| Norbornene base cyclic olefin copolymers(COCs)have excellent properties such as good thermal stability,high optical transparency,and corrosion resistance,the non-polar property and the high birefringence COCs limit their further application.Therefore,the development of the norbornene copolymerization abilities by using novel olefin polymerization catalysts are useful methods to improve the performance of COCs.In this paper,a series of imidazoline(imidazolidine)-2 imine nickel catalysts was synthesized,and their application in the copolymerization of norbornene with olefin,or polar comonomer was studied.First,diamine nickel complexes bearing imidazolidine 2-imine ligand with different substituents exhibited different metal center spatial topologies(VBur,Ni1:47.1%;Ni2:53.1%;Ni3:46.1%).Indicating that the sterically bulky phenyl substituent(Ni2)is effective for the blockage of the axial position of the nickel center.The electronic properties of the imidazolidine ligand was also analyzed by the Tolman electronic parameters(TEP,Ni1:TEP-2052.0 cm-1;Ni2:TEP-2057.0 cm-1),indicating that methy substituted Ni1 has higher electron donating ability than substituted Ni3.Ni3 still kept high activity(1.68×106g·mol-1·h-1)even at 100°C,demonstrating good thermal stability of nickel catalyst.However,Ni2 showed a relatively low activity of 0.7×106g·mol-1·h-1,indicating that the coordination ability of norbornene to the central metal was reduced due to higher steric hindrance of phenyl substituent on the imidazolidine ligand.The catalysts can catalyze the copolymerization of norbornene with styrene(with activity of 2.04×105g·mol-1·h-1)to give high molecular weight copolymers.The monomer reactivity ratios evaluated by Fineman-Ross plots testified that the steric hindrance and electronic effects of the substituents affect the copolymerization ability of the nickel catalyst,where the highest styrene incorporation(17.5mol%)in the copolymer was abserved with Ni3.The transmittance of the copolymer film was about 90%in visible light region,indicating that the norbornene/styrene copolymer can be used as a transparent optical material.Secondly,Dis N-acylated(imidazoline-2-imine)nickel complexs(Ni7-Ni10)were synthesized with different substituent and characterized with phenyl by NMR and single crystal X-ray diffraction.The two six-membered chelate rings were linked though a nickel center,and the geometry at the nicke center showed a square planar with a stans-configuration of each nitrogen and oxygen atoms.In addition,two tetrahydrofurans were vertically coordinated to the nickel from both square planar.Finally,Ni7-Ni10 alone catalyzed ethylene oligomerization and norbornene polymerization with very low activity(?104 g·mol-1·h-1).The activity of ethylene oligomerization with Ni7increased remarkably by the addition of 4 equivalents of B(C6F5)3(2.9×106g·mol-1·h-1).The substituents of ligand affected the structure of oligomers,where content of hexene(C6)in the product obtained by Ni9 was 53.61%,while the product catalyzed by Ni7 contained 20%C6,C8,and C10,respectively,which indicated that the introduction of phenyl substituent reduces chain transfer.The catalysts can also catalyze the homopolymerization of norbornene,and the activity increased with the addition of B(C6F5)3(2.6×106g·mol-1·h-1).Interestingly,the catalyst also maintains a very high polymerization activity even in the presence of water([M]=2 mol·L-1),indicating that the nickel catalyst has good tolerance toward the polar monomer.Therefore,Ni9 can catalyze the copolymerization of norbornene with methyl acrylate or 5-hexenyl acetate with a copolymerization activity of 6.8×105 g·mol-1·h-1,affording functional COCs with controllable polar monomer incorporation. |
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