High Intermediate Polymerization of Styrene with Rare Earth Catalysts

Abstract Express

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

ACS Catalina., 2016, 6 (1), pp 176 - 185

DOI: 10.1021/acscatal.5b02334

Publication Date (Web): November 23, 2015

Because of the steric bulkiness and the η 5/kappa 1-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, (Fluo-CH2-Py) 2 La (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 11exhibited 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 (Delta H) or an entropic effect (Delta S) 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 activities, 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 [ (Fluo-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.

Mesotactic polystyrene is a new type of thermoplastic engineering plastic. It retains the advantages of low density and easy molding of random polystyrene. Its special structure endows it with heat resistance (melting point up to 270 ° C), chemical corrosion resistance and good insulation. It has broad application prospects. The catalysts for the mesotactic polymerization of styrene are mainly Ti and Sc series. Exploring the use of other more abundant and inexpensive non-Sc rare earth metal catalysts to achieve the mesotactic selective polymerization of styrene is a major challenge for styrene polymerization catalysts. At the same time, it is important to study the intrinsic relationship between catalysts and the mesotactic selective polymerization of styrene. It is of great significance to design and develop new catalytic systems.

Cui Dongmei's research group at the Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, prepared for the first time a rare earth complex (Fluoro-CH2-Py) Ln (CH2SiMe3) 2 (THF) x (Flu = fluorenyl; Py = pyridyl; 1: Ln = Sc, x = 0; 2 - 11: Lu, Tm, Er, Ho, Y, Dy, Tb, Gd, Nd, Pr, x = 1).

Under the excitation of [Ph3C] [B (C6F5) 4] and AliBu3, complexes 1-9 were able to achieve high activity and high mesotactic selectivity for styrene (rrrr> 99%), while Pr (10) and Nd (11) with larger ionic radii had slightly worse mesotactic selectivity and lower polymerization activity. Through further experiments, they found that with the decrease of the radius of the central metal ion of the catalyst, the polymerization activity of the catalyst for styrene increased, which was contrary to the relationship between the radius of the central metal ion and the polymerization activity reported in the literature. In order to reveal the laws more deeply, the research group studied the effects of boron salts, alkyl aluminum, electron donors and solvents on the polymerization reaction. Through cooperation with the research group of Professor Luo Yi of Dalian University of Technology, combined with DFT calculations, it was concluded that styrene coordination is the rate-determining step of rare earth catalyst catalyzed styrene polymerization. Based on this understanding, they used [ (Flu- CH2-Py) Ln − nC17H19] + as the model active center, and found that the ligand interacted with the central metal and affected the LUMO of the active center through DFT calculations. More importantly, the lower the LUMO energy of the active center, the higher the polymerization activity of styrene. This shows that the LUMO energy of the active center is the intrinsic reason that affects the coordination between styrene and the central metal and then affects the polymerization activity of styrene. The research group achieved for the first time the intermodal selective polymerization of styrene by rare earth catalysts and revealed the fundamental factors affecting the polymerization activity of rare earth catalysts for polystyrene. The work was published in full text in the first issue of ACSCatalysis in 2016.