structure and conductivity in substituted polypyrroles. part 1. synthesis and electropolymerization...

Polymer International 47 (1998) 43È49

Structure and Conductivity inSubstituted Polypyrroles.

Part 1. Synthesis andElectropolymerization of

N-Trimethylsilylethoxymethyl-3-methyl-4-pyrrole

Carboxylate Ethyl Ester

Yu Chen,1 Corrie T. Imrie,1 Jon M. Cooper,2 Andrew Glidle,2 David G. Morris3

& Karl S. Ryder1,*

1 Department of Chemistry, University of Aberdeen, Meston Walk, Old Aberdeen AB24 3UE, UK2 Department of Electronics & Electrical Engineering, Rankine Building, University of Glasgow, Glasgow G12 8LT, UK

3 Department of Chemistry, Joseph Black Building, University of Glasgow, Glasgow G12 8QQ, UK

(Received 19 February 1998 ; accepted 6 April 1998)

Abstract : The electrochemical polymerization of N-trimethylsilylethoxymethyl-3-methyl-4-pyrrole carboxylate ethyl ester (MPCE-SEM) in the presence ofpyrrole, to give free-standing copolymer Ðlms is described. These Ðlms have beencharacterized by surface conductivity measurements, reÑectance FTIR spectros-copy, X-ray photoelectron spectroscopy and scanning electron microscopy.Increasing the relative concentration of MPCE-SEM in the polymerization solu-tion resulted in an increase in the proportion of these units in the copolymer Ðlm.Increasing the proportion of MPCE-SEM units in the copolymer resulted in adecrease in surface conductivity. This is consistent with expectation because N-substituted polypyrroles tend to have lower conductivity values thanunsubstituted polypyrrole. SigniÐcantly, it has also been shown that the N-protecting group of the MPCE-SEM unit can be removed after polymerizationunder mild conditions in a solid phase deprotection procedure. 1998 Society(of Chemical Industry

Polym. Int. 47, 43È49 (1998)

Key words : conducting polymer ; pyrrole ; conductivity ; XPS; four-point probe

* Author to whom all correspondence should be addressed.Contract/grant sponsor : EPSRC.Contract/grant number : GR/L10185.Contract/grant sponsor : CCLRC.Contract/grant number : GR168.

431998 Society of Chemical Industry. Polymer International 0959È8103/98/$17.50 Printed in Great Britain(

44 Y . Chen et al.

INTRODUCTION

Electronically conducting polymers continue to be thefocus of considerable research activity.1 This situationarises not only as a result of their exciting applicationpotential in a wide range of advanced technologies suchas optoelectronics,2 molecular electronics,3 and bio-electronics,4 but also because these materials provide ademanding challenge to our understanding of chargetransport processes.5 Conducting polymers for practicalapplications will require a high intrinsic conductivitytogether with good chemical and physical stabilities.This combination of properties, however, has yet to befully realized.

Much interest has focused on pyrrole-based polymersbecause the substitution chemistry of the monomers isparticularly versatile, and their chemical or electro-chemical polymerization is generally facile in a range ofsolvents. Polypyrroles exhibit a wide range of surfaceconductivities (10~3\ p \ 100 S cm~1) depending onthe functionality and substitution pattern of themonomer, and the nature of the dopant counterion.Electron transport occurs via a combination of mecha-nisms involving movement of charge along the polymerchains and electron hopping between chains. Theamorphous structures exhibited by conducting poly-mers contribute to their relatively low conductivity, andthis is exacerbated by defect formation or crosslinkingacross the b-positions of the pyrrole monomers. TheseaÈb and bÈb linkages decrease the planarity of thebackbone, and hence reduce the polymerÏs conductivi-ty.5 The aim of this work is to develop novel pyrrole-based monomers carrying substituents at the 3- and4-positions and on the nitrogen, in the expectation thatpolymerization should yield a polymer that is exclu-sively aÈa linked. These studies in turn will allow us toestablish correlations between the structure of the func-tionalized pyrrole monomer and the surface conductivi-ty of the corresponding polymer. Such a relationshipwill help to facilitate the rational design of conductingpolymers with improved properties.

The monomer described here is N-trimethyl-silylethoxymethyl-3-methyl-4-pyrrole carboxylateethyl ester 1, and the acronym used to refer to thismonomer is MPCE-SEM. This molecule has severalimportant features that deÐne its relevance to ourstudies.

molecule is substituted at both b positions of theI The

pyrrole ring and this is expected to provide animprovement in the long-term stability of the poly-mers, rendering them less susceptible to atmosphericoxidation, whilst reducing defect formation duringpolymerisation.6

carbonyl group provides functional and syntheticI Theversatility.

trimethylsilylethoxymethyl group (SEM) isI Thechosen as a convenient protecting group to inhibitcrosslinking at the nitrogen site, whilst exhibiting arelatively weak inÑuence on the electronic propertiesof the pyrrole ring. Here it is shown that this can beeasily removed after polymerization.

EXPERIMENTAL

N-Trimethylsilylethoxymethyl-3-methyl-4-pyrrole car-boxylate ethyl ester (1) was prepared according to thesynthetic route shown in Scheme 1. The synthesis of 3-methyl-4-pyrrole carboxylate ethyl ester (MPCE) hasbeen described in detail elsewhere.7

N-Trimethylsilylethoxymethyl-3-methyl-4-pyrrolecarboxylate ethyl ester (MPCE-SEM, 1)

The SEM group was grafted onto the MPCE precursorusing an adaptation of the established literaturemethod.8 Thus, a 100 ml three-necked Ñask, Ðtted with acondenser, gas inlet and outlet and a dropping funnel,was charged with dry dimethylformamide (DMF, 8 ml)and dry dimethylsulphoxide (DMSO, 1É6 ml). These sol-vents were cooled to 0¡C and sodium hydride (0É13 g,5É3 mmol, prewashed using petroleum ether (40È60¡C,3 ] 10 ml) from a 60% suspension in mineral oil) wasadded. 3-Methyl-4-pyrrole carboxylate ethyl ester(MPCE) (0É5 g, 3É3 mol) in dry DMF (8 ml) and dryDMSO (1É6 ml) was added by dropping funnel. Themixture was left at room temperature with stirring untilevolution of hydrogen gas had ceased. Subsequently, thesolution was cooled to 0¡C and 2-trimethylsilylethoxymethyl chloride (0É88 ml, 5É0 mmol)was added dropwise from a syringe. The solution wasthen stirred at room temperature for 2 h. The resultingreaction mixture was poured into iced water (60 ml) andthe product was extracted with diethylether (4] 50 ml).The combined organic layer was dried over magnesiumsulphate and concentrated to give a yellow oil. Theproduct was puriÐed by silica gel column chromatog-raphy using petroleum ether (40È60¡C)/diethyl ether6 : 1 v/v as the eluent. Yield : 0É53 g, 57%.

1H NMR. d : 7É3 (1H, d, J \ 2 Hz, pyrrole), 6É5(CDCl3)(1H, m, pyrrole), 5É1 (2H, s, 4É2 (2H, q,NCH2O),J \ 7 Hz 3É4 (2H, t, J \ 8 Hz, 2É3CH2CH3), OCH2),(3H, m, 1É3 (3H, t, J \ 7 Hz, 0É9 (2H, t,CH3), CH2CH3),J \ 8 Hz, 0É0 (9H, s, (Fig. 1a).CH2Si), Si(CH3)3)

POLYMER INTERNATIONAL VOL. 47, NO. 1, 1998

Synthesis and electropolymerization of MPCE-SEM 45

Scheme 1

Fig. 1. Spectral data for molecule MPCE-SEM: (a) 1H NMR spectrum recorded in (the peaks labelled with asterisksCDCl3represent solvent impurities), and (b) the transmission FTIR spectrum in NaCl pellet.


46 Y . Chen et al.

FT IR. (NaCl plate), 2956, 1707, 1527, 1243,lmax :1088 cm~1 (Fig. 1b).

Mass spectrum. FW calculated from C14H25O3NSi,283É4 ; found, 284É1.

Polymerization

Electrochemical polymerization from acetonitrile solu-tions containing 1 and pyrrole with 0É1 M

was performed both under galvanostatic[NBu4][BF4]control, at a constant current of 0É1 mA cm~2, and bypotentiostatic control, at E\ ]1É25 V versus AgCl/Ag.Gold foil (1É5 cm2) and ITO glass (2 cm2) were used aselectrode substrates, and the solutions were thoroughlydegassed with Ar before polymerization. During poten-tiostatic polymerization particular care was taken toensure the applied potential did not rise so high as tocause over-oxidation of the polymer Ðlm. The thicknessof polymer Ðlms was determined by scanning electronmicroscopy using a Jeol JSM-5200 instrument. Typicalthicknesses were of the order of 4 lm (Fig. 2). All elec-trochemical procedures were carried out on an E.G. &G. M263A potentiostat/galvanostat driven by the M270software package.

Conductivity measurements

The surface conductivities of electrochemically grownfree standing Ðlms were measured using a custom builtd.c. linear four point probe apparatus with a KiethlyInstruments high impedance electrometer. The thin Ðlmapproximation was used in determining the conductivi-ties of the Ðlms ; speciÐcally, the Ðlm thickness was lessthan one-tenth of the separation of the probes and allthe ÐlmÏs edges were at least twenty times the spacing

Fig. 2. Scanning electron micrograph of a polymer Ðlm growngalvanostatically (I\ 0É1 mA cm~2) from a solution contain-ing and The thickness (t) wasXpyrrole\ 0É5 XMPCvSEM\ 0É5.determined from the edge proÐle of the fractured Ðlm, in this

example t \ 4É6 lm.

between the probes from where the probes contactedthe Ðlm. Under these conditions the surface conductivi-ty p can be calculated using eqn (1) :

p \CA n

ln 2BtAV

IBD~1

(1)

where t is the Ðlm thickness, V is the voltage dropacross the inner two probes and I the current driventhrough the sample.9 Care was taken to ensure thatover the range of measurements the plots of V against Iwere linear. The conductivity values obtained for Ðlmsgrown from solutions containing only pyrrole were con-sistent with the literature.

Characterization

The proposed structures of MPCE-SEM and its inter-mediates were veriÐed using 1H NMR and IR spectros-copy. 1H NMR spectra were measured in on aCDCl3Bruker AC-F 250 MHz NMR spectrometer. IR spectrawere recorded using a Nicolet 205 FTIR spectrometer.Mass spectra were recorded on a Finnigan MassLabNavigator.

The composition of the polymer Ðlms was examinedusing the Scienta ESCA 300 X-ray photoelectronspectrometer (slit width\ 0É8 mm, take o† angle\ 90¡)at the CCLRC RUSTI facility at Daresbury.

RESULTS AND DISCUSSION

MPCE-SEM (1) was obtained in good yield using thesynthetic protocol outline in Scheme 1. The 1H NMRand mass spectra of 1 shown in Fig. 1 are consistentwith the proposed structure.

The electrochemical oxidation of 1 failed to producea free standing Ðlm suitable for conductivity measure-ments. This may be because the radical cation of 1 isstabilized by steric constraints about the pyrrole ring,inhibiting the polymerization process, or, alternativelythis may be because oxidation of 1 produces a solublepolymer. To investigate these possibilities free standingÐlms were prepared by electrochemical oxidation ofsolutions containing both MPCE-SEM andunsubstituted pyrrole. Five polymer Ðlms were preparedfrom MPCE-SEM solutions containing pyrrole in therange 0É2È1É0 mole fraction to establish the(Xpyrrole)dependence of conductivity on composition. A furtherthree copolymer Ðlms were also prepared from solu-tions containing 0É08, 0É14 and 0É20 mole fraction ofpyrrole for elemental analysis using X-ray photoelec-tron spectroscopy (XPS). The presence of MPCE-SEMunits within the polymer Ðlms was established by reÑec-tance FTIR spectroscopy. ReÑectance FTIR spectra (at45¡) of the copolymer Ðlms on a gold coated glass sub-strate showed the appropriate features and in particular



a strong band at 1720 cm~1 which was assigned to thecarbonyl band of the ester group of MPCE-SEM unitsin the polymer. Examination of the free standingcopolymer Ðlms by scanning electron microscopyshowed a smooth surface morphology and the imagingof fracture edges allowed convenient determination ofÐlm thickness data necessary for conductivity measure-ments (Fig. 2).

The dependence of log p on the mole fraction ofpyrrole in the feed mixture for the copolymers is shownin Fig. 3 ; each datum point corresponds to a singlepolymer Ðlm grown electrochemically under chrono-potentiometric, i.e. constant current, conditions.Increasing the mole fraction of MPCE-SEM in the feed,and hence presumably also in the copolymer, decreasesthe conductivity of the Ðlm. Steric interactions involvingthe b-substituents on the MPCE-SEM ring are relievedvia a twisting of the monomer units with respect to oneanother. This departure from planarity compared withpolypyrrole both decreases the extent of conjugationalong the chain and inhibits interchain interactions,resulting in the observed decrease in conductivity. It isnoteworthy that Fig. 3 depicts a linear relationshipbetween log p and whilst other reports of con-Xpyrrole ,ductivity studies in pyrrole copolymers have producedmarkedly non-linear behaviour.10 This may be becauseof the limited range of composition chosen for thisstudy and it seems likely the log p values would dropmore rapidly at smaller values of Xpyrrole .

The interpretation of the decrease in conductivity(Fig. 3) largely in terms of steric interactions assumesthat the mole fraction of MPCE-SEM in the copolymerincreases as the mole fraction of MPCE-SEM isincreased in the feed mixture. To test this assumptionX-ray photoelectron spectroscopy was used to examinethe composition of the copolymer Ðlms.

X-ray photoelectron spectra were acquired for theSi(2p), N(1s), C(1s) and O(2p) regions of the sample

Fig. 3. Dependence of log p on the mole fraction of pyrrole inthe feed mixture for the MCPE-SEM/pyrrole copolymers.

copolymer Ðlms. The elemental composition of thepolymer surface was determined from the integral ofeach peak, taking into account the appropriate elemen-tal sensitivity factors. From these data the ratio ofmonomer units present in the Ðlm was estimated usingthe ratio of the Si and N spectral regions. For example,a Ðlm grown from a solution consisting of XMPCEvSEM \

and gave a polymer containing0É8 Xpyrrole \ 0É2approximately 25% MPCE-SEM units, whereas a Ðlmgrown from a separate solution consisting of

and yielded a Ðlm con-XMPCEvSEM \ 0É92 Xpyrrole \ 0É08taining approximately 50% MPCE-SEM units. Conse-quently, increasing the proportion of MPCE-SEM inthe feed solution resulted in an increased proportion ofthese units in the copolymer Ðlm. (Accurate determi-nation of these ratios is difficult, particularly withrespect to oxygen content, because of rapid contami-nation of the sample surfaces from airborne species.)

Although the proportion of the two monomers in thecopolymer Ðlm does not mirror that in the feed solu-tion, this is not unexpected due to the disparate reacti-vities of the two monomers under these conditions. Thisphenomenon is clearly illustrated by chronocoulometricdata acquired during polymerization experimentscarried out at constant potential. Figure 4 shows thecharge passed during Ðlm growth as a function of time.There are three traces, each corresponding to the forma-tion and growth of a copolymer Ðlm at di†erent relativeconcentrations of pyrrole and MPCE-SEM. In curve 1,pyrrole has a mole fraction henceXpyrrole \ 0É08 ;

In the curve 2, pyrrole was added toXMPCEvSEM \ 0É92.give a solution with andXpyrrole \ 0É14 XMPCEvSEM \

Further addition of pyrrole gave curve 3, in which0É86.and The charge axis inXpyrrole \ 0É2 XMPCEvSEM\ 0É8.

Fig. 4. Chonocoulometric plots showing charge passed duringcopolymerization of pyrrole and MPCE-SEM as a function oftime for three experiments at di†erent relative concentrations.

deÐnes the mole fraction of pyrrole present in the feedXpyrrole ,solution. (The charge in each curve is divided by the total

number of moles of pyrroles present in each solution.)


48 Y . Chen et al.

these plots was normalized in order to compensate forthe e†ect of increase in total concentration of pyrroles.It can be seen from Fig. 4 that adding pyrrole to thesolution results in a greater charge passed in a giventime (at the given potential). This demonstrates thatreaction of pyrrole under these conditions is very muchmore rapid than that of the MPCE-SEM monomer.

The SEM group is often used in the protection stra-tegies of homogeneous synthetic reaction schemes, andthe conditions for its incorporation and removal fromweakly acidic N and O groups are well documented.8,11Using XPS it is shown here for the Ðrst time thatremoval of the SEM group in a solid phase copolymermatrix is also facile (Fig. 5). Thus, whilst the incorpor-ation of the SEM group helps to inhibit crosslinking atthe nitrogen site, the attractive feature of SEM in thissystem is that removal of the group is possible afterpolymerization. As a result the deprotected copolymerhas no N-substituents which tend to lower conductivity.Figure 5 shows the XPS spectra of the Si(2p) region of acopolymer Ðlm (a) before and (b) after the deprotectionprocedure. Soaking the copolymer Ðlm in a 0É1 M tetra-butylammonium Ñuoride solution (in tetrahydrofuran)for 15 min resulted in a pronounced decrease in theintensity of the Si(2p) peak, implying that the SEMgroup had been cleaved in an essentially quantitativemanner. Interestingly, the Ðlms having a larger propor-tion of the MPCE-SEM monomer unit exhibited lessthan complete loss of the Si(2p) peak for the same15 min deprotection procedure, although subsequentprolonged soaking did result in spectra which are indis-tinguishable from that shown in Fig. 5. This result sug-gests that the deprotection reaction may be limited bydi†usion of the reagents within the Ðlm and that thismay occur at a more rapid pace in the copolymers withfewer pendant side chain substituents, i.e. SEM units.

It is noted that XPS is a surface technique, givinginformation about the polymer surface to a depth of

Fig. 5. XPS spectra (at a take-o† angle of 90¡, 0É8 mm slitwidth) showing the silicon 2p region (a) for a copolymer pre-pared from a solution containing 0É1 mole fraction pyrroleand (b) for the same Ðlm after being soaked in a 0É1 M tetra-

butylammonium Ñuoride solution for 15 min.

only a few nanometers, and that it is possible that SEMpersists within the Ðlm. However, our previous work insolid phase functionalization of polypyrroles has shownthat similar reactions, involving sterically bulky species,do propagate throughout the depth of relatively thickpolymer Ðlms (about 10 lm). This was established usingreÑectance FTIR of pyrrole pentaÑuorophenolate estersin combination with XPS to probe both the bulk of theÐlm and its surface.12 The system described here doesnot lend itself well to this type of experiment because ofthe lack of a convenient IR chromophore in the SEMgroup. However, we have examined the SEM deprotec-tion reaction for homopolymers of N-trimethyl-silyl-ethoxymethylpyrrole (pyrroleÈSEM) using bothXPS and reÑectance FTIR. The XPS results from thissimpliÐed system conÐrm our Ðndings here, and theFTIR spectra of the pyrroleÈSEM Ðlms show a cleardecrease in the intensity of the l(CwH) bands around3000 cm~1 following the deprotection reaction. Takingall these factors into consideration, it seems likely thatthe cleavage reaction of the SEM groups in the copoly-mer Ðlms of pyrrole and MPCE-SEM extends through-out the depth of the Ðlm.

CONCLUSIONS

Here the synthesis and characterization of N-tri-methylsilylethoxymethyl-3-methyl-4-pyrrole carboxyl-ate ethyl ester, as a prototype in our strategy towardslow defect, high conductivity, functionalized poly-pyrroles are described. It has been shown that althoughelectrochemical oxidation of this molecule did notproduce polymer Ðlms, copolymerization withunsubstituted pyrrole gave free-standing Ðlms suitablefor conductivity measurements. This result, togetherwith our previous observations that electrochemicaloxidation of similar monomers, 3-methyl-4-pyrrolecarboxylate ethyl ester, and N-(trimethylsilyl-ethoxymethyl) pyrrole (pyrrole-SEM) gave highly insol-uble materials,7 suggests that steric constraintshinder the electrochemical polymerization of MPCE-SEM. The conductivity of the pyrrole MPCE-SEMcopolymer Ðlms shows a strong dependence on thecomposition of the solution from which they weregrown. Increasing the relative concentration of the com-ponent with the N-substituent results in a decrease inconductivity. This is consistent with many literaturereports in which N-substituted pyrroles are generallyshown to have lower conductivities than unsubstitutedpyrrole, or pyrroles substituted at the b-positions.Analysis of the polymer Ðlms, grown from solutions ofMPCE-SEM and pyrrole, by reÑectance FTIR andX-ray photoelectron spectroscopy shows clearly thepresence of MPCE-SEM units within the material. Thisis evidenced by the presence of the carbonyl band (and



other Ðngerprint bands) in the FTIR spectrum, and bythe unique presence of the Si(2p) peak in the XPS spec-trum. Integration of the N(1s) and Si(2p) peaks in theXPS spectrum has provided us with an approximatemeasure of the copolymer ratio.

It has also been shown, using XPS, that the N-substituents can be removed from the copolymer afterpolymerization by a very mild deprotection procedure,although the rate of this reaction appears to be depen-dent on the composition of the copolymer. Where theproportion of MPCE-SEM units in the copolymer isrelatively high (50%) the reaction is slower than forÐlms with a lower proportion of MPCE-SEM units.This observation is rationalized on the basis of hindereddi†usion in the Ðlms that posses a high proportion ofMPCE-SEM units.

ACKNOWLEDGEMENTS

CTI and KSR are grateful to the EPSRC for Ðnancialsupport of this work (GR/L 10185). KSR also acknow-ledges the Royal Society for Ðnancial support (RSRG17906). The authors are grateful to the CCLRC for useof the XPS facility at RUSTI, Daresbury (GR 168). Weare also grateful to Dr Graham Beamson (RUSTI,

Daresbury) for technical insight and valuable discussionand to Dr Eric Lachowski (University of Aberdeen) foracquisition of electron microscopy data.

REFERENCES

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S. M., Timofeeva, O. N. & Khidekel, M. I., Synth. Met., 41 (1991)1143 ; Krieger, Y. G., J. Struct. Chem., 34 (1993) 896 ; Bianco, B. &BonÐglio, A., T hin Solid Films, 285 (1996) 520.

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