Toward the Prediction of Activity in the Ethylene Polymerisation of ansa‐Bis(indenyl) Zirconocenes: Effect of the Stereochemistry and Hydrogenation of the Indenyl Moiety

Abstract

A combined experimental and quantum chemical study has been performed on rac‐ and meso‐[Zr{1‐Me2Si(3‐η5‐C9H5Et)2}Cl2] (rac‐ and meso‐1) and their hydrogenated forms (rac‐ and meso‐2) to understand ligand effects and guide ligand design for more active ansa‐bis(indenyl) zirconocenes for the polymerisation of ethylene. The rac‐ansa‐zirconocene rac‐[Zr(1‐Me2Si{3‐Et‐(η5‐C9H9)}2)Cl2] (rac‐2) has been prepared and fully characterised by NMR spectroscopy and elemental analysis. The molecular structure of rac‐2 has also been determined by single‐crystal XRD. The behaviour of the catalysts was analysed in the polymerisation of ethylene and higher activities were obtained for rac‐1 and its hydrogenated form rac‐2. The influence of the stereochemistry and hydrogenation of the indenyl ligand on the experimental activities has been evaluated by computational studies. The differences along the reaction pathway are dominated by changes in the relative stabilities of the catalytic intermediates. A hybrid density functional B3LYP study, in the presence of toluene as the solvent, indicates that the rac forms give rise to more active species than their meso counterparts. The hydrogenation of the rac forms is a very promising approach to increase activities in polymerisation, in contrast to the meso forms. Finally, the global mechanism rate constants for the polymerisation reaction for each metallocene were calculated by using the thermodynamic formulation of transition‐state theory to complement the computational study.

Experimental and Theoretical Study of Zirconocene-Catalyzed Oligomerization of 1-Octene

“…Zirconocenes hold a significant position among single-site catalysts of α-olefin polymerization due to high catalytic activity, excellent copolymerization homogeneity, and wide boundaries of regio- and sterecontrol [ 1 , 2 , 3 , 4 , 5 , 6 ]. Even in recent years, many of the studies of the reaction mechanisms [ 7 , 8 , 9 , 10 ] and structure–activity relationships [ 11 , 12 , 13 , 14 , 15 , 16 ] for these catalysts were performed using a generally accepted cationic concept ( Scheme 1 A) based on the fundamental research of Cossee and Arlman [ 17 , 18 , 19 , 20 , 21 , 22 , 23 , 24 , 25 , 26 ].…”

Synthesis, characterisation and ethylene polymerisation performance of silyl bridged peralkylated bis(indenyl) zirconocenes

“…[36] 1 is significantly more active than the unbridged analogue rac-( 3-Et Ind # )2ZrCl2 under the same polymerisation conditions (activities of 12423 and 1421 kgPE molZr −1 h −1 bar −1 respectively at 60 °C) [20] due to the ansa-bridge opening up the metal centre and increasing the 'bite angle' of the metallocene complex. [37] 1 also displays higher polymerisation activities than the unmethylated analogue (9150 and 7600 kgPE molZr −1 h −1 bar −1 for rac-Me 2 SB( 3-Et I)ZrCl2 and meso-Me 2 SB( 3-Et I)ZrCl2, respectively, at 70 °C with [AlMAO]0/[Zr]0 of 1000 and 2 bar ethylene) [38] due to the increased inductive donation of 3-Et I* compared to . [27] The molecular weights (Mw) of the polyethylenes produced using 1, determined by gel permeation chromatography (GPC), decreased as polymerisation temperature increased due to increased rates of termination rate relative to the propagation (Figure S15 and Table S2).…”