Influence of ligand substituents of unbridged metallocene complexes on stability of their active centers in ethylene polymerization

Abstract

Four unbridged metallocene catalyst precursors, bis(2,4,7-trimethylindenyl)zirconium dichloride (Cat.1), bis(2,4,6-trimethylindenyl)zirconium dichloride (Cat.2), bis(2,4,5,6-tetramethylindenyl)zirconium dichloride (Cat.3), and bis(2-methyl-4,6-diisopropylindenyl)zirconium dichloride (Cat.4), were synthesized. Cat.4 was found thermodynamically unstable. The correlations between alkyl substituents of the indenyl ligands and olefin polymerization with the metallocene complexes Cat.1, Cat.2 and Cat.3 were investigated via kinetic study on ethylene polymerization. Change of number of active centers ([C⁎]/[Zr]) was determined by quench-labeling the propagation chains with 2-thiophenecarbonyl chloride. According to the kinetic behaviors, kinetic stability of the metallocene complexes was found to follow the order of Cat.1 > Cat.2 > Cat.3. The relationship between ligand substitution mode and kinetic stability of the metallocene complex were discussed.