| Polyolefin materials have been applied widely due to their excellent properities and low cost.The introduction of even low amount of the polar functional groups would effectively improve the polyolefin properties such as superficiality,adhesion,compatibility,toughness,and miscibility etc.to expand the application scope of polyolefins.Direct olefin copolymerization with polar monomers is a most important and simple method for the functionalization of polyolefins.In the 1990s,Brookhart and co-workers first reported that the?-diimine palladium catalysts can directly copolymerize ethylene with polar monomers to produce polar functionalized polyethylene.Since then,much effort has been paid for the development of the late transition metal catalyzed olefin copolymerization with polar monomers.Recent advantage of the palladium catalysts based on?-diimine,phosphine-sulfonate,and N-heterocyclic carbine catalysts indicated that the control of the electronic and steric influence by modifying the structure of the catalysts is most important rule for the improvement of the copolymerization properties.In contract,low-cost nickel catalysts are still very limited,and the most nickel catalysts exhibited much lower copolymerization activity than palladium catalysts to give low molecular weight copolymers with low polar monomer incorporation.Therefore,it is still meaningful to develop strategies for improving the performance of nickel catalysts,where the reduction of the relationship between metal center and polar functional group by the control of the steric and electronic effects of catalysts is necessary.Our work mainly focuses on these aspects:?1?Based on the excellent performance of Ni/Pd catalysts bearing?-ketoimine ligands in olefin polymerization,novel six-membered neutral[N,O]nickel complex combined with N-acylated imidazolin-2-imine fragment was synthesized and applied in ethylene polymerization and copolymerization with 5-hexenyl acetate?HAc?.Single crystal X-ray structure diffraction demonstrated both complexes adopt a slightly distorted square-planar geometry around the nickel center,the phosphine occupies a position trans to the nitrogen atom,and there is the conjugate effect of the six-membered coordination ring.The nickel complexes alone conducted ethylene polymerization with high activity(?1.0×106g·mol-1·h-1)and good thermal stability,affording semicrystalline polyethylene with low molecular weight??2.0×103g/mol?and low branching degrees?6-18/1000 C?.The catalytic activity increased by two times with the addition of Ni?COD?2as an well-known phosphine scavenger.The complex Ni1-2 bearing electron-donating adamantyl substituent showed higher activity than phenyl substituted complex Ni1-1,and highest activity was observed at 60°C(up to?106g?mol-1?h-1).The NMR spectra of the polyethylenes revealed that the carbon-carbon double bond and the branching were formed by?-H elimination and chain walking reactions,respectively,and the predominant branching is methyl branch.Most importantly,the catalysts were also effective for the copolymerization of ethylene and 5-hexenyl acetate moderate activity(?105g?mol-1?h-1).The microstructure analyses by the 13C NMR spectroscopy indicated that the 5-hexenyl acetate was successfully incorporated into the polyethylene main chain with reasonable comonomer content?0.72-1.33 mol%?.In addition,the prepared nickel complexes exhibited high catalytic activities(3.88×105g·mol-1·h-1)for copolymerization of norbornene?N?and 1-hexene?H?in the presence of Et Al Cl2.In particular,the active species was thermally stable and showed high polymerization activities even at 120?.The results indicated that the copolymerization activity,polymer molecular weight?Mn?and glass transition temperature?Tg?decreased with the raising of Et Al Cl2and 1-hexene,on the contrary,1-hexene content increased up to 7.11 mol%.Moreover,raising temperature was favorable to copolymerization activity and 1-hexene incorporation in the copolymerization but went against Mn.Furthermore,Tgwas significantly affected by temperature,and the linear relationship between the Tgvalues and H content was observed.?2?Bis?N-acylated imidazolin-2-imine?nickel complexes were synthesized and used in norbornene polymerization and copolymerization with polar monomers in the presence of B?C6F5?3.Most importantly,the installation of the phenyl group on the imidazolin ligand exhibit higher tolerance toward the polar monomer to copolymerize norbornene with commercial alkyl acrylates,affording high molecular weight functionalized COCs.According to single crystal X-ray diffraction analysis,the two six-membered chelate rings were linked through a nickel center in the solid state of nickel complexes.In the complexes Ni2-1 and Ni2-2,the geometry at the nickel center exhibited a square planar,where each nitrogen and oxygen atoms adopted a trans-configuration,respectively.In contrast,both chelate rings in Ni2-3 and Ni2-4 adopted a twisted geometry with a cis-configuration,indicating the installation of a sterically bulky phenyl group on the imidazoline-2-imine fragment is efficient for the effective blockage of the axial position of the nickel center.All the catalytic systems showed a relatively high reactivity(?1.7×106g·mol-1·h-1)and the polymers were the formation of the vinyl addition type polynorbornenes.The copolymerizations of norbornene with 5-hexenyl acetate?HAc?,allyl acetate?AAc?,methyl acrylate?MA?and butyl acrylate?BA?were investigated with the systems of Ni2-1-Ni2-4/B?C6F5?3.Special polar monomer HAc containing long methylene chain between double bond and polar group showed highest activity in each catalytic system.The reduction of the methylene chain length resulted in a little decrease in the activity for AAc.Interestingly,fundamental polar monomer MA and BA could also be copolymerized to give high molecular weight copolymers?Mn?5.0×104g/mol?with unimodle GPC curves.Moreover,the complexes Ni2-3 and Ni2-4 bearing phenyl substituent on the imidazolin ligand exhibited higher copolymerization activities with each polar comonomer(HAc up to 1.9×105g·mol-1·h-1,AAc up to 1.2×105g·mol-1·h-1,MA up to 5.6×104g·mol-1·h-1,and BA up to 6.0×104g·mol-1·h-1),and the each comonomer content increased by rising the comonomer feed ratio?up to 2.81 mol%for HAc,3.26 mol%for AAc,2.54 mol%for MA and 4.49 mol%for BA?.Water contact angle?WCA?indicated that the introduction of a small amount of ester group can efficiently improve the surface property of COCs.?3?In order to obtain norbornene based functionalized cycloolefin copolymers and improve catalytic performances,four kinds of quinolinyl imidazolidin-2-imine nickel complexes bearing an imidazolidin-2-imine group combined with quinolone framework have been synthesized based on the different substituents on the 1,3-positions of imidazole ring with methyl?Ni3-1?,tert-butyl?Ni3-2?,phenyl?Ni3-3?and 2,6-diisopropylphenyl?Ni3-4?,and poly?norbornene-co-styrene??PNS?and poly?norbornene-co-para-chlorostyrene??PNCS?with Cl groups were obtained.Single crystal X-ray structure diffraction demonstrated the molecular structures of all the nickel complexes Ni3-1-Ni3-4 displayed a distorted tetrahedral geometry at nickel center,and the imidazolidin ring is almost perpendicular to the quinoline plan regardless of the substituent on the nitrogen atoms.The polymerization date indicated that the systems of nickel complexes/MAO exhibited high catalytic activities in norbornene polymerization(9.17×106g·mol-1·h-1)as well as styrene(3.8×105g·mol-1·h-1),and proceeded with moderate catalytic activities for para-chlorostyrene(8.4×104g·mol-1·h-1).These catalysts were also remarkably active for the copolymerizations of norbornene with styrene(4.8×105g·mol-1·h-1)and para-chlorostyrene(?1.0×106g·mol-1·h-1)to yield high monomer incroporaton?S,16.54 mol%;CS,11.7 mol%?and molecular weight?PNS,Mn=1.31×105g/mol,PNCS,Mn=4.24×105g/mol?,and the NMR spectra,single Tgvalue and single-peak distribution of molecular weight demonstrated the random incorporation of comonomers in these copolymers.Moreover,the monomer reactivity ratioswere then evaluated for each system by Fineman-Ross plots,and the values were obtained to be r N?20and r S?0.05 in the copolymerization of norbornene with styrene and to be r N?43 and r CS?0.02in the copolymerization of norbornene with para-chlorostyrene(r N/r CS?2.0×103),and all the catalytic systems give copolymers with a preference for the insertion of norbornene;besides,the transmittance of copolymer films were about 90%in the visible light region?350-750 nm?regardless of the comonomer used,indicating that these copolymers could be used as an optical transparent polymer material. |
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