Dinuclear Half-Titanocene Complex Bearing an Anthracene-Bridged Bifunctional Alkoxide Ligand: Unprecedented Cooperativity toward Copolymerization of Ethylene with 1-Octene or Norbornene
Ze Liu, Yukai Gong, Jinbo Zhang, Shaofeng Liu, Zhibo Li
Abstract
Dinuclear half-titanocene complex Ti 2 bearing an anthracene-bridged bifunctional alkoxide ligand ( LH 2 ) has been synthesized and fully characterized by NMR spectroscopy and single-crystal X-ray diffraction, displaying a separation of 7.433 Å between two Ti centers. The mononuclear complex Me 5 CpTiMe 2 -(OC 6 H 3 –2,6- i Pr 2 ) ( Ti 1 ) was also prepared for control experiments. In the presence of [Ph 3 C][B(C 6 F 5 ) 4 ] as a cocatalyst, Ti 2 shows high activity and thermal stability toward ethylene homopolymerization, similar to those of Ti 1 . For ethylene/1-octene copolymerization, Ti 2 exhibits an extremely high activity of 112800 kg(polymer)·mol –1 (Ti)·h –1 at 120 °C that is almost twice that for ethylene homopolymerization, revealing a dramatically positive effect for copolymerization. For ethylene/norbornene (NBE) copolymerization, it is remarkable that Ti 2 shows an activity of 9180 kg(polymer)·mol –1 (Ti)·h –1 with [NBE] = 2 M under 4 atm of C 2 H 4 and produces the poly(ethylene- co -NBE) copolymer with an M w of 168 × 10 4 g·mol –1 and an NBE incorporation of 30.1 mol %, which are 15.3×, 1.2×, and 2.6× those by Ti 1 under otherwise identical conditions. In order to study the possible dinuclear cooperativity, the activations of dinuclear Ti 2 by [Ph 3 C][B(C 6 F 5 ) 4 ] and B(C 6 F 5 ) 3 have been carried out and studied by density functional theory (DFT) and afford an unusual μ-Me bridging dinuclear monocationic species for both cases, revealing that two Ti centers in Ti 2 that can possibly interact together with a (co)monomer or propagation chain could lead to synergetic catalysis.