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Electrochemical polymerization of 4-allylanisole
European Polymer Journal 37 (2001) 1747±1752
www.elsevier.nl/locate/europolj
Electrochemical polymerization of 4-allylanisole c,* A. Cihaner a, H.N. Testereci b, A.M. Onal a
c
Department of Industrial Engineering, Atilim University, 06836 Ankara, Turkey b Department of Chemistry, Kõrõkkale University, 71450 Kõrõkkale, Turkey Department of Chemistry, Middle East Technical University, 06531 Ankara, Turkey
Received 30 August 2000; received in revised form 5 March 2001; accepted 7 March 2001
Abstract Electrochemical polymerization of 4-allylanisole (4AA), via constant potential electrolysis, has been investigated in acetonitrile using two dierent supporting electrolytes. Redox behavior of the monomer was also studied in the same solvent±electrolyte couples at room temperature. Electrochemical polymerization of the monomer yielded insoluble polymer ®lms on the electrode surface, which bears a very low conductivity, together with the low molecular weight polymers in the bulk of the solution. The decrease in the monomer concentration, during the electrochemical polymerization, was monitored by taking the cyclic voltammogram of the electrolysis solution. The eect of temperature on the rate of electrochemical polymerization was also studied. The polymers were characterized by taking the 1 H-NMR and FTIR spectra. Molecular weight of the soluble polymer was determined by vapor pressure osmometry. Thermal analysis of the polymer ®lm and soluble polymer were done by DSC. Ó 2001 Elsevier Science Ltd. All rights reserved. Keywords: 4-Allylanisole; Electrochemical polymerization; Cyclic voltammetry; Constant potential electrolysis
1. Introduction The polymerization of allyl monomers have received far less attention than that of the corresponding vinyl anologues, because in their polymerization chain transfer to monomer is of greater importance, which hinders formation of high molecular weight polymers. Although, intramolecular rearrangements of simple molecules are well documented, there are few examples of intramolecular rearrangements in polymerization in the literature [1±3]. It is known that some allyl monomers like allylbenzene and allylphenylether undergo isomerization reaction during the polymerization. Kennedy [1] reported that allyl benzene isomerize to b-methylstyrene during the polymerization using aluminiumchloride. Hui and Yip [2] and Endo [3] reported that allylphenylether undergoes Claisen rearrangement during the polymer-
*
Corresponding author. Tel.: +90-312-2103188; fax: +90312-2101280. E-mail address: [email protected] (A.M. Onal).
ization using BF3 OEt2 . We have previously reported electrochemical and radiation induced polymerization of allylthiourea [4] and allylbenzene [5] and electrochemical polymerization of allylphenylether [6]. We have also noted such isomerizations are taking place during the course of electrochemical polymerization of allylbenzene [5] and allylphenylether [6], the former isomerize to b-methylstyrene and the later to 2-allylphenol. Recently, we have also investigated the electrochemical polymerization of 2-allylphenol [7] and we found that 2allylphenol isomerize to 2-propenylphenol during the electrochemical polymerization. 4AA, which contains an allylic group, is also of interest, since it undergoes photochemical cyclization [8] and isomerizes via base catalyzed reactions to anethole [9,10]. It is reported that quaternary ammonium salts, under phase transfer catalytic conditions, undergo Homan degradation and catalyze the isomerization of 4AA to p-methoxy-bmethylstyrene [9]. Since the supporting electrolyte which we use in most of our electrochemical work is also a quaternary ammonium salt, we investigated the electrochemical polymerization of 4AA using two dierent
0014-3057/01/$ - see front matter Ó 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 4 - 3 0 5 7 ( 0 1 ) 0 0 0 6 5 - 9
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A. Cihaner et al. / European Polymer Journal 37 (2001) 1747±1752
electrolytes, one of which was also a quaternary ammonium salt. The present study is concerned with the electrochemical polymerization of 4AA in acetonitrile. The electrochemical behavior of the monomer was investigated by cyclic voltammetry (CV), and the rate of polymerization at various temperatures was followed by monitoring the ®rst oxidation peak heights.
2. Experimental 2.1. Materials The monomer 4AA (Aldrich Chemical Co., 98%) was dried over CaH2 for 48 h and distilled over fresh CaH2 under reduced pressure (5 mmHg). Tetrabutylammonium-tetra¯uoroborate (TBAFB) (Aldrich Chemical Co.) and sodium perchlorate (Aldrich Chemical Co.) were used without further puri®cation as supporting electrolyte for CV measurements and for constant potential electrolysis. Acetonitrile was puri®ed by drying over CaH2 followed by fractional distillation. It was stored under nitrogen atmosphere over Linde 40 nm molecular sieves. 2.2. Cyclic voltammetry The oxidation±reduction behavior of the monomer was determined by CV. The system consisted of a potentiostat (Bank Pos 73), an X±Y recorder (Lloyd PL-3), and a CV cell containing a Pt-wire working electrode, a Pt-wire counter-electrode, and a SCE as reference electrode. Measurements were done under N2 atmosphere in acetonitrile at room temperature. 2.3. Polymer synthesis Electrochemical polymerization of 4AA was achieved by constant potential electrolysis. The supporting electrolyte was dissolved in freshly distilled acetonitrile and then introduced together with monomer solution into Htype cell with two separated compartments. At the end of the electrolysis, the solution from the anode compartment was concentrated by removing solvent under vacuum. When TBAFB was used as electrolyte in the polymerization, it was removed from the remaining mixture by dissolving in methanol containing 10% (v/v) of concentrated ammonia. On adding this solution to water, a yellowish ¯uy polymer was obtained. On the other hand, when sodium perchlorate was used, the electrolyte from the mixture was removed by using extraction method with dichloromethane and water. The black polymer ®lms were separated by peeling o from the electrode surface.
The kinetics of electrolysis were followed by taking the cyclic voltammogram of electrolysis solution at different time intervals. 2.4. Polymer characterization 1
H-NMR spectra of the polymers were taken by a Bruker Instrument-NMR spectrometer (DPX-400) in CDCl3 and d-acetone. FTIR spectra of the polymers were obtained on a Nicolet 510 FTIR Spectrometer using KBr pellets. DSC of samples were taken with a DSC 910S/TA 2000 instrument. Molecular weight of the polymer obtained from the anode compartment was determined using Knauer V7187 vapor pressure osmometry. ESR spectrum of polymer ®lms were recorded on a Varian E12 ESR spectrometer. 3. Results and discussion Electrochemical polymerization of 4AA was studied in acetonitrile with two dierent supporting electrolytes using Pt-electrodes. Prior to constant potential electrolysis, the oxidation±reduction behavior of 4AA was studied by CV in acetonitrile±TBAFB and in acetonitrile±sodium perchlorate solvent±electrolyte couples, separately, at room temperature under N2 atmosphere. 4AA have two oxidation peaks at 1:55 and 2.10 V vs SCE and all peaks represent an irreversible electron transfer. A voltammogram of 4AA is given Fig. 1a. The variation of peak current with voltage scan rate was also studied and a negative slope was obtained when current function I=CV 1=2 for the ®rst oxidation peak was plotted against log V , where I is the peak current, V is the voltage scan rate and C is the concentration (Fig. 1b). The negative slope indicates a reversible one-electron transfer, at 1.55 V vs SCE, followed by a chemical reaction according to Nicholson±Shain criteria [11]. Appearance of an irreversible peak in the voltammogram of 4AA instead of a reversible one can be explained due to higher rate of chemical reaction following the electrochemical oxidation. The eect of temperature on the rate of polymerization of 4AA was studied by recording the cyclic voltammograms of the electrolysis solution during the electrolysis. Since the peak current is directly proportional to the concentration of substrate, monomer concentrations were found from the anodic peak currents and percent conversion to polymer as a function of time was plotted. Fig. 2 includes the results obtained at three dierent temperatures. As seen from Fig. 2 an increase in the initial rate was observed with increasing temperature. Initial rate constants for the electrochemical polymerization of 4AA were found from the initial slopes and an apparent activation energy of 7.3 kJ mol 1 was obtained from the Arrhenius plot. This low value for
A. Cihaner et al. / European Polymer Journal 37 (2001) 1747±1752
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Fig. 1. (a) Cyclic voltammogram of 1:6 10 3 M 4AA measured in 0.1 M NaClO4 ±acetonitrile solution at room temperature (VSR 150 mV s 1 ). (b) Variation of current function I=CV 1=2 with voltage scan rate, where I is current in mA, C is concentration in mol l 1 and V is scan rate in mV s 1 .
Fig. 2. Percent conversion versus time plot for the polymerization of 4AA at (a) 313 K, (b) 300 K and (c) 263 K.
apparent activation energy together with the increase in the initial rates with increasing temperature suggests a radicalic mechanism for the electrochemical polymerization of 4AA. In fact, we also carried out ESR measurements during constant current electrolysis of the electrolysis solution containing the monomer at low temperature (220 K) and a singlet, indicating formation of a radicalic intermediate, was observed. 3.1. Polymer characterization The electrochemical polymerization of 4AA in acetonitrile±TBAFB or in acetonitrile±NaClO4 yielded two dierent products in the anode compartment by CPE at 2.1 V vs SCE. One was insoluble polymer ®lm on the electrode surface and the other was obtained from the bulk of the anolyte after removal of unreacted
monomer, solvent and electrolyte. No polymer was obtained in the cathode compartment. IR spectra of polymers obtained from 4AA by electrochemical polymerization and that of monomer 4AA are given in Fig. 3. In general, for the polymers obtained from the electrode surface and from the bulk of the anolyte in acetonitrile by CPE, methyl group peak at 1375 cm 1 was observed in the spectra as shown in Fig. 3b and c, but not in monomer spectrum (Fig. 3a). Peaks at 913 and 995 cm 1 due to the allylic double bond appear slightly in the polymer spectra. Also, the absorption peak at 756 cm 1 indicates 1,2-disubstituted or 1,2,3-trisubstituted benzene ring and similarly, the peak at 820 cm 1 shows the possibility of trisubstituted (1,4 or 1,2,4) benzene ring. If the monomer were only polymerized by allyl double bond addition, these peaks would not be observed. Peak at 2937 cm 1 is the evidence for the presence of methylene groups. Unexpectedly, the IR spectra of the polymer products showed the presence of the phenolic OH group (about 3400 cm 1 ). In addition, IR spectra show the presence of C±O±C etheric peaks at about 1035 and 1250 cm 1 . Strong peaks at about 1100 and 625 cm 1 in the spectra are due to ClO4 ion. On the other hand, anethole was also polymerized in the same medium, containing acetonitrile±TBAFB, by CPE at predetermined potential. When the IR spectrum of the polymer obtained from 4AA (Fig. 3d) was compared with that of the anethole (Fig. 3e) (827, 1035, 1108, 1178, 1249, 1300, 1375, 1462, 1510, 1611, 2833 and 2959 cm 1 ), they were found to have the same principal peaks. These same principal peaks (except for OH
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Fig. 3. FTIR spectra of (a) 4AA, (b) polymer ®lm, (c) polymer from the bulk of the anolyte, (d) polymer from the bulk of the anolyte (TBAFB used as electrolyte) and (e) polymer obtained from anethole.
group) could be mostly due to the isomerization of 4AA to anethole during the polymerization. In accordance with the IR spectra, polymers obtained from the bulk of the anolyte, the presence of methyl group (d 1:2 ppm) and hydroxyl group (d 4:6 ppm) was con®rmed by 1 H-NMR spectroscopy as shown in Fig. 4. A comparison of the 1 H-NMR spectrum of 4AA with that of polymer shows the appearance of new signals at d 1:2, 2.3, 2.6 ppm and a broad signal at 4.6 ppm in addition to the original signals of monomer. The signal in the monomer spectrum at d 7:3 ppm due to the solvent CDCl3 and the signal in the polymer spectrum at d 2 ppm is due to the solvent acetone. The doublet peaks at 1.2 ppm and the multiplet peaks at 2.3 and 2.6 ppm can be assigned, respectively, to C±CH3 , ±CH, /±CH. Further, the broad peaks at 3.7 and 4.6 ppm show the presence of ±OCH3 and ±OH groups. In addition, ±CH@, @CH2 and ±CH2 groups slightly ap-
pear at 6.0, 5.1 and 3.5 ppm, respectively. On the other hand, multiplet peaks around at 7 ppm show the di- and tri-substituted benzene ring. Although, other small peaks indicate that some side reactions took place, all the main peaks could be assigned. The polymer ®lm with 60 lm thickness obtained from the electrode surface surprisingly found to exhibit a low conducting behavior. Its conductivity was measured by four-probe technique and found to be 4 10 2 S cm 1 . It was also found that this ®lm also exhibits a singlet DHpp 12:5 G (Fig. 5). When the polymer ®lm was doped with iodine an increase in the conductivity and ESR signal intensity was observed. Although the origin of the ESR signal could be the defects formed during the polymerization, the increase in the ESR signal intensity could be an evidence for the formation of polarons during iodine doping. Further explanation of the formation of conducting ®lms needs more works.
A. Cihaner et al. / European Polymer Journal 37 (2001) 1747±1752
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Fig. 4. 1 H-NMR spectra of (a) 4AA and (b) polymer from the bulk of the anolyte.
Fig. 5. ESR spectrum of polymer ®lm after iodine doping.
Also, due to the electroactive property of polymer ®lm, its reduction±oxidation behavior was studied by CV and recycled between 1.8 and 1.6 V as shown in Fig. 6. Peaks belonging to reduction of polymer ®lm decrease as the peaks belonging to oxidation increase during the recycling. Thermal behaviors of the polymer ®lm and the soluble polymer ®lm obtained from the anolyte were investigated by DSC. Thermograms show that these two polymer products show an exothermic behavior and then start to decompose after 200°C. Molecular weight of the polymer obtained from the anolyte was determined by vapor pressure osmometry and it was found about 2000. The allyl radical produced has less tendency to initiate a new polymer chain because the allyl radical stabilizes itself by resonance; therefore, this chain transfer will be essentially a termination reaction. This is called ``degradative chain transfer'' and is the major reason for the low-molecular weight products obtained in the polymerization of allyltype monomers. Based upon above results, it is apparent that the polymerization proceeds via polyalkylation to the aromatic
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NaClO4 solvent±electrolyte couples by CPE. It was shown that the electrochemical polymerization of 4AA gives a mixture of dierent polymers as suggested in our previous studies [6,7] instead of expected polymer product. The polymer obtained from the bulk of anolyte exhibits the same principal IR peaks (except for OH group) as that of polyanethole obtained under the same condition, which indicates that 4AA also undergoes isomerization to anethole during the electrochemical polymerization. The low apparent activation energy and temperature dependence of the rate of the polymerization reaction suggested a radicalic mechanism which is con®rmed the presence of a singlet obtained during low temperature in-situ ESR studies. It is also found that polymer ®lm bears a low conductivity behavior, which can be enhanced by iodine doping.
Acknowledgements The authors are grateful to Middle East Technical University Research Fund for support of this work. Fig. 6. Cyclic voltammogram of polymer ®lm in NaClO4 ± acetonitrile at room temperature (VSR 150 mV s 1 ).
ring, and a conventional double bond opening also take place. On the other hand, polymer chain, especially polymer ®lm, must contain conjugation due to its conductivity.
4. Conclusion The polymerization of 4AA was carried out at room temperature in acetonitrile±TBAFB and in acetonitrile±
References [1] [2] [3] [4] [5] [6] [7] [8] [9]
Kennedy JP. J Polym Sci A 1964;2:5171. Hui KM, Yip LC. J Chem Soc Chem Commun 1971:825. Endo K, Otsu T. Polymer 1991;32(15):2856. Ozkan H, Usanmaz A, Onal AM. Polym Int 1994;35:293. Sousy K, Usanmaz A, Onal AM. Polymer 1990;31:1564. S AM. J Polym Sci A 1995;33:1817. ß en S ß , Usanmaz A, Onal Cihaner A, Onal AM. Polym Bull 2000;45(1):45. Clark T, Watkins TAM. J C S Perkin I 1974:2124. Zerda J, Neumann R, Sasson Y. J Chem Soc Perkin Trans II 1986:823. [10] Neumann R, Sasson Y. J Org Chem 1984;49:3448. [11] Nicholson RS, Shain I. Anal Chem 1964;36:706.
www.elsevier.nl/locate/europolj
Electrochemical polymerization of 4-allylanisole c,* A. Cihaner a, H.N. Testereci b, A.M. Onal a
c
Department of Industrial Engineering, Atilim University, 06836 Ankara, Turkey b Department of Chemistry, Kõrõkkale University, 71450 Kõrõkkale, Turkey Department of Chemistry, Middle East Technical University, 06531 Ankara, Turkey
Received 30 August 2000; received in revised form 5 March 2001; accepted 7 March 2001
Abstract Electrochemical polymerization of 4-allylanisole (4AA), via constant potential electrolysis, has been investigated in acetonitrile using two dierent supporting electrolytes. Redox behavior of the monomer was also studied in the same solvent±electrolyte couples at room temperature. Electrochemical polymerization of the monomer yielded insoluble polymer ®lms on the electrode surface, which bears a very low conductivity, together with the low molecular weight polymers in the bulk of the solution. The decrease in the monomer concentration, during the electrochemical polymerization, was monitored by taking the cyclic voltammogram of the electrolysis solution. The eect of temperature on the rate of electrochemical polymerization was also studied. The polymers were characterized by taking the 1 H-NMR and FTIR spectra. Molecular weight of the soluble polymer was determined by vapor pressure osmometry. Thermal analysis of the polymer ®lm and soluble polymer were done by DSC. Ó 2001 Elsevier Science Ltd. All rights reserved. Keywords: 4-Allylanisole; Electrochemical polymerization; Cyclic voltammetry; Constant potential electrolysis
1. Introduction The polymerization of allyl monomers have received far less attention than that of the corresponding vinyl anologues, because in their polymerization chain transfer to monomer is of greater importance, which hinders formation of high molecular weight polymers. Although, intramolecular rearrangements of simple molecules are well documented, there are few examples of intramolecular rearrangements in polymerization in the literature [1±3]. It is known that some allyl monomers like allylbenzene and allylphenylether undergo isomerization reaction during the polymerization. Kennedy [1] reported that allyl benzene isomerize to b-methylstyrene during the polymerization using aluminiumchloride. Hui and Yip [2] and Endo [3] reported that allylphenylether undergoes Claisen rearrangement during the polymer-
*
Corresponding author. Tel.: +90-312-2103188; fax: +90312-2101280. E-mail address: [email protected] (A.M. Onal).
ization using BF3 OEt2 . We have previously reported electrochemical and radiation induced polymerization of allylthiourea [4] and allylbenzene [5] and electrochemical polymerization of allylphenylether [6]. We have also noted such isomerizations are taking place during the course of electrochemical polymerization of allylbenzene [5] and allylphenylether [6], the former isomerize to b-methylstyrene and the later to 2-allylphenol. Recently, we have also investigated the electrochemical polymerization of 2-allylphenol [7] and we found that 2allylphenol isomerize to 2-propenylphenol during the electrochemical polymerization. 4AA, which contains an allylic group, is also of interest, since it undergoes photochemical cyclization [8] and isomerizes via base catalyzed reactions to anethole [9,10]. It is reported that quaternary ammonium salts, under phase transfer catalytic conditions, undergo Homan degradation and catalyze the isomerization of 4AA to p-methoxy-bmethylstyrene [9]. Since the supporting electrolyte which we use in most of our electrochemical work is also a quaternary ammonium salt, we investigated the electrochemical polymerization of 4AA using two dierent
0014-3057/01/$ - see front matter Ó 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 4 - 3 0 5 7 ( 0 1 ) 0 0 0 6 5 - 9
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A. Cihaner et al. / European Polymer Journal 37 (2001) 1747±1752
electrolytes, one of which was also a quaternary ammonium salt. The present study is concerned with the electrochemical polymerization of 4AA in acetonitrile. The electrochemical behavior of the monomer was investigated by cyclic voltammetry (CV), and the rate of polymerization at various temperatures was followed by monitoring the ®rst oxidation peak heights.
2. Experimental 2.1. Materials The monomer 4AA (Aldrich Chemical Co., 98%) was dried over CaH2 for 48 h and distilled over fresh CaH2 under reduced pressure (5 mmHg). Tetrabutylammonium-tetra¯uoroborate (TBAFB) (Aldrich Chemical Co.) and sodium perchlorate (Aldrich Chemical Co.) were used without further puri®cation as supporting electrolyte for CV measurements and for constant potential electrolysis. Acetonitrile was puri®ed by drying over CaH2 followed by fractional distillation. It was stored under nitrogen atmosphere over Linde 40 nm molecular sieves. 2.2. Cyclic voltammetry The oxidation±reduction behavior of the monomer was determined by CV. The system consisted of a potentiostat (Bank Pos 73), an X±Y recorder (Lloyd PL-3), and a CV cell containing a Pt-wire working electrode, a Pt-wire counter-electrode, and a SCE as reference electrode. Measurements were done under N2 atmosphere in acetonitrile at room temperature. 2.3. Polymer synthesis Electrochemical polymerization of 4AA was achieved by constant potential electrolysis. The supporting electrolyte was dissolved in freshly distilled acetonitrile and then introduced together with monomer solution into Htype cell with two separated compartments. At the end of the electrolysis, the solution from the anode compartment was concentrated by removing solvent under vacuum. When TBAFB was used as electrolyte in the polymerization, it was removed from the remaining mixture by dissolving in methanol containing 10% (v/v) of concentrated ammonia. On adding this solution to water, a yellowish ¯uy polymer was obtained. On the other hand, when sodium perchlorate was used, the electrolyte from the mixture was removed by using extraction method with dichloromethane and water. The black polymer ®lms were separated by peeling o from the electrode surface.
The kinetics of electrolysis were followed by taking the cyclic voltammogram of electrolysis solution at different time intervals. 2.4. Polymer characterization 1
H-NMR spectra of the polymers were taken by a Bruker Instrument-NMR spectrometer (DPX-400) in CDCl3 and d-acetone. FTIR spectra of the polymers were obtained on a Nicolet 510 FTIR Spectrometer using KBr pellets. DSC of samples were taken with a DSC 910S/TA 2000 instrument. Molecular weight of the polymer obtained from the anode compartment was determined using Knauer V7187 vapor pressure osmometry. ESR spectrum of polymer ®lms were recorded on a Varian E12 ESR spectrometer. 3. Results and discussion Electrochemical polymerization of 4AA was studied in acetonitrile with two dierent supporting electrolytes using Pt-electrodes. Prior to constant potential electrolysis, the oxidation±reduction behavior of 4AA was studied by CV in acetonitrile±TBAFB and in acetonitrile±sodium perchlorate solvent±electrolyte couples, separately, at room temperature under N2 atmosphere. 4AA have two oxidation peaks at 1:55 and 2.10 V vs SCE and all peaks represent an irreversible electron transfer. A voltammogram of 4AA is given Fig. 1a. The variation of peak current with voltage scan rate was also studied and a negative slope was obtained when current function I=CV 1=2 for the ®rst oxidation peak was plotted against log V , where I is the peak current, V is the voltage scan rate and C is the concentration (Fig. 1b). The negative slope indicates a reversible one-electron transfer, at 1.55 V vs SCE, followed by a chemical reaction according to Nicholson±Shain criteria [11]. Appearance of an irreversible peak in the voltammogram of 4AA instead of a reversible one can be explained due to higher rate of chemical reaction following the electrochemical oxidation. The eect of temperature on the rate of polymerization of 4AA was studied by recording the cyclic voltammograms of the electrolysis solution during the electrolysis. Since the peak current is directly proportional to the concentration of substrate, monomer concentrations were found from the anodic peak currents and percent conversion to polymer as a function of time was plotted. Fig. 2 includes the results obtained at three dierent temperatures. As seen from Fig. 2 an increase in the initial rate was observed with increasing temperature. Initial rate constants for the electrochemical polymerization of 4AA were found from the initial slopes and an apparent activation energy of 7.3 kJ mol 1 was obtained from the Arrhenius plot. This low value for
A. Cihaner et al. / European Polymer Journal 37 (2001) 1747±1752
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Fig. 1. (a) Cyclic voltammogram of 1:6 10 3 M 4AA measured in 0.1 M NaClO4 ±acetonitrile solution at room temperature (VSR 150 mV s 1 ). (b) Variation of current function I=CV 1=2 with voltage scan rate, where I is current in mA, C is concentration in mol l 1 and V is scan rate in mV s 1 .
Fig. 2. Percent conversion versus time plot for the polymerization of 4AA at (a) 313 K, (b) 300 K and (c) 263 K.
apparent activation energy together with the increase in the initial rates with increasing temperature suggests a radicalic mechanism for the electrochemical polymerization of 4AA. In fact, we also carried out ESR measurements during constant current electrolysis of the electrolysis solution containing the monomer at low temperature (220 K) and a singlet, indicating formation of a radicalic intermediate, was observed. 3.1. Polymer characterization The electrochemical polymerization of 4AA in acetonitrile±TBAFB or in acetonitrile±NaClO4 yielded two dierent products in the anode compartment by CPE at 2.1 V vs SCE. One was insoluble polymer ®lm on the electrode surface and the other was obtained from the bulk of the anolyte after removal of unreacted
monomer, solvent and electrolyte. No polymer was obtained in the cathode compartment. IR spectra of polymers obtained from 4AA by electrochemical polymerization and that of monomer 4AA are given in Fig. 3. In general, for the polymers obtained from the electrode surface and from the bulk of the anolyte in acetonitrile by CPE, methyl group peak at 1375 cm 1 was observed in the spectra as shown in Fig. 3b and c, but not in monomer spectrum (Fig. 3a). Peaks at 913 and 995 cm 1 due to the allylic double bond appear slightly in the polymer spectra. Also, the absorption peak at 756 cm 1 indicates 1,2-disubstituted or 1,2,3-trisubstituted benzene ring and similarly, the peak at 820 cm 1 shows the possibility of trisubstituted (1,4 or 1,2,4) benzene ring. If the monomer were only polymerized by allyl double bond addition, these peaks would not be observed. Peak at 2937 cm 1 is the evidence for the presence of methylene groups. Unexpectedly, the IR spectra of the polymer products showed the presence of the phenolic OH group (about 3400 cm 1 ). In addition, IR spectra show the presence of C±O±C etheric peaks at about 1035 and 1250 cm 1 . Strong peaks at about 1100 and 625 cm 1 in the spectra are due to ClO4 ion. On the other hand, anethole was also polymerized in the same medium, containing acetonitrile±TBAFB, by CPE at predetermined potential. When the IR spectrum of the polymer obtained from 4AA (Fig. 3d) was compared with that of the anethole (Fig. 3e) (827, 1035, 1108, 1178, 1249, 1300, 1375, 1462, 1510, 1611, 2833 and 2959 cm 1 ), they were found to have the same principal peaks. These same principal peaks (except for OH
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Fig. 3. FTIR spectra of (a) 4AA, (b) polymer ®lm, (c) polymer from the bulk of the anolyte, (d) polymer from the bulk of the anolyte (TBAFB used as electrolyte) and (e) polymer obtained from anethole.
group) could be mostly due to the isomerization of 4AA to anethole during the polymerization. In accordance with the IR spectra, polymers obtained from the bulk of the anolyte, the presence of methyl group (d 1:2 ppm) and hydroxyl group (d 4:6 ppm) was con®rmed by 1 H-NMR spectroscopy as shown in Fig. 4. A comparison of the 1 H-NMR spectrum of 4AA with that of polymer shows the appearance of new signals at d 1:2, 2.3, 2.6 ppm and a broad signal at 4.6 ppm in addition to the original signals of monomer. The signal in the monomer spectrum at d 7:3 ppm due to the solvent CDCl3 and the signal in the polymer spectrum at d 2 ppm is due to the solvent acetone. The doublet peaks at 1.2 ppm and the multiplet peaks at 2.3 and 2.6 ppm can be assigned, respectively, to C±CH3 , ±CH, /±CH. Further, the broad peaks at 3.7 and 4.6 ppm show the presence of ±OCH3 and ±OH groups. In addition, ±CH@, @CH2 and ±CH2 groups slightly ap-
pear at 6.0, 5.1 and 3.5 ppm, respectively. On the other hand, multiplet peaks around at 7 ppm show the di- and tri-substituted benzene ring. Although, other small peaks indicate that some side reactions took place, all the main peaks could be assigned. The polymer ®lm with 60 lm thickness obtained from the electrode surface surprisingly found to exhibit a low conducting behavior. Its conductivity was measured by four-probe technique and found to be 4 10 2 S cm 1 . It was also found that this ®lm also exhibits a singlet DHpp 12:5 G (Fig. 5). When the polymer ®lm was doped with iodine an increase in the conductivity and ESR signal intensity was observed. Although the origin of the ESR signal could be the defects formed during the polymerization, the increase in the ESR signal intensity could be an evidence for the formation of polarons during iodine doping. Further explanation of the formation of conducting ®lms needs more works.
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Fig. 4. 1 H-NMR spectra of (a) 4AA and (b) polymer from the bulk of the anolyte.
Fig. 5. ESR spectrum of polymer ®lm after iodine doping.
Also, due to the electroactive property of polymer ®lm, its reduction±oxidation behavior was studied by CV and recycled between 1.8 and 1.6 V as shown in Fig. 6. Peaks belonging to reduction of polymer ®lm decrease as the peaks belonging to oxidation increase during the recycling. Thermal behaviors of the polymer ®lm and the soluble polymer ®lm obtained from the anolyte were investigated by DSC. Thermograms show that these two polymer products show an exothermic behavior and then start to decompose after 200°C. Molecular weight of the polymer obtained from the anolyte was determined by vapor pressure osmometry and it was found about 2000. The allyl radical produced has less tendency to initiate a new polymer chain because the allyl radical stabilizes itself by resonance; therefore, this chain transfer will be essentially a termination reaction. This is called ``degradative chain transfer'' and is the major reason for the low-molecular weight products obtained in the polymerization of allyltype monomers. Based upon above results, it is apparent that the polymerization proceeds via polyalkylation to the aromatic
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A. Cihaner et al. / European Polymer Journal 37 (2001) 1747±1752
NaClO4 solvent±electrolyte couples by CPE. It was shown that the electrochemical polymerization of 4AA gives a mixture of dierent polymers as suggested in our previous studies [6,7] instead of expected polymer product. The polymer obtained from the bulk of anolyte exhibits the same principal IR peaks (except for OH group) as that of polyanethole obtained under the same condition, which indicates that 4AA also undergoes isomerization to anethole during the electrochemical polymerization. The low apparent activation energy and temperature dependence of the rate of the polymerization reaction suggested a radicalic mechanism which is con®rmed the presence of a singlet obtained during low temperature in-situ ESR studies. It is also found that polymer ®lm bears a low conductivity behavior, which can be enhanced by iodine doping.
Acknowledgements The authors are grateful to Middle East Technical University Research Fund for support of this work. Fig. 6. Cyclic voltammogram of polymer ®lm in NaClO4 ± acetonitrile at room temperature (VSR 150 mV s 1 ).
ring, and a conventional double bond opening also take place. On the other hand, polymer chain, especially polymer ®lm, must contain conjugation due to its conductivity.
4. Conclusion The polymerization of 4AA was carried out at room temperature in acetonitrile±TBAFB and in acetonitrile±
References [1] [2] [3] [4] [5] [6] [7] [8] [9]
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