Polyacetylene trans isomer

The discovery of junipal focused the attention of Sorensen, who had been investigating the occurrence of polyacetylenes in Com-positae, on the possibility that these acetylenes were accompanied by thiophenes. From Coreopsis grandiflora Hogg ex sweet, 2-phenyl 5-(1-propynyl) thiophene (240) was isolated and its structure confirmed by synthesis of the tetrahydro compound, 2-phenyl-5-n-propyl-thiophene. From the root of tansy, the cis and trans isomers of methyl 5-(l-propynyl)-2-thienylacrylate (241) have been isolated. The total synthesis of trans (241) was achieved by reacting junipal with methylcarbethoxy triphenylphosphonium bromide (Wittig reaction) Several monosubstituted thiophenes, (242), (243), and... 

The acetylene reaction has been studied also at 77 K [409]. It has been reported that at this temperature the reaction occurs at 12.5 GPa and that the reaction proceeds to saturation and then accelerates on pressure release. In this low-temperature experiment, it has been found that the product contains also c -polyacetylene and that a transformation to the trans isomer occurs on heating. 

The reaction of dienophiles with polyacetylene (33) was investigated for limiting the decomposition of the polyene. Previously, intermolecular Diels-Alder reactions involving ds bonds were believed to form cross-links and render polyacetylene intractable, and that their elimination would lead to soluble products (34). No reaction of maleic anhydride or benzoquinone with polyacetylene was observed. In contrast, a second study (35) described the successful formation of the Diels-Alder adduct of maleic anhydride with remnant cis bonds in thermally isomerized samples of fran -polyacetylene. These data were used to support the notion that the thermal isomerization process used to convert cw-polyacetylene to the trans isomer does not go to completion. Unfortunately, the effect of this transformation on the tract-ability or stability of the polymer was not noted. 

Both cis and trans forms of polyacetylene can be made with the trans isomer being the most stable. 

Polyacetylene is a colorful polymer. The cis isomer transmits red light, the trans isomer blue, and because these polymers often come in mixtures of cis and trans, various shades of purple result. If well formed films of polyacetylene are made, the surfaces reflect silver and, sometimes, gold. Polyacetylene in powder form appears black. The vacuum line experiments produced membranes which appeared purple when wet with solvent (transmitted light - the membrane is transparent when wet) and silver when dry (reflected light-the membrane is opaque when dry). 

The electronic structures of poiy(fluoroacetylene) and poly(difluoroacetylene) have been investigated previously using the ab initio Hartree-Fock crystal orbital method with a minimum basis set (42). Only the cis and trans isomers with assumed, planar geometries were studied. The trans isomer was calculated to be more stable in both cases, and the trans compounds were predicted to be better intrinsic semiconductors and more conductive upon reductive doping than trans polyacetylene. However, our results show that head-to-tail poly(fluoroacetylene) prefers the cis structure and that the trans structure for poly(difluoroacetylene) will not be stable. Thus the conclusions reached previously need to be re-evaluated based on our new structural information. Furthermore, as noted above, addition of electrons to these polymers may lead to structural deformations that could significantly change the conductive nature of the materials. 

The first work reported was done at the beginning of the eighties and dealt with polyacetylene. (CH)x has been characterised under different forms foam [32] iodine doped [33] AsFs doped [34] IrClg doped (which exhibits a giant dielectric constant) [35] encapsulated [36] cis and trans isomers [37] anisotropic [38]. The volution of the transport mechanism with doping level has been studied by measuring evolutions of Oj)c and (75.5 GHz with temperature [39]. An analysis based on the fibrillar structure of polyacetylene has been given. Recent works have been published, as in the case of n-doped polyacetylene [40]. 

The synthesis of the polyacetylene powder has been known since the late 1950s, when Natta used transition metal derivatives that have since become known as Ziegler-Natta catalysts. The characterization of this powder was difficult until Shirakawa and coworkers [18] succeeded in synthesizing lustrous, silvery, polycrystalline films of polyacetylene (which has become known as Shirakawa polyacetylene) and in developing techniques for controlling the content of cis and trans isomers ... 

As discussed in A Word About... Polyacetylene and Conducting Polymers (p. 421), thin films of all-cis or all-trans polyacetylene have different appearances, being coppery or silvery, respectively. Draw structures of the all-cis or aW-trans isomers of polyacetylene as well as the alternating cis-trans isomer. 

The polymerization of acetylene by using [Rh(l,5-Cod)Cl]2, where 1,5-Cod is c/j,cw-cycloocta-l,5-diene, or [Rh(NBD)Cl]2, where NBD is bicyclo[2,2,I]hepta-2,5-diene, was studied by UV-vis spectroscopy [79,80]. The growing polyacetylene chains were identified by three maxima at 500, 544, and 590 nm as a result of subtracting the spectrum of the catalyst from that of the reaction mixture. The first-order derivative of the absorption spectrum of the growing polyacetylene exhibited vibrational maxima at 480, 515, 550, and 600 nm for the cis-isomer and at 640, 670, and 710 nm for the trans-isomer. UV-vis and FTIR spectroscopies were used in the study of the structure of thin freestanding films of cis- and trans-PA obtained by using Rh(I) complexes. The absorption spectrum shows no vibrational structure, which was detected in acetylene polymerization in ethanol. The microstructure of PA is very similar to that of PA synthesized with a Luttinger catalyst in terms of sp defects in the polymer chains detected by FTIR spectroscopy. 

As shown in Figure 21.2, four steric (geometric) structures are theoretically possible for polyacetylenes, that is, cis-cisoid, cis-transoid, trans-cisoid, and trans-transoid, because the rotation of the single bond between two main chain double bonds in the main chain is more or less restricted. Polyacetylene can be obtained in the membrane form by use of a mixed catalyst composed of Ti(0-n-Bu)4 and EtsAl, the so-called Shirakawa catalyst (1) both the cis- and trans-isomers are known, which are thought to have cis-transoidal and trans-transoidal structures, respectively (Table 21.1). Phenylacetylene can be polymerized with a Ziegler-type catalyst, Fe(acac)3/Et3Al (2) (acac = acet-ylacetonate), Rh catalysts (7), and metathesis catalysts (3-5) that contain Mo and W as the central metals, to provide cis-cisoidal, cis-transoidal, cis-rich, or trans-rich polymers, respectively. 

Figure 5.57. Synthetic routes for cis- and trans-polyacetylene. It should be noted that the trans-isomer of PA is more stable than the cis-isomer since the former has two degenerate ground states (two energetically-equivalent arrangements of alternating double bonds).

The situation is more intriguing if we go from sp to sp molecules, that is, to hydrocarbons. Polyacetylene C2 2)x is the prototype of the whole family of conjugated polymers. It has two isomers, the trans and cis forms (see Fig. 1). The trans isomer is more stable than the cis one. The experimentally observed bond lengths are 136/144 and 137/144 pm for trans and cis isomers, respectively. [Note that these bond lengths differ from that of true double bond (133 pm) and single bond (154 pm)]. 

Electronically intermediate between the polyacetylene example with one Jt electron per center and the planar analog of the sulfur chain with two such electrons per center is the (SN) , polymer with three tt electrons per SN atom pair [10]. Let us approach this problem from the Hiickel perspective. The band structure of the trans isomer 13.32 is shown in Figure 13.12 where we have chosen a unit cell containing... 

The Starting point for the production of oriented films is the Durham precursor route (figure 1) which was developed by Feast and coworkers [3], This route utilises a soluble, non-conjugated precursor polymer, polymer B, which can be converted to a fully dense form of polyacetylene by the thermal elimination of hexafluoroorthoxylene [4-6], This conversion process occurs in three stages [6,15,16], The initial elimination of aromatic units from the polymer (transformation) is followed by evaporation of the volatile product from the polymer and, finally, isomerisation of the cis-rich material formed after transformation to the trans-isomer. The kinetics of these reactions have been extensively studied using DSC, i,r, spectroscopy and weight-loss measurements [6],... 

Table 1 Destabilization of cis-transoid and trans-cisoid polyacetylene with respect to the all-trans isomer. Energy values are given in kcal/mol per...

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