Polyethylene-polyacetylene structure

Acetylene (ethyne), C2H2, can be polymerized, (a) Draw the Lewis structure for acetylene and draw a Lewis structure for the polymer that results when acetylene is polymerized. The polymer has formula (CH), where n is large, (b) Consider the polymers polyacetylene and polyethylene. The latter has the formula (CH2)W and is an insulating material (plastic wrap is made of polyethylene), whereas polyacetylene is a darkly colored material that can conduct electricity when properly treated. On the basis of your answer to part (a), suggest an explanation for the difference in the two polymers. 

FIGURE 1.2 Electronic and molecular structures of (a) polyethylene and (b) polyacetylene. 

Polyacetylene retains multiple bonds along the chain. Electrons can be conducted through the series of tt-orbitals. A resonance form of the Lewis structure can be drawn, showing that the electrons may be delocalized along the polyacetylene chain. No such resonance form is possible for polyethylene. 

Fig. 4.1 The chemical structures of several relevant polymers are illustrated. There is a carbon atom at each vertex, and the hydrogen atoms are not shown. PE(CHf polyethylene, or PE, with only the C-H single bonds shown PA trans -polyacetylene PPV poly(p t -phenylenevinylene) and PPP poly(pita -phenylene). The lower three polymers are conjugated, according to the alternating single and double bond system.

The first conducting polymer was trans-polyacetylene which was doped with bromine and was produced at 1970s. Soon other conjugated polymers such as poly (p-phenylene), polypyrrole (PPy), polyethylene dioxythiophene (PEDOT) and polyaniline (PANi) and their derivatives which are stable and processable were synthesized. The molecular structures of a few ICPs are shown in Figurel. 

In the case of a polymer with a saturated chemical structure, such as polyethylene, the strength of a-bonding is such that the band gap will be comparable to that in diamond. However, for a polymer with a conjugated structure, such as polyacetylene, the chemical binding of the re-electrons is much weaker and a gap of a few eV, comparable to those in inorganic semiconductors, is anticipated. 

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. 

The simplest polymer with a conjugated backbone is polyacetylene. Its structure is similar to that of the saturated polymer polyethylene, but has one of the hydrogen atoms removed from each carbon of the polyethylene chain. Each carbon atom in the polyacetylene chain thus has one excess electron which is not involved in the basic chemical binding. And if the separation of the carbon were constant, polyacetylene would conduct along the chain in other words it would behave like a metal in one dimension. But unfortunately this is not true as the free electrons tend to get localized in shorter double bonds. Conjugated polymers can at best be expected to display semiconducting properties. 

Most polymers (typified by polystyrene and polyethylene) are electrically insulating and have conductivities doped with iodine to become electrically conducting (values have now been reported up to olO Scm ) represented a pivotal discovery in polymer science that ultimately resulted in the award of the Nobel Prize for Chemistry in 2000 [4]. The study of electrically conducting polymers is now well advanced and two extremes in the continuum of transport mechanisms exist. If the charge carriers are present in delocalized orbitals that form a band structure along the polymer backbone, they conduct by a delocalization mechanism. In contrast, isolated groups in a polymer can function as acceptors or donors of electrons and can permit... 

Ma, J., et al. 1991. Intrachain dynamics and interchain structures of polymers—a comparison of polyacetylene, polyethylene, polyaniline and poly(p-phenylene vinylene). Phys Rev B 44 11609. 

Figure 12.9. Schematic representation of single straight chains of polyacetylene (top) and polyethylene (bottom). Black circles represent the C atoms white spheres denote H atoms and single or double lines represent the single or double covalent bonds between atoms. The corresponding stable molecules, ethylene and ethane, are shown on the left. In polyacetylene all atoms lie on the same plane. In polyethylene the H atoms lie in front and behind the plane of the C-atom chain. Note the different angles between C-C atoms in the two structures 120° for the polyacetylene, 109° for polyethylene.

It is worthwhile emphasizing the difference in flexibility between polyacetylene and polyethylene. In polyacetylene, the planarity of the ideal structure makes it possible to have strong tt-bonds between the orbitals in adjacent C atoms. These... 

As mentioned above, the cr bands can loosely be referred to as C-C and C-H. In polyethylene, the upper bands are essentially C-H and the lower C-C [52). Because of the large dispersion in polyacetylene, however, this separation in energy does not occur. This is demonstrated in Fig. 23.11, where the evolution of the two cr bands is shown, going from an unfolded band structure,... 

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