Synthesis Of Triphenylphosphinoimidazolidine-2-imine Nickel Complexes And Their Application In Olefin Polymerization

Polyolefins are widely used in our life,due to their excellent properties,easy processing,and low cost.The introduction of a small amounts of polar groups into the polyolefin backbone can significantly improve their non-polarity to expand further application range.The production of functional polyolefins via direct copolymerization of olefin and polar monomer catalyzed by late-transition metal catalysts can precisely control the polar monomer incorporation and its distribution in the copolymer with keeping the original properties of polyolefins.In this paper,the triphenylphosphonimidazolidin-2-imine nickel complexes were designed and synthesized,and their application in homopolymerization of ethylene,propylene,and norbornene and copolymerization of norbornene with methyl acrylate（MA）was studied.First,the triphenylphosphonimidazoline-2-imide ligand was synthesized by the reaction of N-（2-bromophenyl）-1,3-dimethylimidazole-2-imide with disubstituted arylphosphine chloride,the nickel complexes Ni1 and Ni2 were synthesized by the reaction of corresponding ligand and nickel precursor（Ni（dme）Br₂）at room temperature.The conversion reach about 90%and 85%,respectively,and the nickel complexes were characterized by elemental analysis and MALDI-TOF-MS.The results of single crystal X-ray diffraction showed that the nickel metal centers are tetrahedral and form a five-element chelating ring structure with P and N atoms regardless of the substituent introduced.Compared with non-substituted Ni1 complex,the isopropyl substituted Ni2on the phenyl phosphine ligand can effectively increase the axial shielding effect on the nickel metal center,which will reduce the interaction between the heteroatoms and the metal center in the system.Secondly,the olefin homopolymerizations were performed using Ni1 and Ni2 under the activation of methylaluminoxane（MAO）and dichloroethylaluminum（Et Al Cl₂）.Ni1 can catalyze ethylene oligomerization,and the Et Al Cl₂ system exhibited highest oligomerization activity of1.32×10⁶ g·mol^-1·h^-1.The main product was isobutene with selectivity of 78%.The oligomerization activity(0.76×10⁶ g·mol^-1·h^-1)of Ni2 was lower than that of Ni1,and the selectivity of the product was also lower than that of Ni1,indicating that the introduction of isopropyl had a greater effect on the oligomerization activity and selectivity of ethylene.Ni1 with cheap Et Al Cl₂ as a cocatalyst showed much higher polymerization activity(3.54×10⁷ g·mol^-1·h^-1)for the addition type polymerization of norbornene to produce high molecular weight polynorbornene with number average molecular weight of～10⁵ g·mol^-1.Differing with ethylene oligomerization,Ni2 bearing higher steric hinderance showed higher polymerization activity(4.53×10⁷ g·mol^-1·h^-1)than Ni1,indicating that the electronic effect of the nickel complex substituent plays a more important role in the polymerization activity.However,neither Ni1 nor Ni2 could catalyze the polymerization of propylene under the activation of MAO or Et Al Cl₂,respectively.Finally,the copolymerization of norbornene catalyzed by Ni1 and Ni2 under Et Al Cl₂ activation was studied.When Ni1 and Ni2 catalyzed the copolymerization of norbornene with methyl acrylate（MA）,Ni1 exhibited high activity of 4.07×10⁵ g·mol^-1·h^-1 to produce the copolymers with a single-peak distribution of GPC curve and a number average molecular weight of～10⁴ g·mol^-1.The incorporation of MA in the copolymer can be effectively controlled by changing the monomer feed ratio up to 50 mol%.The copolymerization activity of Ni2 is an order of magnitude lower than that of Ni1(0.57×10⁵ g·mol^-1·h^-1)with decreasing MA incorporation.The results indicated that the steric hindrance of the nickel complex affects the coordination and insertion of the MA monomer to reduce the copolymerization activity and the comonomer incorporation.When Ni1 and Ni2 catalyzed the copolymerization of norbornene and styrene,the copolymerization activity of Ni1 reached up to2.14×10⁶ g·mol^-1·h^-1.Under the same conditions,the copolymerization activity of Ni2 was lower than that of Ni1(4.8 VS 3.6×10⁵ g·mol^-1·h^-1),which indicated that compared with the steric hindrance effect of isopropyl substituent,The effect of electron on polymerization activity is more significant.