Fluorenyl based syndiotactic specific metallocene catalysts structural features, origin of syndiospecificity



The stereochemistry of propylene insertion/propagation reactions with a variety of Cs symmetric fluorenyl- containing single site catalysts is discussed. Our recent results indicate that independent of the chemical composition of the ancillary ligand fragments, or nature of the transition metal, active sites with local Cs symmetry and enantiotopic coordination positions behave syndioselectively in the general context of chain migratory insertion mechanism. Perfect bilateral symmetry neither exists nor is required in these processes. In this context the mechanism of syndiospecific polymerization is revisited by taking into account the structural characteristics and catalytic behavior of the original metallocene based (η5-C5H4-CMe25-C13H8) MCl2/ MAO; M = Zr (1), Hf (2) catalyst systems and new syndiotactic specific systems including (η5-C5H4-CPh2-η5-3,6-di-tBut-C13H6)ZrCl2 (3), η15-(μMe2Si)(3,6-di-tBut-Flu)(t-ButN)MCl2/ MAO; M =Ti (4), Zr (5) and η15-(μMe2Si)(2,7-di-tBut-Flu)(t-ButN)MCl2/ MAO; M = Ti (6), Zr (7).