Influence of Metal Identity and Complex Nuclearity in Kumada Cross-Coupling Polymerizations with a Pyridine Diimine-Based Ligand Scaffold

Cross-coupling polymerizations
have fundamentally changed the field
of conjugated polymers (CPs) by expanding the scope of accessible
materials. Despite the prevalence of cross-coupling in CP synthesis,
almost all polymerizations rely on mononuclear Ni or Pd catalysts.
Here, we report a systematic exploration of mono- and dinuclear Fe
and Ni precatalysts with a pyridine diimine ligand scaffold for Kumada
cross-coupling polymerization of a donor thiophene and an acceptor
benzotriazole monomers. We observe that variation of the metal identity
from Ni to Fe produces contrasting polymerization mechanisms, while
complex nuclearity has a minimal impact on reactivity. Specifically,
Fe complexes appear to catalyze step-growth Kumada polymerizations
and can readily access both Csp2–Csp3 and Csp2–Csp2 cross-couplings, while
Ni complexes catalyze chain-growth polymerizations and predominantly
Csp2–Csp2 cross-couplings. Thus, our
work sheds light on important design parameters for transition metal
complexes used in cross-coupling polymerizations, demonstrates the
viability of iron catalysis in Kumada polymerization, and opens the
door to novel polymer compositions.