Experimental And Theoretical Studies On Homogeneous/Heterogeneous Hafnium And Chromium-based Ethylene Polymerization Catalysts

As the core of the polyolefin industry,olefin polymerization catalysts play a key role in increasing the capacity of production plants and developing high performance polyolefin resins.Since the discovery of the Ziegler-Natta catalysts and silica-supported chromium catalysts in the 1950s,a variety of novel olefin polymerization catalysts have been developed.For heterogeneous olefin polymerization catalysts,despite the great research efforts,the academic progresses are lagging far behind their successful commercial applications.The complex coordination environment of catalyst surface,low percentage and difficult separation of active sites,have caused slow progress in the study of core issues such as the nature of the active species,polymerization mechanism,and the relationship between catalyst structures and catalytic properties.The development of well-defined single-site supported catalysts and the molecular simulation of heterogeneous polymerization catalystic system,will help to have a deeper understanding of polymerization behavior,active species and polymerization mechanisms,which will be of great scientific and economic importance for the development of novel olefin polymerization catalysts.First of all,a single-site supported catalyst Catl-ZrS,was synthesized by chemisorption of pyridylamine Hf complex Catl(L1-HfMe2,L1=2,6-diisopropyl-N-{(2-isopropylphenyl)[6-(naphthalen-1-yl)pyridin-2-yl]methyl}aniline)on Bronsted acidic sulfated zirconia(ZrS).The 13C-CPMAS NMR,IR and DFT results indicate formation of L1-HfMe+organohafnium cation having a largely electrostatic pyridylamido-Hf-CH3+…ZrS-interaction with elongated Hf…Ozrs distances of?2.14 ?.The supported catalyst exhibits moderate activity for ethylene homopolymerization,and ethylene/1-octene copolymerization with no activator/co-catalyst.Catl-derived supported catalyst retains the capacity to copolymerize ethylene with a-olefins such as 1-octene(2.6 mol%).The active site titration results indicate that up to 80%of the catalyst surface species can catalyze ethylene polymerization.To obtain a better understanding of the active site in this heterogeneous case,DFT calculations were performed on the initial ethylene activation/insertion process.On the basis of the DFT analysis,ethylene insertion at the chemisorbed molecular catalyst occurs preferentially at the Hf-methyl bond in most cases,in contrast to the analogous catalyst in homogeneous solution.This inverted trend on passing from the naked cation to the supported catalyst mainly reflects the cation…surface interactions:ethylene insertion into the Hf-aryl bond involves a greater displacement of the cation from the surface with respect to the ethylene insertion into the Hf-methyl bond due to steric constraints.Finally,the kinetically and thermodynamically preferred site B@S 1 is proposed to be the most plausible active site for ethylene polymerization.In order to achieve a better understanding of the structure-activity relationships in the organohafnium supported olefin polymerization catalyst system,two novel pyridylamido trimethyl hafnium catalysts(Cat2,Cat3)and three sulfated alumina(AlS)supported catalysts(Catl-AlS,Cat2-AlS and Cat3-AlS)were successfully synthesized and studied.By the application of X-ray single crystal diffraction,13C-CPMAS NMR,DRIFT and DFT calculations,the homogenous catalyst crystal structure and heterogeneous catalyst surface structure were characterized and defined.New designed hafnium catalsyts(Cat2,Cat3),show high capacity to copolymerize ethylene with 1-octene(-30 mol%)and potential catalysts for the production of polyolefin elastomers.Supported catalsyts,Catl-AlS,Cat2-AlS and Cat3-AlS show higher ethylene polymerization activity with no cocatalysts.Subsequently,DFT calculations were conducted to study the active sites structure and polymerization mechanism in these three supported catalyst systems.The theoretical information of the catalyst surface structures(Hf coordination evironment etc.)and reaction energies(ethylene coordination energy,ethylene insertion barrier and ion-pair separation energy,etc.)were combined olefin polymerization results to preliminarily establish the relationship between the catalyst structure and catalytic performance in the heterogeneous organohafnium catalyst system.In the last part,DFT calculations were employed to study the nature of the active species and their switching mechanism between ethylene polymerization and ethylene nonselective oligomerization over the triphenylsiloxy complex of chromium(II)1([(Ph3SiO)Cr?.(THF)]2(?-OSiPh3)2)/MAO catalyst system.DFT calculations provided some essential insights into the nature of the active chromium species and mechanistic aspects in the 1/MAO system.It was demonstrated that the trivalent[(Ph3SiO)Cr?Me]+and[(?6-toluene)Cr?(Me)2]+models generated from the disproportionation reaction might be the most plausible polymerization active species at lower Al/Cr ratios(?100),and the divalent[(?-toluene)Cr?Me]+model was proposed to lead to ethylene nonselective oligomerization at higher Al/Cr molar ratios(?200),which could rationally explain well the switching behavior in the 1/MAO catalyst system.Combining experiments(novel catalyst design,catalyst structure characterization,olefin polymerization)and theoretical calculations(structure optimization,energy calculations),this thesis provides an effective approach to explore and solve chemical problems associated with olefin polymerization.The deeper understanding of the active species and polymerization mechanism will promote the development of novel olefin polymerization catalysts.