Preparation of dinuclear, constrained geometry zirconium complexes with polymethylene bridges and an investigation of their polymerization behavior

Abstract

We have prepared the polymethylene-bridged, dinuclear, half-sandwich constrained geometry catalysts (CGC) [Zr(η⁵:η¹-C₉H₅SiMe₂NCMe₂NCMe₃)]₂ [(CH₂)_n] [n=6 (9),n=12 (10)] by treating 2 equivalents of ZrCl4 with the corresponding tetralithium salts of the ligands in toluene.¹H and¹³C NMR spectra of the synthesized complexes provide firm evidence for the anticipated dinuclear structure. In¹H NMR spectra, two singlets representing the methyl group protons bonded at the Si atom of the CGC are present at 0.88 and 0.64 ppm, which are considerably downfield positions relative to the shifts of 0.02 and 0.05 ppm of the corresponding ligands. To investigate the catalytic behavior of the prepared dinuclear catalysts, we conducted copolymerizations of ethylene and styrene in the presence of MMAO. The prime observation is that the two dinuclear CGCs9 and10 are not efficient for copolymerization, which definitely distinguishes them from the corresponding titanium-based dinuclear CGC. These species are active catalysts, however, for ethylene homopolymerization; the activity of catalyst10, which contains a 12-methylene bridge, is larger than that of9 (6-methylene bridge), which indicates that the presence of the longer bridge between the two active sites contributes more effectively to facilitate the polymerization activity of the dinuclear CGC. The activities increase as the polymerization temperature increases from 40 to 70 °C. On the other hand, the molecular weights of the polyethylenes are reduced when the polymerization temperature is increased. We observe that dinuclear metallocenes having different-length bridges give different polymerization results, which reconfirms the significant role that the nature of the bridging ligand has in controlling the polymerization properties of dinuclear catalysts.