Ethylene-bridged Indenyl-fluorenyl Metallocene Complexes for Efficient Preparation of Allyl-terminated Propylene Oligomers and Polymers via Selective β-Methyl Transfer

Four C₁-symmetric ansa-metallocene complexes, C₂H₄(Ind)(2,7-^tBu₂-Flu)ZrCl₂ (4), C₂H₄(3-Bn-Ind)(2,7-^tBu₂-Flu)ZrCl₂ (5), C₂H₄(3-Bn-Ind)(3,6-^tBu₂-Flu)ZrCl₂ (6), and C₂H₄(3-Bn-Ind)(2,7-^tBu₂-Flu)HfCl₂ (7), were synthesized and characterized. The structures of complexes 4, 5, and 7 were further determined via X-ray diffraction studies. Upon activation with modified methylaluminoxane (MMAO) or AlⁱBu₃/[Ph₃C][B(C₆F₅)₄] (TIBA/TrB), most of these complexes showed high efficiency in catalyzing propylene oligomerization/polymerization to afford products dominantly with allyl terminals via selective β-methyl transfer (β-Me transfer). The introduction of 3-benzyl group on the indenyl ring of the complexes was found to be crucial in enabling highly selective β-Me transfer during the polymerization process, leading to selectivities up to 89% obtained by zirconocene complexes 5 and 6, and up to 91% obtained by hafnocene complex 7. Detailed chain-end analysis by ¹H-NMR, ¹³C-NMR, and MALDI-TOF mass spectroscopy revealed that the allyl chain-ends of the polymeric products resulted from a selective β-Me transfer process after two successively primary insertions of the monomer. Further studies concerning the dependence of chain release selectivity as well as the molecular weight of products on monomer concentration suggested that both β-Me transfer (major) and β-hydrogen transfer (β-H transfer) (minor) mediated by 5/MMAO and 6/MMAO systems may mainly operate in a bimolecular pathway.