Optimization of oxidant for polymerization of indole in water-ethanol medium

Influence of oxidant nature and monomer:oxidant ratio was studied systematically for finding optimal conditions for indole polymerization in water-ethanol medium. Conventional oxidizing agents applied for the preparation of conducting polymers, such as iron (III) chloride, iron (III) nitrate, cerium (IV) sulfate and ammonium peroxydisulfate, were used for the optimization. For the oxidant electrons:monomer ratio = 2.5, all studied oxidants resulted in successful conversion of monomer to polyindole, which was confirmed by ultraviolet–visible spectroscopy, MALDI TOF spectrometry, NMR and infrared spectroscopy. The NMR spectroscopy confirmed 2,3-coupling of units in polyindole chains. According to the infrared spectroscopy, using iron (III) nitrate, compared to other oxidants, resulted in the product with the least amount of carbonyl groups connected with oxidative degradation of polyindole, which correlated with the highest conductivity (1.5 × 10 −7 S cm −1 ) and thermal stability of the material. Optimization of the oxidant:monomer ratio performed with iron (III) nitrate as an oxidizing agent showed that the ratio = 1 led to the product with the lowest yield but allowed further decrease of the carbonyl content in polyindole molecular structure, while showing enhancement of the material electroactivity, thermal stability and conductivity up to 1.2 × 10 −5 S cm −1 . The prepared materials can be potentially used in electrorheology. • Indole was polymerized in water-ethanol medium using various oxidants. • Using Fe(NO 3 ) 3 led to the product with the highest conductivity and thermal stability. • Optimization of Fe(NO 3 ) 3 :indole ratio further enhanced the polyindole properties.