Highly Active Immobilized Catalyst for Ethylene Polymerization: Neutral Single Site Y(III) Complex Bearing Bulky...

Exploring the surface organometallic chemistry on silica of highly electrophilic yttrium complexes is a relatively uncommon endeavor, particularly when focusing on tris-alkyl complexes characterized by Y-C σ-alkyl bonds. A drawback with this class of complexes once grafted on silica, is the frequent occurrence of alkyl transfer by ring opening of siloxane groups, resulting in a mixture of species. Herein, we employed a more stable homoleptic yttrium allyl complex bearing bulky η³-1,3-bis(trimethylsilyl)allyl ligand to limit this transfer reaction. This strategy has been validated by comparing the reactivity between [Y{ η³-1,3-C₃H₃(SiMe₃)₂}₃] and [Y(o-CH₂PhNMe₂)₃] with SiO_2-700, where the undesired alkyl transfer reaction occurred for [Y(o-CH₂PhNMe₂)₃] leading to a bipodal [(≡SiO)₂Y(o-CH₂PhNMe₂)] as major surface species, 2, while [Y{ η ³-1,3-C₃H₃(SiMe₃)₂}₃] resulted selectively in a monopodal species, [(≡SiO)Y{η³-1,3-C₃H₃(SiMe₃)₂}₂], 1. The materials obtained were characterized by DRIFT, solid state NMR, mass balance analysis and EXAFS. Catalyst 1 showed high activity compared to 2 in ethylene polymerization. The catalytic performance of this neutral catalyst 1 was extended to pre-industrial scale in the presence of hydrogen and 1-hexene. An unprecedented activity, up to 7400 g_PE g_cat ^-1 h^-1 was obtained even with very low concentration of scavenger AliBu₃ (TIBA/Y=1.2). The obtained HDPE exhibited desired spherical particle morphology with broad molar mass distribution.