Nature of the Entire Range of Rare Earth Metal-Based Cationic Catalysts for Highly Active and Syndioselective Styrene Polymerization

Because of the steric bulkiness and the η⁵/κ¹-constrained-geometry-configuration (CGC) geometry, the entire range of pyridyl-methylene-fluorenyl-stabilized rare earth metal bisalkyl complexes, (Flu-CH₂-Py)­Ln­(CH₂SiMe₃)₂(THF)_x (Flu = fluorenyl; Py = pyridyl; for 1, Ln = Sc and x = 0; for 2–11, Ln = Lu, Tm, Er, Ho, Y, Dy, Tb, Gd, Nd, or Pr and x = 1), and monoalkyl complex, (Flu-CH₂-Py)₂La­(CH₂SiMe₃) (THF) (12), has been successfully achieved for the first time via the sequential salt metathesis reactions. Activated by [Ph₃C]­[B­(C₆F₅)₄] and AlⁱBu₃, complexes 1–9 showed high activity and perfect syndioselectivity for styrene polymerization, while the large Nd- and Pr-attached precursors 10 and 11 exhibited slightly decreased syndioselectivity but rather low activity; the monoalkyl La precursor 12 was completely inert. The activity increased with the decrease in the rare earth metal size, in striking contrast to the literature that has shown that a large metal facilitates a high activity, which was also not a result of an enthalpic effect (ΔH^⧧) or an entropic effect (ΔS^⧧) according to Eyring plots. The types of organoborates and the aluminum alkyls, the electron donors, and the polarity of the reaction medium, which affected the coordination of styrene to the active species, aroused significantly different catalytic activity, indicating that styrene coordination played the key role in the polymerization process. On the basis of this, the density functional theory calculation of the active species in the model of [(Flu-CH₂-Py)­Ln-nC₁₇H₁₉]⁺ revealed whenever the orbitals of the pyridyl-methylene fluorenyl ligand overlapped with those of the rare earth metals, the LUMO energy of the active species was lowered and thus the catalytic activity was high. Therefore, the LUMO energy of the active species could be adopted as a potential criterion to estimate the activity of a catalytic system for styrene polymerization. This work reveals for the first time the power of the pyridyl-methylene fluorenyl ligand and the nature of the factors influencing the catalytic performance.