|Ph.D Student||Shaviv Ella|
|Subject||Early Transition Metal Complexes Containing Oxygen and|
Nitrogen Based Ligands in the Polymerization of
|Department||Department of Chemistry||Supervisor||Professor Moris Eisen|
|Full Thesis text|
Homogeneous metallocene and metallocene-like catalysts enable precise control of the primary properties of the polymer such as molecular weight, co-monomer sequence and stereoregularity. The polymerization of a-olefins involves the reaction of the metal complex with a Lewis-acidic co-catalyst, usually methylalumoxane (MAO), which generates an active cationic species. Homogeneous catalytic systems enabled the formation of unique polymers, such as elastomeric or hemiisotactic polypropylene, not available with calasical Ziegler-Natta catalysts.
In order to utilize homogeneous catalysts in common commercial processes, heterogenization of the catalyst on a solid support is required. Silica-based supports, with a wide range of surface areas, porosities and pore volumes, are the most commonly used for immobilization of homogeneous catalysts. Mesoporous molecular sieves, with high surface areas and pore diameter of 20 to 100 Å, appear promising as supports. Among these, the short range ordered hexagonal molecular sieves (HMS) is potentially favorable for catalytic activity.
In this work we have studied the immobilization of homogeneous catalytic systems containing oxygen and nitrogen based ligands on HMS silica.
The oxygen-based dibenzoylmethane ligand, bearing phenyl substituents on the diketonate backbone, and the corresponding nitrogen-based b-diketiminate ligand, with trimethylsilyl groups on the nitrogen, were reacted with group IV transition metals. The titanium complexes were characterized and studied in the homogeneous and heterogeneous polymerization of a-olefins. The b-diketiminate ligand was found to undergo a metal induces isomerization to the amidinate isomer, producing a titanium bis-amidinate complex. The titanium dibenzoylmethane complex exhibited dynamic behavior in solution, caused by equilibrium between h1 and h2 coordination modes.
The reaction of both complexes with MAO was studied in detail and the active species characterized by various spectroscopic methods. The activation was found to proceed via a paramagnetic Ti(III) species. Plausible activation mechanisms are presented. Both complexes were catalyticaly active in the polymerization of propylene, producing elastomeric polymers under different reaction conditions.
HMS mesoporous molecular sieves were synthesized and characterized. The complexes were immobilized on MAO-modified HMS molecular sieves, and studied in the heterogeneous polymerization of propylene. The immobilized catalysts exhibited higher activity than the parent homogeneous system, and were less sensitive to solvent influence.
The catalytic activity of the homogeneous and heterogeneous catalysts in the isomerization of allyl benzene was studied as well.
In addition, new b-diketiminate ligands with aromatic groups on the nitrogen, and their Litium complexes were synthesized and characterized.