Polyacetylenes conformations

In a nondoped state, the basic polymers have low conductivity (Figure 5.14). The two polyacetylene conformations cis and trans are in the semiconductor range PPP is a good insulator. 

Polyacetylenes are the most important class of synthetic polymers containing conjugated carbon-carbon double bonds. Some optically active monomers have been used with the following conclusions. Polymers of 1-alkynes having a branched side-chain assume in solution a helical conformation. A chiral side-chain induces a predominant screw sense in these helices. In particular, for alkyl branching, it has been shown that (S) monomers lead to a left-handed screw sense. 

Schottky photovoltaic elements were proposed because of the good conformity of the solar spectrum with the polyacetylene forbidden gap. The photovoltaic spectra for (CH)n-Al structure and volt-current characteristic for (CH) -nCdS heterojunction are presented in Fig. 19 and Fig. 20 respectively. 

Several systems have been examined in the context of potential tris- and higher homoaromatic systems21. These include cis. cis, cis-1,4,7-cyclonon atriene (128), triquinacene (129), hexaquinacene and the cyclic polyacetylenes such as 130. The conformations of some of these systems are such that they could be considered to be examples of in plane homoaromatic systems266. 

The Durham precursor route to polyacetylene is an excellent example of the application of organic synthesis to produce a precursor polymer whose structure is designed for facile conversion to polyacetylene. Durham polyacetylene was first disclosed by Edwards and Feast, working at the University of Durham, in 1980 227). The polymer (Fig. 6 (I)) is effectively the Diels-Alder adduct of an aromatic residue across alternate double bonds of polyacetylene. The Diels-Alder reaction is not feasible, partly for thermodynamic reasons and partly because it would require the polymer to be in the all m-conformation to give the required geometry for the addition to take placed 228). However, the polymer can be synthesised by metathesis polymerization of the appropriate monomer. 

Pleated sheet conformation, 30,31 PLEDs (polymeric light-emitting diodes), 218 Plexiglas, 62 Plunkett, Roy, 65-66 PMMA. See Poly(methyl methacrylate) Polartec (Polar Fleece), 194 Poly(6-aminohexanoic acid), 25 Poly(a methyl styrene), 20 Polyacetylene, 72, 73 Polyacrylamide, 20 Polyamides, 22, 28, 61, 146 biodegradable, 185 Polyaramids, 77, 86 Polybutadiene, 70,109,148,155 Poly butyl acrylate), 20 Poly(butylene isophthalate), 25 Polycaprolactam, 21 Polycarbonate (PC), 17, 48, 86, 140 biodegradable, 185 density of, 247 impact strength of, 143 permeability of, 163 Polychloroprene, 65 Polycondensation, 85, 90-91 interfacial, 91-92... 

Finally, it must be mentioned that the above discussion refers only to pure polyacetylene samples. According to the results of calculations on halogen-doped polyacetylenes87 the presence of impurity atoms not only significantly changes the band structures but may completely reverse the conformations. 

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