Rethinking Catalyst Trapping in Ni-Catalyzed Thieno[3,2- b ]thiophene Polymerization.

Catalyst-transfer polymerization (CTP) is a chain-growth method used to synthesize conjugated polymers. Although CTP works well for most donor-type monomers, the polymerization stalls with thieno[3,2- b ]thiophene when using Ni catalysts. Previous reports have rationalized this result by suggesting that the catalyst is trapped in a Ni 0 π -complex with the highly electron-rich arene. In this study, evidence is provided that the catalyst trap is more likely a Ni II complex that arises from oxidative insertion of Ni 0 into the C-S bonds of thieno[3,2- b ]thiophene. This result is consistent with the known reactivity of Ni 0 complexes toward S -heteroarenes and is supported herein by 31 P nuclear magnetic resonance spectra acquired in situ, as well as data collected from small-molecule model reactions and density-functional theory simulations of the polymerization. We propose that this C-S insertion pathway and related off-cycle reactions may be relevant to understanding or enabling the CTP of other monomers with fused thiophenes.