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  • 1
    ISSN: 0025-116X
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: In the first part of this series the individual rate constants of the radical polymerization of ethylene are reported in dependency of pressure at 405 K. By further measurements under 500 - 1750 bar and 400 - 470 K, using again the method of the rotating sector, the dependency of the rate constants on both pressure and temperature was determined. With increasing temperature the rate constant of chain growth kp as well as that of chain termination kt increases. With increasing pressure, kp increases, whereas kt decreases because of the diffusion controlled termination reaction. For the chain growth an activation energy of Ep = 37 kJ/mol and an activiation volume of Δvp≠ = -25,5 ml/mol was found. The activiation parameters of the chain termination are Et = 9,9 kJ/mol and Δvt≠ = 7 ml/mol. Based on these results and literature data the rate constants of different chain transfer reactions involved in the high pressure polymerization of ethylene and their dependency on temperature and pressure were evaluated.
    Additional Material: 6 Ill.
    Type of Medium: Electronic Resource
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  • 2
    ISSN: 0025-116X
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: In order to calculate the average molecular weight, the molecular weight distribution or the branching of low density polyethylene, the individual values of the rate constants of the different reactions involved in the free radical polymerization are required. The method of rotating sector was applied to determine the rate constants of chain propagation and chain termination in a wide range of temperatures and pressures. This first part of the work reports the influence of the pressure. It was found that the rate constant kp of the chain propagation increases with rising pressure whereas the rate constant kt of chain termination decreases. The effect of the pressure on kp can be explained in terms of the activation volume. The decrease of kt with pressure is caused by a diffusion controlled termination reaction.
    Additional Material: 7 Ill.
    Type of Medium: Electronic Resource
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  • 3
    ISSN: 0947-3440
    Keywords: Furans ; Thiophens ; Palladium catalysis ; Coupling reactions ; Heterocycles ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The regiospecific synthesis of 3,4-disubstituted furans and thiophenes is reviewed. Our initial approach used 3,4-bis(tri-n-butylstannyl)furan as a building block, which led to many structurally diverse 3,4-disubstituted furans. Another avenue was explored by using 3,4-bis(trimethylsilyl)furan and 3,4-bis(trimethylsilyl)thiophene as precursors. A variety of unsymmetrically 3,4-disubstituted furans and thiophenes were prepared in this way. The advantages of our strategy lie in its stepwise manner as well as its prospect of yielding 3,4-disubstituted furans and thiophenes with rather different substitution pattern.
    Additional Material: 5 Tab.
    Type of Medium: Electronic Resource
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