A NEW FORM OF POLYACETYLENE

Transcription

1 A NEW FORM OF POLYACETYLENE D. Bott, C. Chai, J. Edwards, W. Feast, R. Friend, M. Horton To cite this version: D. Bott, C. Chai, J. Edwards, W. Feast, R. Friend, et al.. A NEW FORM OF POLYACETYLENE. Journal de Physique Colloques, 1983, 44 (C3), pp.c3143c3146. < /jphyscol: >. <jpa > HAL Id: jpa Submitted on 1 Jan 1983 HAL is a multidisciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

2 JOURNAL DE PHYSIQUE Colloque C3, supplément au n 6, Tome 44, juin 1983 page C3143 A NEW FORM OF POLYACETYLENE B.C. Bott \ C.K. Chai, J.H. Edwards, W.J. Feast, R.H. Friend ) and M.E. HortonO) (1) B.P. Research Centre, Chertsey Road, SunburyonThames, Middlesex TU16 7LN, U.K. (2) Chemistry Department, Durham University, South Road, Durham DH1 3LE, U.K. (3) Cavendish Laboratory, Madingley Road, Cambridge, U.K. Résumé Nous décrivons la préparation et la caractérisation d'une nouvelle forme de polyacétylène. Ce matériau semble avoir une très faible concentration en défauts et être principalement amorphe ; quelques aspects de ses propriétés, en particulier électriques (non dopé et dopé) sont discutés. Abstract We report the preparation and characterization of a new form of polyacetylene. This material appears to have a very low concentration of defects and to be largely amorphous; some aspects of its properties, in particular electrical behaviour (pristine and doped), are discussed. We have previously described a novel approach to the synthesis of polyacetylene via a two stage eliminative process (Scheme 1). As reported at that time, the Scheme l relatively low thermal stability of the polymer II led us to suppose that the sequence shown in Scheme 1 was unlikely to provide a satisfactory route to high quality polyacetylene. However, since our earlier report, we have refined the techniques required both for the purification of the precursor polymer II and for its conversion to a high purity form of polyacetylene. At the same time we have developed a second route in which the precursor polymer is significantly more Article published online by EDP Sciences and available at

3 C3144 JOURNAL DE PHYSIQUE stable (Scheme 2). The syntheses of the monomers used in this work have been Scheme 2 described previously, Compound I2 and Compound 18. The first step in both schemes involves ring opening polymerization at the cyclobutene ring, a reaction which is readily accomplished using a typical metathesis catalyst, such as that produced by reaction of tungsten hexachloride with tetramethyltin.* There are, of course, two CH=CH units in both monomers I and IV but only the cyclobutene unit will undergo ring opening since the other double bond is contained in a six membered ring and, for thermodynamic reasons, such systems do not undergo ring opening polymerization. The precursor polymers I1 and V were purified by repeated dissolutionreprecipitation sequences in the conventional manner, they were then cast Q@v as films from solution and heated to complete the eliminati~n.~ The elimination reactions involved in the conversions I and V t VI were studied by differential scanning calorimetry which showed that a significant rate of conversion commenced at z. 310K for polymer 11, whereas for polymer V the onset of the conversion reaction occurs at E. 380K. It is interesting to note that studies of the related precursor polymer VII (Scheme 3) indicate a temperature of I_) + VII ++ Scheme 3 ca. 500K for the onset of the anthracene elimination reaction. Thus, the polymers 11, V and VII show a progressive increase in thermal stability which mirrors the progressive decrease in delocalization energy gained in the formation of the new aromatic rings in the respective elimination reactions.

4 Careful regulation of the precursor polymer's purification and the conversion step5 allows the production of polyacetylene of high purity possessing a novel morphology. The data reviewed briefly below refers, for the most part, to thin films produced by the route outlined in Scheme 1. 7 Structure. In contrast to Shirakawa6 and Luttinger polyacetylene, this new material shows virtually no evidence of crystallinity when examined by Xray and electron diffraction methods. Scanning and transmission electron microscopy studies revealed no detectable structura Fig film 11 um film S. E.M. Micrographs of transpozyacety Zene produced by the route out Zined in Scheme 2 ix 2000). This shows a lack of fibrillar morphology. Increasing magnification to X shows no structuraz units. obtained as a coherent film of high density, p = 1.05 g.cm. 3 (by flotation). Elemental analysis indicates that total elimination of the aromatic unit can be achieved, and i.r. and n.m.r. defects attributable to sp3 carbons. spectroscopy give no evidence for the presence of The sample was examined by Resonance Raman Spectroscopy; when irradiated with a laser line of nm the sample gave a 1 1 C=C stretching frequency at 1480 cm., this compares with a value of 1460 cm. for polyacetylene prepared by conventional techniques, and indicates markedly shorter conjugation lengths. Electrical Properties. Pristine polyacetylene films produced by the route outlined 7 1 in Scheme 1 show a room temperature d.c. conductivity of s. 10 (R cm.), this is a factor of 50 to 100 times lower than typical values reported for Shirakawa transpolyacetylene.8 This observation probably indicates a lower defect

5 C3146 JOURNAL DE PHYSIQUE concentration in the sample described here, this hypothesis is supported by ammonia compensation experiments which reveal a decrease in conductivity by a factor of z. 50 for our material compared with a change of more than four orders of magnitude for similarly compensated Shirakawa transpolyacetylene.9 The 1 temperature dependence of the conductivity is given by En5 a for T > 300K and T TO % by Enu a (; ) for T < 300K. The frequency dependence of the conductivity was measured up to lo5 Hz between room temperature and 10K; a relation of the form rs a T~u', where n > 1 and s < 1 was observed. Thls behaviour is characteristic of many amorphous semiconductors, although it differs from that reported for Shirakawa transpolyacetylene.8 Doping of the films of this new material results in a rapid uptake of iodine, and for example at a composition represented by 1 (CH10.27)x, values of o (300K) of 10 (a cm.) were recorded. Arsenic penta fluoride has a rapid initial uptake but the rate of uptake falls sharply; at a 1 Composition represented by ( C H [ A ~ F ~ values ] ~ ~ ~ of ~ 0 ~ (300K) ) ~ of 2 (a cm.) were recorded. Conclusion. We have described a method of obtaining polyacetylene of high purity in an apparently amorphous, or at least very low crystallinity, form. Space does not allow a detailed presentation of the evidence in this communication but the various aspects of our investigations of this novel material will be thoroughly documented in a series of forthcoming publications. References EDWARDS, J.H. and FEAST, W.J., PolyrtIer, 21, (1980), 595. LIU, R.S.H. and KRESPAN, C.G., J. Org. Chem., 2, (1969), PAQUETTE, L.A., KUKLA, M.J. and STOWELL, J.C., J. Amer. Chem. Soc., 94, (1972), ELSAAFIN, I.F.A.F. and FEAST, W.J., J. Mol. Cat., 15, (1982), 61. BOTT, D.C., EDWARDS, J.H. and FEAST, W.J., European Patent Application Number *5,'18th November ITO, T., SHIRAKAWA, H. and IKEDA, S., J. Polym. Sci. Polym. Chem. ~dn., 12, (1974), 11; 13, (1975), LUTTINGER, L.B., J. Org. Chem., 27, (1962), EPSTEIN, H., ROMMELMANN, H., ABKOWITZ, M. and GIBSON, H.W., M0l. Cryst. Liq. Cryst., 77, (1981), 81. HEEGER, A.J. and MacDIARMID, A.G. in 'The Physics and Chemistry of LOW Dimensional Solids', Edited by ALCALER, L., (1980), 353.