Binuclear titanium complexes coordinated by rigid p-phenylene linked bis-beta-carbonylenamine: Synthesis, structure, ethylene polymerization and copolymerization with 1,5-hexadiene

Binuclear complexes for olefin polymerization have attracted great attention due to their unique catalytic properties compared with their mononuclear counterparts. Here a series of p-phenylene-bridgedbis-beta-carbonylenamine ligands and their binuclear Ti complexes Ti2L1-Ti2L3 were prepared and characterized by H-1 NMR, C-13 NMR, Fourier transform infrared spectroscopy, and elemental analysis. The binuclear complex Ti2L3 bearing an octylthio sidearm was further investigated by single-crystalX-ray diffraction, which revealed that the ligand was of beta-imino enol form, with one titanium atom ligated with six other atoms, forming a deformed octahedral configuration. Furthermore, the ligand in Ti2L3 adopted a cis configuration, which was different from the trans configuration of its m-phenylene-bridged derivatives. These binuclear complexes (Ti2L1-Ti2L3) could catalyze ethylene polymerization and copolymerization with 1,5-hexadiene(1,5-HD) efficiently under modified methylaluminoxane activation. Compared with the mononuclear complex TiL5, the binuclear catalysts were thermally more stable and showed higher activity for ethylene polymerization at higher temperatures. The activity of these titanium complexes for the copolymerization of ethylene with 1,5-HD were over 10(6) g/mol Ti.h.atm, almost twice as high as for homopolymerization. Compared with the mononuclear analogue TiL5 and the m-substituted binuclear derivative Ti2L4, binuclear catalyst Ti2L2 showed higher activity and insertion rate of the comonomer. The activity of Ti2L2 was two to three times higher than that of TiL5 and Ti2L4, indicating that p-substituted binuclear catalysts generate clear bimetallic synergistic effect for the copolymerization of ethylene and 1,5-HD. Meanwhile, 1,5-HD takes 1,3-cyclopentyl form in the polymer by 1,3-insertion. The copolymer prepared by binuclear catalysts had higher molecular weight and wider molecular weight distribution than that prepared by the mononuclear catalyst.