The stereoselective and sequence controlled coordination copolymerizations of butadiene (BD) and styrene (St) with diblock, tapered, gradient, and random sequence distributions were achieved for the first time through varying the central metals of the rare-earth metal bis(alkyl) catalyst precursors (Flu-CH2-Py)Ln(CH2SiMe3)2(THF)x (Flu = fluorenyl, Py = pyridyl, x = 1: Ln = Nd(1), Y(2), Tm(3); x = 0, Ln = Sc(4)). The thermal behavior, morphology, and the mechanical property of these copolymers were analyzed, and their relationships were established for the first time. The mechanism of central metal size tuning the sequence distribution was discussed based on DFT calculations.

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.Keywords: DFT calculation; ligand design; rare-earth metal complexes; styrene polymerization; syndiotactic

A series of aluminum complexes LaAl2Me4 (1), Lb2Al4Me4 (2), and LcAl2Me4 (3) have been prepared from the reaction of AlMe3 with Salan- and Salen-type ligands (LaH2 = [2-OH-3,5-tBu2-C6H2CH2N(CH3)]2-(m-phenylene); LbH4 = [2-OH-3,5-tBu2-C6H2CH2NH]2-(m-phenylene); LcH2 = [2-OH-3,5-tBu2-C6H2CH═N]2-(m-phenylene)), respectively. All these complexes were characterized by NMR spectroscopy, X-ray diffraction, and elemental analyses, with complexes 1 and 3 adopting binuclear structures, while complex 2 being tetranuclear. In the presence of alcohol, the binuclear complexes 1 and 3 catalyzed controlled ring-opening homopolymerizations of both ε-CL and l-LA. In the copolymerization experiments, complexes 1 and 2 produced tapered copolymers of ε-CL and l-LA, while complex 3 was able to provide ε-CL-co-l-LA with tendentially random structure indicated by the average lengths of the caproyl and lactidyl sequences (LCL = 1.4; LLA = 2.6). Particularly, addition of excess alcohol into the catalytic system of complex 3 established the first “immortal” copolymerization of ε-CL/l-LA, which accelerated the polymerization rates of both monomers and, thus, afforded random copolymers with predictable molecular weights and narrow molecular weight distributions.