Mechanisms of stereocontrol for doubly silylene-bridged C(s)- and C1- symmetric zirconocene catalysts for propylene polymerization. Synthesis and molecular structure of a dilithio double silylene-bridged bis(cyclopentadienyl) complex and the corresponding zirconium dichloride complex

Journal of the American Chemical Society

Volumn 121, Issue 3, 1999, Pages 564-573

  Veghini, Dario ^a   Henling, Lawrence M ^a   Burkhardt, Terry J ^b   Bercaw, John E ^a  

^a CALIFORNIA INSTITUTE OF TECHNOLOGY   (United States)

^b EXXONMOBIL UPSTREAM RESEARCH COMPANY   (United States)

Author keywords

[No Author keywords available]

Indexed keywords

LIGAND; LITHIUM DERIVATIVE; POLYMER; PROPYLENE; SILANE DERIVATIVE; UNCLASSIFIED DRUG; ZIRCONIUM DERIVATIVE; ZIRCONOCENE;

ARTICLE; CATALYST; CHEMICAL STRUCTURE; POLYMERIZATION; STEREOSPECIFICITY; SYNTHESIS; TEMPERATURE SENSITIVITY;

EID: 0033608107     PISSN: 00027863     EISSN: None     Source Type: Journal    
DOI: 10.1021/ja982868j     Document Type: Article

References (56)

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44. centroid-Li1-O1 = 168°.
45. -3. Relevant bond distances and angles (average): Zr-Cp1 = 2.22 Å, Zr-Cp2 = 2.24 Å, Zr-Cl = 2.43 Å, Cp1-Zr-Cp2 = 122°, Cl-Zr-Cl = 105°. Both 1e and 1b (ref 3b,c) have similar conformations.
46. The pentad distributions (average, %) for polypropenes produced by 2b in the propene concentration range 0.8-4.6 M with [r] = 55.3 and 49.2, respectively, are [mmmm] = 14, [mmmr] = 12, [rmmr] = 6, [mmrr] = 25, [rmrr] ÷ [mrmm] = 2, [rmrm] absent. [rrrr] = 21, [mrrr] = 14, [mrrm] = 5. The polymer microstructure is essentially identical at 0 °C in neat propene.
47. Since these pentads may arise from site epimerization, it may be the case that site epimerization does occasionally occur, even for 2b/MAO.
48. A full analysis of the possible pathways to the "allowed" pentads is given in the Supporting Information.
49. 13C NMR spectra for polypropylenes produced by 1d/MAO in neat propylene and in toluene solutions having differing [propylene] can be fit very well to a three-parameter model: one parameter (α) represents the monomer enantiofacial selectivity when the chain is on the less crowded side of the metallocene wedge, a second (β) represents (the same) monomer enantiofacial preference with the chain on the more crowded side of the metallocene wedge, and the third (∈) represents the probability of site epimerization wherein the chain moves from the less selective side to the more selective side (in the favored direction of Scheme 6). As required by the model in Scheme 6, least-squares fits give α ≈ 0.99, β ≈ 0.17, and ∈ increases steadily as the [propylene] decreases from that in liquid propylene (∈ ≈ 0.12) to 0.5 M (∈ ≈ 0.91). Details of this statistical model will be presented in a forthcoming manuscript (Miller S. A.; Bercaw, J. E., manuscript in preparation).
50. -4 for the other catalysts. These data suggest dominant monomolecular chain termination for 1a-c and 1e and partial contribution of a bimolecular process for 1d. For analogous discussions, see: Stehling, U.; Diebold, J.; Kirsten, R.; Röll, W.; Brintzinger, H. H.; Jüngling, S.; Mülhaupt, R.; Langhauser, F. Organometallics 1994, 13, 964 and ref 8.
51. u = 101 000 at 23 °C and 157 000 at 0 °C (PDI = 1.5 for both).
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