Comprehensive studies of ligand electronic effect on unsymmetrical α-diimine nickel(II) promoted ethylene (co)polymerizations
Ligand electronic effect plays a significant role in tuning catalytic activity, polymer molecular weight and topology, and comonomer incorporation in the ethylene (co)polymerization; however, study is rather limited in milestone α-diimine late transition metal catalysts. In this contribution, by tailoring a sterically encumbered pentiptycenyl/dibenzhydryl substituted framework, ligand electronic effect derived from both the para-position of N-aryl group (horizontal axis: Me, MeO, and Cl) and the para-position of dibenzhydryl moiety (vertical axis: Me, H, and F) is comprehensively investigated in unsymmetrical α-diimine Ni(II) promoted ethylene (co)polymerizations for the first time. In the ethylene polymerization, the electron-withdrawing Cl group (horizontal axis) prefers to give higher branching density (145/1000C) in higher catalytic activity (29200 kg mol-1 h-1), while the electron-donating Me group affords higher molecular weight (2573 kDa). Moreover, the electron-withdrawing F group (vertical axis) again generates higher branching density but lower molecular weight in lower catalytic activity. By contrast, in the ethylene copolymerization with methyl 10-undecenoate, the electron-donating Me group derived from both horizontal axis and vertical axis concurrently is beneficial to give higher polymer molecular weight (374 kDa) and comonomer imcoporation in higher catalytic activity. However, all the electron-withdrawing groups come from either horizontal axis (Cl) or vertical axis (F) are not good for copolymerization. This work significantly sheds light on the differed impact of electronic substituents on between ethylene polymerization and ethylene-polar monomer copolymerization.