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621.
Oskar Friedrich Olaj Andreas Kornherr Philipp Vana Monika Zoder Gerhard Zifferer 《Macromolecular Symposia》2002,182(1):15-30
A survey is given on a selection of recently developed methods for the evaluation of the rate coefficient kt of termination and its chain-length dependence. In particular these are the time-resolved single-pulse pulsed laser polymerization (TR-SP-PLP), the single pulse pulsed laser polymerization in combination with the analysis of the molecular weight distribution produced (SP-PLP-MWD), the methods yielding an average kt either from the second moment of the chain-length distribution (CLD) or from the rate of polymerization, and a method focusing on the chain-length dependence of kt consisting in an analysis of the CLD resulting from PLP experiments carried out at low pulse frequencies (LF-PLP). The results obtained by these methods are compared and discussed. The role of the shielding of the two radical chains by their appendant coils is emphasized. 相似文献
622.
The values for the rate coefficient of chain termination kt in the bulk polymerization of methyl methacrylate at 25°C were formally calculated (i) from the second moment of the chain-length distribution and (ii) from the rate equation for laser-initiated pseudostationary polymerization (both expressions were originally derived for chain-length independent termination) by inserting the appropriate experimental data including the rate constant of chain propagation kp. These values were treated as average values, k and k , respectively. They exhibited good mutual agreement, even the predicted gradation (k < k by about 20%) was recovered. The log-log plot of kt vs. the average degree of polymerization of the chains at the moment of their termination v′ yielded exponents b of 0.16–0.17 in the power-law k t = A · v′−b, A ranging from 1.1 × 108 to 1.3 × 108 (L · mol−1 · s−1). A 70% contribution of disproportionation to overall termination has been assumed in the calculations. 相似文献
623.
Multibody systems such as legged robots require sophisticated and efficient methods for their modeling, control and simulation. This paper discusses the development of a software library based on modern tools such as C++, OpenGL and XML for highly efficient dynamics modeling. A primary focus lies on modularity permitting its easy extensibility in connection with different actuation and contact models, optimization algorithms, localized and centralized on‐line control schemes as well as animation and simulation environments. 相似文献
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626.
A method is presented which allows to simulate surface properties of a model polymer system (in addition to bulk properties). The decisive parameter which controls the equilibrium concentration of empty sites within the bulk phase as well as the surface properties is the attractive energy operating between nonbonded neighbouring polymer segments. In effect, by varying this parameter between -0,5 and -1,5 the volume fraction of vacancies in the bulk phase changed from 0,183 to 0,003. All the parameters defined in order to characterize the “roughness” of the surface indicate that the surface gets smoother and smoother with increasing attractive segment-segment interaction and, accordingly, also with decreasing bulk equilibrium concentration of vacancies. 相似文献
627.
Oskar Nuyken Ralf Weberskirch Martin Bortenschlager Daniel Schönfelder 《Macromolecular Symposia》2004,215(1):215-230
In this presentation we give an overview on our results in the field of poly(2-oxazolines). By means of living cationic polymerization in combination with the initiation and termination method, functionalized poly(2-oxazoline)s have been prepared, that were used as (i) macromonomers to form graft copolymers, (ii) lipopolymers to prepare stealth liposomes, (iii) ABC like polymers to form two compartment micellar networks and (iv) macroligand for micellar catalysis application. Within this report, we will discuss in detail the synthesis and characterization of various poly(2-oxazoline)s for the above mentioned research areas. 相似文献
628.
Markus Antonietti Rimantas Kublickas Oskar Nuyken Brigitte Voit 《Macromolecular rapid communications》1997,18(4):287-294
Low-molar-mass surfactants (S) and polyelectrolytes (PE) containing diazosulfonate chromophores which are photo-labile and electrically charged were used for the formation of polyelectrolyte/surfactant complexes (PESC) in combination with commonly used polyelectrolytes and surfactants. The stable complexes are no longer water-soluble and were characterized by means of nuclear magnetic resonance spectroscopy and in some cases by means of gel-permeation chromatography measurements. Complexes with a 1 : 1 stoichiomctry with respect to charge are formed. Ultraviolet irradiation of the photo-labile PESC resulted in the loss of the ionic interactions. 相似文献
629.
Oskar Friedrich Olaj Andreas Kornherr Gerhard Zifferer 《Macromolecular theory and simulations》2001,10(9):881-890
Chain length distributions have been calculated for polymers prepared by pulsed laser polymerization (PLP) under the condition that not only chain termination but also chain propagation is subject to chain length dependence. The interplay between these two features is analyzed with the chain length dependence of the rate coefficient of termination kt introduced in the form of a power law and that of propagation kp modeled by a Langmuir‐type decrease from an initial value for zero chain length to a constant value for infinite chain lengths. The rather complex situation is governed by two important factors: the first is the extent of the decay of radical concentration [R] during one period under pseudostationary conditions, while the second is that termination events are governed by [R]2 while the propagation goes directly with [R]. As a consequence there is no general recommendation possible as to which experimental value of kp is best taken as a substitute for the correct average of kp characterizing a specific experiment. The second point, however, is apparently responsible for the pleasant effect that the methods used so far for the determination of kt and its chain length dependence (i.e., plotting some average of kt versus the mean chain‐length of terminating radicals on a double‐logarithmic scale) are only subtly wrong with regard to a realistic chain length dependence. This is especially so for the quantity kt* (the average rate coefficient of termination derived from the rate of polymerization in a PLP system) and its chain length dependence. 相似文献
630.
An amphiphilic poly(2‐oxazoline) block copolymer consisting of a water‐soluble poly(2‐methyloxazoline) block and a hydrophobic block bearing bipyridine moieties in the side chain was synthesized by living cationic polymerization. This macroligand was applied to atom‐transfer radical polymerization (ATRP) of methyl methacrylate in aqueous solution in the presence of Cu(I)Br and ethyl 2‐bromoisobutyrate as the initiator. High monomer conversion up to 96% was achieved after 3 h of polymerization at 60°C. 相似文献