Trans-polyacetylene forms

The process results in a cis-transoid structure. The formation of trans-polyacetylene is suggested to take place through isomerization of the new segment formed by cis insertion before it can crystallize.412... 

Woemer et al. 373) produced polyacetylene with locally oriented regions and an optical anisotropy of 2 x by polymerization on crystals of biphenyl. Yamashita and co-workers 374,375) have recently reported epitaxial polymerization of acetylene on crystals of anthracene, naphthalene and biphenyl where fibrils of cis- or trans-polymer formed, crystallographically aligned with the substrate. Fincher et al. 376) produced a 3 x extension which gave a 4 x optical anisotropy. 

The cis-trans content of polyacetylene formed with coordination catalysts depends on the polymerisation temperature the increasing trans content with increasing polymerisation temperature has been explained in terms of a thermally induced cis-trans isomerisation which occurs before crystallisation of the chain segment [10,76 78],... 

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). 

Figure 1. TYie trans isomeric form of polyacetylene shovn in its planar trans and cis conformations, with pertinent si )stituents numbered.

Acetylene selectivity polymerizes in the presence of Ziegler catalysts whose components have low Lewis acidity [e.g., Ti(0-n-Bu)4—Et3Al(l 4)J. Cis-polyacetylene forms at low temperature, and trans-polyacetylene at high temperature (Eq. (1)). When doped, a polyacetylene film shows metallic conductivity, and hence the application of polyacetylene to polymer batteries and solar cells is now under intensive study 1-3). 

Transition-metal containing zeolites such as CoY and NiY (but not the Cu, Mn and Zn forms) polymerize acetylene to give trans-polyacetylene with relatively short conjugation length, as indicated by resonance Raman spectroscopy.70 The pol3nmerization products appeared to be restricted to the zeolite crystal surfaces. The authors also point to die importance of Lewis acidic centers for the polymerization. 

Suspensions of polyacetylene were prepared as burrs or fibers (46) by using a vanadium catalyst. When the solvent was removed, films of polyacetylene were formed with densities greater than that prepared by the Shirakawa method. These suspensions were mixed with various fillers to yield composite materials. Coatings were prepared by similar techniques. Blends of polypyrrole, polyacetylene, and phthalocyanines with thermoplastics were prepared (47) by using the compounding techniques typically used to disperse colorants and stabilizers in conventional thermoplastics. Materials with useful antistatic properties were obtained with conductivities from 10" to 10" S/cm. The blends were transparent and had colors characteristic of the conducting polymer. For example, plaques containing frans-polyacetylene had the characteristic violet color exhibited by thin films of solid trans-polyacetylene. 

The work of Su Shrieffer and Heeger (11) focussed on conjugated polymers having two valence bond structures, say A and B. In the case of trans polyacetylene, where A and B are degenerate, they showed intrinsic defects can occur, in which A phase goes continuously over to B phase, forming a soliton kink. 

Fig. 97. Resonance forms for the different lattices of tran -polyacetylene.

As an example we examine some results obtained for trans-polyacetylene. The Wannier function of this polymer can be expressed in the LCAO form... 

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