_NA_ru__ ~_v_o_L_ . ~__ ~I_A_u_a_u~--~~----------------------NN~E~W~S~A~N~D~V~IE~W~S~-----------------------------------~~7 Materials science Preparation of soluble conducting polymers from Paul Calvert CONDUCTING polymers have been the focus of a great deal of research in the last H c1 ~ c1 ~ few years. The interest started with the -t--5=-c-c-c- ) discovery that acetylene could be polym­ H H A H H erized to a shiny black film, polyacetylene n (I in the figure), which became highly con­ ducting when 'doped' by reaction with large amounts of strong oxidizing or reduc­ ing agent l-J. The conductivity is at­ tributable to the ease of electron transport along the chain. Many other polymers with similarly conjugated double bonds have been found to conduct when doped. m The promise offered by these materials is of a class of conductors or semiconductors that would carry the benefits associated with polymers: cheapness, toughness and easy processing to fibres, films and coatings on a large scale. Unfortunately most of these convenient properties are lost y in the conjugated systems whose chain rigidity and strong interchain forces make IY them insoluble infusible black powders. Various routes recently used in attempts to solubilize conducting polymers. I, polyacetylene; II, Recent months have seen the publication of polyvinylchloride; Ill, precursor polymer; IV, esterified polycyclohexene; V, polyphenylene. a number of routes to solubilizing conduc­ ting polymers (see the figure). One route is to prepare a flexible soluble 20,000 molecular weight attached to poly­ arsenic pentafluoride contaminated with precursor polymer which can be readily isoprene6. These soluble polymers have a arsenic trifluoride. AsF 3 had little effect on cast into films before conversion to poly­ separate acetylene phase in the solid state the polymer itself but seemed to be a strong acetylene. In principle, polyvinylchloride and yield interesting spectroscopic data but plasticizing agent for the AsF 5-doped state. (PVC; II in the figure) should do this there is no mention of conductivity. Galvin Using liquid AsF3 Frommer and his col­ because it is degraded on heating by loss of and Wnek 7 have described previously a leagues now report that the doped polymer HCl to leave a black polyacetylene-like similar system where acetylene is polymer­ dissolves to a blue solution from which they material. However the result is not con­ ized inside polyethylene to produce a con­ can cast tough conducting films. When cast ducting when doped, apparently because ducting composite. the film will not redissolve because of removal of adjoining H, Cl pairs at random Polyacetylene has the disadvantage that cross-linking. It seems often to be the case will leave many isolated Cl or H atoms it is not very stable to oxidative degradation. that such powerful dopants also cause side which break up the conjugated sequences. From this point of view polyphenylene is a reactions and cross-linking. A way of avoiding this is to pair up the leav­ much more attractive polymer but it has on­ This year a mood of realism has been ing groups beforehand. Jim Feast and his ly been available so far as an intractable spreading over this rapidly growing field. co-workers have produced a number of powder of very short chain length. Ballard Various applications for conducting poly­ precursor polymers which lose cyclic com­ and co-workers at ICI Runcorn are now mers have been proposed in the past, in­ pounds on heating and transform to poly­ reporting 8 a route through a soluble precur­ cluding solar cells, semiconductor devices acetylene4·5. The unnameable precursor sor polymer, an esterified polycyclohexene and batteries, but these are still a long way polymer (III) is soluble in acetone and (IV) which converts to polyphenylene (V) on off and a polymeric replacement for cop­ forms transparent films which, at heating to 240°C. per wires looks unlikely. Those interested S0-80°C, lose hexafluoro-o-xylene and 70 This will be of great interest because the in silicon semiconductors have been point­ per cent of their weight to leave black shiny polyphenylenes studied to date have been ing out that the polymers are impure dopable amorphous polyacetylene. In­ . poorly characterized materials with short unstable uncharacterized junk compared terest is centring on the fact that this chain lengths and it offers the exciting pro­ with single-crystal silicon. The availability material is different in structure and pro­ spect of being able to control the chain of soluble materials that can be purified perties from polymer prepared by direct length and composition and study the ef­ and characterized should improve this state polymerization, but characterization of fect on properties. An added bonus is that ofaffairs. 0 black insoluble films is no easy task. there is a bacterial fermentation route from Another approach to solubility is to Paul Calvert is at the School of Chemistry and benzene to the precursor. This may not Molecular Sciences, University of Sussex, modify the polymer to build enough flexi­ materially affect the final polymer but it Fa/mer, Brighton BNI 9QJ. bility into the chains to allow them to has enabled Ballard and colleagues to per­ dissolve without losing conductivity. The form the remarkable feat of joining both I. See Calvert, P. NaTure 284, 213 (1980). results of copolymerization with small the biotechnology and conducting polymer 2. Wegner, G. Agnew. Chern . in£. Edn 20,361 (1981) . 3. Baughman, R.H. e£ al. CMm. Rev. 82, 209 (1982). amounts of polymethylacetylene have been bandwagons at once. 4. Edwards, J.H. & Feast, W.J. Po(vmer2J, 595 (1980). disappointing because conductivity rapidly Recently published studies on another of 5. Feast, J . Polymer(submittcd). 6. Ba les, F.S. & Baker, G.L. Macromolecules 16, 704(1983). decreased as side groups were added to the these polymers, polyphenylenesulphide, at 7. Galvin. M .E. & Wnek, G.E. Polymer 23, 795 (1982). chain. Allied Chemical, illustrates the complexity 8. Ballard, D.G.H .. Courl is , A .. Shirley, I.M . & Taylor. 9 S.C. Chem. Commun. (submitted). Workers at Bell Laboratories now report of solubilizing them • Strange effects were 9. Frommer, J.E. , Elsenbaumcr, R.L. & Chance, R. R. ACS graft copolymers with acetylene chains of found when this polymer was doped with ORPL Preprinfs 48, 552 (1983). IJ028-1JS36/ 831320487·0l$01 .1JO @ J9KJ Macmilla n Journals ltd .