Evidence for Catalyst Association in the Catalyst Transfer Polymerization of Thieno[3,2-b]thiophene

The performance of catalyst transfer polymerization (CTP) reactions which depend on catalyst association was studied on 3,6-dioctylthiothieno[3,2-b]thiophene (TT) monomers. This monomer was selected because a strong association of the catalyst is expected, since the aromaticity of thienothiophene is largely maintained when the catalyst and thienothiophene associate. This study includes both reported and unreported Ni- and Pd-catalyst systems. It is found that no polymer formation can be observed using Ni-catalysts, whereas Pd-catalysts show a similar behavior as for other monomer systems. During the study of the Ni-catalyzed CTPs, the π-associated Ni0-complex has been isolated in situ and displayed a high stability in solution. It is shown that the associated complex interferes with the polymerization reaction and even prevents polymer formation. Furthermore, this complex prevented any Kumada-coupling reaction in the presence of the TT unit, as it serves as a “trapping site” for free Ni0 catalyst entities. Ni0-trapping2does not occur during polymerization of 3-alkylthiophene, confirming the presence of the π-associated Ni0-complex in this polymerization. This introduces a new convenient method of probing Ni0-association during all Ni-catalyzed reactions. Furthermore, these results establish the presence of an upper limit to the catalyst association strength -above which oxidative addition is prevented and the polymerization is inhibited- and they therefore add extra considerations for optimal catalyst design.status: publishe