New Metallocene Catalysts: Synthesis, Characterization And Catalytic Performance For Olefin Polymerization

Polyolefin has been one of the most important materials in our daily life. Theprogress on olefin polymerization catalysts is a key factor for the vigorous developmentof polyolefin industry. Metallocene catalysts have attracted extensive interest in the pastdecades due to their unique properties and advantages as olefin polymerization catalysts.Many research efforts have been focused on the development of new homogeneousmetallocene catalysts for producing a variety of high performance polyolefin materialsand understanding the relationship between the structure and their catalytic property of atype of catalysts with respect to polymer chain composition and architecture as well asstereospecificity.Half-sandwich rare-earth complexes have been studied as good catalysts for thestereo-and regio-selective (co)polymerizations of olefins. In particular, it was foundthat a side arm on the Cp ring of the half-sandwich rare-earth complexes wouldsignificantly affect their catalytic activity and stereoselectivity. A number of newhalf-sandwich scandium(III) complexes bearing imine-cyclopentadienyl ligands1,2,3and4were synthesized by different methods. Cs-symmetric complexes2can beobtained by heating the unchelated complexes1under vaccum. Complexes1b and1cwere directly converted to complexes2b and2c by heating at120oC under vaccumwithout purification and characterization. Typical scandium(III) complexes werecharacterized by1H and13C NMR spectroscopy and elemental analyses. The structuresof complexes1a,2c,3and4were determined by single crystal X-ray crystallography,which indicates that the imine N atoms in complexes1a and3do not coordinate to the central scandium atoms. Complexes2a–c and4were found to exhibit moderatecatalytic activity for propylene and1-hexene polymerization upon activation withAlR3/Ph3CB(C6F5)4or methylaluminoxane(MAO), and produce atactic polypropyleneand isotactic poly(1-hexene). The effects of molecular structures and reaction conditionson the catalytic behavior of these complexes were examined and the possible catalyticmechanism was discussed.Based on the above mentioned unique catalytic performance of the Cs-symmetricscandium(III) complexes on stereo specific polymerization of1-hexene, as well as thatof group4CGC metallocene on ethylene/α-olefin copolymerization. It is therefore ofinterest to develop a type of titanium(IV) complexes chelated with similar structures. Aseries of new half-sandwich titanium(IV) complexes chelated with2-tetramethylcyclopentadienyl-benzylamido ligands,2-Me4CpC6H4CH2(R)NTiCl2[R=iPr (5), Cy (6),nPr (7),4-MePh (8)], have been synthesized from thechlorotrimethylsilane elimination reactions of TiCl4with the doubletrimethylsilyl-substituted preligands. The titanium complexes were characterized by1Hand13C NMR, elemental analyses and single crystal X-ray crystallography. The X-raycrystallography analysis reveals that these titanium complexes possess a three-leggedpiano stool geometry with the amide N atom in a mitered six-membered chelating ringand the two chloride atoms as the legs. The angle between the cyclopentadienyl planeand the attached phenyl plane in these complexes (59.2,62.7, and61.9o for complexes1,2, and4, respectively) is much less than90o in the solid state. Upon activation withAliBu3and Ph3CB(C6F5)4, complexes5–8exhibit reasonable catalytic activity forethylene homopolymerization and copolymerization with1-hexene at110oC, producinghigh molecular weight polyethylenes and poly(ethylene-co-1-hexene)s with relativelyhigh comonomer incorporation. Complex4was found to show higher catalytic activityfor ethylene/1-hexene copolymerization than complexes1–3under similar conditions,while complexes1–3produce poly(ethylene-co-1-hexene)s with higher comonomerincorporation. The above mentioned thermo stability of the new catalyst systems isfavorable for the application of this type in industry.Half-sandwich titanium(IV) complexes bearing an anilide ligand showed goodcatalytic performance in ethylene/α-olefin copolymerization. However, the poor thermo stability limited their application in industry. In order to improve the thermo stability ofthis type of catalysts, it is therefore of interest to develop a type of titanium complexwith more sterically bulky ligand. A series of new half-sandwich titanium(IV)complexes with anilinopyridito ligands, Cp*TiCl2[NArPy][Ar=2,6-Me2Ph (9);2,6-iPr2Ph (10);4-MePh (11)], have been synthesized from the reactions of Cp*TiCl3with the lithium salt of the corresponding ligand. The titanium complexes werecharacterized by1H and13C NMR, elemental analyses and single crystal X-raycrystallography. The X-ray crystallography analysis on complex10reveals that thesetitanium complexes possess a four-legged piano stool geometry with the two chelatingN atoms and the two chloride atoms as the legs. Upon activation with AliBu3andPh3CB(C6F5)4, complexes9–11exhibit high catalytic activity for ethylenehomopolymerization producing high molecular weight polyethylenes.