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-(Bu2C13H7)-Bu-t, n = 1; 3, Flu' = 9-SiMe3C13H8, n = 1; 4, Flu' = 2,7-Bu-t(2)-9-SiMe3C13H6, n = 1; 5, Flu' = 9-CH2CH2NMe2C13H6, n = 0; 6, Flu' = 2,7-Bu-t(2)-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 (AlBu3)-Bu-t, 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 All3u3 is present, affording syndiotactic polystyrenes with broad molecular weight distributions. The DFT calculations of the activation mechanism by using the representative catalysts suggest that Arl3u3 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.