Toward a trace-free oxidant - Insight into unexpected high yields of vapor phase polymerized polyterthiophene

Vapor phase polymerization (VPP) of conducting polymers has been widely reported as a versatile technique to obtain thin films with high conductivity and order. One oxidant commonly used for VPP is iron(III) para-toluenesulfonate (Fe(III)PTS) owing to its outstanding film-forming properties. The polymerization mechanism using Fe(III)PTS has been believed to follow the normal route, where (at least) two Fe(III) is needed for oxidizing one monomer unit to form the polymer. Surprisingly, when polymerizing terthiophene (TTh) monomers at 100 °C using Fe(III)PTS, it was found that ∼2.3 times more the expected amount of PTTh was produced, when long polymerization times were used. An explanation for the phenomenon was found when VPP of TTh was successful using only para-toluenesulfonic acid (PTSa) as oxidant. It is believed that PTSa is gradually converted to SO 3 and toluene at elevated temperature and that reduction of SO 3 is driving the oxidation of the TTh monomers. This finding also opens a useful oxidative polymerization route where the oxidant can be converted to volatile compounds leaving no trace in the end product.