Higher-order Kinetics in Propene Polymerizations by Zirconocene Catalysts. Analysis of Alternative Reaction Mechanisms via a Genetic Algorithm

Schaper, Frank; Brintzinger, Hans-H; Kleinschmidt, Ralph; Leek, Yolanda van der; Reffke, Mathias; Fink, Gerhard

doi:10.1007/978-3-642-59465-6_5

Cited by 3 publications

(4 citation statements)

References 20 publications

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“…For example, no major revision of the Cossee−Arlman mechanism, e.g., through inclusion of a second monomer molecule, is required. The model is purely kinetic in nature, but the authors suggest that the two states of the active center “could differ in the conformation of the growing polymer chain”, an explanation in agreement with the suggestions cited above, , i.e., that the reasons for the broken rate order are more likely due to mechanistic details such as equilibria between different species, or states, of the active catalyst. In principle, it should be possible to locate candidates for such species, for example by quantum chemical modeling.…”

Section: Introductionsupporting

confidence: 65%

“… where [C] is the concentration of active centers and [M] the concentration of monomer. However, the observed reaction order with respect to monomer concentration is not restricted to unity, and rate orders higher than unity have been reported for a broad spectrum of catalysts and monomers. − For metallocene-based polymerizations in the homogeneous phase, all conceivable indirect effects such as mass- or heat-transfer limitations have been ruled out. , This led Mülhaupt et al to claim that the observed broken rate order must be caused by “equilibria involving the active species” and that the monomer “might be involved in an equilibrium between dormant and active catalyst sites” whereas Schaper et al pointed out that this must be due to the “intrinsic mechanisms of the polymerization catalysis”. One possible mechanistic explanation for a rate order higher than unity in monomer concentration would be the simultaneous participation of more than one monomer in the propagation cycle.…”

Section: Introductionmentioning

confidence: 99%

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DFT Investigation of the Single-Center, Two-State Model for the Broken Rate Order of Transition Metal Catalyzed Olefin Polymerization

et al. 2005

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Density functional calculations have been performed on different conformers and isomers of the propyl group in alkyl cations [L2Zr−Pr]+ (L = Cp, Cp*; Pr = n-propyl), corresponding to two catalysts with different observed rate orders (n) for ethylene polymerization, to explore the single-center, two-state kinetic model for olefin polymerization. For L = Cp (n ≈ 1), the β-agostic conformer is found to be the most stable structure and also the most reactive with respect to ethylene coordination, which is commensurate with unity rate order. For L = Cp* (n ≈ 1.4), the favored propagation route involves the γ- and α-agostic conformations of the alkyl complex, with coordination taking place to an α-agostic conformation in order to minimize the steric hindrance experienced by the incoming ethylene. The barriers to rearrangement from the α- and γ-agostic conformers to the more stable β-agostic structure are significantly lower than those of propagation. Moreover, no structure was found to be of lower energy than the β-agostic conformation, and the latter thus takes the role of the resting state for both catalysts in the present study. Assuming that the single-center, two-state model applies to zirconocene-catalyzed ethylene polymerization, our calculations thus suggest that the corresponding fast and slow, or dormant, states do not originate from equilibria of the alkyl group as investigated here.

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Section: Introductionsupporting

confidence: 65%

Section: Introductionmentioning

confidence: 99%

DFT Investigation of the Single-Center, Two-State Model for the Broken Rate Order of Transition Metal Catalyzed Olefin Polymerization

et al. 2005

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“…The rate with which the model ligand DBE is substituted by the methylborate anion, k - 1 = k / K , can be obtained from the experimental data in Table to be in the range 3 × 10 -5 to 0.8 s -1 for complexes 1a − 7a . Even though olefins, as weaker Lewis bases, are probably replaced more rapidly than dibutyl ether, it appears unlikely that substitution of the olefin by the methylborate anion (and even more so by less coordinating cocatalyst anions such as B(C 6 F 5 ) 4 - or X-MAO - ) can compete with the insertion reaction, for which rates of approximately 10−100 s -1 can be estimated from overall polymerization activities.…”

Section: Discussionmentioning

confidence: 99%

Displacement of H₃CB(C₆F₅)₃^- Anions from Zirconocene Methyl Cations by Neutral Ligand Molecules: Equilibria, Kinetics, and Mechanisms

2002

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The displacement of the MeB(C6F5)3 - anion from seven different zirconocene methyl cations by neutral Lewis bases, such as dimethylaniline, benzyldimethylamine, and dinbutyl ether, was investigated by 1D and 2D NMR spectroscopy. Equilibrium constants for reactions with dinbutyl ether change by factors of less than 5 between the zirconocene contact ion pairs studied, despite substantial steric differences. Rate constants of this displacement reaction, however, change by a factor of more than 105 between Me2Si(C5H4)2ZrMe+MeB(C6F5)3 -, the most “open” complex, and rac-Me2Si(2-Me-BzInd)2ZrMe+MeB(C6F5)3 -, the most highly substituted species studied. Kinetic and stereochemical data indicate that Lewis base−anion exchange proceeds by way of an associative mechanism, which occurs without side change of the zirconium-bound methyl group. DFT calculations support an associative substitution mechanism and propose five-coordinated reaction intermediates with the Lewis base coordinated to the central coordination site.

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“…Even more recently, Schaper et al analyzed propene polymerization data for a number of ansa -zirconocene catalysts making use of a genetic algoritm and concluded not only that one or more catalyst “resting” states need to be included in the reaction scheme in order to reproduce the experimental rate law but also that the possibility of monomer “trigger” cannot be ruled out.…”

Section: Introductionmentioning

confidence: 99%

Metallocene-Catalyzed Propene Polymerization: From Microstructure to Kinetics. 1. C₂-Symmetric ansa-Metallocenes and the "Trigger" Hypothesis

et al. 2001

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A microstructural method for the determination of the reaction order with respect to the monomer in coordinated 1-alkene polymerizations is proposed. The method is applicable whenever enantioselective monomer polyinsertion competes with an intramolecular process introducing stereodefects of known and recognizable nature in the growing polymer chains at a rate independent of monomer concentration, [M] (e.g., chain or site epimerization for C 2-symmetric and C s-symmetric ansa-metallocene catalysts, respectively). Indeed, in such cases, the stereoregularity of the polymer obtained at a given temperature results from the balance between the rates of the two competing processes, and is obviously a function of [M], because so is the rate of polyinsertion, 〈v p〉. Therefore, from the way polymer stereoregularity changes with [M], the corresponding change of 〈v p〉 can be inferred. Compared with the conventional macroscopic approach based on measurements of monomer consumption, the microstructural route is more reproducible, and allows one to discriminate between “authentic” and apparent higher-order polyinsertion rate laws. In this paper, in particular, we show that the “trigger” hypothesis invoked to explain the observed pseudo-second-order kinetics of propene polymerization promoted by C 2-symmetric ansa-metallocene catalysts is inconsistent with the experimental results and that an alternative explanation assuming first-order kinetics at catalytic species which interconvert between a propagating and a “resting” state is more plausible.

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Higher-order Kinetics in Propene Polymerizations by Zirconocene Catalysts. Analysis of Alternative Reaction Mechanisms via a Genetic Algorithm

Cited by 3 publications

References 20 publications

DFT Investigation of the Single-Center, Two-State Model for the Broken Rate Order of Transition Metal Catalyzed Olefin Polymerization

DFT Investigation of the Single-Center, Two-State Model for the Broken Rate Order of Transition Metal Catalyzed Olefin Polymerization

Displacement of H₃CB(C₆F₅)₃^- Anions from Zirconocene Methyl Cations by Neutral Ligand Molecules: Equilibria, Kinetics, and Mechanisms

Metallocene-Catalyzed Propene Polymerization: From Microstructure to Kinetics. 1. C₂-Symmetric ansa-Metallocenes and the "Trigger" Hypothesis

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Higher-order Kinetics in Propene Polymerizations by Zirconocene Catalysts. Analysis of Alternative Reaction Mechanisms via a Genetic Algorithm

Cited by 3 publications

References 20 publications

DFT Investigation of the Single-Center, Two-State Model for the Broken Rate Order of Transition Metal Catalyzed Olefin Polymerization

DFT Investigation of the Single-Center, Two-State Model for the Broken Rate Order of Transition Metal Catalyzed Olefin Polymerization

Displacement of H3CB(C6F5)3- Anions from Zirconocene Methyl Cations by Neutral Ligand Molecules: Equilibria, Kinetics, and Mechanisms

Metallocene-Catalyzed Propene Polymerization: From Microstructure to Kinetics. 1. C2-Symmetric ansa-Metallocenes and the "Trigger" Hypothesis

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Displacement of H₃CB(C₆F₅)₃^- Anions from Zirconocene Methyl Cations by Neutral Ligand Molecules: Equilibria, Kinetics, and Mechanisms

Metallocene-Catalyzed Propene Polymerization: From Microstructure to Kinetics. 1. C₂-Symmetric ansa-Metallocenes and the "Trigger" Hypothesis