Connection of Stereoselectivity, Regioselectivity, and Molecular Weight Capability in rac-R′₂Si(2-Me-4-R-indenyl)₂ZrCl₂ Type Catalysts

Abstract

A set of 19 silicon-bridged C 2 -symmetric zirconocenes rac-R′ 2 Si(2-Me-4-R-indenyl) 2 ZrCl 2 of varying steric demand in position 4 were synthesized and screened in propene homopolymerization in a high-throughput experimental setup. The size and accuracy of the experimental data set allow to identify surprisingly good correlations among stereoselectivity, regioselectivity, and molecular weight capability (R 2 ≈ 0.8−0.9) over a broad range. We rationalize this trend by assuming that steric tuning in the 4-position affects both preferred insertion and stereoerror formation similarly but leaves other barriers largely unaffected. A quantitative structure−activity relationship based on one single computational descriptor, Δ%V Bur using the difference in the percent of buried volume between the "blocked" and "open" quadrants of the catalyst precursoris established. Provided that a large sphere of 5.0 Å is used, stereoselectivity can be predicted with unprecedented accuracy, i.e., a mean average deviation (MAD) of 0.18 kcal/mol (ΔΔG ‡ enantio ), 0.0007 (σ, probability that the preferred propene enantioface is selected at an active site of given chirality), or 0.3% (mmmm pentads). On the basis of this empirical model, we predicted that the catalyst with R = o-tolyl is an ideal candidate for high stereoselectivity/high MW capability. Ad hoc synthesis and testing of the precursor confirmed the expectations: the catalyst shows the highest stereoselectivity reported so far (σ = 0.9999) for metallocenes at 60 °C, while maintaining a high MW capability (M w > 1 MDa) and relatively high regioselectivity.

Structure‐Activity Relationships for Bis(phenolate‐ether) Zr/Hf Propene Polymerization Catalysts

“…The former trend (Ph < 3,5‐Me 2 C 6 H 3 < 2,6‐Me 2 C 6 H 3 < 3,5‐ t Bu 2 ‐4‐MeO‐C 6 H 2 < Cbz ≈ t Bu 2 Cbz; Cbz = carbazole) is perhaps somewhat surprising since several of these substituent modifications occur rather far away from the active site, but in a general sense it agrees with ligand effects observed for e.g. metallocenes . In any case, L15 Zr was the most stereoselective catalyst in our library and comparable in stereoselectivity to L22 Zr and Hf catalysts reported by Waymouth (Figure ) which lack the p ‐methoxy groups at the phenoxy ortho‐substituents …”

Extraction of Reliable Molecular Information from Diffusion NMR Spectroscopy: Hydrodynamic Volume or Molecular Mass?

“…Bis-cyclopentadienyl complexes, triphenylmethane, POSS, trimethylaluminium, styrene, BHT,a nd bis-BHT were purchased from Sigma-Aldrich, Ti(OiPr) 4 was obtained from Abcr, and PS1 and PS2 from Agilent Te chnologies, and used without further purifications. Me 2 Al(bht), [34] MeAl(bht) 2 , [34] (Me)Ind 2 ZrMe 2 , [35] (tBuPh)Ind 2 ZrMe 2 , [36] and POSS-Ti-POSS [19] were synthesized according to previously established procedures. NMR experiments were performed by using aB ruker Avance III HD 400 spectrometer equipped with as martprobe (400 MHz for 1 H) with a z gradient coil.…”

Separating Electronic from Steric Effects in Ethene/α-Olefin Copolymerization: A Case Study on Octahedral [ONNO] Zr-Catalysts

“…To describe the steric hindrance imparted by the ancillary ligand framework, the percentage buried volume in the catalyst active pocket (%V bur , see also Supporting Information) can be used, as proposed by Cavallo and coworkers [85][86][87]. Scanning spheres having 5 Å radius have been reported to be suitable to explore steric effects in olefin polymerization [19]. Maps of the steric bulk, as shown in Figure S1 and Supporting Information, as well as the %V bur values (total and separately for the four quadrants of the active pocket), as shown in Table 2, clearly indicate that the steric hindrance provided by the ancillary ligand is small and virtually identical for all complexes R 1 R 2 .…”

“…Computational chemistry has contributed significantly to the identification of some rationale in the relationships between catalyst structure and properties [10][11][12]. The demystification of the origins of stereoselectivity is generally considered as a successful example of rational understanding [13][14][15][16], although some aspects are still debated [17][18][19][20][21][22]. Conversely, other catalyst properties, including comonomer affinity, are far less understood.…”