Application Of Novel Late-Transition-Metal Nickel And Palladium Catalysts In Olefin(Co) Polymerization

Since the first report ofα-diimine nickel and palladium catalysts for the coordination-insertion polymerization of olefins by Brookhart and their coworkers,late transition metal catalysts have always been one of the research hotspots in the field of olefin polymerization catalysts.Among them,scholars have reported many methods to control the olefin polymerization process in order to synthesize polyolefins with excellent properties.For example,modifying ligands,selecting co-catalysts,screening polymerization conditions,and so on.In this work,we used the method of modifying ligands to control the steric hindrance and electronic effects of the catalyst,as well as used catalysts with special structures to prepare high-performance polyolefin materials.The following studies were carried out:In chapter 1,we briefly introduced transition metal catalysts based on different ligand frameworks.In addition,the characteristics of various polymer materials and the main synthesis methods of functionalized polyolefin materials are also introduced.In chapter 2,we described the synthesis and characterization of a series of bulkyα-diimine ligands and the corresponding nickel and palladium catalysts bearing diarylmethyl moiety with different electronic effects.The bulky nickel complexes show great thermal stability,and achieve the highest activity at 80 ^oC in ethylene polymerization.The generated polyethylene possesses very high molecular weight,moderate branching density and moderate melting temperature even at 80 ^oC.The corresponding palladium complexes display moderate activity and generate high molecular weight semicrystalline polyethylene with low branching density.The high molecular weight polar functionalized polyethylene can also be obtained using these palladium complexes.Surprisingly,the remote nonconjugated electronic perturbations exert great influence on the ethylene polymerization in terms of the polymerization activities and polymer microstructures.In chapter 3,we described the synthesis and characterization of a series of bulky yet flexible cycloalkyl substitutedα-diimine ligands and the corresponding nickel and palladium catalysts.The thermostable nickel complexes in this system show high activity(up to 4.89×10⁶ g mol^-1 h^-1)and can generate highly branched polyethylene（up to 112/1000C）with high molecular weight(up to 54.4×10⁴ g mol^-1).The obtained polyethylene displays good elastic properties（SR value up to 77%）characteristic of thermoplastic elastomers.The flexible cycloalkyl substituted palladium complexes exhibit low to moderate catalytic activities(0.6-43.9×10⁶ g mol^-1 h^-1),low to morderate molecular weights(0.93-31.23×10⁴ g mol^-1),and high branching density（87-122/1000C）for ethylene polymerization,meanwhile allowing appreciable comonomer incorporation（1.0-7.7%）for ethylene/MA copolymerization.Most interestingly,compared to the flexible cyclohexyl substitutedα-diimine nickel and palladium catalysts,the rigid phenyl substituted catalysts generated much lower branching density polyethylene in ethylene polymerization.In chapter 4,we described the synthesis and characterization of a series of unsymmetricalα-diimine ligands bearing n-butyl moiety and diarylmethyl or 8-arylnaphthyl moiety as well as the corresponding nickel and palladium complexes.These unsymmetricalα-diimine nickel and palladium complexes were investigated for ethylene polymerization and copolymerization with methyl acrylate（MA）.Under the synergistic effect of compact alkyl substituents and bulky aryl substituents,the nickel complexes showed moderate to high activities and generated low to high molecular weight polyethylene with various branching densities.Similar polymerization results were also observed in the corresponding palladium system.The aryl orientation in rigid bulky aryl substituents has significant effects on the polymerizations and copolymerizations in terms of activity,the molecular weight of obtained polyethylene and copolymer,and the incorporation ratio of MA.In chapter 5,the synthesis of polyethylene thermoplastic elastomers viaα-diimine-nickel-catalyzed ethylene polymerization using industrial preferred polymerization conditions such as elevated temperatures and alkane solvent is highly desirable and it would be of great general interest.In this contribution,we address these issues by constructing a series of highly sterically demandingα-diimine Ni（II）catalysts with abundant ^tBu substituents.These nickel catalysts were examined for ethylene polymerization in hexanes at elevated temperatures（up to 90°C）and proved to be thermally robust at temperatures as high as 90°C.Generally,these nickel catalysts can generate highly branched（ca.70-80/1000C）polyethylene with very high molecular weight（M_n up to 55.79×10⁴g/mol）.Furthermore,the resultant polyethylenes displayed characteristics of thermoplastic elastomers with excellent elastic recovery（SR up to84%）.Compared with some similarα-diimine Ni（II）catalysts,it is shown that the presence of axial remote ^tBu is not only conducive to the dissolution of the catalyst in alkanes,but also improves the elastic recovery value of the obtained polyethylene.In chapter 6,thermoplastic elastomers（TPE）can be easily molded,extruded and reused like plastics and exhibit typical elastic properties like rubbers.They are widely used in the automotive sector and household appliances sector.The synthesis of thermoplastic polyolefin elastomers using only ethylene as feedstock in a single step is a fascinating and challenging way.The synthesis of polyethylene thermoplastic elastomer throughα-diimine-nickel-catalyzed ethylene polymerization have been reported extensively in recent years.In this contribution,we report the direct synthesis of polyethylene thermoplastic elastomer throughα-diimine-palladium-catalyzed ethylene polymerization.Most importantly,biomass functionalized polyethylene thermoplastic elastomer with excellent elastic properties could be generated throughα-diimine-palladium-catalyzed ethylene-biomass monomer copolymerization.In chapter 7,steric effects are the key factors in producing high molecular-weight polymers by the mediation of most late transition-metal catalysts.Therefore,metal ions such as nickel and palladium are prone toβ-hydride elimination and subsequent chain transfer in olefin polymerization reactions.Size and position of the catalysts’substituents significantly affect efficiency of the olefin polymerization.Herein,synthesis and characterization of a series of bulky aryl substituted iminopyridyl ligands and the corresponding palladium and nickel complexes are described in detail.The as-synthesized palladium complexes show moderate activity(1.17-4.80×10⁴g mol^-1 h^-1)in the ethylene polymerization and generate highly-branched oligoethylene or polyethylene（72-126/1000 C）with low molecular-weights(395-13401 g mol^-1).Meanwhile,the prepared palladium complexes perform a high degree of comonomer incorporation（5.6-17.4%）in ethylene/methyl acrylate（MA）or ethylene/acrylic acid（AA）copolymerizations.The corresponding nickel complexes are all highly active(well above 10⁶ g mol^-1 h^-1)in ethylene polymerization and produce branched oligoethylene and/or polyethylene with low to moderate molecular weights.Intriguingly,the orientation of aryl substituents plays a decisive role in the molecular weight of the attained polymer or copolymer under the performed reaction conditions.In chapter 8,a series of iminopyridyl Pd（II）catalysts containing bulky diarylmethyl substituents with various remote nonconjugated electron-withdrawing or-donating groups were synthesized and characterized.These catalysts possessing high catalytic activities and good thermal stability,are capable of producing hyperbranched ethylene oligomers in ethylene oligomerization.More importantly,hyperbranched polar functional ethylene oligomers with very high incorporations can be accessible from ethylene-MA co-oligomerization in the presence of iminopyridyl Pd（II）catalysts.In terms of catalysts,the remote nonconjugated electronic substituents of bulky diarylmethyl moieties have a significant effect on the molecular weight and branching density of ethylene oligomers/ethylene-MA co-oligomers,as well as the incorporations of MA in ethylene（co）oligomerization.In chapter 9,we developed a series of highly sterically hindered iminopyridyl Pd（II）catalysts bearing the 8-arylnaphthyl substituents or/and dibenzosuberyl substituents.These substituents can provide an effective shield on the axial sites of the active center,thus allowing for the formation of high-molecular-weight polyethylene or functionalized polyethylene.Unexpectedly,these catalysts were also highly efficient for ethylene and various polar monomers copolymerization,giving access to high-molecular-weight functionalized polyethylene with very high incorporation ratio（up to24%）.