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Boletín de la Sociedad Chilena de Química

versión impresa ISSN 0366-1644

Bol. Soc. Chil. Quím. v.46 n.2 Concepción jun. 2001

http://dx.doi.org/10.4067/S0366-16442001000200017 

ELECTROPOLYMERIZATION OF ANILINE AND PYRROLE IN
THE PRESENCE OF COPPER(II)

L. BASÁEZ*, 0. ARREDONDO, F. BUSTOS, C. SÁNCHEZ and B.L. RIVAS.

Facultad de Ciencias Químicas, Universidad de Concepción, Casilla 160-C,
Concepción, Chile. * e-mail: lbasaez@udec.cl *
(Received: December 29, 2000 - Accepted: April 11, 2001)
* To whom correspondence should be addressed

RESUMEN

Se estudió la polimerización de anilina y pirrol en presencia de diferentes sales de cobre(II) por voltametría cíclica. Los sistemas empleados fueron: polímero/CuCl2/HCl, polímero/CuSO4/H2SO 4 y polímero/Cu(NO3)2/HNO 3). En ambos casos la relación polímero/metal fue 10/1.

Los voltamogramas fueron registrados a pH = 0 y velocidad de barrido de 100 mV. s-1. Se encontró que la anilina polimeriza en presencia de Cu(II) y, de todos los ácidos empleados (HCl, H2SO4 and HNO3) el mejor para obtener polianilina fue Cu(II) in 1 M HCI.

Tanto la polianilina como el polipirrol mostraron un mejor comportamiento voltamétrico en medio de HCl. Esto se atribuyó a la formación de los complejos CuCl2- o CUCl32- los cuales pueden ser incorporados en el polímero cuando este deja sitios positivos al ser electrooxidado. No fue posible obtener polipirrol en presencia de CuSO4/H2SO4.

Palabras claves: Electropolimerización; anilina; pirrol; complejos de cobre; cobre(II).

SUMMARY

Polymerization of aniline and pyrrole in the presence of copper(II) coming from different salts were studied by cyclic voltammetry. The systems used are: polymer/CuCl2/HCl, polymer/CuSO4/H2SO4 and polymer/Cu(NO3)2/HNO3 ). The polymer/metal mol ratios were 10/1 in both cases. The voltammograms were recorded at pH = 0 under constant potentiodynamic regime of 100 mV. s-1. It was found that aniline polymerizes in presence of Cu(II) and of all the strong acids tested (HCl, H2SO4 and HNO3). The best medium to obtain polyaniline was Cu(II) in 1 M HCI.

Both polyaniline and poly(pyrrole) showed a better voltammetric behavior in HCl medium. It was attributed to the CuCl2- or CuCl32- complex formation which can be incorporated to the polymer when it is electro-oxidized. It was not possible polymerize pyrrole in H2SO4.

Keywords: Electropolymerization; aniline; pyrrole; copper complexes; copper (II).

INTRODUCTION

Polymeric materials have a great importance due to their very wide application fields. One of them corresponds to the electrical conducting properties. However, most of these polymers have a band structure with a large energy gap [1-3]. Nevertheless, there are polymers such as poly(aniline), Pani [4] and poly(pyrrole), Ppy [5] which show electrical conducting properties when chemically or electrochemically synthesized [6-10]. These materials have applications in devices such as rechargeable batteries [11], sensors [12], diodes [13], catalysts [14], etc. The development of these polymers has been increased during the last decades, specially functional polymers. To achieve that, two main routes have been developed: the first one is to substitute covalently the structure and the second one is to use the anion exchange property of the polymer during the oxidation process [15]. According to that, Ppy has been substituted with metal centers such as 2,2-bipyridine-Ru [16], porphyrines-Co(II) [17] and porphyrines-Ni(II) [18]. Moreover, metal anion complexes such as PtCI 6= have been incorporated into Pani [19], and Co(II) and Fe(III) metal ions into functionalized Ppy with carboxylate groups were also incorporated [20,21].

The aim of this paper is to study the effect of Cu(II) on the reversibility of the redox process associated to polyaniline and poly(pyrrole), which can be useful to understand the electrical properties of these polymers and to propose its future utility in devices such as analytical sensors or others.

EXPERIMENTAL

Reagents. All the reagents used were analytical grade and the solutions were deaerated with high purity argon, both before and during the measurements. Aniline (Aldrich, > 99.9 %) and pyrrole (Aldrich, > 99 %) monomers were purified by distillation under vacuum. The metal ion solutions employed were CuCl2.2H20, CuSO4.5H20, CuNO3.3H20 (Merck. p.a.), in 1 M HCl, 1 M HNO3 or 1 M H2SO4 (Mallinckrodt p.a.) respectively. Bidistilled water was used to prepare all the solutions. The films were synthesized electrochemically from an aqueous solution of 0.1M aniline and pyrrole contaning 0.01 M copper(II) ions. The pH value chosen for the study was a compromise between acidic conditions which are favorable for an easy polymerization.

Measurements. Standard three-electrode cells were used for Pani and Ppy growth. The working electrode (WE) was a Pt disc (0.5 mm diam.); the counter electrode was a large-area Pt wire, and the reference electrode was Ag/AgCl. All potentials in this paper are reported vs Ag/AgCl. The experiments were performed at room temperature (20°C). Cyclic voltammograms were recorded with a CV-50W BAS (Bio Analytical System, U.S.A.) coupled to Cell Stand C3, at a scanning rate of 100 mV.s-1.

RESULTS AND DISCUSSION

Figure 1 shows Cu(II) voltammograms in the presence of (a) HCl ; (b) H2SO4 and (c) HNO3. It can be observed that in presence of HCl and copper(II) ion a quasireversible behavior is obtained (@ 90 mV peak to peak), probably due to the stabilization of the Cu2+/Cu+ redox couple by effect of the CuCl2- or CuCl32- complex formation:

Cu+ + 2Cl- ® CuCl2- bbbbbbbbKf = 1 05.5

Cu+ + 3Cl- ® CuCl32- bbbbbbbKf = 105.9


Fig.1. Cyclic voltammogram without aniline and pyrrole under different experimental conditions: a) 0.01 M CuCl2 / 1 M HCI, b) 0.01M CuSO4 / 1M H2SO4, and c) 0.01M CuNO3 / 1M HNO3. Scan rate: 100 mV s-1.

Cu(I) and Cu(II) did not form completes in HNO3 media. Furthermore, less current was appreciated in the presence of HCl, which confirms that the charge involved in this redox process is equivalent only to one electron. In nitric and sulfuric acid the process occurs irreversibly, and the major current is proper of the metallic copper formed during the cathodic cycle.

Polyaniline Pani.

The reaction of two monomer molecules yielding dimer species, neutral, cationic, and cationic radical, has been previously describes [22,23]. These dimer species can also react with aniline and / or dimer species, increasing the polymer chain. It is suggested that in this step, 2 plays a more important role than 1, which can be explained by the anionic effect observed during the electropolymerization process [24-28].

Figure 2a shows the voltammograms obtained during aniline polymerization in the presence of 0.1 M Cu(II) and 1 M H2SO4 ranging between -0.30 and +1.00 V. It is important to notice that in this case the electropolymerization potential was not affected by the presence of Cu(II).

Figure 2b shows that in 1 M HNO3 a similar behavior to that observed in H2SO4 is obtained. Nevertheless, in the presence of HCl (see Figure 2c) a shifting of potential to more positive values occurs and most of the anodic processes disappear. An important improvement of the reversibility of Pani / Cu(II/I) system can be appreciated (@ 60 mV between the anodic and cathodic peaks). The potential peak for aniline without copper(II) in HCI medium is @ 140 mV. This significant difference can be attributed to the incorporation of the anionic metal complex of copper(I) into the polymer backbone, as indicated above, which favors the charge transference processes during the conducting polymer-metal formation [29].


 

 

 

 

 

 

 

 

 

 

 

 

 

Fig. 2. Cyclic voltammogram of 0 1 M aniline / 0.01 M Cu(II) in a) 1 M H2SO4, b) 1M HNO3, and c) 1 M HCI.

Poly(pyrrole) Ppy.

The electro-oxidation mechanism of pyrrole involves a positive charge on the nitrogen atom and a negative charge on the neighbor carbon. This species reacts with other similar ones yielding a dimer, and by successive steps the polymer is obtained.

It is known that the oxidation process produces positive charges on the polymer, increasing the current intensity but not the reversibility of the process [30].

Figure 3 shows the voltammetric response of pyrrole in HNO3. It is clearly appreciated that the presence of copper(II) ions produces the following: (a) there is an anodic process at around +200 mV which disappears in the absence of this ion and b) nevertheless, the response is not reversible, which would indicate that it is necessary the presence of a specie that stabilizes Cu(I) so that the cathodic process may occur. It is corroborated by the response of figure 3b where it is observed that the process is sharply reversible when Cu(II) is found in presence of 1 M HCl. This would indicate that the presence of CuCl2- or CuCl32- complexes would allow the conditions to achieve the reversibility. The potential difference between the anodic and cathodic peaks was near 60 mV.


Fig. 3. Cyclic voltammogram of 0.1 M Pyrrol / 0.01 M Cu(II) in a) 1M HNO3 and b) 1M HCl.

It was not possible to obtain poly(pyrrole) in 1 M H2SO4, appearing a precipitate probably due to the decomposition of the monomer or to the presence of a neutral complex [31]

Cu2++ S042- ® CuSO4 Kf = 102.3

CONCLUSIONS

The electropolymerization of both compounds depended on the metal salt. The results of this work showed that the presence of Cu(II) ions change the electrode behavior during the electropolymerization of aniline and pyrrole. The formation of charged species such as copper-chloride complex, are probably the responsible of this behavior. This confirms that the monomer oxidation is the key to its electropolymerization, which would allow to obtain positive sites, and hence, the incorporation of the metal ion-complex into the polymer backbone.

ACKNOWLEDGEMENTS

The authors are gratefully acknowledged to FONDECYT (Grant Líneas Complementarias N 8990011) for the financial support of this work.

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