Edith Valder

An enormous progress was done, long since the Nobel Prize laureates Karl Ziegler and Giulio Natta first polymerized ethylene and propylene by heterogeneous titanium chloride catalyst. Since then, a variety of metallocene based homogeneous catalysts were developed. Beyond the metallocenes, new complexes were design containing a miscellany of ligands based on nitrogen, oxygen, phosphorous and sulfur atoms.

We have chosen to focus on a chiral benzamidinate ligand in a titanium complex with MAO (methyl alomoxane) as cocatalyst.

Our goal was to study the influence, if any, of a chiral ligand. The existence of a chiral ligand enabled us to receive a single diastereomer complex and not a racemic mixture at the metal center. Our assumption was that the existence of a chiral center will direct the approaching monomer to the active site in a specific way, which is energetically preferred from other naturally existing approaches for a prochiral monomer.

Two complexes were synthesized: [PhC(NTMS)(N-Myrtanyl)]TiCl₃·THF and [PhC(NTMS)(N-Myrtanyl)]₂TiCl₂, and their catalytic activity was studied in the polymerization of propylene and styrene. The complexes showed a good catalytic activity. For propylene, the polymerization gives elastomeric polypropylene with significant stereoselectivity which was attributed to the existence of both, an isospecific site and an aspecific site.

In the polymerization of styrene, only complex 1 showed activity to produce highly syndiotactic polystyrene. In these experiments, at least two fraction of syndiotactic polystyrene were received with different molecular weights, a fact resulting from the existence of a chiral ligand which causes to a unequivalency of the polymerization sites.