Studies On The Immobilization Of ?-Diimine Nickel Catalysts For Ethylene Polymerization

Late-transition-metal catalysts form a new generation of catalysts for olefin polymerization after Ziegler-Natta catalysts and Metallocene catalysts.In comparison with the former,late-transition-metal catalysts exhibit some unique catalytic properties,which have led to great interest in the last few decades.A significant impediment to the industrial application of homogeneous catalysts is the reactor fouling and extremely exothermic in polymerization process,and lack of morphology control of the polymers.In general,the way to solve these problems is to immobilize the catalysts on suitable carriers.In this thesis,a series of novel?-diimine nickel???late-transition-metal catalysts containing hydroxyl groups were synthesized.Then the silica-supported catalysts were successfully prepared.In the next part,these homogeneous catalysts and immobilized catalysts were used in the polymerization of ethylene.The main results were as follows:1.We synthesized five different novel?-diimine nickel???catalysts containing hydroxyl groups?Cat1,Cat2,Cat3,Cat4 and Cat5?,and were characterized by NMR analysis,FT-IR and element analysis.The silica-supported catalysts were successfully prepared by immobilizing the?-diimine catalysts on the surface of AlMe₃ or MAO treated SiO₂.The morphology of silica support and the silica-supported catalysts were observed with SEM.The content of Ni in supported catalyst were determined by ICP.2.The polymerization of ethylene was carried out under homogeneous nickel catalysts Cat1-Cat3,and the resulting polyethylene was branched polyethylene.It was obvious that the Cat2 with bulkier diphenylmethyl substituent at the ortho-aryl position exhibited much lower acticity than Cat1,but demonstrated much greater thermal stability than Cat1,reached about 2×10⁶g PE/mol Ni·h even at 90°C.The catalyst Cat4 bearing bulkier backbone than Cat5 showed higher activity and better thermal stability towards ethylene polymerization.The molecular weight of the product obtained by Cat1 is only 168kg/mol,which lower than these prepared by Cat2 and Cat3 under the same conditions.Compared with Cat2 and Cat3,the product obtained by Cat1 has a higher degree of branching,reaching to 96¹14 branches/1000C.The low melting points of the polyethylenes obtained by Cat1 presupposed polyethylenes with high branching content.In particular,when the temperature reaches to 70^oC,there is no melting peak on its DSC curve.At the same time,the polymerization conditions also have an effect on the catalytic activity and the properties of polymers?such as,thermal performance and chain structure?.3.The performance of the silica-supported catalysts in ethylene polymerization were investigated.When performed under the same polymerization conditions,sightly reduced productivities can be observed compared with homogeneous polymerizations.But the polyethylenes obtained via silica-supported catalysts showed better particle morphology than that using homogeneous catalyst with the SEM images.And the branching densities lower for the supported catalysts under same conditions.In general,the similar behavior had been observed for supported catalysts with the unsupported catalysts upon changed polymerization.