Polyacetylene geometric isomers

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. 

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 ). 

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 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. 

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