J. Chil. Chem. Soc., 48, N 4 (2003) ISSN 0717-9324

ELECTROPOLYMERIZATION OF CARBAZOLE USING TEMPO
AS RADICAL TRAPPING

M. Angélica del Valle*, Fernando R. Díaz, Fabiola E. Bustos, Liezel Guerra

Facultad de Química, Pontificia Universidad Católica de Chile, Santiago, Chile. e-mail: mdvalle@puc.cl
(Received: October: 25, 2002 - Accepted: May 10, 2003)

ABSTRACT

Considering that electropolymerization process occurs via radical intermediates, it was used a stable nitroxide radical such as TEMPO, with the aim to stabilize the species generated in the electrode/solution interface. The obtained results show that TEMPO really acts as a "radical trapping". Therefore, applying an adequate electrochemical perturbation it should be possible the progressive elongation of polymeric chains, and with that, the control over the structure and polydispersity of the deposit.

The development of conducting polymers as new materials, has attracted the interest of many chemists and physicists, and its study has allowed, on one hand, the postulation of new concepts in solid state science, and on the other, a tremendous technological potential. At the same time, has produced a great expansion in electrochemistry, being this technique an excellent tool for synthesis, characterization and application of this type of materials.

The general mechanism that has been proposed for the electropolymerization process is represented in the following scheme[1] :

Generally, an increase in the chain length involves a higher electronic delocalization, so that the oxidation potentials follow the order [1]:

E_monomer > E_dimer > E_trimer > ..... > E_polymer

In this way, the potential required to oxidize the monomer, is enough to oxidize any of the oligomeric species, which can be generated. In other words, when the monomer is oxidized and the radical generated, this can couple to other, and immediately be oxidized to generate a radical dimer, which in turn can couple to another radical monomer or dimer, and go on successively. This is why the interface will not be composed by only one type of oligomer, but a mixture of them, which means that the chain length of the clusters deposited on the electrode is variable, as it is the mode in which these clusters will be growing.

All this explain on one hand, the contribution of different nucleation and growth mechanisms to the global electropolymerization process [2] and, on the other, the impossibility of achieving a control of the deposit structure.

In summary, the radical character of the electropolymerization mechanism and the properties of these intermediate radical species, have hindered a real control of electropolymerization, even though all other variables involved are controlled (support electrolyte, solvent, electrochemical perturbation, etc.), which makes impossible the electroobtention of conducting polymers of adequate polydispersity and well define structure.

Traditionally, macromolecules of well define structures, could only be obtained by a chemical method, such as living polymerization. The control of the structure had been considered difficult, due to secondary reactions, such as radical coupling, intermolecular and disproportionation reactions. However, this idea of uncontrolled reaction has changed, due to the rapid progress in systems of living radical polymerization, LRP, with important advances in polymerization processes in the presence of nitroxides.

Twenty years ago, Otsu et al. [3,4] gave a great impulse to LRP, when he indicated that "in order to achieve a LRP in homogeneous solution, polymeric chains should be formed capable of dissociating in others ending in radicals and small radicals, which have to be stable, to avoid the initiation of a new polymeric chain". Thus, he presented a formal scheme considering the step by step growth, by which it is possible to control the polymer molecular weight. In his first work in this area, Otsu used phenylazotrimethylmethane as initiator, to polymerize methylmetacrylate at 60 ºC. The molecular weight increased linearly with the conversion, as expected for controlled processes.

Subsequently, Rizzardo [5] reported the use of alcoxyamines as regulating initiators of LRP and block copolymerization of vynilic monomers, postulating a mechanism analogue to that proposed by Otsu. Afterwards, Georges et al. [6] showed that moderate molecular weights of polystyrene, with low polydispersity, can be obtained by radicalary polymerization with nitroxides, at temperatures closed to 120 ºC. They also showed that, in such a case, the polydispersity decreases as the polymerization rate also decreases.

In the first works carried out with nitroxide radicals, such as Tempo (2,2,6,6-tetramethyl-1-piperydiniloxy), it is shown that in LRP the addition of initiators radicals generate free radicals centered on carbon, which react with the nitroxide radical, at diffusion controlled rates. The resultant alcoxyamine derivatives are essentially stable at the working temperatures and do not participate in follow-up reactions, acting only as "radical trapping, RT".

Nitroxide radicals are stable and give CO bond enthalpies consistent with the experimental evidence of being in the presence of LRP. By polymerization in the presence of nitroxide type radicals, short polydispersities and long reaction times are obtained, with a linear correlation between molecular weight and conversion.

Based on the results presented above, showing the analogy that could exist between LRP and the electropolymerization process, the following hypothesis was postulated: the use of "radical trapping", as Tempo, will allows controlled electropolymerization process, being possible the progressive elongation of polymeric chains, and with that, the control over the structure and polydispersity of the deposit.

In this work, are presented the first results obtained for carbazole, Cz, electropolymerization in the presence and absence of Tempo, by cyclic voltammetry, which allows a preliminary analysis of the system behavior in the presence of RT. The work was done on steel (SS) disk in acetonitrile, using tetrabutylammonium hexafluorophosphate (TBAPF₆) as electrolyte, under experimental conditions previously established for the electropolymerization of Cz [7] .

Fig. 1 shows the voltammetric profiles obtained during successive potentiodynamic scans, for both systems, which indicate a big difference between them. This is demonstrated mainly by a reduction peak near 0.2 V versus SCE, which is observed only when Tempo is used. Furthermore, the potential shift towards positive values when oligomers are being produced during the electro-oxidation process is not so evident when RT is present.

Another noticeable aspect is the "polymerization time": while in the absence of Tempo, 5 cycles are enough to observe deposit formation on the electrode surface, at least 150 cycles are required in the presence of Tempo under the same experimental conditions.

Considering that nucleation and subsequent growth of the deposit is produced from oligomers of certain chain length, which makes them insoluble in the interface, the above observation would explain why, in the presence of Tempo, such chain length is reached after a much longer time, which would demonstrate that in fact, the RT is allowing a better control of the chain elongation process.

Fig. 1 Successive voltammetric scans of carbazole in CH3CN: (a) SS/10-2 M Cz + 10-2 M TBAPF6; (b) SS/10-2 M Cz + 10-2 M TBAPF6 + 2·10-2 M Tempo. Numbers on the curves are the voltammetric scan cycle number. Scan rate: 100 mV·s-1.

This is also evident in Fig. 1: in absence of TEMPO there is a shift of the oxidation potential towards positive values on increasing voltammetric scan number, attributable to longer oligomers, but in the presence of TEMPO this phenomenon is not observed.

Nevertheless, the use of an electrode of small area does not allow this phenomenon to take place in the whole solution, which means that the control is not complete.

Figure 2 shows the evolution of the UV-Vis spectra in both previous cases. Considering that, as the chain becomes longer, the absorption band is shifted towards the visible region [8] , it becomes evident that such lengthening takes place much earlier when the RT is not used.

The macroscopic morphology of the deposits can be considered as another evidence of the RT effect: when TEMPO is used a very homogeneous thin film is obtained on the electrode, but in it absence, a great quantity of powder is produced, which give rises of more or less homogeneous structure, respectively. Furthermore, FTIR spectra of both solids show that it is forming an analogue structure, because the same vibration frequencies are obtained. Anyway, these results will be contrasted by SEM and XPS analysis.

Fig. 2 UV-Vis spectra of electrolytic solutions during Cz electropolymerization in the same conditions of figure 1: (a) without Tempo; (b) with Tempo. Numbers on the curves are the corresponding voltammetric scan cycle number.

In principle, these results confirm our hypothesis. However, it is necessary to work on electrodes of large area and by potentiostatic method, in order to generate first the radical-RT species, and then produce the breaking of that union, to generate throughout the bulk electrolytic medium the radical-radical coupling.

Although the electropolymerization in the presence of Tempo takes longer times, the same as the chemical polymerization in the presence of RT, one can also obtain lower polydispersities and a better structure control. Moreover, compared with LRP, it has the advantage of not requiring high temperatures, or other external agents, to produce the coupling and the breaking of the radical-RT union, or to act as initiators or catalysts.

Based on the conditions established here, we are currently working in the detection of the intermediate species and to obtain deposits large enough to allow the determination of their polydispersity, with the aim of contrasting definitively our proposed hypothesis. Nevertheless, the results obtained up to now, show that the electropolymerization can be carried out in an analog manner as LRP, which could have an enormous impact in the development of new materials in the area of conducting or semiconducting polymers.

ACKNOWLEDGEMENTS

The authors acknowledge FONDECYT-Chile (Grant 1020520) for financial support of this investigation.

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