|Ph.D Student||Gueta Neyroud Tal|
|Subject||Titanium Complexes for the Polymerization of Alpha-Olefins|
and Cyclic Esters: Homogeneous and
|Department||Department of Chemistry||Supervisor||Professor Moris Eisen|
|Full Thesis text|
The production of polyolefins and polyesters has increased continuously over the last decades due to the outstanding properties of these compounds. An extraordinary amount of research has been directed toward the development of homogeneous single-site polymerization catalysts but in order to apply these catalysts for the production of polyolefins in the common commercial processes and prevent reactor fouling, heterogeneization of the catalyst on a support material is necessary.
We present the synthesis, characterization and catalytic activity in the polymerization of propylene of a bis(dimethyl malonate) titanium bis(diethylamine) complex (37). The complex exhibits in solution a dynamical isomerization following an internal Bailar twist. The complex was activated by lewis-acidic methylalumoxane (MAO). At least two activated complexes were found in solution involving the binding of the dimethyl malonate ligand to the metal center. The slow equilibrium between these complexes was found responsible for the formation of, at least, two different polymers. The active species in the polymerization were studied by NMR and ESR spectroscopies. The resulting polypropylene showed elastomeric properties with low tacticites.
We present as well the synthesis, characterization, and catalytic activity of a [(HOEt)Ti(m-OEt)OEt(Cl)2]2 (38) dimeric complex. Complex 38 was found to undergo, in solution, three fluxional processes involving the different ligands. The complex, when activated by MAO, undergoes an abstraction of the ligands through an heterobimetallic intermediate with ethoxide bridges between the aluminum and the titanium atoms. We propose that this intermediate is the active species in the polymerization of a-olefins. The complex displays a single site behavior in contradiction to other simple titanium alkoxide complexes. The resulting polymers were found to show elastomeric properties with intermediate tacticities.
Complex 38 was studied as well in the ring opening polymerization of e-caprolactone and L-lactide. Interestingly, the coordinative neutral ethanol molecule was found to remain attached to the metal atom at a 38-monomer ratio of 1:5 respectively. The poly-caprolactone obtained displayed large polydispersities, thus implying the occurrence of major transesterification side reaction. Whereas, the polylactide showed a narrow polydispersity, most probably due to a difficult approach of the large polylactide chain to the metal center encumbered by the coordinating ethanol molecule.
Complexes 37 and 38 were heterogeneized by a reaction with MAO-modified HMS silica. The features of the polymers achieved in the homogeneous systems could be preserved upon heterogenization and the activity of the system was increased.