Soluble Polymers and Latices

Polyacetylene prepared by the Shirakawa route pyrolyses on heating, before showing any detectable crystal melting point. At the same time, it is insoluble in all known solvents. For these reasons it is essentially unprocessable. Until recently it has seemed to be a general rule that all conducting polymers were insoluble, which follows naturally from the conjugation of the double bonds along the chain which results in chain stiffness. 

In 1982, Soga et al. 256 showed that exposure of acetylene to AsFs at low temperatures leads to rapid polymerization (in our experience this reaction can be explosively violent). The product is a solid polymer which is heavily arsenic-doped and has a conductivity several orders of magnitude lower than a conventional sample of polyacetylene saturation-doped from the gas phase. Aldissi and Liepins 2S7) have adapted this reaction to the preparation of soluble polyacetylene by adopting AsF3 as the reaction solvent. They claim that polymerization of acetylene with AsF5 is very rapid, giving a polymer which is soluble in common solvents. However, elemental analysis shows that the polyacetylene formed contains about one As atom per 10 CH units and this is not removed on repeated reprecipitations. It seems likely that the As atoms form part of the chain backbone, conferring sufficient flexibility to allow dissolution. It is claimed that films of soluble polyacetylene can be doped but very little information has been published. 

The use of AsF3/AsF5 mixtures as solvents for heavily doped conducting polymers 

The availability of soluble polymers has permitted measurements of molecular weight, as discussed below. The glass transition temperatures are quite low, from 145 °C for polymethylthiophene to 41 °C for polybutylthiophene. X-ray diffraction shows a broad peak which has been interpreted by different groups as showing a structure which is either partly crystalline or is amorphous. The temperature of fusion decreases from 280 °C to 80 °C as the length of the alkyl chain increases from 4 to 22 carbons 263). 

When doped, these polymers show conductivities in the range from 0.5 to50Scm 1, as illustrated in Table 1 (page 17) Kaeriyama and co-workers 264 show from optical spectroscopy that the band gaps increase with the size of the side group. In the doped states the spectra were all similar, but the conductivity drops with the size of the side group. 

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