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

Abstract

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 °C, to a moderate polyethylene formation (70–80 wt%, 60–70 kg_polyethylene g_Ti⁻¹ h⁻¹) at a higher reaction temperature (T > 60 °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 ¹³C NMR analyses, polyethylenes were found to exhibit high molar masses (>10⁵ 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 °C but a parallel process may occur at a higher temperature. The increase of dispersity to 4.6 at 80 °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.