Ethene polymerization catalyzed by monoalkyl-substituted zirconocenes. Possible effects of ligand-metal agostic interaction

Macromolecules

Volumn 33, Issue 22, 2000, Pages 8136-8145

  Thorshaug, Knut ^a   Støvneng, Jon Andreas ^a   Rytter, Erling ^a,b  

^a NORWEGIAN UNIVERSITY OF SCIENCE AND TECHNOLOGY   (Norway)

^b STATOIL   (Norway)

Author keywords

[No Author keywords available]

Indexed keywords

CATALYSIS; CATALYSTS; ETHYLENE; FOURIER TRANSFORM INFRARED SPECTROSCOPY; GEL PERMEATION CHROMATOGRAPHY; INFRARED SPECTROSCOPY; ISOMERIZATION; MOLECULAR STRUCTURE; REACTION KINETICS; SUBSTITUTION REACTIONS; THERMAL EFFECTS; ZIRCONIUM COMPOUNDS;

CYCLOPENTADIENYL LIGAND; DENSITY FUNCTIONAL; LIGAND METAL AGOSTIC INTERACTION; METHYLALUMINOXANE; POLYMER UNSATURATION; TERMINATION; ZIRCONOCENES;

POLYMERIZATION;

EID: 0034291944     PISSN: 00249297     EISSN: None     Source Type: Journal    
DOI: 10.1021/ma990963m     Document Type: Article

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45. 2, containing two methyl bridges between Zr and Al. Certainly, such a complexation with TMA (which is always present to some extent in a MAO cocatalytic solution) will also slow insertion of the first monomer. However, the metalligand agostic interaction represents an additional effect that makes the catalysts with R = Et and longer qualitatively different from the catalysts with R = H or R = Me.
46. R interaction with R = Bu than with R = n-Pr is in agreement with our previous experience, namely that a hydrogen atom on a secondary carbon forms a stronger agostic bond than one on a primary carbon.
47. 3 (i.e., without a metal-ligand agostic interaction) confirm this picture: 0.736, 0.658, 0.656, and 0.654 for R = H, Me, Et, and n-Pr, respectively. Apparently, the insertion barrier is not affected accordingly. The exact values of the insertion barriers in these systems depend on the choice of gradient corrections to the density functional. In ref 6, we found barriers of 26 and 10 kJ/mol for the first and second insertion with R = H, using so-called BLYP gradient corrections. With the BP91 gradient corrections used in the present work, the corresponding insertion barriers are 4 and 0 kJ/mol. In both cases, the first insertion has the highest energy barrier.
48. The insertion product after passing through an α-agostic transition state has an agostic bond between Zr and γ-H on the growing polymer chain. Therefore, γ-agostic conformaj tions were chosen as the basis for the QSAR analysis.
49. With R = n-Pr, β-H transfer to Zr (starting from conformation D in Figure 11) is found to have an energy barrier of 116 kJ/mol, whereas β-H transfer to a coordinated monomer (starting from conformation B) has a barrier of 23 kJ/mol. The corresponding termination barriers with R = H are 146 and 34 kJ/mol, respectively.
50. Chain transfer to TMA or MAO results in saturated end groups. When this mechanism is dominating, it is reflected in large deviations between the FTIR- and GPC-determined molecular weights.
51. 6 and is estimated to be in the range 60-80 kJ/mol from DFT reaction pathway calculations (increasing the Hβ-Cβ distance).