Titanium-based phenoxy-imine catalyst for selective ethylene trimerization: effect of temperature on the activity, selectivity and properties of polymeric side products

The reactivity of a phenoxy-imine-ether system (FI)TiCl3/MAO was studied toward selective ethylene trimerization. This system was shown to either trimerize or polymerize ethylene depending on the reaction temperature. Its selectivity switches from a significant production of the trimerization product, 1-hexene (85 wt%, 520-450 kg(1-hexene) g(Ti)(-1) h(-1)) between 30 and 40 degrees C, to a moderate polyethylene formation (70-80 wt%, 60-70 k(gpolyethylene) g(Ti)(-1) h(-1)) at a higher reaction temperature (T > 60 degrees C). Polymerization was investigated based on an original "polymer-to-catalyst" strategy aiming at identifying the active species responsible for this side reaction. Using DSC, SEC and high temperature C-13 NMR analyses, polyethylenes were found to exhibit high molar masses (>10(5) g mol(-1)) and a low 1-hexene content (<1 mol%) at any temperature. Kinetic studies support that trimerization and polymerization species are generated from the catalyst precursor at 40 degrees C but a parallel process may occur at a higher temperature. The increase of dispersity to 4.6 at 80 degrees C suggests a change from single to multi-site catalysis. The poor comonomer incorporation ability of the active species is reminiscent of a molecular Ziegler-Natta or a bulky post-metallocene catalyst.