Synthesis And Characterization Of Bulky Phosphino-Phenolate Neutral Nickel Catalysts And Copolymerization Of Ethylene With Polar Monomers

Direct coordination copolymerization of nonpolar olefin with various polar vinyl comonomers was the most desired method to prepare well-defined functionalized polyolefins,and this method has long been a scientific challenge in the field of polymer chemistry.Up to date,major advances have only been achieved by using a limited scope of palladium catalysts,mainly including?-diimine cationic palladium catalysts and neutral phosphine-sulfonato palladium catalysts.Nevertheless,the high cost and relatively low catalytic activity of palladium catalysts limited their practical utility.Considering the low cost and generally higher catalytic activity of nickel catalysts relative to palladium catalysts,development of nickel-based catalysts toward ethylene/polar vinyl monomer copolymerization is eagerly desirable and of great scientific and industrial interest.However,nickel catalysts suffer from some serious poisoning processes,such as?-coordination of polar group,back-biting reaction between polar group and nickel center,and?-X(X:polar group)elimination reaction.In order to overcome poisoning processes,we designed several neutral nickel catalysts based on axially bulky phosphino-phenolate ligands,and explored their properties for ethylene polymerization and copolymerizations of ethylene with various challenging vinyl polar monomers:(1)Introduction of an axially bulky group or two bulky group on the phosphorus atom could shield the catalytic active center from above and below,which effectively suppress associative chain transfer or chain transfer to monomer.Bulky phosphino-phenolate nickel catalysts exhibited excellent thermal stability,long life of catalyst,catalytic activity and molecular weight of polymer.In the absence of an activator,these nickel complexes showed high catalytic activity(up to 10⁷ g/mol _Ni·h)and produced high molecular-weight polyethylene(up to 770000)for ethylene polymerization.Additionally,catalytic activity and molecular weight of polyethylene of bulky nickel catalysts were nearly 1 or 2 orders of magnitude higher than that of small catalyst under the same conditions.(2)All the bulky phosphino-phenolate neutral nickel catalysts displayed unexpectedly high tolerance to polar group,and exhibited excellent catalytic activity(up to 5.76×10⁶ g/mol _Ni×h)for ethylene polymerization in polar solvents like DMSO,even in water.Moreover,highly efficient copolymerization between ethylene and acrylates(such as MA and BA)was readily achieved with these complexes under mild conditions.High-molecular-weight(M_w up to 1.08×10⁵)copolymers,greatly improved catalytic activity(up to1.9×10⁵ g/mol _Ni×h),and good polar group incorporation(about3 mol%)were simultaneously achieved by introducing bulky phosphorus substituents in the axial position.(3)To further reveal the effect of electronic or steric modification at different position of this catalyst,we designed a series of bulky nickel complexes bearing different substituents on ortho-phenoxy position or phosphorus atom.Introducing an electron-withdrawing group(–F,–CF₃,and–C₆F₅)or a bulky group(^tBu)on ortho-phenoxy position not only efficiently improved catalytic activities toward both ethylene polymerization and ethylene/methyl acrylate copolymerization,but also enhanced the tolerance of catalyst to acrylates.Conversely,nickel complex bearing an electronic-donating substituent on P-aryl moiety exhibited increased catalytic activity toward ethylene(co)polymerization and enhanced catalyst tolerance to acrylates.(4)Besides acrylates,various promising but very challenging polar vinyl monomers could also be efficiently copolymerized with ethylene by bulky neutral phosphino-phenolate nickel catalyts,yielding linear high-molecular-weight copolymers with good incorporation of polar group(0.5–5.9 mol%),such as acrylamides,acrylic acid,hydroxybutyl acrylate,methoxyethyl acrylate,acrylonitrile,triethoxyvinylsilane,polyethylene glycol monomethyl ether acrylate macromonomer,diethyl vinylphosphonate,vinyl diphenyl phosphine oxide,vinyl sulfone and phenylvinyl sulfoxide.Phosphino-phenolate neutral nickel catalysts showed high catalytic activity and produced high molecular weight copolymers with linear structure.Polar comonomer mainly inserted into the main chain,and it was very different from the copolymer produced by neutral phosphine-sulfonato palladium catalysts,in which polar comonomer mainly located at chain end.