Electrochemical synthesis of intrinsically conducting polymers of 3-alkylpyrroles

In the present work the electrochemical polymerization of pyrrole (Py) derivatives has been performed in order to identify the conditions for obtaining intrinsically conducting polymers, checking film morphology and conductivity. The study mainly concerns 3-alkylsubstituted pyrroles with different chain lengths (3-hexylpyrrole (3HP), 3-decylpyrrole (3DP), 3-hexadecylpyrrole (3HDP)). In the case of 3DP (0.01 M) various experimental conditions have been adopted: different solvents (propylene carbonate, acetonitrile), different counterions (ClO4-, BF4-, NO3-, PF6-, TsO-) with the same cation Bu4N+ and different current densities (0.05, 0.1, 0.2, 0.4 mA cm(-2)) in the case of BF4- in propylene carbonate. Porous and elastic films are obtained when the substituent alkyl side chain is longer and the supporting electrolytes are ClO4-,BF4-, PF6-; with these last electrolytes the best conductivity is also obtained (0.1-0.8 S cm(-1) for poly(3-decylpyrrole), P3DP). P3DP(TsO) and P3DP(NO3), aside from the type of solvent, show poor mechanical properties and low electrical conductivity (10(-3) and 10(-6) S cm(-1) for P3DP(TsO) and P3DP(NO3), respectively). The conductivity decreases as the alkyl side chain becomes longer. When the polymerization is performed at low monomer concentration (of the order of 0.001 M), the surface properties of P3DP(ClO4) are very poor and the conductivity is only 10(-3) S cm(-1). The obtained polymers have also been electrochemically characterized through cyclic voltammetry. The voltammetric evolution observed during cycling is studied under different experimental parameters (counterion type, solvent nature and electrochemical cycling). The cyclic voltammetry change, regarding the gradual shift to negative potentials of the redox process on P3DP with ClO4-, BF4- and PF6- as counterions, seems to be ascribed to a gradual deprotonation process. Analysing the voltammetric profile change from the point of view of a deprotonation process, it can be concluded that this process is speeded up by the basic character of the solvent (propylene carbonate) and in the presence of a basic counterion (tosylate) this is almost complete and immediate. P3DPNO(3), although conductive (10(-6) S cm(-1)), can be switched from the oxidized form to the reduced one only a few times in acetonitrile and never in propylene carbonate, showing the difficulty of the polymer in being reduced and oxidized several times. (C) 1998 Elsevier Science S.A.