| Polyethylene is a polymeric material derived from low-cost ethylene raw materials,and is widely used because of its cheapness,light weight and durability.It can replace traditional materials such as wood,metal and glass in many occasions.The"key"for the preparation of polyethylene is the catalyst.The late transition metal catalystα-diimide nickel catalyst has attracted much attention because of its low price,high activity,and the ability to catalyze the homopolymerization of ethylene monomer to form branched polyethylene.Researchers have done a lot of work on the structure and performance of catalysts,but this catalyst has greatly limited its industrial application due to problems such as product stickiness and the difficulty in controlling the shape,size and degree of adhesion of polymer particles.In order to improve these defects,the catalyst needs to be supported.In this paper,two series of new covalently bonded loaded catalysts were prepared and catalyzed ethylene polymerization usingα-diimino nickel catalyst and bisimino pyridine iron catalyst with large site resistance substituents and multiple hydroxyl groups by reacting with activated carriers,respectively.The effects of catalyst structure,carrier,polymerization conditions and other factors on the polymerization activity and structural properties of polyethylenes were investigated.The specific research contents are as follows:1.Theα-diimide nickel catalyst CatA with large sterically hindered groups and multiple hydroxyl groups was reacted with modified 757#Si O2and Si O2Aerogel to obtainα-diimine nickel catalysts S-CatA,S-CatA-1,S-CatA1and S-CatA1-1 with different loading rates.Ethylene polymerization experiments were carried out to explore the effects of support,polymerization conditions,and loading rate on catalyst activity and polyethylene structural properties.The experiments found that the activities of CatA,S-CatA and S-CatA1-1 catalysts were similar and all had higher activities at the polymerization temperature of 70°C,while S-CatA-1 with high loading rate and S-CatA1with low loading rate had similar activities.The activity of S-CatA1catalyst is higher,and the activity of S-CatA-1 reaches 4.07×106g PE/mol Ni·h.Compared with the homogeneous catalyst,the heat resistance and life of the supported catalyst have been greatly improved,and the polyethylene obtained by its catalysis has higher molecular weight and melting point,as well as a lower degree of branching and a narrower molecular weight distribution.At the same time the resulting polyethylene particles replicates the morphology of the carrier,forming dispersive polyethylene spherical particles.2.The supported catalysts S-CatB and S-CatB-1 with different loading rates were prepared by the reaction of diimino pyridine iron catalyst CatB with large hydroxyl groups with modified 757#Si O2,and ethylene polymerization experiments were carried out.The effects of support,loading rate,and polymerization conditions on catalyst activity and polyethylene structural properties were investigated.The study found that the activity of S-CatB catalyst with low loading rate was higher than that of CatB,which could reach1.09×106g PE/mol Fe·h at 70℃,but the activity decreased after increasing the loading rate.Compared with homogeneous catalyst,S-CatB supported catalyst has better heat resistance and longer life,and the polyethylene obtained by S-CatB catalyst is a powder with good fluidity.3.The catalyst S-CatB was added to S-CatA to catalyze ethylene polymerization.It was found that the polymer cauldron problem could be improved by using the composite system with n（S-CatB/S-CatA）of 2:1 at 70℃,and DSC showed that the polyethylene prepared by the composite system was only an in-situ blend of branched polyethylene and linear polyethylene. |
