Doped Polymers by Oxidative Polymerization. 4. Oxidative Coupling of Methylated Oligothiophenes by FeCl<INF>3</INF>·6H<INF>2</INF>O as a Model Reaction for the Oxidative Polymerization of Thiophene Derivatives

Methylated bi- and terthiophenes (monomers, <BO>1</BO>, <BO>3</BO>, <BO>5</BO>, <BO>8</BO>) with one free α-position can be oxidized with FeCl<INF>3</INF>·6H<INF>2</INF>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 (<BO>6</BO>, <BO>9</BO>) having two methylated α-positions. The products of these reactions show α,β‘ linkages (<BO>7</BO>, <BO>10</BO>, <BO>11</BO>). 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 10⁹ 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.