Ph2Si-bridged constrained-geometry complexes of hafnium and titanium for copolymerization of ethylene and 1-octene: an experimental and computational comparison

Abstract

A series of new constrained-geometry hafnium and titanium complexes, including {Me₂Si(2,7-tBu₂Flu)NtBu}HfCl₂ (1-HfCl₂), {Ph₂Si(2,7-tBu₂Flu)NtBu}HfCl₂ (2-HfCl₂) and {Ph₂Si(2,7-tBu₂Flu)NtBu}TiCl₂ (2-TiCl₂), was synthesized and fully characterized using solution NMR spectroscopy and X-ray crystallography. These complexes were subsequently evaluated in the copolymerization of ethylene with 1-octene ([C8]₀ = 0.48–1.88 M), revealing striking differences in catalytic performance and properties of the resulting polyolefin elastomers/plastomers (POE/POP). The hafnium complexes 1-HfCl₂ and 2-HfCl₂ exhibited low catalytic activity, yielding polymers with broad molecular weight distributions and only modest incorporation of 1-octene. In contrast, the titanium complex 2-TiCl₂ demonstrated significantly higher catalytic activity and achieved substantial 1-octene incorporation at an ethylene/1-octene molar ratio of ca. 6 : 1, with values reaching up to 36.6 mol%. 2-TiCl₂ exhibited approximately half the productivity of benchmark CGC {Me₄CpSiMe₂NtBu}TiCl₂ (3-TiCl₂), yet yielding POE with a much higher molecular weight and a density of 0.8547 g cm⁻³. DFT calculations were also performed for the cationic catalytic species derived from the pre-catalysts 2-HfCl₂ and 2-TiCl₂ to rationalize the relative performance of the titled systems in incorporating ethylene and 1-octene and undergoing termination and transfer reactions. A +5.6 kcal mol⁻¹ energy difference (in terms of ΔΔH^‡) was computed between the transition states for the third ethylene insertion into the M–C(polymeryl) bond for the Hf-based system compared to the Ti-based system; other phenomena possibly plaguing efficiency of the Hf-system are discussed.