Leveraging Earth-Abundant Nickel for Orthogonal Polymerizations: Synthesizing Polar Polyolefin Block Copolymers

The selectivity of diimine Pd^II complexes toward a radical polymerization pathway upon irradiation and coordination-insertion polymerization in the dark enabled the development of a one-pot access route to polar polyolefin block copolymers. So far, the replacement of the Pd^II complex with an analogous complex containing earth-abundant Ni has been unattainable. In this work, we demonstrate that carbyl iminopyridyl Ni^II complexes exhibit unprecedented and unique selectivity toward radical and coordination-insertion polymerization pathways, which is a key feature to obtain polar polyolefin block copolymers through the metal-organic insertion light-initiated radical polymerization (MILRad). Investigations into key intermediate steps revealed a high tolerance of this class of Ni^II complexes toward acrylic monomers, forming a highly reactive Ni^II acryl chelate with the ability to form a macroradical through blue light irradiation. A series of carbyl iminopyridyl Ni^II complexes with different ligands was studied and yielded block copolymers of M_n = 49-244 kg/mol, with low crystallinity polyethylene segments ranging from 2.5 to 88 kg/mol and acrylic blocks of MA, EA, n-BuA, and BnA monomers. TEMPO trapping experiments and DFT calculations provide further insights into the mechanistic pathway of the catalytic system. A kinetically controlled 2,1-insertion of methyl acrylate occurs on both Ni-o-Tol and Ni-PE bonds. While this is irreversible and proceeds to chain-walking in the latter case, it is followed by slow conversion to the thermodynamic 1,2-insertion product in the former case. This study demonstrated that carbyl iminopyridyl Ni^II complexes provide a viable one-pot synthesis pathway toward polar polyolefin block copolymers.