ansa-Metallocene derivatives. 31. Origins of stereoselectivity and stereoerror formation in ansa-zirconocene-catalyzed isotactic propene polymerization. A deuterium labeling study.

Leclerc, Margarete K.; Brintzinger, Hans‐Herbert

doi:10.1021/ja00110a027

Cited by 151 publications

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“…Similar explanations have been used for isomerization during propene polymerization. 26,27,30,31 Our data indicate that chain isomerization will be followed by termination rather than propagation. Monomer insertion after isomerization would result in singlemethyl branches along the polymer backbone, but the FTIR analysis did not show any absorbance at ν ) 1150 cm -1 .…”

Section: Chain Termination and Isomerization As Listed Inmentioning

confidence: 73%

Termination, Isomerization, and Propagation Reactions during Ethene Polymerization Catalyzed by Cp₂Zr−R⁺ and Cp*₂Zr−R⁺. An Experimental and Theoretical Investigation

et al. 1998

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Ethene polymerization in toluene has been studied in the temperature range -7 to +97°C and pressure range 0.28 to 9 bar, using two different L 2ZrCl2/methylaluminoxane (MAO) catalyst systems. With bis(cyclopentadienyl)zirconium dichloride (Cp2ZrCl2, L ) Cp), the average activity over 1 h increases with temperature between 10 and 97°C. With bis(pentamethylcyclopentadienyl)zirconium dichloride (Cp*2ZrCl2, L ) Cp*), a maximum average activity over 1 h is observed at 45°C. If propagation and deactivation effects are separated through kinetic modeling, the activity corresponding to chain propagation is found to increase in the whole temperature range for both catalysts. The molecular weight is higher with L ) Cp* than with L ) Cp below 80°C. Above 80°C, the opposite is observed. With L ) Cp*, the molecular weight increases with increasing ethene pressure up to about 2 bar, where it levels off. With L ) Cp, the molecular weight is independent of pressure between 0.28 and 9 bar. The ratio between vinyl and trans-vinylene unsaturation is approximately 6:1 with L ) Cp and 1:1 with L ) Cp*, both slightly increasing with increasing ethene pressure. As the temperature is increased, the relative vinyl content decreases with L ) Cp and increases with L ) Cp*. On the basis of density-functional calculations, we present a reaction scheme consistent with most of the experimental results. This reaction scheme, in which different agostic interactions play a crucial role, assumes a Cossee-like mechanism for chain propagation, chain termination via hydrogen transfer to a coordinated monomer (for both catalysts) or to the metal (for L ) Cp*), and chain isomerization via partial hydrogen transfer to the metal, relative rotation of the olefin and the hydride, and reinsertion of the coordinated olefin. The calculated activation energy for propagation is 25-35 kJ/mol for L ) Cp*, in fair agreement with the experimental value of 17 kJ/mol. For L ) Cp, we calculate an activation energy of 10-20 kJ/mol, whereas the experimentally derived value is 61 kJ/mol. The poor agreement for L ) Cp may indicate that the polymerization is influenced by the surrounding solvent and MAO. The calculated difference in activation energy between chain propagation and termination is larger for L ) Cp* than for L ) Cp, in qualitative agreement with the stronger temperature dependence of the molecular weight observed with L ) Cp*. Chain isomerization is found to be easier, relative to termination, with L ) Cp* than with L ) Cp. This may account for the large amount of trans-vinylene unsaturation observed when Cp* 2ZrCl2 is used as catalyst.

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Section: Chain Termination and Isomerization As Listed Inmentioning

confidence: 73%

Termination, Isomerization, and Propagation Reactions during Ethene Polymerization Catalyzed by Cp₂Zr−R⁺ and Cp*₂Zr−R⁺. An Experimental and Theoretical Investigation

et al. 1998

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“…Apparently, NMR studies reveal that the chain end isomerization of growing polymer was the reason for stereoerror formation, meaning that other possibilities of propylene attack give misinsertions. 60 Hence, we assume that the skeleton carbon chain which represents polymer is in the same plane (Scheme 2) and its two CH 3 groups should be up or down (shown in red) in identifying the syndiotactic and isotactic products. Therefore, first and second insertions are not enough in the cases of the A and C approaches to judge the tacticity, since resting intermediates formed after second insertions contain only one chiral carbon.…”

Section: Resultsmentioning

confidence: 99%

Revisiting the Stereochemistry of Propylene Isotactic Polymerization Reaction Mechanism on C2 Symmetric [SiH2(Ind)2ZrCH3]+ and [SiH2(Ind)2ZrCH3]+[CH3B(C6F5)3]−

Sandhya¹,

Koga²,

Nagaoka³

2016

Bulletin of the Chemical Society of Japan

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The [SiH 2 (Ind) 2 ZrCH 3 ] + (Ind = indenyl) catalyzed stereoregularity of propylene polymerization mechanism has been investigated at M06 level of theory. Four different approaches of propylene to the reactive catalyst lead to four isomeric products due to the C 2 symmetry of bridged Ind ligand of the catalyst. Consequently, various possibilities of propylene attack as well as orientation of polymer chain yield numerous stereoisomers. The calculations of the first and second insertions with various conformers clarified the most favorable reaction pathway and showed that isotactic propagation is more favorable (3.5 kcal mol ¹1 ) than syndiotactic propagation. The structures of resting state catalysts displayed various agostic interactions of the CH bond with the Zr center which stabilize the catalytic systems and play important roles in determining the favorable reaction pathway. The influence of counter anion¹ on the reactivity of the catalyst was also studied. The results also confirm that the trans orientation of the counter anion with respect to propylene is more favorable than its cis orientation and clarify the most favorable reaction pathway in the first and second insertion. Because agostic interactions are involved in various aspects, AIM analysis has been used to find the bonding nature of agostic interactions as well as ion-pair bonds. The overall results suggest that rigidity of ansa-zirconocene, unique structure of C 2 symmetric ansa ligand, influence of [CH 3 B(C 6 F 5 ) 3 ]¹ and β agostic interaction may restrict the attack of propylene only to isotactic polymerization and not to syndiotactic polymerization.

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“…Especially in the field of olefin polymerization catalysis, kinetic isotope effects reveal the presence or absence of agostic interactions in intermediates of the catalytic cycle. a-Agostic interactions in the transition state of the olefin insertion were discussed for metallocene catalysts, leading to a KIE in the range of 1.2-1.3 [3][4][5][6][7]. A theoretical study confirmed that this interaction lowers the barrier for the olefin insertion by stabilization of the transition state and contributes to the stereospecificity of the reaction [8].…”

Section: Introductionmentioning

confidence: 99%

The mechanism of ethylene polymerization by nickel salicylaldiminato catalysts – Agostic interactions and their kinetic isotope effects

Zeller

Straßner

2006

Journal of Organometallic Chemistry

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ansa-Metallocene derivatives. 31. Origins of stereoselectivity and stereoerror formation in ansa-zirconocene-catalyzed isotactic propene polymerization. A deuterium labeling study.

Cited by 151 publications

References 0 publications

Termination, Isomerization, and Propagation Reactions during Ethene Polymerization Catalyzed by Cp₂Zr−R⁺ and Cp*₂Zr−R⁺. An Experimental and Theoretical Investigation

Termination, Isomerization, and Propagation Reactions during Ethene Polymerization Catalyzed by Cp₂Zr−R⁺ and Cp*₂Zr−R⁺. An Experimental and Theoretical Investigation

Revisiting the Stereochemistry of Propylene Isotactic Polymerization Reaction Mechanism on C2 Symmetric [SiH2(Ind)2ZrCH3]+ and [SiH2(Ind)2ZrCH3]+[CH3B(C6F5)3]−

The mechanism of ethylene polymerization by nickel salicylaldiminato catalysts – Agostic interactions and their kinetic isotope effects

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ansa-Metallocene derivatives. 31. Origins of stereoselectivity and stereoerror formation in ansa-zirconocene-catalyzed isotactic propene polymerization. A deuterium labeling study.

Cited by 151 publications

References 0 publications

Termination, Isomerization, and Propagation Reactions during Ethene Polymerization Catalyzed by Cp2Zr−R+ and Cp*2Zr−R+. An Experimental and Theoretical Investigation

Termination, Isomerization, and Propagation Reactions during Ethene Polymerization Catalyzed by Cp2Zr−R+ and Cp*2Zr−R+. An Experimental and Theoretical Investigation

Revisiting the Stereochemistry of Propylene Isotactic Polymerization Reaction Mechanism on C2 Symmetric [SiH2(Ind)2ZrCH3]+ and [SiH2(Ind)2ZrCH3]+[CH3B(C6F5)3]−

The mechanism of ethylene polymerization by nickel salicylaldiminato catalysts – Agostic interactions and their kinetic isotope effects

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Termination, Isomerization, and Propagation Reactions during Ethene Polymerization Catalyzed by Cp₂Zr−R⁺ and Cp*₂Zr−R⁺. An Experimental and Theoretical Investigation

Termination, Isomerization, and Propagation Reactions during Ethene Polymerization Catalyzed by Cp₂Zr−R⁺ and Cp*₂Zr−R⁺. An Experimental and Theoretical Investigation