Doped Polymers by Oxidative Polymerization. 4. Oxidative Coupling of Methylated Oligothiophenes by FeCl₃·6H₂O as a Model Reaction for the Oxidative Polymerization of Thiophene Derivatives

Methylated bi- and terthiophenes (monomers, 1, 3, 5, 8 ) with one free α-position can be oxidized with FeCl 3 ·6H 2 O. An irreversible dimerization reaction forms dimers of the expected structure. The radical cations of the monomers also react with free β-positions of the formed dimers ( 6, 9 ) having two methylated α-positions. The products of these reactions show α,β‘ linkages ( 7, 10, 11 ). Different β-positions in end-capped dimers show a graduated reactivity against radical cations of the monomers; the free β-position with the higher electron spin density can be attacked preferably by an α-position of a radical cation of a monomer molecule. No products with β,β‘ linkages were found. Electrochemical investigations using fast-scan voltammetry show the formation of α,α‘ linkages of methylated oligothiophenes. For bithiophene derivatives, rate constants of the dimerization process in the range of 10 9 L·mol -1 ·s -1 were determined. In comparison with a terthiophene derivative, it was found that the dimerization rates decrease with increasing chain length of the oligomers.