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-CH2CH2NMe2C13H8, n = 0; 6, Flu′ = 2,7-tBu2-9-CH2CH2NMe2C13H6, n = 0) have been synthesized
and structurally characterized. In comparison with the well-known
cyclopentadienyl-ligated scandium catalyst system [(C5Me4SiMe3)­Sc­(CH2SiMe3)2(THF)]/[Ph3C]­[B­(C6F5)4], the analogous combinations of the fluorenyl-ligated, THF-containing
complexes 1–4 with [Ph3C]­[B­(C6F5)4] show relatively low
activities, albeit with similar syndioselectivities for styrene polymerization
and styrene–ethylene copolymerization. However, on treatment
with 15 equiv of AliBu3, 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(CH2SiMe3)]­[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(C6F5)4] have
also been carried out, thus shedding new light on the mechanistic
aspects of the (co)­polymerization processes.