fully doped, while the one with a cation at alternate segments is semidoped. (From Ref. 107 by permission of the publishers.) 

When a local defect is created in the lattice, it is accompanied by a displacement of the surrounding atoms, thereby removing at least some of the translational and point group symmetry that is a property of the perfect lattice. Even if the effect of the defect is so localized that it is imperceptible on the structure a few tens of A from the defect center (thereby permitting the application of the lattice-defect method described in Section 2.5), relaxing symmetry restrictions effectively introduces additional degrees of freedom into the permitted atomic displacements. 

The results of applying the lattice-defect method to a single vacancy in 

Billmeyer in Textbook of Polymer Science, Interscience, Oxford University Press, New York, NY, USA, 1984. 

Wendorff, H. Finkelmann and H. Ringsdorf,/owmia/ of Polymer Science Polymer Symposium, 1978, 63, 245. 

Shirakawa, E.J. Louis, A.G. MacDiarmid, C.K. Chiang and A.J. Heeger, Journal of Chemical Society Chemical Communication, 1977,16, 578. 

Kenawy, F.I. Abdel-Hay, M.H. EI-Newehy and G.E. Wnek, Material Science of Engineering, 2007b, 459, 390. 

Polyenes, CH2(=CH-CH) =CH2, have been discussed in previous chapters. The transitions to the first and second excited state are either strongly allowed ( A or forbidden ( A 2 Ap and go into the visible region for long chains (red, brown, and yellow autumn colors). If the number of carbou atoms teuds to infinity, there appears to be a remaining gap of 1.5-2 eV, contrary to the case of graphene, where the gap has closed. This is directly connected to Peierls distortion to alternating bond lengths in the one-dimensional case. Peierls distortion cannot take place in the two-dimensional case. 

Long chains without any specified number of carbon atoms are referred to as a polyacetylene polymer. Oxidative (I2) or reductive (alkali) doping leads to conductivity. During the 1970s, Japanese chemists, including H. Shirakawa, were able to polymerize PA (-CH=CH)x-CH=CH2. In 1978, H. Shirakawa, A. G. MacDiarmid, and A. J. Heeger added I2 as a dopant and found a high conductivity. 

In calculations, it has been found that each alkali atom donates one electron to a PA chain. The electron delocalizes itself on about 20 carbon atoms. The alkali ion is attracted to the center of the distortion. There is no reason to believe that the electron has free mobility along the PA chain, at least not at low doping levels. The conductivity as a function of temperature shows a typical activated behavior for low doping levels. The conductivity probably arises when the electron jumps along a PA chain, or possibly between PA chains, from the neighborhood of an alkali atom to another without a near electron. The electron is not fully delocalized for low doping levels. 

Naarman and N. Theophilou improved PA in several ways. At high alkali doping levels, the conductivity was almost the same as that of copper (lO il cm ). As was found by M. Winokur et al., at high doping levels, three or four PA chains form a channel in which the positive alkali atoms are located. The bonding of one alkali atom to one PA chain is both ionic and covalent. A strong dipole is formed with a direction perpendicular to the PA chain. The dipole attracts another PA chain. Each doping alkali atom thus connects two PA chains. Next, an alkali atom may connect to another chain, and in this way the channels are formed. 

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In considering the molecules in Table 5.2 it should be remembered that the method of detection filters out any molecules with zero dipole moment. There is known to be large quantities of FI2 and, no doubt, there are such molecules as C2, N2, O2, FI—C=C—FI and polyacetylenes to be found in the clouds, but these escape detection by radioffequency, millimetre wave or microwave spectroscopy. 

A common example of the Peieds distortion is the linear polyene, polyacetylene. A simple molecular orbital approach would predict S hybddization at each carbon and metallic behavior as a result of a half-filled delocalized TT-orbital along the chain. Uniform bond lengths would be expected (as in benzene) as a result of the delocalization. However, a Peieds distortion leads to alternating single and double bonds (Fig. 3) and the opening up of a band gap. As a result, undoped polyacetylene is a semiconductor. 

A second type of soHd ionic conductors based around polyether compounds such as poly(ethylene oxide) [25322-68-3] (PEO) has been discovered (24) and characterized. These materials foUow equations 23—31 as opposed to the electronically conducting polyacetylene [26571-64-2] and polyaniline type materials. The polyethers can complex and stabilize lithium ions in organic media. They also dissolve salts such as LiClO to produce conducting soHd solutions. The use of these materials in rechargeable lithium batteries has been proposed (25). 

Polyacetylenes. The first report of the synthesis of a strong, flexible, free-standing film of the simplest conjugated polymer, polyacetylene [26571-64-2] (CH), was made in 1974 (16). The process, known as the Shirakawa technique, involves polymerization of acetylene on a thin-film coating of a heterogeneous Ziegler-Natta initiator system in a glass reactor, as shown in equation 1. 

The resulting porous, fibrillar polyacetylene film is highly crystalline, so is therefore insoluble, infusible, and otherwise nonprocessible. It is also unstable in air in both the conducting and insulating form. 

Much effort has been expended toward the improvement of the properties of polyacetylenes made by the direct polymerization of acetylene. Variation of the type of initiator systems (17—19), annealing or aging of the catalyst (20,21), and stretch orientation of the films (22,23) has resulted in increases in conductivity and improvement in the oxidative stabiHty of the material. The improvement in properties is likely the result of a polymer with fewer defects. 

Even with improvement in properties of polyacetylenes prepared from acetylene, the materials remained intractable. To avoid this problem, soluble precursor polymer methods for the production of polyacetylene have been developed. The most highly studied system utilizing this method, the Durham technique, is shown in equation 2. 

A drawback to the Durham method for the synthesis of polyacetylene is the necessity of elimination of a relatively large molecule during conversion. This can be overcome by the inclusion of strained rings into the precursor polymer stmcture. This technique was developed in the investigation of the ring-opening metathesis polymerization (ROMP) of benzvalene as shown in equation 3 (31). 

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. 

There are several approaches to the preparation of multicomponent materials, and the method utilized depends largely on the nature of the conductor used. In the case of polyacetylene blends, in situ polymerization of acetylene into a polymeric matrix has been a successful technique. A film of the matrix polymer is initially swelled in a solution of a typical Ziegler-Natta type initiator and, after washing, the impregnated swollen matrix is exposed to acetylene gas. Polymerization occurs as acetylene diffuses into the membrane. The composite material is then oxidatively doped to form a conductor. Low density polyethylene (136,137) and polybutadiene (138) have both been used in this manner. 

Eig. 3. Lattice distortions associated with the neutral and soHton states in polyacetylene. 

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. 

GLASER - CHODKIEWCZ Acetylene Coupling Polyacetylenes from monoacetylenes in the presence of copper salts. 

STEPHENS CASTRO Acetylene cycloptiane synthesis Polyacetylene cyclophane synthesis from an iodophenyl copper acetylide... 

The polymers which have stimulated the greatest interest are the polyacetylenes, poly-p-phenylene, poly(p-phenylene sulphide), polypyrrole and poly-1,6-heptadiyne. The mechanisms by which they function are not fully understood, and the materials available to date are still inferior, in terms of conductivity, to most metal conductors. If, however, the differences in density are taken into account, the polymers become comparable with some of the moderately conductive metals. Unfortunately, most of these polymers also have other disadvantages such as improcessability, poor mechanical strength, instability of the doped materials, sensitivity to oxygen, poor storage stability leading to a loss in conductivity, and poor stability in the presence of electrolytes. Whilst many industrial companies have been active in their development (including Allied, BSASF, IBM and Rohm and Haas,) they have to date remained as developmental products. For a further discussion see Chapter 31. 

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. 

Growth mechanism of a (9n,0) tubule, over 24n coordination sites of the catalyst. The growth of a general (9 ,0) tubule on the catalyst surface is illustrated by that of the (9,0) tubule in Fig. 16 which shows the unsaturated end of a (9,0) tubule in a planar representation. At that end, the carbons bearing a vacant bond are coordinatively bonded to the catalyst (grey circles) or to a growing cis-polyacetylene chain (oblique bold lines in Fig. 16). Tlie vacant bonds of the six c/s-polyacetylene chains involved are taken to be coordinatively bonded to the catalyst [Fig. 16(b)]. These polyacetylene chains are continuously extruded from the catalyst particle where they are formed by polymerization of C2 units assisted by the catalyst coordination sites. Note that in order to reduce the number of representations of important steps, Fig. 16(b) includes nine new Cj units with respect to Fig. 16(a). 

The 12 catalyst coordination sites — drawn further away from the surface of the particle (closer to the tubule) — are acting in pairs, each pair being always coordinatively bonded to one carbon of an inserted (F) or of a to-be-inserted (2 ) Cj unit and to two other carbons which are members of two neighbouring cis-polyacetylene chains (3°). It should be emphasized that, as against the (5n,5n) tubule growth, the C2 units extruded from the catalyst particle are positioned in this case parallel to the tubule axis before their insertion. 

Once the pair of sites is disconnected from the growing tubule [Fig. 17(d)], an orthogonal unit is inserted below the five membered ring [4° in Fig. 17(e)]. The latter inserted unit and the remaining two ci.s-polyacetylene C2 segments are finally displaced by the arrival two orthogonal C-, units [.r in Fig. 18(f)]. 

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