Titanium and Zirconium Complexes for Polymerization of Propylene and Cyclic Esters

The new group IV complexes ((CH3)3CCOCHCONEt2)2Ti(OMe)2 (3), ((CH3)3CCOCHCONMe2)2Ti(OMe)2 (4), (CH3COCHCONMe2)2Ti(OC(CH3)3)2 (5), and ((CH3)3CCOCHCONEt2)2ZrCl2 (6) were synthesized and characterized. The titanium dialkoxide complexes 3−5 were produced during the attempts to synthesize the corresponding titanium diamido complexes via an intramolecular metathesis. A mechanism for the intramolecular metathesis is presented. Complexes 3 and 4 exhibit dynamic behavior in solution as a function of temperature. This dynamic behavior is due to a disconnection and recoordination of the β-ketoamidate chelating ligands through the weaker Ti−O bond, resulting in the formation of different octahedral stereoisomers in solution. Complexes 3−6, activated with MAO, were found to be active in the polymerization of propylene, producing elastomeric polypropylene. The elastomeric properties of the polymers obtained using complex 3 as catalyst are due to a dynamic interconversion between two C2-symmetric enantiomeric structures via an open C2v-symmetric intermediate complex. Additionally, complexes 3−6 were found to be active in the polymerization of ε-caprolactone and rac-lactide. The activity of the zirconium dichloride complex 6 is higher than the activities of any of the titanium dialkoxide complexes 3−5 both in ε-caprolactone and in rac-lactide polymerizations.