Chain polyacetylene

All conducting polymers have a common feature a long chain of sp2 hybridized carbon atoms, often with nitrogen or sulfur atoms included in the chains. Polyacetylene, the first conducting polymer, is also the simplest, consisting of thousands of —CH=CH -units ... 

Fig. 3. Raman spectra of ethyne (in polymerized form as a long-chain polyacetylene) on (A) Rh/Al203 and (B) a gold electrode. [(A) Reprinted with permission from Ref. 94. Copyright 1985 American Chemical Society (B) reprinted with permission from Ref. 83. Copyright 1985 American Chemical Society.]...

Trans-polyacetylene, tra 5-(CH) was the first highly conducting organic polymer [1,2]. The simple chemical structure, -CH- units repeated (see Fig. IVB-la), would imply that each carbon contributes a single p electron to the tr-band. As a result, the rr-band would be half-filled. Thus, based upon this stmcture, an individual chain of neutral polyacetylene would be a metal since the electrons in this idealized metal could move only along the chain, polyacetylene would be a one-dimensional (Id) metal. However, experimental studies show clearly that neutral polyacetylene is a semiconductor with an energy gap greater than 1.5 eV. Rudolf Peierls [86] showed many years ago that Id metals are... 

Selective hydrogenation of isolated triple bonds in polyacetylenic compounds is possible with preference for a terminal triple bond to an internal one. Partial reduction of long-chain polyacetylenes affords all-c long-chain unsaturated acids, e.g., arachi-donic acid, is prepared by hydrogenation of 5,8,11,14-eicosatetraynoic acid over a deactivated Pd-on-CaCOj catalyst" ... 

Kirtman et al.149 show how low frequency collective modes contribute to the dynamic vibrational hyperpolarizabilities of different linear chains (polyacetylene, polyyne, polyethylene and polysilane). In another work Champagne et al.150 have calculated the static vibrational yvL/N of polyacetylene and they have found it to be 30% larger than the electronic contribution, while in the case of polyyne yj[/yL is 0.92151 at the extrapolated infinite chain length. For polydiacetylene and polybutatriene both the static electronic and the vibrational contributions to a /N and ctvL/N, and yl/N and yj JN, respectively, were computed. Also in these cases for both chains the vibrational contributions are of the same order as the electronic ones. One should mention, however, that in these cases the yL/N values extrapolated to infinity are about 10% larger than the values obtained for seven or ten units, respectively, in the two chains. Finally for d -trans polysilane154 the static y /N and yvL/N and values were also calculated. Their values have been found also to be of comparable magnitude. 

Another possibility has been suggested (Bohlmann, Bornowski and Arndt I962), namely, that sequences of this kind may be due to the formation of the straight chain polyacetylenes and corresponding thiophenes from common precursors. The idea is supported by isolation from an Artemesia species, of the ketone (LIII) for which a jff-diketone may be considered the biological precursor. 

Using dehydromatricaria ester-1(XLIII) as precursor, it was demonstrated for the first time that an open chain polyacetylene could be converted biologically to an aromatic compound (Bohlmann, Bohm and Rybak, 1965). In this instance, the reaction apparently involved, in addition, migration of the end methyl group. Thus, when XLIII was fed to Anthemis tinctoria, not only was the thioether XLII which was isolated, radioactive, but also the aromatic compound LXXXIII. All of the activity was in the carboxyl carbon. [See also addendum ]. 

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. 

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. 

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. 

In the context of fra/u-polyacetylene cjia and c are, respectively, the creation and annihilation operators of an electron with spin projection a in the n-orbital of the nth carbon atom (n= l,...,N) that is perpendicular to the chain plane (see Fig. 3-3). Furthermore, u is the displacement along the chain of the nth CH unit from its position in the undimerized chain, P denotes the momentum of this unit, and M is its mass. 

The Coulomb interaction between the re-electrons is neglected. The standard tra/is-polyacetylene parameters are ta=2.5 eV for the hopping amplitude in the undimcrizcd chain, u-4. cV/A for the electron-phonon coupling, and K= 21 eV/A2 for the spring constant [1,4, 8]. 

An interesting example of regioselective CM with ethylene as a tool in natural product degradation was recently disclosed by Hawaiian authors [149]. Thus, CM using catalyst C and ethylene gas was used to degrade the plant polyacetylene oxylipin (+)-falcarindiol (342) with uncertain stereochemistry at C3. As the reaction provided a meso product (343) in 81% yield by regioselective attack at the aliphatic side chain, the natural compound 342, isolated from a Hawaiian endemic plant, had the 3R,8S configuration shown in Scheme 66. 

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. 

In the following Section we present results of the application of the method to two model prototype systems, namely molecular hydrogen chains and all-trans polyacetylene. 

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