J. MATER. CHEM., 1994, 4( 12), 1805-1810 Electrochemistry of Poly(3-thiopheneacetic acid) in Aqueous Solution: Evidence for an Intramolecular Chemical Reaction Philip N. BartIett* and Darryl H. Dawsont Department of Chemistry, University of Southampton, Southampton, UK SO9 5NH Electrochemical oxidation of 3-thiopheneacetic acid in dry acetonitrile leads to the formation of a conducting polymeric film. These films can be cycled between oxidised and reduced forms in acetonitrile, but on oxidation in water or methanol are converted to a passive film. This process is accompanied by the passage of approximately two electrons for every monomer unit within the film. Based on the electrochemistry and FTIR studies of the process, a mechanism for the electrochemical passivation of the polymer is proposed involving the formation of an intermediate cyclic lactone and subsequent breakdown by reaction with the solvent. Electropolymerised films of the corresponding methyl ester, methyl 34hiopheneacetate, are not subject to the same electrochemically driven passivation reaction in watcsr.Conducting polymer films prepared by the electrochemical polymerisation of heterocyclic monomers have attracted much attention over the past The electrochemical oxi- dation and reduction of these polymers is accompanied by the movement of charge-balancing counter-ions into and out of the film to maintain electroneutrality. In so-called ‘self- doped’ polymers the anion required for electroneutrality in the oxidised, conducting, form of the polymer is covalently bound to the polymer ba~kbone.~ Examples of polymers of this type include p~lypyrroles~,~ withand polythi~phenes~~~ covalently attached sulfonate groups prepared from the corre- sponding sulfonated pyrrole and thiophene monomers.If, instead of using strongly acidic sulfonic acid substituents, conducting polymers are produced with carboxylic acid sub- stituents then this self-doping process will be pH-dependent in aqueous solution because of the weakly acidic nature of the carboxylate. As a result the electrochemistry of the con- ducting polymer film is then pH-dependent and the redox process is, in general, accompanied by the ingress or egress of protons. Pickup” was probably the first to report the aqueous electrochemical behaviour of a polymer with a carboxylic acid substituent. In his study of the electrochemistry of poly(3- methylpyrrole-4-carboxylic acid) the potentials of the oxi- dation and reduction peaks (Epa and EPc)were found to depend upon the pH of the aqueous electrolyte.Subsequently, Delabouglise and Garnier reported a preliminary study on the electrochemical properties of poly( 3-carboxymethylpyr- role) films.” They found that the anodic peak potential for these films shifted on average by ca. 60mV per pH unit in the region between pH 0 and 6, indicating that one proton was lost for each electron removed in the oxidation of the polymer. Other examples of conducting polymers exhibiting pH-dependent aqueous electrochemistry include polyani-line,12,13 which requires protonation of the partially oxidised form in order to attain electronic conductivity, and poly(5- carboxyindole).l4 Our interest has been in the applications of conducting polymers in bioelectro~hemistry,~~~~~and for this reason we are interested in the electrochemistry of these films in aqueous solution.Although polythiophenes have been widely studied, stable electrochemical responses of polythiophene films are not generally observed in aqueous solution and this has proved to be the major barrier in the exploitation of these polymers. The limited aqueous electrochemistry has mainly been attributed to the hydrophobicity of the polythiophene” t Present address: Department of Chemistry, University College, 20 Gordon Street, London, UK WClH OAJ.chains and the relatively high oxidation potential of these polymers when compared to polypyrroles. Sunde et a1.’83’9 were able to perform cyclic voltammetry of poly( 3-mrbthylthio- phene) films in aqueous solutions containing “I3-and C104- but were unable to repeat the measurements in SO,’--and C1 --containing solutions. This presumably reflelcts differ- ences in the nucleophilicity of the anions. Covalent substitution has also been employed as it strategy to improve the aqueous electrochemistry of polythrophenes. For example, when polyether groups2’ are substituted at the b-position of the thiophene ring the overall hydrophobicity of the polythiophene is reduced, facilitating the free niovement of counter-ions within the film.Based on these observations one might expect that covalently bound carboxylic actd groups would also reduce the hydrophobicity of the polyt hiophene and thus facilitate stable aqueous electrochemistry. Ir addition the carboxylic acid substituted polymer would be expected to show pH-dependent electrochemistry. In this paper ~e present results of a study of the growth, in acetonitrile, and the aqueous electrochemistry of poly (3-thiopheneacetic id)"-^^ and its esters using FTIR spectroscopy to charac,erise the resulting films. Experimental Materials 3-Methylthiophene (Aldrich, 99 + %) was distilled under reduced pressure and stored over activated 3 A inolecular sieves. 3-Thiopheneacetic acid (Aldrich, 98YO)was recrystal- lised from distilled water and dried under high vacuum.(Used 3-thiopheneacetic acid was recovered from acetonit rile solu- tion and background electrolyte by reducing the solution under vacuum followed by recrystallising twice from distilled water .) Methanol (Fisons, analytical reagent) was distilled over calcium hydride and used immediately without furthe c storage. Methyl( 3-thiopheneacetate) was synthesized in the following manner. 3-Thiopheneacetic acid (4 x 5 g) was dis.;olved in dried methanol (100 cm3 each) with a drop of concentrated hydrochloric acid and refluxed. The reaction was followed using thin-layer chromatography with a 10:1 dichloro-methane (Aldrich, Reagent Grade)-methanol system with a spray for detecting esters (1% vanillin in concentrated sulfuric acid with a trace of ethanol).After complete reaction the reaction solutions were combined and reduced under vacuum and dissolved in diethyl ether (Fisons, analytical reagent) (50 cm-’). The organic phase was washed with saturated sodium hydrogencarbonate solution (3 x 30 cm -3) tnd dried 1806 over anhydrous magnesium sulfate before being reduced under vacuum and pumped under high vacuum. The ester was obtained as a clear liquid (80-90% yield) and used without further purification. Methyl (3-thiopheneacetate): dH (ppm, 400 MHz, CDCI,, Me&): 3.65 (2 H, s, H5), 3.69 (3 H, s, H7), 7.03 (I H, c, H3), 7.14 (1 H, c, H1), 7.27 (I H, c, H2).6, (ppm, 100 MHz, CDCI,, Me,CI): 35.3 (C'), 51.7 (C'), 122.6 (C'), 125.4 (C3), 128.2 (C'), 133.3 (C'), 171.2 (C6).m/z (EI) 156 (SO%, M") 97 [lOO%, (C,H,S)CH,]. IR (o/cm-', thin layer): 3100 (m, d-H stretch), 2800-3000 (ms, aliphatic C-H stretches), 1741 (vs, C=O stretch), 1440 (s, CH, sym. def.), 1350-1550 (m, aromatic ring stretches), 1150 [s, CC(=O) -0 stretch]. (d= 1.26 g ern-,). The corresponding ethyl, propyl and butyl esters of thiophene acetic acid were prepared by similar routes. Acetonitrile (Aldrich, BDH and Rathburn, HPLC grade) was distilled over calcium hydride, under a dry nitrogen blanket, for at least 24 h and was used immediately without further storage. Methanol (Aldrich, anhydrous 99 + %) was used without any further purification.Aqueous solutions were prepared using water from the Whatman WR50 RO purifi- cation system pumped through a carbon filter (Whatman Still Plus) giving a conductivity in the range 0.1-1.0 pS cm-'. Tetraethylammonium tetrafluoroborate (TEAT) (Aldrich, 98%) was recrystallised from methanol (Fisons, analytical reagent) and dried under high vacuum. Electrochemical Equipment Electrochemical experiments were carried out using a commer- cial potentiostat (Thompson Electrochem Ltd., Ministat) coupled with an external 16-bit digital potential sweep gener- ator (Thompson Electrochem. Ltd., Miniscan). Electrochemical data were recorded using an X Y-t chart recorder (Bryans/Gould, 60 000 series) or a digital voltmeter (Keithley model: 197). All electrochemical experiments were performed inside a water-jacketed electrochemical cell thermo- statted at 25 & 0.2 'C.Aqueous solutions were degassed directly in the cell by bubbling oxygen-free nitrogen through the solution for at least 20 min. A platinum rotating disc working electrode (A=0.385 cm2, Oxford Electrodes) encased in Kel-F was employed in most studies. Initial polishing was achieved using individual Hyprocel lapping cloths (Engis) sprayed with 6, 3 and 1 pm diamond lapping spray (Engis). Before each experiment the electrodes were initially polished with 1 pm alumina-water slurry followed by 0.3 pm alumina- water slurry, both on medical cotton wool, to give a mirror finish. All potentials were measured against a in-house con- structed saturated calomel electrode (SCE) incorporating a low-porosity ceramic frit (gift from Kent Industrial Measurements Ltd.).The reference electrodes were checked against a commercial SCE (Radiometer) for deviations >6 mV. Counter-electrodes were constructed from (1x 4) cm2 platinum gauze spot-welded a thick platinum wire. The coun- ter-electrode was regularly washed in Whatman RO water and was cleaned before each experiment by heating it over a blue Bunsen flame until the platinum glowed orange and the flame had no colours corresponding to contaminants. FTIR Spectroscopy Two FTIR spectrometers were utilised in this work, a Perkin- Elmer 1720X (maximum resolution 2 cm-') and a Nicolet 510P (maximum resolution 0.8 cm-l) interfaced with a Philips 386 microcomputer running PCIR software (Nicolet Computers Ltd.).Ex situ reflection-absorption spectra of polymer films deposited on electrodes were recorded using specular reflectance accessory (Specac) at 21" to the surface J. MATER. CHEM , 1994, VOL. 4 normal. A special electrode support was designed to hold the disc electrode in the correct position within the reflectance accessory. Results and Discussion Growth and Non-aqueous Electrochemistry of Poly (3-methylthiophene), Poly (3-thiopheneacetic acid) and Poly [methyl( 3-thiophene acetate)] Films of poly( 3-methylthiophene) were prepared for compari- son with the substituted polythiophenes. The> were grown from solutions of 3-methylthiophene (0.1 rnol dm-,) in aceto-nitrile containing TEAT (0.1 mol dmP3) by stepping the potential from 0.0 to 1.65 V at a polished stationary platinum disc electrode.The cyclic voltammetry of the films formed in acetonitrile solutions containing TEAT (0.1 mol dm -,) was consistent with previously reported results.23 Films of poly( 3-thiopheneacetic acid) were grown from solutions of 3-thiopheneacetic acid (1.0 mol dni-,) in aceto-nitrile containing TEAT (0.1 rnol drn-,) by cyclic voltammetry between 0.0 and 1.8 V at a polished stationary platinum disc electrode. Films were generally grown by sweeping the poten- tial between the preset limits four times at 100 mV s-' (Fig. 1). The success of growth depended heavily upon the purity of the solution, with aged solutions producing poorer quality films, presumably because of the presence of increased concen- trations of water.Cycling the potential of the films between 0.0 and 1.4 V in solutions of acetonitrile containing TEAT (0.1 mol dm-3) resulted in passivation after several (cu. 10) scans. However, cyclic voltammetric data recorded on the first scan immediately after growth gave an approximately linear correlation between sweep rate (v) and peak height (ips) (r=0.999, n=4 with E,, between 1.2 and 1.3 V). The anodic peak potential, Epa,for oxidatioii of the mon- omeric 3-thiopheneacetic acid in acetonitrile (1.99 V) was found to be similar to that for 3-methylthiophene under the same conditions (1.96 V), indicating that the acetic acid group has very little direct inductive effect on the oxidation potential of the thiophene ring.In contrast, the peak potential for the oxidation of poly( 3-thiopheneacetic acid) is shifted around 0.4 V anodic of the corresponding oxidation potential for poly( 3-methylthiophene) films under the same conditions. This suggests that there are significant steric effects acting within the poly(3-thiopheneacetic acid) which increase the oxidation potential of the film' over and above any effects which might be accounted for by the electron-withdrawing effect of the substituent. Films of poly [methyl( 3-thiophene acetate)] were grown I I I 0.0 0.5 1.0 1.5 2.0 EN vs. SCE Fig. 1 Growth of a poly( 3-thiopheneacetic acid) film by cyclic voltam- metry at a platinum electrode (A=0.385 cm') in a solution of 3-thiopheneacetic acid (1.0 rnol dm-3) in dry acetonitrile containing TEAT (0.1 mol dm-3) (sweep rate, I)= 100 mV s-') J.MATER. CHEM., 1994, VOL. 4 1807 -1.5 I I I 0.0 0.5 1.o 1.5 EN vs. SCE Fig. 2 Cyclic voltammetry of a poly(methy1 3-thiopheneacetate) film in acetonitrile containing TEAT (0.1 rnol dm-3) at sweep rates, u= 20,40, 60, 80 and 100 mV s-' from acetonitrile solutions of methyl( 3-thiophene acetate) (0.1 mol dm-3) containing TEAT (0.1 mol dm-3) by cycling the potential between 0.0 and 1.7 V at a polished stationary platinum disc electrode. Films could also be grown by stepping the potential of the electrode to 1.7 V in acetonitrile solutions of the monomer at concentrations of as little as 15 mmol dm-3.However, the best quality films were obtained by potential cycling. In contrast to the behaviour found for films of poly(3- thiopheneacetic acid), the cyclic voltammetry of poly(methy1 3-thiopheneacetate) films is very stable in acetonitrile, (Fig. 2), giving a linear i,, us. u relationship (r=0.999, n=5). The limiting values for the E,, and E,, for poly(methy1 3-thio- pheneacetate) are Epa=1.16 V and EPC=1.11 V, which are as high as the corresponding poly( 3-thiopheneacetic acid) values. This again suggests low planarity of the heterocyclic rings' of the monomers within the polymer. The ethyl, propyl and butyl esters, and their corresponding polymers, exhibit very similar electrochemistry to the methyl ester.25 Characterisation of Poly (3-thiopheneacetic acid) and its Esters by FTIR The ex situ reflection-absorption FTIR spectrum of a poly( 3-thiopheneacetic acid) film held at 0.0 V in acetonitrile containing TEAT (0.1 mol drn-,) and then removed from solution and dried is shown in Fig.3. The broad absorption extending down from 4000cm-' is probably the result of direct electronic transitions within the conducting polymer. Below 2000 cm-' there are a number of IRAV bands. A very strong carbonyl band is observed at 1709 cm-' ,corresponding to the H-bonded acid dimer in the polymer. Medium-strength bands are observed between 1300 and 1590 cm-', correspond- ing to aromatic and ring-ring stretches. The PC-H out-of-plane bend is observed as a weak band at 835 cm-' which is m0.3 2 0,0 e5 0.2 I0 2l 0.1m shifted to higher wavenumber than the corresponding PC-H out-of-plane bend for poly( 3-methylthi0phene).'~ There do not appear to be any significant aC-H out-of-plane bending absorptions in the spectrum (observed at 690 and 790cm-' in p~lythiophene~~), indicating that there are few P or P-P defects in the polymer, even when grown at relatively high potentials.This may be due to the steric directing effects of the attached acetic acid groups.' The reflection-absorption FTIR spectrum of fully reduced poly(methy1 3-thiopheneacetate) is shown in Fig. 4. Similar spectra are obtained from electropolymerised films of the ethyl, propyl and butyl esters25 except that they shoh increas-ing intensities for aliphatic C-H stretches between 2800 and 3000 cm-' as the aliphatic chain length increases.Very little aliphatic C-H stretching is observed in the poly(methy1 3- thiopheneacetate) spectrum. The poly(methy1 3-thio-pheneacetate) spectrum also contains a band at 1440 cm-' corresponding to a symmetric CH, deformation which is not observed in the other spectra. All the ester-substituted poly- mers have carbonyl peaks corresponding to the ester groups25 between 1740 and 1730cm-' and a peak at 1150cm-' corresponding to an acetate C-C(=0)-0 symmetric stretch. It is evident that there are also weaker underlying bands between 1300 and 1500 cm-', corresponding to aro- matic ring stretching and ring-ring stretching.The PC-H out-of-plane bending in all the spectra occurred as a weak band at 835 cm-' with no visible aC-H out-of-plane bending. The spectra described above clearly demonstrate the pres- ence of the relevant functional groups within the polymer structure. A slight difference in the carbonyl peaks from monomer to polymer is obtained. This is quite comnion and occurs in other types of polymers due to steric packing effec ts.26 Aqueous Electrochemistry of Poly (3-rnethylthiophene), Poly( 3-thiopheneacetic acid) and Poly(methy1 3-thiopheneacetate) The voltammetric behaviour of a film of poly( 3-methylthio- phene) cycled between 0.0 and 0.9 V at 20 mV s-' in degassed aqueous potassium nitrate (0.1 rnol dm-3) was consistent with that in the literat~re;'~,~~ over the first few cycles the amount of charge passed during oxidation or reduction of the film (Qcv) decreased and then, with repeated cycling, stabilised.In contrast, when a film of poly( 3-thiopheneacetjc acid) was placed in a degassed aqueous solution of pogassium nitrate (0.1 rnol dmP3) and cycled between 0.0 and 1.4 V at 10 mV s-' a large current was passed on the first anodnc cycle and then subsequent cycles showed passivated behaviour (Fig. 5). The ratio of the charge passed in the first anodic sweep in aqueous solution Qcv(aqueous) to the charge passed for oxidation of the same film in acetonitrile Q,,(acetonitrile) was 9.1k0.2: 1. This corresponds to around 2 e for each monomer unit within the polymer film, based on a doipancy' 0.0 10.0: 4000 3000 2000 1000 2000 1600 1200 800 400 waven u m be r/cm-' waven umberkm-' Fig.3 Reflection-absorption FTIR spectrum of a fully reduced film Fig. 4 Reflection-absorption FTTR spectrum of a fully reduced film of poly( 3-thiopheneacetic acid) of poly(methy1 3-thiopheneacetate) 1808 J. MATER. CHEM . 1994, VOL. 4 between 0.0 and 1.2 V, with a sweep rate of 10 mV s-', in degassed aqueous potassium nitrate (0.1 rnol dnxP3) solution. 0.6i Again passivation was observed upon the second cycle. However, in this case the amount of charge passed in passiv- ation of the film was approximately the same as the charge passed in oxidation of the film in acetonitrile, i.e. ca. 1 e for a every four monomer units within the polymer chain.The E2 reflection-absorption FTIR spectra of poly(methy1 3-thio- 0.21 pheneacetate) after cycling in aqueous solution shows the presence of a new band at 1650cm-' which we ascribe to a,p-unsaturated ketone groups formed by nucleophilic attack on the oxidised polymer by ~ater.~~,~* Comparison of our 0.0Iresults for poly(methy1 3-thiopheneacetate) and poly( 3-thio- pheneacetic acid) films indicates that the large charge associ- 0.c 0.5 1 .o 1.5 EN vs. SCE Fig. 5 Passivation of a poly( 3-thiopheneacetic acid) film during cyclic voltammetry between 0 and 1.5 V (us. SCE) in degassed aqueous potassium nitrate (0.1 mol dm-3) solution (sweep rate, o= 10 mV s-') of 6=0.25 for the polymer.It is clear that upon oxidation in the aqueous solution an electrochemical reaction occurs which destroys the conductivity of the polymer. The reflection-absorption FTTR of poly( 3-thiopheneacetic acid) before and after cyclic voltammetry in aqueous potass- ium nitrate is shown in Fig. 6. Following voltammetry in aqueous solution the intensity of the carbonyl band at 1709 cm-' is significantly decreased and the intensity of adsorption at 1650 cm-' is enhanced relative to the intensities of the aromatic and ring-ring stretches between 1300 and 1500 cm -'. The band at 1650 cm -which increases in relative intensity with passivation probably corresponds to an a$-unsaturated ketone group within the polymer formed by nucleophilic attack of water upon the polymer during oxi- dati~n.~~,~*Based on the amount of charge passed in the oxidation of the polymer in aqueous solution we estimate that almost every monomer has been converted to the a$-unsaturated ketone form during the initial cycle.This is a remarkable finding because we might have expected a lower degree of passivation to have destroyed sufficient of the film conductivity to prevent total oxidation within the film during the first cycle. We return to this point below. The electrochemistry of poly(methy1 3-thiopheneacetate) was studied in degassed aqueous solution to provide a com- parison to poly( 3-thiopheneacetic acid) since both have simi- lar E,, values in acetonitrile solutions. A film of poly(methy1 3-thiopheneacetate) was studied by cyclic voltammetry r ated with passivation of the poly( 3-thiopheneacetic acid) films is directly related to the presence of the carboxylic acid substituent on the chain.Electrochemistry of Poly (3-methylthiophene) and Poly (3-thiopheneacetic acid) in Methanol To investigate the electrochemical passivation of poly( 3-thiopheneacetic acid) further we examined the electrochemis- try of films of the polymer, grown in acetonitrile, in methanol containing TEAT (0.1 mol drnp3). Methanol has similar properties to acetonitrile but is more nucleophilic. This pre- vents polymer growth but not polymer electrochemistry. A film of poly(3-methylthiophene) grown for 60s was studied by cyclic voltammetry in a degassed solution of methanol between -0.3 and 1.0 V.The electrochemistry was stable and consistent with the electrochemistry of the film in acetonitrile (once allowance was made for the for difference in liquid-junction potentials for the SCE in the two solvents, this shifted the voltammetry by ca. 0.1 V). There were no changes in the reflection-absorption FTIR spectra of the polymer before and after cyclic voltammetry in methanol and the subsequent electrochemistry in acetonitrile remained unchanged. This demonstrates that, on the timescale of our experiments, methanol does not attack poly( 3-methylthio- phene) during oxidation and reduction. In direct contrast when a film of poly( 3-thiopheneacetic acid), grown in the manner previously described for four cycles, was studied by cyclic voltammetry between 0.0 and 0.3r 0.0 I I 0.00i2000 1600 1200 8002000 1600 1200 800 400 wavenumbedcm-' waven um ber/cm-' Fig.7 Reflection-absorption FTIR spectra of poly ( 3-thiopheneaceticFig. 6 Reflection-absorption FTIR spectra of poly (3-thiopheneacetic acid) before (a) and after (b)passivation by electrochemical oxidation acid) before (a) and after (b)the passivation shown in Fig. 5 in methanol J. MATER. CHEM., 1994, VOL. 4 0 0 I -2e, -H'I /-. -*H+ /e-, -2H+ 111 0 0 /-, -2H' 0 0 N 0 0 0 0 OR 0 HOR v further solvdysis leadingto passive film Fig. 8 Reaction scheme proposed to account for the electrochemical passivation of poly(3-thiopheneacetic acid) (I) in water or mcthanol 1.6 V at 20 mV s-l in a degassed solution of methanol contain- ing TEAT, passivation was observed following the first anodic scan.The passivation of the film was again, as in the experi- ments with this polymer in aqueous solution, accompanied by the passage of a significant charge corresponding to ca. 2 e for each monomer unit within the polymer. Inspection of the reflection-absorption FTIR spectra of the film before and after cyclic voltammetry in methanol (Fig. 7) showed that the carboxylic acid functionality had been totally replaced by a methyl ester functionality with a C=O stretch at 1734 cm-I and a CH, symmetric deformation band at 1440 cm-' as well as other bands associated with the methyl ester. We assign the additional band at 1098 cm-' to C-OH stretch of residual methanol within the film.Thus it appears that in methanol the electrochemical passivation of the film is accompanied by the esterification of the carboxylic acid groups. This is unexpected since esterifications in methanol usually require more forcing conditions such as low pH and prolonged heating. Discussion The electrochemistry of poly( 3-thiopheneacetic acid) shows some unusual features which are not found in the electrochem- istry of poly( 3-methylthiophene) or poly(methy1 3-thio-pheneacetate). We suggest that this behaviour can be explained by the reaction scheme shown in Fig. 8. In dry acetonitrile poly( 3-thiopheneacetic acid) can be cycled electrochemically between its reduced (I) and oxidised (11) forms.However, if the carboxylic acid groups can be deprotonated, for example in water or methanol, we postulate that oxidation of the polymer can be followed by intramolecu- lar reaction and further oxidation to form a cyclic lactone (TIT). This process does not destroy the conductivity of the polymer so that the process can continue until all the monomer units within the film are converted into the postulated lactone form (IV). Overall this consumes two electrons for every monomer unit in the film so that in total nine times as much charge is passed to oxidise the film in water or methanol as is passed in dry acetonitrile. The poor stability of poly(3- thiopheneacetic acid) electrochemistry in acetonitrile, as com- pared to that for poly( 3-methylthiophene) or poly(methy1 3- thiopheneacetate), is presumably the result of traces of water leading to some deprotonation and lactone formation.We then suggest that the postulated cyclic lactone (IV) is itself unstable with respect to solvolysis and that the lactone rings open to give unsaturated ketones (V) and to destroy the conductivity of the film. In water this produces carboxylic acid groups, but in methanol the solvolysis of the lactone would yield the corresponding methyl ester. This scheme is consistent with the ex situ reflection-absorption FTIR studies of the films before and after passivation and with the charge passed to passivate the film, although we have no direct evidence from this work for the intermediacy of the cyclic lactone.The fact that almost all the monomer units within the film undergo reaction indicates that the electrochemical oxidation and cyclic lactone formation must occur more rapidly than the subsequent solvolysis which destroys the J. MATER. CHEM., 1994, VOL. 4 conductivity of the film. If this were not the case the loss of conjugation in the polymer would prevent further charge propagation and electrochemical oxidation in the bulk of the film. In the case of both poly( 3-methylthiophene) and poly(me- thy1 3-thiopheneacetate) the formation of a cyclic lactone is not possible and therefore these polymers are not subject to the same type of electrochemical passivation. To the best of our knowledge this is the first example of this type of electrochemically driven passiva tion, and substi- tution, of a conducting polymer to be reported.In principle other reactions of this type should be possible and offer an interesting way to prepare thick, insulating polymer films at electrode surfaces. References 1 Handbook of Conducting Polymers, ed. T. A. Skotheim, Marcel Dekker, New York, 1986. 2 S. Bruckenstein and A. R. Hillman, J. Phqs Chem., 1988,92,4837. 3 G. Bidan, B. Ehui and M. Lupowski, J. Phys. D, 1988,21, 1043. 4 A. Patil, Y. I. Kewone, F. Wudl and A. Heeger. J. Am. Chem. Soc., 1987,109,1858. 5 A. Patil, Y. I. Kewone, N. Basescu, N. Colaneri, J. Chen, F. Wudl and A. Heeger, Synth. Met., 1987,20, 151. 6 W. Wernet, M.Monkenbusch and G. Wegner. Mukromol. Chem. Rapid Commun., 1984,5, 157. 7 A. Patil, Y. I. Kewone, F. Wudl and A. Heeger, J. Am. Chem. Soc., 1987,109,327. 8 S. Basak, K. Rajeshwar and M. Kaneko, Ancil. Chem., 1990, 62, 1407. 9 N. S. 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B. Li and W. J. Albery, Electrochim. Actu, 1991,37, 293. 23 F. B. Li and W. J. Albery, Langmuir, 1992,8, 645. 24 J. L. Sauvajol, D. Chenouni, J. P. Lere-Porte, C. Chorro, B. Moukala and J. Petrissans, Synth. Met., 1990.38, 1. 25 D. H. Dawson. Ph. D. Thesis, University of Warwick, 1992. 26 H. W. Siesler and K. Holland-Moritz, Infrtired and Raman Spectroscopy of Polymers, Marcel Dekker, New York, 1980. 27 F. Beck, P. Brown and M. Oberst, Ber. Bunsenges. Phys. Chem., 1987,91,967. 28 E. W. Tsai, S. Basak, J. P. Ruiz, J. R. Reynolds and K. Rajeshwar, J. Electrochem. Soc., 1989,136, 3683. 29 J. Heinze, Synth. Met., 1991,43, 2805. Paper 4/02095B; Received 8th April, 1994