Ž . Synthetic Metals 99 1999 69–77 The formation of oligomers in the electrolyte upon polymerization of pyrrole G. Appel a , D. Schmeißer a, ) , J. Bauer b , M. Bauer b , H.J. Egelhaaf c , D. Oelkrug c a Technische UniÕersitat Cottbus, Angewandte Physik-Sensorik, P.O. Box 101344, 03013 Cottbus, Germany ¨ b Fraunhofer Institut fur ZuÕerlassigkeit und Mikrointegration, Außenstelle Polymermaterialien und Composite, P.O. Box 126, 14513 Teltow, Germany ¨ ¨ c UniÕersitat Tubingen, Institut fur Physikalische und Theoretische Chemie, Auf der Morgenstelle 8, 72076 Tubingen, Germany ¨ ¨ ¨ ¨ Received 20 August 1998; accepted 2 November 1998 Abstract Pyrrole was polymerized by electrochemical and chemical oxidation of a solution of pyrrole and p-toluene sulfonic acid in acetonitrile. We have investigated the components formed within that electrolyte solution during the polymerization. Both, UV–VIS Ž . spectroscopy and size exclusion chromatography give evidence that pyrrole oligomers nP n F30 are formed in the solutions during the polymerization process. A model is proposed in which these oligomers act as intermediates of the polymerization reaction. Within that model the deviations from Faradayic behaviour at elevated current densities are explained. It also rationalizes the fact that polymer films with completely different properties can be realized by varying the experimental parameters. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Polypyrrole; Polymerization mechanism; Oligomers; SEC; Optical spectroscopy 1. Introduction The mechanism of the electrochemical polymerization of polypyrrole is usually described by the formation of radical cations at the electrode and the recombination of w x two of such species 1–4 . Two pyrrole radical cations recombine under dissociation of two protons to form a dimer. Higher oligomers are obtained by the same mecha- nism with the participation of radical cations of lower oligomers or monomers. The anode is supposed to play a crucial role for electron transport in all these oxidation reactions. However, Beck already found that intermediates w x may leave the electrode and dissolve in the electrolyte 1,2 but he could not give information about the nature and size of the intermediates. There are several experimental observations that in the past have caused doubts on the pre-dominance of the electrode reactions assumed in the electrochemical poly- merization mechanism. Recently, we have been able to ) Corresponding author. Tel.: q49-355693073; Fax: q49-355693073; E-mail: dsch@tu-cottbus.de obtain polypyrrole films and pellets by chemical oxidation also. These films show the same characteristic temperature dependencies as those prepared by electro-polymerization, although no electrodes are involved in that process at all. Further evidence for deviations from the general accepted mechanism are based on the conductivity of the resulting polymer is strongly dependent on this reaction parameter w x Ž 2 . 5,6 : for low current densities j F 0.3 mArcm one-di- mensional polypyrrole with conductivities in the range of 0.1 Srcm result, whereas conductivities of about 100 Ž Srcm are obtained using high current densities j G 3 2 . mArcm . The first preparation conditions are believed to cause a one-dimensional polymer which consists of exclu- sively a-a X -coupled monomer units. In the other case, a two-dimensional polymer is obtained if the films are pre- pared at high current densities which is attributed to the X wx occurrence of a-b -coupling of the monomer units 5 . The differences of both are visualized in Fig. 1 in which the a-a X -linked pyrroles are shown together with an idealized 1-dim structure as well as a corresponding 2-dim sheet like planar arrangement formed by a-b X -linked tri-pyrrolino units. The differences between these preparation conditions and the materials respective properties have been summa- wx rized recently 6 . 0379-6779r99r$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. Ž . PII: S0379-6779 98 00200-8