Polyacetylene geometric structure

Polyacetylene is a polymer which has a degenerate ground state it possesses two geometric structures having exactly the same energy and differing only in the sequence of the carbon-carbon single and double bonds (Fig. 9.6). 

The geometrical structure of rrawx-polyacetylene, which is the stable polyacetylene stereoisomer at room temperature, is illustrated in Fig. 4.1. Of particular interest is the series of alternating single and double bonds. Since the carbon atoms are sp2-hybridized (each carbon atom has three nearest neighbours), the remaining pz atomic... 

The polymers whose geometric structures have been quantitatively evaluated are polyacetylene 89), poly(ferf-butylacetylene)19), poly(isopropylacetylene)14), and poly-(phenylacetylene)88). In the case of polymers from aromatic monosubstituted acetylenes, qualitative evaluation of geometric structure is possible by means of IR spectroscopy, differential thermal analysis, and X-ray diffraction 66,90). In contrast, no information has been obtained on the geometric structure of disubstituted acetylene polymers. This is due to the fact that their main chain comprises fully substituted ethylene units, the difference between cis and trans structures being small. 

Table 26 shows X-ray diffraction data of polyacetylenes and those of polyethylenes for comparison. The ratios of half-height width to diffraction angle (A28/28) for the substituted polyacetylenes are all larger than 0.20. The value for amorphous polyethylene is similar to these values, while those of crystalline polyethylene and cis-polyacetylene89) are much smaller. Therefore, it is concluded that the present polymers are amorphous. This must be due to the presence of bulky substituents and/or the non-selective geometric structure of the main chain. 

Figure 1.7 Top schematic illustration of the geometric structure of a neutral soliton on a ra s-polyacetylene chain. Bottom band structure for a traws-polyacetylene chain containing (a) a neutral soliton, (b) a positively charged soliton and (c) a negatively charged soliton. (Reprinted with permission from Accounts of Chemical Research, 18, 309. Copyright (1985) American Chemical Society.)...

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. 

Polyacetylenes are the most typical and basic r-conjugated polymers, and can ideally take four geometrical structures (trans-transoid, trans-cisoid, cis-transoid, cis-cisoid). At present, not only early transition metals, but also many late transition metals are used as catalysts for the polymerization of substituted acetylenes. However, the effective catalysts are restricted to some extent, and Ta, Nd, Mo, and W of transition metal groups 5 and 6, and Fe and Rh of transition metal groups 8 and 9 are mainly used. The polymerization mechanism of Ta, Nd, W, and Mo based catalysts is a metathesis mechanism, and that of Ti, Fe, and Rh based catalysts is an insertion mechanism. Most of the substituted polyacetylenes prepared with W and Mo catalysts provide trans-rich and cis-rich geometries respectively. Polymers formed with Fe and Rh catalysts selectively possess stereoregular cis main chains. 

The term A plays the opposite role the polarization by the end groups induces a reduction of the bond alternation in both the ground and excited states [151,1521, thus producing more similar geometrical structures in the two relevant electronic states. The fact that the intensities are even larger with respect to those of the apolar species indicates that the structure is far from a perfectly equalized CC chain even in the ca.se of very efficient end groups. Notice, however, that the zwitterionic structure can be stabilized by solvent effects [151-153]. It is shown in Section X.C that the chain structure of compounds VI and Vll can be equalized by the choice of a suitable polar solvent in this case a marked decrease of the Raman cross section (by virtue of the reduction of AQk ) is observed. This finding is very relevant, especially if compared with the experimental data from Raman spectra of heavily doped polyacetylene, where a drastic reduction in the Raman cross section was observed (Section V). 

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. 

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. 

Also shown in Table lO-l is an (alkylcyclohexylaryloxy)-substituted polyacetylene [77]. Polymers of this general structure have been found to display liquid-crystalline behavior. In contrast to vinyl-based liquid-crystalline polymers, the geometric isomerism of the main-chain double bonds plays a role in determining the type of phase that is found. Advincula et al. have examined Langmuir films of polyacetylenes at the air-water interface [78]. Polyacetylene derivatives are unusual in that the polymer backbone itself acts as a chromophore therefore, in studies such as these, UV-visible spectroscopy can be a sensitive probe of polymer conformation. 

Self-localized excitations and corresponding chemical terminologies are listed in Table 4-2. Schematic structures of the self-localized excitations in poly(p-phenylene) and tranj-polyacetylene are depicted in Figure 4-3. In these illustrations, the charge and spin are localized on one carbon atom. In real polymers, however, they are considered to be localized over several repeating units with geometric changes. 

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