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Gerhard Fink - One of the best experts on this subject based on the ideXlab platform.
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13C NMR Studies of ethene norbornene copolymers using 13C enriched monomers signal assignments of copolymers containing norbornene microblocks of up to a length of three norbornene units
Macromolecular Chemistry and Physics, 2001Co-Authors: Ralf Alexander Wendt, Gerhard FinkAbstract:Ethene and norbornene were copolymerized at 0.8 bar ethene absolute pressure using metallocene catalysts that produce microstructures containing norbornene blocks, particularly rac-Me 2 Si[Ind] 2 ZrCl 2 /MAO, iPr[Cp-Flu]ZrCl 2 /MAO and iPr[CpInd]ZrCl 2 /MAO. Intensive 13 C NMR spectroscopic investigations of the obtained copolymers were performed and norbornene di- and tri-block sequences detected clearly. Several copolymerization experiments were carried out by using 13 C 1 -enriched ethane, or 13 C 5/6 -enriched norbornene, respectively. Thus, resonances of the norbornene carbon atoms C5/C6 and the ethene carbon atoms Ca/Cb, which overlap extensively in the 13 C NMR spectra, could clearly be differentiated. In addition, further copolymerizations with natural monomers were performed at 2 bar ethene excess pressure to investigate the influence of the monomer concentration on the microstructure. By comparing the 13 C NMR spectra of copolymers obtained under different conditions assignments of most resonances were achieved. These 13 C NMR Studies made it possible to assign norbornene triblock resonances with differing stereochemical connections such as meso, meso and rac, meso which have not been mentioned in previous works. The results led to new insights about the copolymer microstructure and allow development of a scheme showing the ethene/norbornene microstructures depending on the various catalyst systems utilized, including the metallocenes iPr[(3-iPr-Cp)Flu]ZrCl 2 and iPr[(3-tert-But-Cp)Ind]ZrCl 2 which have been applied in previous publications.
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13C NMR Studies of ethene norbornene copolymers assignment of sequence distributions using 13C enriched monomers and determination of the copolymerization parameters
Macromolecular Chemistry and Physics, 1999Co-Authors: Ralf Alexander Wendt, Richard Mynott, Klaus Hauschild, Dieter Ruchatz, Gerhard FinkAbstract:Ethene and norbornene were copolymerized using metallocene catalysts that produce copolymers having isolated norbornene units or microblocks with a maximum of two norbornene units. The resonances of the norbornene C 5/6 and the ethene carbon atoms, which overlap extensively in the 13 C NMR spectrum, were differentiated and assigned by comparing the 13 C NMR spectra of the copolymers obtained from monomers having 13 C at natural abundance with those prepared from feedstocks containing 13 C 1 -enriched ethene or 13 C 5/6 -enriched norbornene. The NMR analysis revealed that the chemical shifts of the norbornene C 5/6 carbon atoms are triad sensitive and those of the ethene carbon atoms are pentad sensitive. 13 C NMR analysis of copolymers containing isolated norbornene units in various proportions allowed the resonances of the norbornene C 5/6 and the ethene carbon atoms to be assigned to the respective triads and pentads. The complete triad distributions of these copolymers determined in this way were used to calculate the copolymerization parameters for a representative metallocene catalyst.
Kevin P Wainwright - One of the best experts on this subject based on the ideXlab platform.
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diastereomeric 1 4 7 10 tetrakis s 2 hydroxypropyl 1 4 7 10 tetraazacyclododecane and its alkali metal complex ions a nuclear magnetic resonance potentiometric titration and molecular orbital study
Journal of the American Chemical Society, 1997Co-Authors: Ramesh Dhillon, Samer E Madbak, Frank G Ciccone, Mark A Buntine, Stephen F Lincoln, Kevin P WainwrightAbstract:13C NMR Studies are consistent with 1,4,7,10-tetrakis((S)-2-hydroxypropyl)-1,4,7,10-tetraazacyclododecane (ΛS-thpc12) and its eight-coordinate alkali metal complex ions (Λ[M(S-thpc12)]+) existing predominantly as single distorted cubic diastereomers in methanol in accord with structures predicted through molecular orbital calculations. Intramolecular exchange in ΛS-thpc12 is characterized by k(298.2 K) = 34 800 ± 1600 s-1, ΔH⧧ = 53.9 ± 0.6 kJ mol-1, and ΔS⧧ = 22.8 ± 2.5 J K-1 mol-1 in methanol. This process is slowed in [M(S-thpc12)]+, for which k(298.2 K) = 332 ± 6, 125 ± 2, and 3020 ± 30 s-1, ΔH⧧ = 21.4 ± 0.2, 26.3 ± 0.5, and 46.3 ± 0.2 kJ mol-1, and ΔS⧧ = −125 ± 1, −116 ± 2, and −23.1 ± 0.9 J K-1 mol-1, respectively, when M+ = Li+, Na+, and K+. For intermolecular ligand exchange on Λ[M(S-thpc12)]+, decomplexation is characterized by kd(298.2 K) = 2200 ± 10, 64.3 ± 1.6, and 11 900 ± 300 s-1, ΔHd⧧ = 35.3 ± 0.5, 62.8 ± 0.5, and 41.8 ± 0.4 kJ mol-1, and ΔSd⧧ = −62.6 ± 2.1, 0.3 ± 2.0, and −26.8 ± 1.6 J K-1 ...
Robin D Rogers - One of the best experts on this subject based on the ideXlab platform.
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high resolution 13C NMR Studies of cellulose and cellulose oligomers in ionic liquid solutions
Chemical Communications, 2005Co-Authors: Jason S Moulthrop, Richard P Swatloski, Guillermo Moyna, Robin D RogersAbstract:High-resolution 13C NMR Studies of cellulose and cellulose oligomers dissolved in the ionic liquid (IL) 1-butyl-3-methylimidazolium chloride ([C4mim]Cl) show that the β-(1→4)-linked glucose oligomers are disordered in this medium and have a conformational behavior which parallels the one observed in water, and thus, reveal that the polymer is disordered in IL solution as well.
Rita Delgado - One of the best experts on this subject based on the ideXlab platform.
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Study of the cyclen derivative 2-[1,4,7,10-tetraazacyclododecan-1-yl]-ethanethiol and its complexation behaviour towards d-transition metal ions
Polyhedron, 2007Co-Authors: Sara Lacerda, Maria Paula Campello, Isabel Santos, Rita DelgadoAbstract:The compound 2-[1,4,7,10-tetraazacyclododecan-1-yl]-ethanethiol (L2) has been synthesized and characterized by multinuclear NMR spectroscopy and mass spectrometry. Its thiol-protected precursor L1 has also been isolated and characterized, including by X-ray structural analysis. The protonation constants of L2 were determined by potentiometric methods at 25.0 °C and 0.10 mol dm−3 KNO3 ionic strength. 13C NMR Studies and 2D NMR spectra recorded at different pD values have been used to analyse its protonation scheme. Stability constants of L2 with Cu2+, Zn2+ and Cd2+ were also determined by potentiometry, and the Zn(II) and Cu(II) complexes were studied in solution by NMR, UV–Vis, and EPR spectroscopies. The pM values (pH 7.4) calculated for the metal complexes of L2 are higher than the corresponding values found for cyclen and cyclam, but the selectivity of L2 for Cu2+ is low.
Ralf Alexander Wendt - One of the best experts on this subject based on the ideXlab platform.
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13C NMR Studies of ethene norbornene copolymers using 13C enriched monomers signal assignments of copolymers containing norbornene microblocks of up to a length of three norbornene units
Macromolecular Chemistry and Physics, 2001Co-Authors: Ralf Alexander Wendt, Gerhard FinkAbstract:Ethene and norbornene were copolymerized at 0.8 bar ethene absolute pressure using metallocene catalysts that produce microstructures containing norbornene blocks, particularly rac-Me 2 Si[Ind] 2 ZrCl 2 /MAO, iPr[Cp-Flu]ZrCl 2 /MAO and iPr[CpInd]ZrCl 2 /MAO. Intensive 13 C NMR spectroscopic investigations of the obtained copolymers were performed and norbornene di- and tri-block sequences detected clearly. Several copolymerization experiments were carried out by using 13 C 1 -enriched ethane, or 13 C 5/6 -enriched norbornene, respectively. Thus, resonances of the norbornene carbon atoms C5/C6 and the ethene carbon atoms Ca/Cb, which overlap extensively in the 13 C NMR spectra, could clearly be differentiated. In addition, further copolymerizations with natural monomers were performed at 2 bar ethene excess pressure to investigate the influence of the monomer concentration on the microstructure. By comparing the 13 C NMR spectra of copolymers obtained under different conditions assignments of most resonances were achieved. These 13 C NMR Studies made it possible to assign norbornene triblock resonances with differing stereochemical connections such as meso, meso and rac, meso which have not been mentioned in previous works. The results led to new insights about the copolymer microstructure and allow development of a scheme showing the ethene/norbornene microstructures depending on the various catalyst systems utilized, including the metallocenes iPr[(3-iPr-Cp)Flu]ZrCl 2 and iPr[(3-tert-But-Cp)Ind]ZrCl 2 which have been applied in previous publications.
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13C NMR Studies of ethene norbornene copolymers assignment of sequence distributions using 13C enriched monomers and determination of the copolymerization parameters
Macromolecular Chemistry and Physics, 1999Co-Authors: Ralf Alexander Wendt, Richard Mynott, Klaus Hauschild, Dieter Ruchatz, Gerhard FinkAbstract:Ethene and norbornene were copolymerized using metallocene catalysts that produce copolymers having isolated norbornene units or microblocks with a maximum of two norbornene units. The resonances of the norbornene C 5/6 and the ethene carbon atoms, which overlap extensively in the 13 C NMR spectrum, were differentiated and assigned by comparing the 13 C NMR spectra of the copolymers obtained from monomers having 13 C at natural abundance with those prepared from feedstocks containing 13 C 1 -enriched ethene or 13 C 5/6 -enriched norbornene. The NMR analysis revealed that the chemical shifts of the norbornene C 5/6 carbon atoms are triad sensitive and those of the ethene carbon atoms are pentad sensitive. 13 C NMR analysis of copolymers containing isolated norbornene units in various proportions allowed the resonances of the norbornene C 5/6 and the ethene carbon atoms to be assigned to the respective triads and pentads. The complete triad distributions of these copolymers determined in this way were used to calculate the copolymerization parameters for a representative metallocene catalyst.
