Design And Synthesis Of Phosphine-phenolate Nickel Catalysts And Their Application In Ethylene Polymerization

Polyolefins（e.g.polyethylene,polypropylene）occupy an extremely important position in the field of polymer materials,and the development of polyolefin materials can’t be separated from the development of olefin polymerization catalysts.Since the introduction of Ziegler-Natta catalysts,the development of early transition metal catalytic systems（Ti,Zr,Hf,etc.）has given a great boost to the field of olefin polymerization,however,there are great limits in the copolymerization of olefins with polar monomers caused by monomer tolerance and other problems.In contrast,late transition metal catalysts（Ni and Pd）has shown good tolerance to polar monomers and has great potential for copolymerization of olefins with polar monomers,and therefore has attracted a lot of attention from researchers.In the late transition metal catalyst system,it suffers from the tendency ofβ-H elimination resulting in low molecular weight of the polymer.For instance,SHOP type nickel catalysts are mostly used in industry for the preparation of oligomers and have limited applications in copolymerization.Until 2017,a major breakthrough in the copolymerization of ethylene with polar monomers of acrylate by SHOP type phosphine-phenolate nickel catalysts made this type of catalysts gain the attention of researchers once again.Researches on phosphine-phenolate nickel catalytic systems have focused on the modification of the catalytic performance by the electronic and site barrier effects of the phenoxy o-substituents and the phosphine substituents.This thesis focuses on the structural design of new phosphine-phenolate nickel catalysts and investigates their application to ethylene polymerization and copolymerization of ethylene with polar monomers.（1）In this work,a novel catalyst structure with a second coordination domain was constructed by introducing an imine structure with weak coordination in the phenoxy neighboring position of the phosphine-phenolate ligand.These catalysts enable the preparation of highly linear（brs<2/1000C）,high molecular weight(M_w:10⁵ g·mol^-1)polyethylene at high polymerization activity(10⁶ g·mol^-1·h^-1)under mild conditions and without the auxiliary of any costly cocatalyst.Significantly,the preferred catalysts were able to synthesize ultra-high molecular weight linear polyethylene with molecular weights up to 149.6×10⁴ g·mol^-1（brs:2/1000C;T_m:136.0°C）.In addition,the addition of borohydride reagent can clearly improve the ethylene polymerization activity and obtain polyethylene with different microstructures.In the copolymerization with MA,all catalysts showed good insertion rates,with the largest Ni3 incorporation to 1.34 mol%.（2）In this work,a series of dinuclear phosphine-phenolate nickel catalysts with different linkage structures were designed and synthesized.Compared with the mononuclear catalysts,the dinuclear catalyst with an xanthene backbone was able to remarkably increase the ethylene polymerization activity(2.6 vs 0.5×10⁶ g·mol^-1·h^-1)at a higher polymerization temperature（70°C）Furthermore,when the alkali metal Zn Br₂ has been added,the occurrence ofβ-H elimination by the dinuclear catalyst has been exacerbated,resulting in a polymer with a lower molecular weight.This ability of the alkali metal to modulate the molecular weight of the polymer did not play a significant role in the mononuclear nickel catalyst system.In copolymerization with polar monomer MA,the dinuclear catalyst Ni7 can obtain 0.91 mol%MA incorporation.