Investigation of catalytic effects of the proton and

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oligomerization and chemical polymerization of pyrrole ... It was observed that pyrrole interacts with proton and Lewis acids on similar mechanisms to produce ...
Polymer 45 (2004) 7011–7016 www.elsevier.com/locate/polymer

Investigation of catalytic effects of the proton and Lewis acids on oligomerization and chemical polymerization of pyrrole ¨ mer Is¸ıldaka, Nuran O ¨ Pekmezb, Attila Yıldızb ¨ zaslana, O Muzaffer Cana,*, Hayri O a

Department of Chemistry, Gaziosmanpas¸a University, Campus of Tas¸lıc¸iftlik, Tokat 60250, Turkey b Department of Chemistry, Hacettepe University, Beytepe, Ankara 06532, Turkey Received 14 May 2004; received in revised form 3 August 2004; accepted 3 August 2004 Available online 20 August 2004

Abstract Protonation and oligomerization mechanisms of pyrrole monomer were elucidated in the presence of acid such as HBF4 and Lewis acid such as AlCl3, FeCl3 in acetonitrile solutions, using experimental methods such as UV–vis and AA spectroscopy and semi-empirical AM1 calculations. It was observed that pyrrole interacts with proton and Lewis acids on similar mechanisms to produce pyrrole oligomers. The theoretical studies predicted that the both types of acids add to the pyrrole ring on Cb atom. These studies also showed that FeCl3, which is used as oxidative agent in the chemical polymerizations, adds to the polypyrrole chain as well. q 2004 Elsevier Ltd. All rights reserved. Keywords: Polypyrrole; Protonation; Oligomerization

1. Introduction The conducting polymers such as polypyrrole and polythiophene are prepared either by chemical [1–5] and electrochemical [6–10] synthesis or, alternatively, by polymerization inside some 2D and 3D microporous solids (in zeolitic channels) [11,12]. In the polymerization performed inside microporous solids, the relative role of transition metal ions such as Fe3C, Cu2C and of acidic centres in polymerization mechanism has not been understood. In the chemical and electrochemical polymerizations, the influence of several parameters such as nature of the electrolyte, solvent, pH, temperature, and substituents on polymerization and their conductive properties have been investigated [13–18]. There remain some important unexplained points in polymerization mechanism in acidic media. In this work, protonation and oligomerization reactions of pyrrole were investigated in acidic media, experimentally and theoretically. The effect of the Lewis acids (AlCl3,

* Corresponding author. Tel.: C90-3562521616; fax: C90-3562521585 E-mail address: [email protected] (M. Can). 0032-3861/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2004.08.003

FeCl3) on chemical polymerization of pyrrole monomer was also elucidated.

2. Experimental Pyrrole (97%, Aldrich) was distilled before use. Anhydrous acetonitrile (99.8%, Aldrich), diethylether complex of tetrafluoroboric (HBF4) (85%, Aldrich), anhydrous aluminium chloride (AlCl3) (99%, Aldrich), anhydrous ferric chloride (FeCl3) (99%, Fuluka) and 2,6-di-tertbutylpyridine (Aldrich) were used as received. All of the studies were performed at room temperature. The UV–vis spectra were obtained using Jasco V-530 spectrometer. Electrothermal Atomic Absorption Spectrometry (An ATI Unicam 939 Model) was used for determining the amount of Al and Fe in the polymer samples. The dry conductivity values were measured using a four-probe technique. At least 10 different current values were used in the measurement of the potential drop. In the theoretical calculations, semi-empirical AM1 method in Hyperchem package programs [19], was used for the calculation of the total energies and heats of formation of species. All calculations related to the study

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were performed using HyperChem package program, run on a Pentium III computer. In the synthesis of polymers, 0.01 mol pyrrole, 0.01 mol pyrroleC0.01 mol of AlCl3, 0.01 mol pyrroleC0.01 mol HBF4 and 0.01 mol pyrroleC0.01 mol 2,6-di-tert-butylpyridine were mixed and equivalent amount of ferric chloride (0.01 mol) was then added to each these mixtures. The solutions were stirred at room temperature for 3 h. The resulting black polymers were filtered and then dried under vacuum.

3. Results and discussion Acetonitrile solutions containing pyrrole–HBF4 (1:1) and pyrrole–AlCl3 (1:1) were prepared and examined using UV–vis spectrometer. As can be seen in Fig. 1(a) and (b), there are two broad absorption bands at 310 and 456 nm, which belong to pyrrole oligomers [14]. These results show that similar products are formed by the reaction of pyrrole with HBF4 and AlCl3 through the same mechanisms. The absorption bands of pyrrole itself were reported to lie below 250 nm [12,14]. In order to investigate the interaction of FeCl3 with pyrrole, the UV–vis spectra of solutions consisting pyrroleCFeCl3 and FeCl3 in acetonitrile were obtained (Fig. 2). As is well known, polypyrrole black is synthesized by chemical oxidation of pyrrole with FeCl3. The band maximum observed at 800 nm in Fig. 2(a) belongs to soluble forms of polypyrrole formed in acetonitrile. This band was assigned to the transition from the valence band into the bipolaron band [20]. There are no absorption bands in the UV–vis spectrum of acetonitrile solution containing FeCl3 in a wavelength region between 400 and 800 nm (Fig. 2(b)). The Lewis acid effect observed for AlCl3 should also be observed in the presence of FeCl3 which is a Lewis acid as well. But, this effect was not observed probably because of its more dominant oxidative property. The intensity of the

Fig. 1. UV–vis spectra of (a) pyrrole–AlCl3 (1:1) mixture, (b) pyrrole– HBF4 (1:1) mixture and (c) pyrrole solution, (The concentrations of species, CZ0.001 M).

Fig. 2. UV–vis spectra of (a) pyrrole–FeCl3 mixture (1:1), (b) FeCl3 solution, (The concentrations of species, CZ0.001 M).

broad absorption band observed at 800 nm increases up to the equivalent amount of the added FeCl3 (Fig. 3(a)). When the amount of the FeCl3 added is more than that of pyrrole monomer a new absorption band at 672 nm appears (Fig. 3(b)). This new absorption band can only be explained by the addition of FeCl3 to the polymer backbone. Upon this addition, the conjugation in the presence of polymer chain is apparently broken and so, the band observed at 800 nm is shifted to lower wavelengths and the absorption intensity diminishes as a result of this type of degradation. In addition, the increase of the bands intensities at 359 and 313 nm for FeCl3 (Fig. 2(b)) upon the addition of pyrrole gives further evidence for the fact that there is an interaction between FeCl3 and polypyrrole chain. The amount of Fe and Al in polymer samples determined by AAS indicates that both FeCl3 and AlCl3 add to the polypyrrole chain. The amount of Fe in the polypyrrole polymer synthesized in pyrrole–FeCl3 (1:1) and pyrrole– FeCl3 (1:3) mixtures are 4.8% (correspondence to 14.0% FeCl3) and 13.5% (correspondence to 39.2% FeCl3), respectively. The amount of Al in the polypyrrole synthesized in pyrroleCAlCl3CFeCl3 (1:1:1) mixture is 5.7% (correspondence to 28.18% AlCl3). When the amount of the ferric chloride added to the polymerization solution is higher than that of the pyrrole, the amount of ferric chloride in polymer increases due to the addition of ferric chloride as Lewis acid to the polymer chain. The additions leading to a lower degree of conjugation along the polymer backbone, result in a decrease in conductivity value. Dry conductivity values of polymers synthesized using ferric chloride as oxidative agent in neutral (pyrrole), acidic (pyrrole–HBF4 (1:1)) and pyrrole– AlCl3 (1:1) and basic (pyrrole-2,6-di-tert-butylpyridine (1:1)) media are given Table 1. The conductivity of the polymer decreases in the presence of proton or Lewis acids in polymerization medium due to addition of these acids to the polymer chain. In the presence of a base such as 2,6-ditert-butylpyridine in polymerization solution, the conductivity of polypyrrole increases. The conductivity values of

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Fig. 3. UV–vis spectra of (a) pyrrole–FeCl3 mixture (1:1) (CpyrroleZ0.001 M, CFeCl3Z0.001 M) (b) pyrrole–FeCl3 mixture (1:3) (CpyrroleZ0.001 M, CFeCl3Z 0.003 M).

polypyrrole, synthesized by using FeCl3 as an oxidant varies between 10K5 and 200 S cmK1 depending upon the solvent used [21]. The conductivity values are usually smaller when the polymerization is carried out in an organic solvent [1, 21].

4. Theoretical discussion In order to explain the protonation and oligomerization mechanisms of pyrrole theoretically, the proton was added to N (NC–H), Ca ðCa –HÞ and Cb ðCb –HÞ atoms in the pyrrole, and then geometrical optimization of pyrrole and its Table 1 Dry conductivities of the polymer films prepared from various acetonitrile solutions consisting of pyrrole, pyrrole–AlCl3 (1:1), pyrrole–HBF4 (1:1) and pyrrole–2,6-di-tert-butylpyridine (1:1) via chemical polymerization of pyrrole with FeCl3 Solution combination

Dry conductivity (S cmK1)

Pyrrole PyrroleCAlCl3 (1.1) PyrroleCHBF4 (1:1) PyrroleC2,6-di-tert-butylpyridine (1:1)

0.32 0.018 0.019 1.2

protonated species were performed using AM1 method. Total energy (Etot) and heats of formation values (Hf) obtained from these optimizations are given in Table 2. The calculated results showed that the protonation of pyrrole should occur on C atoms, not on N atom. The pyrrole is highly reactive towards protons on both the -2 ðCa Þ and -3 ðCb Þ positions [22] since the transition state for substitution at each position is strongly stabilized by accommodation of the positive charge by nitrogen atom. If the calculated Etot and Hf values for Ca –H and Cb –H formations are compared, it will be seen that the protonation of pyrrole occurs preferably on Cb atoms. The electron densities calculated using AM1 method for each atom in pyrrole ring indicate that the charge on Cb atom is higher than those on Ca and N.

Table 2 The calculated total eneries (Etot) and heats of formation (Hf) using AM1 method for the structures formed with addition of HC to N, Ca and Cb atoms in the pyrrole ring

Pyrrole NC–H Ca –H Cb –H

Etot (kcal/mol)

Hf (kcal/mol)

K18143.62 K18285.77 K18297.99 K18298.64

39.75 212.50 200.29 199.63

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Scheme 1.

Unprotonated pyrrole has the following resonance structures,

This can also be seen in the chemical shift value given in literature [23] for Ca –H (6.708 ppm) and Cb –H (6.187 ppm) protons. The chemical shift value of Ca –H proton is higher than that of Cb –H proton due to withdrawal of electrons on Ca atoms by electronegative nitrogen atom. In other words, the shielding effect of electrons on Ca atom is lower than that of Cb atom. This can also be inferred from the 13C chemical shift values (117.3 ppm for Ca and 107.6 ppm for Cb [24]. In order to explain the interaction of pyrrole with Lewis acid, the similar theoretical calculations were performed for the interaction of pyrrole with AlCl3. The results are given in Table 3. It is clear that AlCl3 also adds to the pyrrole ring on Cb atom, as is the case with the addition of proton. With the addition of proton or Lewis acids to the pyrrole ring on Cb atom, pyrrole cation, which has a cationic core on Ca atom, can further react with the excess free pyrrole to produce pyrrole oligomers. According to the results inferred from these experimental results and theoretical considerations, the mechanism related to the protonation of pyrrole and the formation of the oligomers can be given as follows. Table 3 The calculated total energies (Etot) and heats of formation (Hf) using AM1 method for the structures formed with addition of AlCl3 to N, Ca and Cb atoms in the pyrrole ring

N–AlCl3 Ca –AlCl3 Cb –AlCl3

Etot (kcal/mol)

Hf (kcal/mol)

K45272.61 K45266.83 K45275.44

K102.49 K96.712 K105.32

The protonation of pyrrole or addition of Lewis acid to pyrrole occurs as follows

The formation of pyrrole oligomers is given in Scheme 1. According to the calculated heats of formations values of monoprotonated dimer (B) (in Scheme 1) in Table 4, the protonation of dimer B should occur on C2 atom.

Table 4 The heats of formation calculated for monoprotonated dimer (B) in Scheme 1 on C1, C2, C3, C4 and C5 atoms Hf (kcal/mol) C1 C2 C3 C4 C5

254.82 206.27 218.48 218.33 216.24

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Scheme 2.

Table 5 The heats of formation calculated for monoprotonated dimer (C) in Scheme 2 on C1, C2, C3 C4 and C5 atoms Hf (kcal/mol) C1 C2 C3 C4 C5

255.20 233.64 224.81 222.23 219.66

of polymer synthesized is much higher than that of the polymer synthesized in neutral and acidic media. These results suggest that the transition metal cations present in zeolitic channels behave as Lewis acids and interact with pyrrole to form polymeric thin films [11] based on the same mechanism proposed above.

Acknowledgements If the protonation of pyrrole occurs on nitrogen atom or Ca atom, the protonated pyrrole on nitrogen or Ca atoms (Scheme 2) would not react with free pyrrole to produce pyrrole trimer as proposed [17,22]. According to the calculated heats of formation values (Table 5), protonation of dimer C occurs on C5 atom. This type of protonation prevents the formation of trimer, thus, stops the further growth of the oligomers.

5. Conclusions The results obtained from experimental studies and theoretical calculations can be summarized as follows. The interactions of pyrrole with proton and Lewis acids occur on Cb : The same adduct are formed with proton or Lewis acids. FeCl3 also adds to the polypyrrole backbone as Lewis acid. In the presence of a base, which has no nucleophilic character such as 2,6-di-tert-butylpyridine the conductivity

The authors are indebted to the Department of Chemistry (Gaziosmanpasa University) for financial support of this study (Grant No. 2000/07 University Research Found).

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