A Study Of Homogeneous Models For SiO₂-Supported Chromium-Based Polyethylene Catalysts And Their Polymerization Mechanisms

Phillips CrOx/SiO2 catalyst and UCC S-2 organo-silyl chromate catalyst are two of the most important industrial polyethylene catalysts, which nowadays are still producing more than 10 million tons of the world's high density polyethylene (HDPE). Despite the great research efforts, the academic progresses about these catalysts are lagging far behind their successful commercial applications. Most importantly, the nature of active sites and polymerization mechanism are still in debate. In this work, two homogeneous model catalysts were used to study on their catalytic performance, active sites and their oligo-/polymerization mechanisms mimicing the heterogeneous Cr-based polyethylene catalysts by experimental as well as theoretical studies.First of all, bis(triphenylsilyl)chromate [(Ph3SiO)2Cr(Ⅵ)O2] (1), as a homogeneous model for the Cr-based polyethylene catalysts, was investigated for catalystic performance upon activation with aluminum alkyl cocatalysts under low Al/Cr molar ratios (Al/Cr:0.5-30).1 demonstrates a relatively low activity for ethylene polymerization. Butyl radical and [Cr(η6-toluene)2]+ cation were produced during the activation, and [Cr(η6-toluene)2]+ cation was proved to be inactive for ethylene polymerization by in-situ ESR experiments. Additionally, the identified [Cr(η6-toluene)2]+ cation was elucidated for its molecular structure and electron configuration by theoretical calculation using density functional theory (DFT) with non-hybrid (BPW91 and BLYP) and hybrid (B3PW91 and B3LYP) functional. The doublet spin state was proved to be the ground spin state, and the stability of four eclipsed conformers are predicted to be in the order of 180°(trans-)>120°>60°>0°(cis-). Hybrid functional was proved to be more suitable for description of molecular geometry and frontier orbital of [Cr(η6-toluene)2]+ cation compared to non-hybrid functional.Then, model catalyst 1 was tested for catalystic performance upon activation with aluminum alkyl cocatalysts under high Al/Cr molar ratios (Al/Cr:100-1500). An unexpected transformation from ethylene polymerization (Al/Cr≤200) to ethylene nonselective oligomerization (Al/Cr≥500) was observed over 1/MAO catalyst system. However, 1/Al('Bu)3 only presents an activity of ethylene polymerization, and a further increasing Al/Cr molar ratio solely deactivated the catalyst resulting into lower polymerization activity.Next, a novel Cr(Ⅱ) complex [(Ph3SiO)Cr(Ⅱ)·(THF)]2(μ-OSiPh3)2 (2), as a homogeneous model for the prereduced Cr-based polyethylene catalyst, has been successfully synthesized and structurally characterized. The X-ray result indicates that 2 is dinuclear oxygen-bridged Cr(II) complex in a tetrahedrally distorted square-planar coordination geometry. Complex 2 was tested for catalystic performance with or without aluminum alkyl cocatalysts.2/MAO exhibits an interesting transformation from ethylene polymerization (Al/Cr≤100) to nonselective oligomerization (Al/Cr≥200) with an increase of Al/Cr molar ratio. However, 2/Al('Bu)3 presents only an activity of ethylene polymerization and further increasing Al/Cr molar ratio solely deactivated the catalyst resulting into lower polymerization activity.The active sites for ethylene polymerization and nonselective oligomerization, as well as poly-/oligomerization mechanisms in 1/MAO and 2/MAO catalyst systems were studied by theoretical calculation using DFT methods. Eight possible active site models (A-H) bearing Cr-Me bond, which should be produced by activation of model catalysts 1 and 2 with MAO, were investigated for ethylene insertion by Cossee-Arlman mechanism. The results suggest that models B ((Ph3SiO)CrMe), C (CrMe)+ and G ([(Ph3SiO)CrMe]+) were the most plausible active sites due to their low insertion energy barriers. Moreover, models BH, CH and GH (containing Cr-H bond), which were obtained byβ-H transfer to the chromium center from models B, C and G, respectively, were further studied for ethylene insertion andβ-H transfer. The results indicate that the model B (BH) may be the active site for ethylene polymerization, and model G (GH) may be the active site for ethylene nonselective oligomerization.