Kumada Catalyst-Transfer Polycondensation of Thiophene-Based Oligomers: Robustness of a Chain-Growth Mechanism

Polymerization of conjugated oligomers is an efficient approach for band gap engineering of conjugated polymers. On the other hand, a recently discovered chain-growth Ni-catalyzed Kumada polycondensation of monothiophenes becomes an important tool in engineering of macromolecular architectures. In this paper we evaluate the feasibility for chain-growth polycondensation of model monomers comprising one, two, or three thiophene rings. We found that Ph-Ni(PPh3)(2)-Br-mediated polycondensations of HT-coupled bi- and terthiophenes lead predominantly to Ph-terminated polymers indicating the chain-growth polymerization mechanism. Although an increase of the monomer molecular length somewhat decreases the fraction of the Ph-terminated products as a result of increased probability of a chain-termination and chain-transfer reactions, chain-growth elementary steps remain dominating even for lengthy monomers. Such results reflect that in the key polycondensation step the Ni catalyst undergoes selective intramolecular "ring walking" along conjugated systems comprising two or even three thiophene rings. The chain-growth performance is sensitive to the substitution pattern of the polymerized oligomers. The best results were obtained with the monomers having alkyl substituents in an ortho (respective to the growing site) position possibly due to higher stabilities of intermediate ortho-substituted aryl-nickel complexes, and this knowledge might be helpful for proper design of conjugated oligomers polymerizable on the chain-growth manner.