ELECTROCHEMICAL COPOLYMERIZATION OF 5-SUBSTITUTED INDOLES

The electrochemical oxidation of mixtures of indole-5-carboxylic acid (ICA) and 5-cyanoindole (CI) produces a film containing free cyclic tr imers and polymeric species consisting of linked cyclic trimer units. Trimers containing 3 ICA molecules and 3 CI molecules are present, tog ether with cotrimers which contain 2 ICA and 1 CI molecule and 1 ICA a nd 2 CI molecules. A mechanism is proposed which explains the observed rate of trimer formation and the relative amounts of trimer and cotri mer formed. In this mechanism, trimer formation occurs via adsorbed IC A and CI radical cations, with the proportion of the trimers in the la yer being governed by the coupling of a statistical distribution of th ese radical cations; the overall rate of formation of each trimer is t hen given by the combined coupling rate of the ICA and CI radical cati ons involved. As for the monosubstituted indoles, further linking of t rimers and cotrimers occurs when forming the polymer species. The oxid ation potentials of the cotrimers lie between those of the (3 ICA) tri mer and the (3 CI) trimer, consistent with the proposal that the oxida tion potential correlates with the combined electron-withdrawing/donat ing power of all three substituents in the trimer. Copolymerisation on an electrode surface by the electrochemical oxidation and coupling of preformed ICA and CI trimers is also possible.