Isomers of polyacetylene

As in any other chemical compound, different geometrical arrangements of substituent groups are possible in a polymer where rigid molecular units are involved. This gives rise to trans- and cis-configurational isomerism in polymers containing double bonds in their repeat units, as in polyacetylene and natural and synthetic rubbers. The structures of the trans- and m-isomers of polyacetylene and polybutadiene are illustrated in Fig. 1.8. 

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. 

Fig. 1 Trans (trans-transoid) and cis (cis-transoid) isomers of polyacetylene, above and below, respectively...

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... 

Polyacetylene aldehydes undergo a curious dimerization with loss of carbon monoxide " - . The reaction occurs spontaneously when concentrated solutions of the aldehydes 230 are allowed to stand at room temperature giving both Z and E isomers of the dimeric aldehydes 231. p-Substituted 5-phenyl-2,4-pcntadiynals (232)... 

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). 

Polyacetylene (CH)X is one of the simplest conjugated organic polymers. A number of quantum-chemical calculations with respect to the electronic structure of this substance have been accumulated up to the present. There can be distinguished two geometrical isomers of (CH) chains, namely, trans and cis. The trans- and cis-type chains are further classified into two and three structural isomers, respectively, in terms of the relative position of the C=C bonds. 

Poly(phenylacetylenes) with ortho substituents on the phenyl ring and poly(2-ethy-nyl-IV-methylpyridium) derivatives [73] form an interesting subclass of substituted polyacetylenes. It has been found that the absorption maxima of the cis isomers of the phenyl polymers shift to longer wavelength as the size of the substituent is increased. For example, poly(o-me-thylphenylacetylene) absorbs at 440 nm, and poIy(o-trimethyIsiIyIphenyIacetylene) absorbs at 520 nm [74]. Evidently, the steric requirements, of the ortho substituents impose a planar conformation on the backbone (Fig. 10-9). 

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 first conducting polymer to be synthesised was polyacetylene. When polymerised, acetylene (ethyne) forms a silvery flexible film of polyacetylene. Acetylene (ethyne) has a formula C2H2. The carbon atoms are linked by a triple bond, consisting of 1 sp-hybrid a bond and two n bonds (Scheme 13.2). Generally, polymerisation leads to the aU-cii polymer. At room temperature this changes to the thermodynamically stable ail-trans form. These two forms are geometrical isomers (see Section S2.1). Both are poor insulators, with the trans form having a conductivity similar to that of silicon (approximately 10 S m ), and the cis form with a conductivity similar to that of water (approximately 0.1 S cm ). 

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 ... 

The morphology of all polyacetylenes made with various Ziegler-Natta catalysts is fibrillar. The conductivity of films of polyacetylene depends on the cis-trans content, varying from 10 (ohm-cm) for the trans material to 10 (ohm-cm) for the cis isomer. Doping increases the conductivity of both cis- and frons-polyacetylene drastically. 

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).

There are two possible isomers of pure polyacetylene (the cis and trans forms) that show characteristic differences in their physical and chemical properties. The cw-isomer is thermodynamically unstable in pure form. Its doped form shows, however, electrical conductivities that exceed in some cases those of the -modification." Early experimen-... 

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