Polyacetylene, repeat unit structure

Polyacetylene, one of the most studied electrically conductive polymers has a simple repeating unit structure (—C=C—) and can be synthesized using a variety of methods [1-4], The nascent polymer can be readily doped to achieve high electrical conductivity... 

In real tran -polyacetylene, the structure is dimerized with two carbon atoms in the repeat unit. Thus the tt band is divided into occupied tt and unoccupied n bands. The bond-alternated structure of polyacetylene is characterishc of conjugated polymers. Consequently, since there are no partially filled bands, conjugated polymers are expected to be semiconductors, as pointed out earlier. However, for conducting polymers the interconnection of chemical and electronic structure is much more complex because of the relevance of non-linear excitations such as solitons (Heeger, 2001). 

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 containing a phenylcyclohexyl meso-gcnic side chain on every repeat unit has been synthesized by Yoshino et al. [158], employing the Araya method for a substituted 1-pentyne. The polymer shows the smectic A phase, and retains the layer structure upon cooling to the solid state. Only a low-angle diffraction corresponding to a spacing of 20-22 A is reported. 

From the viewpoint of its molecular structure, PPV can be considered an alternating copolymer of the repeat units of polyacetylene and poly(p<7r<7-phenylene) (Figure 1.35). With respect to electronic structure, it is one of the simplest non-degenerate ground state polymers, and as such it is attractive for theoretical approaches. 

Scheme 13.2 Schematic structures of polyacetylenes (a) acetylene (ethyne), (h) repeating unit of the trans polymer and (c) repeating unit of the cis polymer...

Figure 1.1 Chemical structures for monomers of the conjugated polymers discussed in this chapter (a) vinylene (the repeat unit of polyacetylene) (b) ethynylene (polydiacetylene is alternating vinylene-ethynylene) (c) phenylene (d) thiophene (e) leucoemeraldine form of polyaniline (f) pernigraniline form ofpolyaniline (the number of protons can vary between the...

Figure 7.1 Structural formulas and repeat units of a few n -conjugated polymers discussed in this chapter. First row polyacetylene (PA), poly(p-phenylene) (PPP), poly(p-phenylenevinylene) (PPV), polythiophene (PTh) and poly(thienylvinylene) (PThV). Second row the two valence tautomeric forms (A = aromatic and Q = quinonoid) of polyisothianaphthene (PITS), thienoacene ladder polymer (C2Sfad), thiohelicene (C2SHel) and thiaheterohelicene (THHel). The subscript x is used for polymers and n for oligomers...

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. 

Within the trans structure there is one acetylene per repeat unit and the factor group is isomorphous to the D2h print group. However, the situation for cis-polyacetylene is quite different. In this case cis-polyacetylene has two monomers per repeat unit with a factor group isomorphous to C2h symmetry. Hence the modes of vibration are different and are described by ... 

Most properties of the substituted polyacetylenes (PA), such as permeability to penetrants, solubility, and thermal stability are strongly related to the number and/or steric hindrance of die substituents 1-4). However, these properties cannot be related in a strai tforward fashion to the kind of substituents or to the mono- or doublesubstitution along the main chain. In fact, the configuration of the repeat units and, consequently, the chain conformation and packing have to be taken into account and might be responsible for the properties of Afferent polymers. For instance, it has been shown that structural differences originating from different cis/trans tactic ratio in... 

After polyacetylene, PPy is one of the simplest ECPs. The i-conjugated backbone of a neutral PPy resembles that of cis-polyacetylene, except PPy has an additional nitrogen heteroatom every four carbon atoms, linking between the 1st and 4th carbon atoms (Fig. 5b) [56]. The nitrogen heteroatom and the ring structure of the repeating unit stabilise the polymer s molecular structure. PPy is very attractive for... 

How much do the results we have obtained here tell us about the fundamental limits to the mobilities of carriers in devices fabricated with polymer that is very much better ordered than the polyacetylene that we have used here There are recent reports of very much improved mobilities for devices based on sexithiophene (the six repeat unit oligomer of polythiophene), with a value of 0.4 cm /Vsec now reported [73], and there is now considerable interest in the development of polymer FETs as large area thin film transistors, with interest in polythiophene derivatives [74] and in poly(arylenevinylenes) such as poly(2,5-thienylene vinylene) [75]. We can see from the optical characterisations of the MIS devices that the surface layer of polyacetylene formed on SiC>2 is very much more disordered than the bulk material, but we have not made FET devices with the polymer insulator layers which give better ordered structures as characterised optically. 

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