Polyacetylenes, halogenated

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... 

Fig. 9.3 Change of conductivity of tram polyacetylene following exposure to halogen oxidising agents (from Chiang et al. (1978)).

POLYACETYLENE. A linear polymer of acetylene having alternate single and double bonds, developed in 1978. It is electrically conductive, but this property can be varied in either direction by appropriate doping either with electron acceptors (arsenic pentaflnoride or a halogen) or with electron donors (lithium, sodium). Thus, it can be made to have a wide range of conductivity from insulators to n- or >-type semiconductors to strongly conductive forms, Polyacetylene can be made in both cis and trans modifications in the form of fibers and thin films, the conductivity... 

Acetylene gas and the acetylide salts of heavy metals silver and copper acetyl-ides are extremely shock-sensitive. Polyacetylenes and some halogenated acetylenes. 

Finally, it must be mentioned that the above discussion refers only to pure polyacetylene samples. According to the results of calculations on halogen-doped polyacetylenes87 the presence of impurity atoms not only significantly changes the band structures but may completely reverse the conformations. 

Impurity and Aperiodicity Effects in Polymers.—The presence of various impurity centres (cations and water in DNA, halogens in polyacetylenes, etc.) contributes basically to the physics of polymeric materials. Many polymers (like proteins or DNA) are, however, by their very nature aperiodic. The inclusion of these effects considerably complicates the electronic structure investigations both from the conceptual and computational points of view. We briefly mentioned earlier the theoretical possibilities of accounting for such effects. Apart from the simplest ones, periodic cluster calculations, virtual crystal approximation, and Dean s method in its simplest form, the application of these theoretical methods [the coherent potential approximation (CPA),103 Dean s method in its SCF form,51 the Hartree-Fock Green s matrix (resolvent) method, etc.] is a tedious work, usually necessitating more computational effort than the periodic calculations... 

Table I. Structural Parameters Used in Calculations for Polyacetylene and Its Halogenated and Silylated Analogues...

The above values were selected, in the case of polyacetylene, from averages taken from available experimental and theoretical structural data. For the halogenated chains, appropriate values were selected from the results of both ab initio and CNDO/2 molecular orbital calculations and from structural data available on small-molecule analogues. Due to the unavailability of structural data for the silylated analogues, values were taken from those used for polyacetylene (except, of course, for those bonds and angles associated with an X atom) so as to deduce the effect of the substitution alone without inclusion of concomitant structural modifications along the polymer backbone resulting from such substitutions. 

When acetylenic halogen compounds are treated with base, dehydrohalogenation occurs followed by rearrangement to yield the more stable conjugated acetylenic compounds exclusively [ 123,124]. This reaction is used for the preparation of polyacetylenic compounds. 

Zeolites. The weak Raman signals arising from the aluminosilicate zeolite framework allow for the detection of vibrational bands of adsorbates, especially below 1200 cm which are not readily accessible to infrared absorption techniques. Raman spectroscopy is an extremely effective characterization method when two or more colored species coexist on the surface, since the spectrum of one of the species may be enhanced selectively by a careful choice of the exciting line. A wide range of adsorbate/zeolite systems have been examined by Raman spectroscopy and include SO2, NO2, acety-lene/polyacetylene, dimethylacetylene, benzene, pyridine, pyrazine, cyclopropane, and halogens. Extensive discussions of these absorbate/zeolite studies are found in a review article by Bartlett and Cooney. ... 

IODINE (7553-56-2) A powerful oxidizer. Material or vapors react violently with reducing agents, combustible materials, alkali metals, acetylene, acetaldehyde, antimony, boron, bromine pentafluoride, bromine trifluoride, calcium hydride, cesium, cesium oxide, chlorine trifluoride, copper hydride, dipropylmercury, fluoride, francium, lithium, metal acetylides, metal carbides, nickel monoxide, nitryl fluoride, perchloryl perchlorate, polyacetylene, powdered metals, rubidium, phosphorus, sodium, sodium phosphinate, sulfur, sulfur trioxide, tetraamine, trioxygen difluoride. Forms heat- or shock-sensitive compounds with ammonia, silver azide, potassium, sodium, oxygen difluoride. Incompatible with aluminum-titanium alloy, barium acetylide, ethanol, formamide, halogens, mercmic oxide, mercurous chloride, oxygen, pyridine, pyrogallic acid, salicylic acid sodium hydride, sodium salicylate, sulfides, and other materials. 

Among the classical (summarized in [11]) reactions of cyanoacetylenes (salt and n-complex formation, nucleophilic addition of, inter alia, amines and alcohols, Diels-Alder additions with 2 (see Section 2.22.2), addition of halogens and hydrogen halides, etc.) their polymerization might deserve a second look since the products formed - polyacetylenes - have attracted much attention during the last two decades. Thus 1, several of its derivatives, and 2 have been polymerized with anionic initiators (triethylamine, sodiiun cyanide, butyllithium) to give black, low-molecular-weight polymers claimed to have structures like 33 [38-40], which is obtained from 2 by treatment with butyllithium. 

Of the various halogen derivatives of the polyacetylenes structures 16-19 have been investigated by PE spectroscopy. As already stated in the previous section, there is a correlation found between the observed orbital energies of the symmetric dihalodiacetylenes 17 and the valence state ionization energies of the free halogen atoms. ... 

Polyacetylene reacts with chlorine rapidly to give a white polymer that is equivalent to poly(l,2-dichloroethylene) and no stereospecificity has been reported on the chlorinated polymer. The addition of halogens is an important general reaction of carbon-carbon double bonds in a stereospecific and regiospecific sense. For instance, the electrophilic additions of chlorine to ethylene, buta-1,3-diene and hexa-l,3,5-triene have been shown to proceed by 1,2-trans (anti), 1,4-cis (syn) and 1,6-trans (anti) attack, respectively. This stereospecificity has been rationalized with a mixing rule of a-n orbital interaction. The reaction of a long conjugated polyene like polyacetylene with chlorine may produce an atactic chlorinated polyene, because random l,2n additions occur to result in a random addition product. ... 

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