Ring Walking versus Trapping of Nickel(0) during Kumada Catalyst Transfer Polycondensation Using Externally Initiated Electron-Accepting Thiophene–Benzothiadiazole–Thiophene Precursors

Abstract

Interactions of Ni(0) and thiophene–benzothiadiazole–thiophene (TBT) units during the Kumada catalyst transfer polycondensation (KCTP) of 2-chloromagnesio-5-bromo-3-hexylthiophene (1) are investigated. Monofunctional TBT initiator precursors are used for the external initiation of KCTP, and the mechanism of initiator activation and polymerization is elucidated using NMR and MALDI-ToF MS. We find that the activation of the TBT-bromide initiator precursor using nickel–diethylbipyridine (NiEt2bipy) occurs via a two-step pathway, in which NiEt2bipy coordinates to benzothiadiazole (B) first, followed by the elimination of butane and oxidative addition of liberated Ni(0)bipy into the terminal TBT–Br bond. It is shown that the B unit traps Ni(0) during the KCTP of 1, which results in significant termination, as derived from the degree of bromine-terminated chains. The ability of B units to trap Ni(0) is further illustrated by using a symmetric bifunctional Br-TBT-Br initiator precursor for the KCTP of 1, during ...