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  • Chemistry  (4)
  • Cell & Developmental Biology  (2)
  • Anaerobiosis
  • Nuclear Reactions
  • 1990-1994  (6)
  • 1
    Electronic Resource
    Electronic Resource
    Effects of procatalyst composition on the stereospecificity of a Ziegler-Natta catalyst systemRevised manuscript of December 20, 1991. (1992)
    New York : Wiley-Blackwell
    Die Makromolekulare Chemie 193 (1992), S. 1463-1468 
    Details
    ISSN: 0025-116X
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: In propylene polymerization with MgCl2/TiCl4/phthalate ester catalysts, using 2,2,6,6-tetramethylpiperidine as external donor, it has been found that the fraction of atactic polymer formed increased with decreasing ester content in the catalyst. At the same time, however, the stereoregularity of the isotactic polymer fraction, as measured by 13C NMR, increased. Polymerization in the absence of the external donor led to lower polymer yield and a lower stereoregularity of the isotactic fraction. The results indicate that the external donor is actively involved in the generation of isospecific sites, particularly in catalysts having relatively low ester contents.
    Additional Material: 1 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/macp.1992.021930622
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  • 2
    Electronic Resource
    Electronic Resource
    Hydrogen activation in propene polymerization with MgCl2-supported Ziegler-Natta catalysts: the effect of the external donor (1994)
    Weinheim : Wiley-Blackwell
    Macromolecular Chemistry and Physics 195 (1994), S. 167-172 
    Details
    ISSN: 1022-1352
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: 13C NMR analysis of relatively low-molecular-weight polypropylenes prepared using the catalyst system MgCl2/TiCl4/phthalate ester-AlEt3-external Lewis base has revealed the presence of up to around 20% butyl (n-Bu) chain ends, indicative of regioirregular (2,1-) monomer insertion followed by chain transfer with hydrogen. The effect of hydrogen on catalyst activity depends on the nature of the external donor, and the results indicate that at least part of the observed increase in catalyst activity in the presence of hydrogen can be ascribed to the regeneration of active species via chain transfer at dormant (2,1-inserted) sites.
    Additional Material: 1 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/macp.1994.021950114
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  • 3
    Electronic Resource
    Electronic Resource
    Application of conservation law to NMR measurements of poly(propylene) stereoregularity (1993)
    Basel : Wiley-Blackwell
    Die Makromolekulare Chemie, Theory and Simulations 2 (1993), S. 385-398 
    Details
    ISSN: 1018-5054
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: A new method is presented for data reconciliation in the 13C NMR determination of the pentad distribution in regioregular poly(propylene). Deviations of experimentally measured data from mathematically derived conservation laws are corrected by a maximum-likelihood method. In this method some assumptions are made about the structure of the erros to obtain better estimates of the correct data. The validity of this procedure was demonstrated with Monte Carlo simulation, after which the method has been applied to real 13C NMR data to show what corrections may be obtained. Conservation laws for hexads and heptads are derived as well, for possible future use.
    Additional Material: 4 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/mats.1993.040020309
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  • 4
    Electronic Resource
    Electronic Resource
    Probabilistic multi-site modelling of propene polymerization using markov models (1993)
    Basel : Wiley-Blackwell
    Die Makromolekulare Chemie, Theory and Simulations 2 (1993), S. 399-420 
    Details
    ISSN: 1018-5054
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Possible propagation mechanisms in propene polymerization using heterogeneous catalysts are described by combinations of Bernoulli and first- or even second-order Markov models. Both NMR-measured pentad fractions and GPC Gel permeation chromatography.-measured molecular weight distributions are directly taken into account to fit an appropriate propagation model for a specific catalyst. The analytical data so obtained have been used for an examination of the applicability of multi-site propagation models and for estimation of the main parameters in each model. The propagation and termination probabilities are estimated, as are “mix” parameters giving the fraction of the polymer population stemming from each kind of propagation mechanism. In this way the subpopulation produced by each site is characterized in terms of tacticity and molecular weight distribution. A three-site model is required to explain the measured data properly.
    Additional Material: 8 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/mats.1993.040020310
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  • 5
    Electronic Resource
    Electronic Resource
    Studies on the energy metabolism of opossum (Didelphis virginiana) erythrocytes: V. Utilization of hypoxanthine for the synthesis of adenine and guanine nucleotides in vitro (1990)
    New York, NY [u.a.] : Wiley-Blackwell
    Journal of Cellular Physiology 143 (1990), S. 563-568 
    Details
    ISSN: 0021-9541
    Keywords: Life and Medical Sciences ; Cell & Developmental Biology
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Biology , Medicine
    Notes: High pressure liquid radiochromatography was used to test the ability of opossum erythrocytes to incorporate tracer amounts of [G-3H] hypoxanthine (Hy) into [3H] labelled triphosphates of adenine and guanine. In the presence of supraphysiologic (30 mM) phosphate which is optimal for PRPP synthesis, both ATP and GTP are extensively labelled. When physiologic (1 mM) medium phosphate is used, red cells incubated under an atmosphere of nitrogen accumulate [3H] ATP in a linear fashion suggesting ongoing PRPP synthesis in red cells whose hemoglobin is deoxygenated. In contrast, a lesser increase of labelled ATP is observed in cells incubated under oxygen, suggesting that conditions for purine nucleotide formation from ambient Hy are more favorable in the venous circulation.
    Additional Material: 3 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/jcp.1041430322
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  • 6
    Electronic Resource
    Electronic Resource
    Dependence of mouse thymocyte-erythrocyte rosette formation on complete identity at class-l-MHC (1991)
    New York, NY [u.a.] : Wiley-Blackwell
    Journal of Cellular Physiology 148 (1991), S. 485-492 
    Details
    ISSN: 0021-9541
    Keywords: Life and Medical Sciences ; Cell & Developmental Biology
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Biology , Medicine
    Notes: Mouse thymocytes and erythrocytes form rosettes when incubated together at 4°. The frequency is much higher when the thymocytes and erythrocytes are MHC-identical. If the indolizidine alkaloid swainsonine (SW) is present during rosette formation at concentrations of 1 μg/ml (5.7 μM) or greater, rosette formation between MHC-identical pairs is inhibited to levels comparable to those observed for MHC-different pairs: rosette formation by MHC-different pairs is not affected. This was confirmed by examining 17 different MHC-identical combinations (9 completely syngeneic and 8 differing in non-MHC genes) and 13 MHC-different combinations (3 of these identical everywhere except at MHC). A SW-inhibitable component of rosette formation was observed only when thymocyte and erythrocyte were completely identical at MHC. Thus F1-parent pairs behaved as if allogeneic, although both F1-F1 and parent-parent had a SW-inhibitable rosetting component. Similarly, inbred strains only partially MHC-identical (B10.BR-B10.A, B10.D2-B10.A) behaved as if allogeneic. The SW-inhibitable component of rosetting could be partially but significantly blocked by including monoclonal antibodies against Thy-1, and anti-CD4 plus anti-CD8(together but not separately); anti-class-l-MHC produced some inhibition of marginal significance. Monoclonal antibodies against class-l-MHC, LFA-1, and CD3 did not block. Pretreatment of erythrocytes with neuraminidase greatly reduced the SW-inhibitable component of rosetting. The SW effect would appear to be due to a direct interaction of SW with a cell surface structure involved in syngeneic rosette formation rather than the known ability of SW to block the processing of N-linked sugar structures. The results are consistent with cell surface lectins and cell surface sugars playing a role in rosette formation.
    Additional Material: 6 Tab.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/jcp.1041480322
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