Double bonds, trans-polyacetylene

POLYACETYLENE. A linear polymer of acetylene having alternate single and double bonds, developed in 1978. It is electrically conductive, but this property can be varied in either direction by appropriate doping either with electron acceptors (arsenic pentaflnoride or a halogen) or with electron donors (lithium, sodium). Thus, it can be made to have a wide range of conductivity from insulators to n- or >-type semiconductors to strongly conductive forms, Polyacetylene can be made in both cis and trans modifications in the form of fibers and thin films, the conductivity... 

The polymer of 138 is more stable than that of 136 but its conversion to polyacetylene is much more exothermic and not so easily controlled406. The double bonds B formed in the retro-Diels-Alder reaction are initially cis, as may be some of the double bonds A formed in the metathesis reaction, but these quickly isomerize to trans above 100 °C. In a DSC... 

We choose as model system the simplest conjugated polymers - the trans-polyacetylene. This compound shows a. dimerized structure with an altemance between double bond (1.35A) and single bond (1.45A) the monomer is then simply a double bond (see figure (8)). The tt electrons are assumed to be effectively describe not by the full PPP Hamiltonian but by the Extended Peierls-Hubbardmodel (EPH) [17, 14], a short version of it) for simplicity... 

Figure 8. Model for the trans-polyacetylene and the system of bonding - antibonding levels associated with each double-bond.

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.

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. 

X,n,ax> indicating a slightly reduced effective conjugation length of 20-25 double bonds. In fact, trans-po y sec-h xty COT) displays the highest observed effective conjugation length for a soluble polyacetylene. 

The soluble poly(RCOT)s undergo a photochemical cis/trans isomerization in solution - a reaction that is not observed for unsubstituted polyacetylene in the solid state. The photochemical cis/trans isomerization of poly(trimethylsilylCOT) can be followed by visible spectroscopy (Fig. 10-26) [124], Here, the chromophore is a collection of double bonds with an ef-... 

Polyacetylene reacts with chlorine rapidly to give a white polymer that is equivalent to poly(l,2-dichloroethylene) and no stereospecificity has been reported on the chlorinated polymer. The addition of halogens is an important general reaction of carbon-carbon double bonds in a stereospecific and regiospecific sense. For instance, the electrophilic additions of chlorine to ethylene, buta-1,3-diene and hexa-l,3,5-triene have been shown to proceed by 1,2-trans (anti), 1,4-cis (syn) and 1,6-trans (anti) attack, respectively. This stereospecificity has been rationalized with a mixing rule of a-n orbital interaction. The reaction of a long conjugated polyene like polyacetylene with chlorine may produce an atactic chlorinated polyene, because random l,2n additions occur to result in a random addition product. ... 

The order of stability of the polymers is 5P < 7P < 6P < 9P ss lOP < IIP. The last two are quite stable at room temperature and must be heated to 100-200°C to effect their conversion to polyacetylene. Polymers made using WCVMe4Sn as catalyst (MW 40 000) contain comparable proportions of cis and trans double bonds (Edwards 1984). Polymer 8P, unlike 6P, is quite stable because the retro-Diels-Alder reaction is less favoured (it would give a cyclohexadiene, not a benzene derivative). Polymers of 8 having various cis contents (6-77%) have been prepared, but no information is available concerning their tacticity (Feast 1987). 

The configuration of the double bonds strongly depends on the temperature of polymerization. The trans content of polyacetylene prepared by the Ziegler-Natta catalysts decreases with decreasing polymerization temperature, as listed in Table 29.3, determined by infrared spectroscopy. 

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