In metallocene-mediated propylene polymerization, β-methyl elimination (β-Me elimination) is considered as the key chain-release step for obtaining allyl-terminated products, which are highly preferred as macro(co)monomers or building blocks for preparing novel polymers. However, for most metallocene catalysts the transfer of a β-methyl is instinctively less favored due to its steric and electronic disadvantages. Up to date, very few cases have been found to be efficient for triggering selective β-methyl elimination. In this work, a series of novel ansa-metallocene complexes, ansa-C₂H₄-{2-Me-3-Bn- 5,6-[1,3-(CH₂)₃]Ind}(Flu)ZrCl₂ (C1), ansa-C₂H₄-{2-Me-3-Bn-5,6-[1,3-(CH₂)₃]Ind}(2,7-^tBu₂-Flu)ZrCl₂ (C2), ansa-C₂H₄-{2-Me-3-Bn-5,6-[1,3-(CH₂)₃]Ind}(3,6-^tBu₂-Flu)ZrCl₂ (C3) and ansa-C₂H₄-{2-Me-3-Bn-5,6-[1,3-(CH₂)₃]Ind}(Flu)HfCl₂ (C4), were synthesized via the reaction of the dilithium salts of the corresponding proligand with 1 equiv. of ZrCl₄ or HfCl₄ in Et₂O. All complexes were characterized by ¹H NMR, ¹³C NMR and elemental analysis. The molecular structures of complexes C1, C2, and C3 were further determined via X-ray diffraction method. In the solid state, these complexes adopted an indenyl-backward orientation with rotation angles (RA: the orientation of the indenyl ring with respect to the fluorenyl ring) ranging from －11.30° to －17.07°. Upon activation with modified methylaluminoxane (MMAO) or AlⁱBu₃/ [Ph₃C][B(C₆F₅)₄] (TIBA/TrB), all these complexes exhibited moderate to high activities for propylene oligomerization at 40～100 ℃, affording propylene oligomers with both allyl and vinylidene chain-ends, which arised from β-Me elimination and β-H eliminations respectively. The methyl group at the 2-position of the indenyl ring turned out to have negative effects on both catalytic activity and β-Me elimination selectivity. Zirconocene complex C1 polymerized propylene to give oligomers with 40%～52% allyl chain-ends. However, further modification of the fluorenyl moiety allowed a great improvement in β-Me elimination selectivity. At 40～100 ℃, zirconocene complexes C2 and C3 bearing a 2,7- or 3,6-di-tert-butyl- substituted fluorenyl moiety showed significantly higher β-Me elimination selectivities (C2, 81%～86%; C3, 68%～77%), affording propylene oligomers (M_n 400～4500 g·mol^－1) with allyl-dominant chain-ends. Moreover the substitution pattern of the fluorenyl moiety also substantially influenced the catalytic activities. The incorporation of an electron-donating 2,7-di-tert-butyl groups on the fluorenyl moiety led to notably increased catalytic activities of complex C2 at higher temper- atures above 60 ℃, while complex C3 bearing a 3,6-di-tert-butyl-substituted fluorenyl moiety showed lowest activities among the zirconocene series due to its overcrowded coordination sites. Compared with its zirconocene analogue, the hafnocene complex C4 activated with TIBA/TrB proved to be even more selective toward β-Me elimination, and meanwhile gave products with much lower molecular weights. At 100 ℃, the hafnocene system mainly oligomerized propylene to dimers and trimers. Studies on the dependence of the product molecular weight and the chain-release selectivity on monomer concentration suggested that both β-Me and β-H elimination involved in these systems mainly operate in a bimolecular pathway.

Key words: metallocene catalyst, propylene oligomerization, allyl chain-end, β-methyl elimination