Polyacetylenes trans -cisoid

Fig. 1. Possible structures for polyacetylene chains showing the two degenerate trans-structures (a) and (b), and the two non-degenerate cis-structures, (c) cis-transoid and (d) trans-cisoid and (e), a soliton defect at a phase boundary between the two degenerate trans-phases of polyacetylene, where the bond alternation has been reversed.

O j-polyacetylene shows 10 reflections in x-ray diffraction 439). The unit cell was identified as orthorhombic, a = 761 pm, b = 447 pm, c = 439 pm with a density of 1.16 g cm-3. In the original x-ray study the b axis was taken to be the chain axis but subsequent electron diffraction studies allowed fibre patterns to be obtained 440). On the principle that, for all polymers, the fibre axis is the chain axis, c was identified as the chain direction although there is some dispute The analysis cannot definitely distinguish between the cis-transoid and trans-cisoid structures. 

Figure 2 cis-Polyacetylene structures regular (Model III) trans-cisoid (Model IV) and cis-transoid (Model V). The unit cells are surrounded by broken lines... 

Figure 4 Relative total energies per elementary cell of four polyacetylene models as function of the number of interacting cells (N). As reference for each value of N the corresponding energy of the cis-transoid model has been chosen. regular trans, O alternating-Xrzm, A regular-cis, trans-cisoid, A cis-transoid...

The Conduction and Valence Band of cis-transoid and trans-cisoid Polyacetylenes and of Polydiacetylene (ideal Acetylene Structure) in eV s... 

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. 

C/5-polyacetylene is very different from trans-poly-acetylene, as far as solitons are concerned. Certainly the band structure is more complicated because there are four atoms in an elementary cell along the chain instead of two. But the crucial difference is related to symmetry. In Figure 1.31 a soliton in trans-polyacety-lene is set in contrast to one in cw-polyacetylene. In rra 5-polyacetylene there is a mirror plane at the soliton, but not in cis. In cw-polyacetylene a soliton separates a cw-transoidal domain from a tran -cisoidal. The latter is richer in energy and the soliton is pushed... 

There could also be polarons in c/5-poIyacetylene, and these could be mobile. Even without interchain interaction there is a soliton-antisoliton attraction in cis-polyacetylene because of the energy difference between the cw-transoid and the trans-cisoid structure. Figure 1.32 shows polarons and bipolarons in cw-polyacety-lene. In polyaraphenylene the situation is very similar... 

Computed cis-trans energy differences should be less influenced by the intrinsic failures of the Hartree Fock approximation. In table 1 we compare energies of all-trans, cis-transoid, and trans-cisoid polyacetylenes. In agreement with the experimental results... 

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...

The experiments showed clear differences of behavior between polyacetylene in the cis and trans forms. Namely, SEE and OE are obtained in the cis and trans forms, respectively. In other words, the electronic spins are fixed in ds-polyacetylene, and they become mobile in the trans form. This result is quite consistent with the soliton picture. In cw-polyacetylene, a bond alternation defect divides the chain into two parts ds-transoid and frans-cisoid, whose energies are different (Fig. 8a). Thus, to minimize energy, the spin defect will be trapped at one end of the chain (Fig. 8b). On the other hand, in trans-polyacetylene the chain is divided into two degenerate parts. The defect should therefore be free to move (Fig. 9). 

PolyaCGtyiGnes. Polyacetylene, one of the most studied of the EAPs, possesses the simplest EAP structure, consisting of alternating single and double carbon-carbon bonds, in either a cisoid or transoid configuration. (Fig. 1). The transoid configuration is thermodynamically favored, but cis-trans isomerization is a reversible process. 

When acetylene is polymerized by the Shirakawa technique at low temperature, c. — 70°C, the product film has a yellow-gold appearance, it is an insulator with a conductivity of c. 10 (Qcm) and it has a low free spin density. Diffraction and spectroscopic studies establish that there is regular single-double bond alternation and that the material is predominantly the cis-cisoid homopolymer. However, this material is stable only at low temperature, and when the polymerization is conducted at room temperature a different material with a silvery appearance and a predominantly trans-transoid structure is obtained essentially the same material is produced when the cis polymer is allowed to warm to room temperature. This form of polyacetylene is a semiconductor with conductivity in the to... 

---