Aluminum effects in the syndiospecific copolymerization of styrene with ethylene by cationic fluorenyl scandium alkyl catalysts

A series of half-sandwich fluorenyl (Flu′) scandium dialkyl complexes Flu′Sc(CH₂SiMe₃)₂(THF)_n (1, Flu′ = C₁₃H₉, n = 1; 2, Flu′ = 2,7- ^tBu₂C₁₃H₇, n = 1; 3, Flu′ = 9-SiMe₃C₁₃H₈, n = 1; 4, Flu′ = 2,7- ^tBu₂-9-SiMe₃C₁₃H₆, n = 1; 5, Flu′ = 9-CH₂CH₂NMe₂C ₁₃H₈, n = 0; 6, Flu′ = 2,7-^tBu ₂-9-CH₂CH₂NMe₂C₁₃H ₆, n = 0) have been synthesized and structurally characterized. In comparison with the well-known cyclopentadienyl-ligated scandium catalyst system [(C₅Me₄SiMe₃)Sc(CH₂SiMe ₃)₂(THF)]/[Ph₃C][B(C₆F ₅)₄], the analogous combinations of the fluorenyl-ligated, THF-containing complexes 1-4 with [Ph₃C][B(C₆F ₅)₄] show relatively low activities, albeit with similar syndioselectivities for styrene polymerization and styrene-ethylene copolymerization. However, on treatment with 15 equiv of AlⁱBu ₃, the 1-4/[Ph₃C][B(C₆F₅) ₄] combinations show a dramatic increase in catalytic activity without changes in the stereoselectivity. In contrast, the combinations of complexes 5 and 6, which have an amino group attached to the fluorenyl ring and intramolecularly bonded to the metal center, exhibit very low activity, no matter whether or not AlⁱBu₃ is present, affording syndiotactic polystyrenes with broad molecular weight distributions. The DFT calculations of the activation mechanism by using the representative catalysts suggest that AlⁱBu₃ can capture the THF molecule from the catalyst precursors 1-4 at first,and then the new, THF-free cationic half-sandwich scandium active species [Flu′Sc(CH₂SiMe ₃)][B(C₆F₅)₄] with less steric hindrance around the metal center is generated in the presence of an activator such as [Ph₃C][B(C₆F₅)₄]. The DFT calculations on the syndioselectivity of styrene (co)polymerization catalyzed by [Flu′Sc(CH₂SiMe₃)][B(C₆F ₅)₄] have also been carried out, thus shedding new light on the mechanistic aspects of the (co)polymerization processes.