Structural features of bridged cyclopentadienyl-fluorenyl based metallocene catalyst: origin of syndiospecificity

The mechanism of syndiospecific polymerization with (η5-C5H4-CMe2-η5-C13H8) MCl2; M=Zr, Hf/MAO catalyst systems is discussed by taking into account the structural characteristics of the metallocene molecules and the chiral particularities of their cationic species within the framework of a chain migratory insertion mechanism. A generally accepted transition state structure that respects the relative importance of different steric interactions of the active participants in the polymerization process, ligand, growing polymer chain and the coordinating monomer is proposed. The model is reexamined on the basis of an isotactic specific (η5-C5H3C4H9-CMe2-η5-C13H8)ZrCl2/MAO and a recently discovered syndiotactic specific catalyst system, η1,η5-tert-butyl(3,6- bis-tert-butylfluorenyl-dimethylsilyl)amidoMCl2/MAO; M=Zr, Ti and its validity is confirmed.