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 ∆∆H≠) 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.