Abstract
Ethylene/polar monomer coordination copolymerization offers an attractive way of making functionalized polyolefins. However, ethylene copolymerization with industrially relevant short chain length alkenoic acid remain a big challenge. Here we report the efficient direct copolymerization of ethylene with vinyl acetic acid by tetranuclear nickel complexes. The protic monomer can be extended to acrylic acid, allylacetic acid, ω-alkenoic acid, allyl alcohol, and homoallyl alcohol. Based on X-ray analysis of precatalysts, control experiments, solvent-assisted electrospray ionization-mass spectrometry detection of key catalytic intermediates, and density functional theory studies, we propose a possible mechanistic scenario that involves a distinctive vinyl acetic acid enchainment enabled by Ni···Ni synergistic effects. Inspired by the mechanistic insights, binuclear nickel catalysts are designed and proved much more efficient for the copolymerization of ethylene with vinyl acetic acid or acrylic acid, achieving the highest turnover frequencies so far for both ethylene and polar monomers simultaneously.
Highlights
Ethylene/polar monomer coordination copolymerization offers an attractive way of making functionalized polyolefins
Even more challenging is to incorporate short chain length alkenoic acid, such as acrylic acid (AA) and vinyl acetic acid (VA) into PEs25–27, since: (i) acidic proton in comonomers such as alkenoic acids can protonate and poison the catalyst[27]; (ii) strong carboxylate coordination can promote the formation of stable chelate complex or β-X elimination[28,29]
A few results of direct copolymerization of ethylene with AA using palladium catalyst based on either phosphinesulfonate or diphosphazane monoxide ligand were reported[25,26,27], AA are more harmful to enchainment than the corresponding aprotic monomers in all the cases
Summary
Synthesis and characterization of tetranuclear nickel complexes. Complexes 1a-c were synthesized in good yields. Following Procedure A, i.e., no polar monomer pretreatment, precatalyst 2a catalyzes ethylene + vinylacetic acid (VA) copolymerization with an activity of 54 kg/(mol Cat)∙h∙atm and great VA incorporation (2.1 mol%, entry 1). Following Procedure B, i.e., mixing VA with MMAO for 24 h, results in a marginal increase of activity to 84 kg/(mol Cat)∙h∙atm with a significant 5.3× decrease in VA incorporation to 0.4 mol% (entry 5 vs 1). This trend is counterintuitive as pretreating polar monomers with aluminoxane or R3Al is an effective strategy for group 43,4,32 and late[38,39] transition metal complexes catalyzed polar monomer copolymerizations. A Computed energy profiles for Ni mediated copolymerization of ethylene with VA anion
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