Ethylene Polymerization And Copolymerization With Polar Monomers By Late Transition Metal Nickel Catalysts

Polyolefins are the most widely used synthetic polymers in the world,and have many excellent properties such as easy molding,light weight and chemical corrosion resistance.Over the past half century,the polyolefin industry has grown rapidly since the pioneering work of Ziegler and Natta.Due to the non-polar characteristics of polyolefin chain structure greatly limit its application scope,the introduction of polar functional groups into the chain to prepare functionalized polyethylene olefins has always been an important research direction in the field of olefin polymerization.Compared with early-transition metal catalysts,late-transition metals catalysts have attracted much attention due to their low oxophilicity and high tolerance to polar groups.In the late-transition nickel catalytic system,in order to enhance the polymerization performance of the catalyst and prepare high-performance polyolefin materials,the following methods are commonly used:increasing the axial steric hindrance of the metal center;regulating the electronic effect;and introducing the secondary coordination effect;and regulating by external stimulus.In this paper,from the perspective of steric hindrance regulation,two different nickel catalyst systems were selected for research:(1)the effect of axial steric hindrance on the α-diimine nickel catalytic system;(2)the effect of steric hindrance and Synergistic effect of orthofluorine effect on salicylaldimine neutral nickel system.(1)Polymer molecular weight is one of the important parameters for evaluating the properties of polyolefins.In this work,a new "sandwich-like" structure of α-diimine nickel catalyst was designed and synthesized by introducing the rotation-constrained benzosuberyl substituent into the ortho-position of the N-aryl rings to increase the axial steric hindrance of the catalyst metal center of the catalyst to obtain ultra-high molecular weight polyethylene.The rotationally constrained benzosuberyl substituents cover the nickel center from two axial sites,acting as a "sandwich" wrapping.Under the same polymerization conditions,the Ni3 catalyst containing benzosuberyl group exhibited the highest polymer molecular weight,polymerization activity and insertion rate of polar monomers.In the study of mechanical properties,the polyethylene(90/1000C)with medium and high branching degree prepared by Ni3 has the characteristics of elastomer.(2)Transition metal catalysts are the core of the whole olefin polymerization process,among which salicylaldimine neutral nickel catalysts are favored due to the nature of both functional-group tolerance and cocatalysts-free.In this work,para-sterically hindered substituents(phenyl,naphthalene and anthracene)and ortho-fluorine substituents were simultaneously introduced into the neutral nickel catalyst of salicylaldimine.The effects of steric hindrance effect,fluorine effect,polymerization temperature and polymerization time on the activity,molecular weight and branched degree of ethylene polymerization were systematically studied.The results showed that ortho-fluorine substituents could significantly improve the activity,catalyst life and molecular weight of the polymer,and reduce the branching degree of the polymer.Increasing axial steric hindrance can improve catalytic activity and molecular weight of polymer,but has little effect on branched degree of polymer.In summary,two kinds of nickel catalyst systems were used to study the role of steric hindrance effect and fluorine effect in olefin polymerization,and high performance polyolefins such as ultra-high molecular weight polyethylene were prepared.We investigated the relationship between the structure and polymerization properties of the catalyst,which provides theoretical guidance for the design and synthesis of catalysts with excellent properties for the preparation of high-performance polyolefin materials.