The Experts below are selected from a list of 150 Experts worldwide ranked by ideXlab platform
Gerhard Fink - One of the best experts on this subject based on the ideXlab platform.
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On the Mechanism of Stereospecific Polymerization—Development of a Universal Model to Demonstrate the Relationship Between Metallocene Structure and Polymer Microstructure
Chemistry: A European Journal, 1997Co-Authors: Y. Van Der Leek, Klaus Angermund, M. Reffke, Ralph Kleinschmidt, R. Goretzki, Gerhard FinkAbstract:With the discovery of stereorigid bridged metallocenes, soluble catalysts became available for the Stereospecific Polymerization of α-olefins. A relatively simple mechanism was used to explain the Stereospecificity, primarily in terms of the catalyst symmetry. In this paper we demonstrate that the simple rule of thumb that C2-symmetric catalysts produce isotactic and Cs-symmetric catalysts syndiotactic polypropylene is too narrow. The introduction of one methyl group at the Cp ring in the [{iPr(CpFlu)}ZrCl2]/ MAO system (Flu = fluorenyl, MAO = methylalumoxan) reduces the Cs symmetry to C1, and the resulting catalyst produces hemiisotactic polypropylene. The analogous catalyst with a bulkier tert-butyl group at the Cp ring gives isotactic polypropylene. When the C2 symmetry of [{Me2Si(Ind)2}ZrCl2] (Ind = indenyl) is reduced to C1, a metallocene can be obtained that produces atactic polypropylene. We have broken away from the symmetry-based model and developed a universal model, which accurately describes the experimental microstructures of the polymers by considering the four lowestenergy conformers of the metallocene species coordinating to prochiral propene (Rre, Sre, Ssi, and Rsi) and the positional changes that the polymer chain undergoes during insertion. The relative energy levels of the four diastereomers can be determined by molecular modeling calculations; these energy gradations, in particular the size of the energy gaps, are decisive in determining the Stereospecificity. Also, the model permits the stereoerrors to be classified and explained. Through this model the stereosequence of a polymer chain can be calculated and predicted.
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on the mechanism of Stereospecific Polymerization development of a universal model to demonstrate the relationship between metallocene structure and polymer microstructure
Chemistry: A European Journal, 1997Co-Authors: Y Van Der Leek, Klaus Angermund, M. Reffke, Ralph Kleinschmidt, R. Goretzki, Gerhard FinkAbstract:With the discovery of stereorigid bridged metallocenes, soluble catalysts became available for the Stereospecific Polymerization of α-olefins. A relatively simple mechanism was used to explain the Stereospecificity, primarily in terms of the catalyst symmetry. In this paper we demonstrate that the simple rule of thumb that C2-symmetric catalysts produce isotactic and Cs-symmetric catalysts syndiotactic polypropylene is too narrow. The introduction of one methyl group at the Cp ring in the [{iPr(CpFlu)}ZrCl2]/ MAO system (Flu = fluorenyl, MAO = methylalumoxan) reduces the Cs symmetry to C1, and the resulting catalyst produces hemiisotactic polypropylene. The analogous catalyst with a bulkier tert-butyl group at the Cp ring gives isotactic polypropylene. When the C2 symmetry of [{Me2Si(Ind)2}ZrCl2] (Ind = indenyl) is reduced to C1, a metallocene can be obtained that produces atactic polypropylene. We have broken away from the symmetry-based model and developed a universal model, which accurately describes the experimental microstructures of the polymers by considering the four lowestenergy conformers of the metallocene species coordinating to prochiral propene (Rre, Sre, Ssi, and Rsi) and the positional changes that the polymer chain undergoes during insertion. The relative energy levels of the four diastereomers can be determined by molecular modeling calculations; these energy gradations, in particular the size of the energy gaps, are decisive in determining the Stereospecificity. Also, the model permits the stereoerrors to be classified and explained. Through this model the stereosequence of a polymer chain can be calculated and predicted.
Xiubo Jiang - One of the best experts on this subject based on the ideXlab platform.
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Stereospecific Polymerization of olefins with supported ziegler natta catalysts
Polymer International, 2014Co-Authors: Xiubo JiangAbstract:The discovery of Ziegler − Natta catalysts has been one of the greatest developments in technology for the synthesis of stereoregular polymers in both academy and industry since the 1950s. In particular, the development of the fourth generation Ziegler − Natta catalyst with MgCl2 as support brings a revolutionary improvement to the properties of manufactured polyolefins and stimulates people to explore the Stereospecific Polymerization of α-olefin and diene monomers, which supplies the power to synthesize a new type of polyolefin materials. Although research on single-site catalysts has attracted a lot of attention in recent years, the ‘old’ and conventional Ziegler − Natta catalysts are still being developed vigorously due to their unique industrial advantages such as low cost, high catalytic efficiency and high Stereospecificity. In this mini-review, we mainly summarize the development of the conventional supported Ziegler − Natta catalyst system and the stereoregular polyolefins synthesized with supported Ziegler − Natta catalysts. © 2013 Society of Chemical Industry
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Stereospecific Polymerization of olefins with supported Ziegler − Natta catalysts
Polymer International, 2013Co-Authors: Xiubo JiangAbstract:The discovery of Ziegler − Natta catalysts has been one of the greatest developments in technology for the synthesis of stereoregular polymers in both academy and industry since the 1950s. In particular, the development of the fourth generation Ziegler − Natta catalyst with MgCl2 as support brings a revolutionary improvement to the properties of manufactured polyolefins and stimulates people to explore the Stereospecific Polymerization of α-olefin and diene monomers, which supplies the power to synthesize a new type of polyolefin materials. Although research on single-site catalysts has attracted a lot of attention in recent years, the ‘old’ and conventional Ziegler − Natta catalysts are still being developed vigorously due to their unique industrial advantages such as low cost, high catalytic efficiency and high Stereospecificity. In this mini-review, we mainly summarize the development of the conventional supported Ziegler − Natta catalyst system and the stereoregular polyolefins synthesized with supported Ziegler − Natta catalysts. © 2013 Society of Chemical Industry
Kooji Mizunuma - One of the best experts on this subject based on the ideXlab platform.
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chain transfer reaction by trialkylaluminum air3 in the Stereospecific Polymerization of propylene with metallocene air3 ph3cb c6f5 4
Polymer, 1998Co-Authors: Naofumi Naga, Kooji MizunumaAbstract:Abstract Stereospecific Polymerization of propylene was carried out with rac-ethylenebis(indenyl)zirconium dichloride (rac-Et(Ind)2ZrCl2) (1), rac-dimethylsilylenebis(indenyl)zirconium dichloride (rac-Me2Si(Ind)2ZrCl2) (2) and isopropylidene(cyclopentadienyl)(9-fluorenyl)zirconium dichloride (i-Pr(Cp)(Flu)ZrCl2) (3) combined with trialkyl-aluminum (AIR3: R = C2H5, i-C4H9)/triphenylcarbenium tetrakis(pentafluorophenyl)borate (Ph3CB(C6F5)4) (4). In isospecific Polymerization with 1 and 2, the molecular weight of polypropylenes decreased with increase in the molar ratio of AlEt3 (Et = C2H5)/Zr, whereas, an effect of AliBu3 (iBu = i-C4H9) concentration on molecular weight was not observed. The microstructures of resulting polypropylenes were studied by 13C n.m.r. and an increase in the molar ratio of ethyl end groups (derived from chain transfer to AlEt3) to n-propyl end groups (derived from β-hydrogen transfer) was observed with increase in the molar ratio of AlEt 3 Zr (1 and 2). The chain transfer reactions by both AlEt3 and AliBu3 were also detected in syndiospecific Polymerization with 3. The molar ratio of alkyl (R) end groups (derived from chain transfer to AIR3) to n-propyl end groups was higher in the polypropylene obtained with AlEt3 than that obtained with AliBu3. The relative constants k trA k p (ktrA = rate constant of chain transfer to AIR3, kp = rate constant of propagation) were determined by kinetic study.
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Chain transfer reaction by trialkylaluminum (AIR3) in the Stereospecific Polymerization of propylene with metallocene — AIR3/Ph3CB(C6F5)4
Polymer, 1998Co-Authors: Naofumi Naga, Kooji MizunumaAbstract:Abstract Stereospecific Polymerization of propylene was carried out with rac-ethylenebis(indenyl)zirconium dichloride (rac-Et(Ind)2ZrCl2) (1), rac-dimethylsilylenebis(indenyl)zirconium dichloride (rac-Me2Si(Ind)2ZrCl2) (2) and isopropylidene(cyclopentadienyl)(9-fluorenyl)zirconium dichloride (i-Pr(Cp)(Flu)ZrCl2) (3) combined with trialkyl-aluminum (AIR3: R = C2H5, i-C4H9)/triphenylcarbenium tetrakis(pentafluorophenyl)borate (Ph3CB(C6F5)4) (4). In isospecific Polymerization with 1 and 2, the molecular weight of polypropylenes decreased with increase in the molar ratio of AlEt3 (Et = C2H5)/Zr, whereas, an effect of AliBu3 (iBu = i-C4H9) concentration on molecular weight was not observed. The microstructures of resulting polypropylenes were studied by 13C n.m.r. and an increase in the molar ratio of ethyl end groups (derived from chain transfer to AlEt3) to n-propyl end groups (derived from β-hydrogen transfer) was observed with increase in the molar ratio of AlEt 3 Zr (1 and 2). The chain transfer reactions by both AlEt3 and AliBu3 were also detected in syndiospecific Polymerization with 3. The molar ratio of alkyl (R) end groups (derived from chain transfer to AIR3) to n-propyl end groups was higher in the polypropylene obtained with AlEt3 than that obtained with AliBu3. The relative constants k trA k p (ktrA = rate constant of chain transfer to AIR3, kp = rate constant of propagation) were determined by kinetic study.
Hartwig Höcker - One of the best experts on this subject based on the ideXlab platform.
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Single Component Zirconocene Catalysts for the Stereospecific Polymerization of MMA
Organometallic Catalysts and Olefin Polymerization, 2001Co-Authors: Holger Frauenrath, Helmut Keul, Hartwig HöckerAbstract:The Stereospecific Polymerization of methyl methacrylate (MMA) with Single component cationic zirconocene catalysts has been investigated, yielding highly isotactic poly(methyl methacrylate) (PMMA) with Me2C(Cp)(Ind)Zr(Me)(thf)+BPh4 - 1 as a catalyst, and syndiotactic PMMA at low temperatures with Me2C(Cp)2Zr(Me)(thf)+BPh4 - 2. Similar cationic complexes with other ligands have been found to be inactive for MMA Polymerization under similar reaction conditions. On the basis of Polymerization kinetics and Stereospecificity control in MMA Polymerization with 1 and 2 a possible Polymerization mechanism is discussed.
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Stereospecific Polymerization of methyl methacrylate with single-component zirconocene complexes: Control of Stereospecificity via catalyst symmetry
Macromolecules, 2001Co-Authors: Holger Frauenrath, Helmut Keul, Hartwig HöckerAbstract:We report the Stereospecific Polymerization of methyl methacrylate (MMA) with single-component cationic zirconocene catalysts. The complexes Me2CCpIndZrMe(thf)+BPh4- (1) and Me2CCp2ZrMe(thf)+BPh4- (2) are active catalysts for the Polymerization of MMA, in remarkable contrast to other cationic zirconocenes. While 1 yields highly isotactic poly(methyl methacrylate) (PMMA), 2 is syndiospecific at low temperatures. This is the first example for a rational control of PMMA microstructure via catalyst symmetry. Polymerization kinetics and Stereospecificity control in MMA Polymerization with 1 and 2 are discussed. On the basis of the experimental data, a possible Polymerization mechanism is proposed.
Naofumi Naga - One of the best experts on this subject based on the ideXlab platform.
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chain transfer reaction by trialkylaluminum air3 in the Stereospecific Polymerization of propylene with metallocene air3 ph3cb c6f5 4
Polymer, 1998Co-Authors: Naofumi Naga, Kooji MizunumaAbstract:Abstract Stereospecific Polymerization of propylene was carried out with rac-ethylenebis(indenyl)zirconium dichloride (rac-Et(Ind)2ZrCl2) (1), rac-dimethylsilylenebis(indenyl)zirconium dichloride (rac-Me2Si(Ind)2ZrCl2) (2) and isopropylidene(cyclopentadienyl)(9-fluorenyl)zirconium dichloride (i-Pr(Cp)(Flu)ZrCl2) (3) combined with trialkyl-aluminum (AIR3: R = C2H5, i-C4H9)/triphenylcarbenium tetrakis(pentafluorophenyl)borate (Ph3CB(C6F5)4) (4). In isospecific Polymerization with 1 and 2, the molecular weight of polypropylenes decreased with increase in the molar ratio of AlEt3 (Et = C2H5)/Zr, whereas, an effect of AliBu3 (iBu = i-C4H9) concentration on molecular weight was not observed. The microstructures of resulting polypropylenes were studied by 13C n.m.r. and an increase in the molar ratio of ethyl end groups (derived from chain transfer to AlEt3) to n-propyl end groups (derived from β-hydrogen transfer) was observed with increase in the molar ratio of AlEt 3 Zr (1 and 2). The chain transfer reactions by both AlEt3 and AliBu3 were also detected in syndiospecific Polymerization with 3. The molar ratio of alkyl (R) end groups (derived from chain transfer to AIR3) to n-propyl end groups was higher in the polypropylene obtained with AlEt3 than that obtained with AliBu3. The relative constants k trA k p (ktrA = rate constant of chain transfer to AIR3, kp = rate constant of propagation) were determined by kinetic study.
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Chain transfer reaction by trialkylaluminum (AIR3) in the Stereospecific Polymerization of propylene with metallocene — AIR3/Ph3CB(C6F5)4
Polymer, 1998Co-Authors: Naofumi Naga, Kooji MizunumaAbstract:Abstract Stereospecific Polymerization of propylene was carried out with rac-ethylenebis(indenyl)zirconium dichloride (rac-Et(Ind)2ZrCl2) (1), rac-dimethylsilylenebis(indenyl)zirconium dichloride (rac-Me2Si(Ind)2ZrCl2) (2) and isopropylidene(cyclopentadienyl)(9-fluorenyl)zirconium dichloride (i-Pr(Cp)(Flu)ZrCl2) (3) combined with trialkyl-aluminum (AIR3: R = C2H5, i-C4H9)/triphenylcarbenium tetrakis(pentafluorophenyl)borate (Ph3CB(C6F5)4) (4). In isospecific Polymerization with 1 and 2, the molecular weight of polypropylenes decreased with increase in the molar ratio of AlEt3 (Et = C2H5)/Zr, whereas, an effect of AliBu3 (iBu = i-C4H9) concentration on molecular weight was not observed. The microstructures of resulting polypropylenes were studied by 13C n.m.r. and an increase in the molar ratio of ethyl end groups (derived from chain transfer to AlEt3) to n-propyl end groups (derived from β-hydrogen transfer) was observed with increase in the molar ratio of AlEt 3 Zr (1 and 2). The chain transfer reactions by both AlEt3 and AliBu3 were also detected in syndiospecific Polymerization with 3. The molar ratio of alkyl (R) end groups (derived from chain transfer to AIR3) to n-propyl end groups was higher in the polypropylene obtained with AlEt3 than that obtained with AliBu3. The relative constants k trA k p (ktrA = rate constant of chain transfer to AIR3, kp = rate constant of propagation) were determined by kinetic study.