THE ROLE OF WATER IN THE ELECTRODEPOSITION AND DOPING OF POLYTHIOPHENE AND 2 OF ITS DERIVATIVES

The galvanostatic electropolymerization of thiophene (1), 3-methylthio phene (2) and 2,2'-bithiophene (3) in acetonitrile, containing 0,002 t o 20 mol/L water, was studied in detail. The current efficiency gamma for the polymer film formation decreased rapidly for 1, but much slowe r for 2 and 3 with increasing water concentration c(H2O). For c(H2O) = 0, 1 mol/L, gamma was found to be 10, 82 and 95% for 1-3. It was poss ible to describe the experimental gamma vs. c(H2O) plots quantitativel y by a model where the electrogenerated radical cations (rc) of the th iophene (derivative) are consumed by two competing second-order follow up reactions, the nucleophilic attack of the rc by water (i) and the rate-determining dimerization of two rc's (ii). Their rates are given by: (i) v1 = k1 C(rc) C(H2O) and (ii) v2 = k2 c(rc)2. The side reactio n (i) leads to soluble products, while reaction (ii) contributes to th e polymer film growth. The ratio k1/k2 was found to be 3 . 10(-7), 2 . 1 0(-8) and 4 . 10(-9) for 1, 2 and 3, respectively, and this shows t hat the electropolymerization of 2,2'-bithiophene is by a factor of ab out 100 less sensitive toward the present water than thiophene. The st eady state concentration c(rc) is in the order of 10(-9) mol/L in all three cases. The relative insensitivity of doping/undoping cycles with respect to the presence of water is discussed in terms of a lessened reactivity of the polarons and a low solubility of water in the hetero aromatic polymer.