Application Of Novel α-Diimine Nickel Catalyst In Preparation Of Branched Polyethylene

The advancement of the polyolefin industry symbolizes the advancement of a country’s petrochemical industry.Polyethylene is the most widely used variety of polymer materials and its applications cover a wide range of fields.The most critical development of polyethylene is the catalyst.The catalyst can control the polymerization activity,and the properties of the polyethylene can be controlled through changing the structure of the catalyst.The late-transition-metalα-diimine catalyst has the advantages of single active center,high activity and good stability,but it has poor thermal stability,and the problem of the sticking pot has always restricted its industrial development.In general,the loading of homogeneous catalysts is considered as an important way to solve these problems.In this paper,a series of novelα-diimine nickel-based transition metal catalysts with different hydroxyl number reactive functional groups and asymmetric steric hindrance were designed and synthesized and silica gel-loaded large sterically hindered catalysts were successfully prepared.The homogeneous and supported catalysts were used to catalyze the polymerization of ethylene.The specific research content is as follows:In this paper,three novelα-diimine nickel（Ⅱ）catalysts Cat1,Cat2 and Cat3 with different number of reactive functional groups were designed and synthesized,and then a large sterically hindered asymmetric catalyst Cat4 and Cat5 with a highly active functional group was synthesized.The structures of the ligands and catalyst were fully characterized by NMR,FT-IR and EA.The polymerization of ethylene with Cat1,Cat2 and Cat3 catalysts as the main catalyst showed that the Cat2 catalyst system with three hydroxyl functional groups had better catalytic activity under the same conditions and the Cat3 catalyst system had better thermal stability at high temperature.With the temperature increasing,the degree of branching of the polymer increases,the melting point decreases,the molecular weight decreases and the product is in a colloidal form.The polymerization of ethylene was carried out with Cat4 and Cat5 catalysts as the main catalyst.The polymerization activity of the catalyst decreased with the increase of steric hindrance of the catalyst,but the thermal stability of the catalyst was good.The melting point and the molecular weight of the obtained polymer were increased and the molecular weight was increased and the polymerization product obtained by the maximum sterically hindered Cat5 catalyst reached a molecular weight of 4.64×10⁵ g mol^-1 at 70°C under a polymerization condition of 0.5 MPa.The homogeneousα-diimine nickel（Ⅱ）catalyst Cat4 and Cat5,was supported on the Al Me₃ treated 757^#/955^#silica support to successfully prepare silica-supportedα-diimine nickel（Ⅱ）catalyst.The apparent morphology of the silica support and the supported catalyst was characterized by SEM and the Ni content of the supported catalyst was determined by ICP and the distribution of the elements on the surface of the silica was measured by EDX.Comparing the results of silica-supported catalyst and homogeneous catalyst catalyzed ethylene polymerization,the catalytic activity is reduced,but the particle morphology of the polymerized product is well adjusted.The polymer is composed of globular small units under scanning electron microscope.The degree of branching is reduced and the molecular weight is increased compared to the homogeneous polymer.Moreover,the influence of the change of polymerization conditions on the catalytic activity and the performance of polyethylene is basically the same as that in the case of homogeneous polymerization.