1,721,131 research outputs found
POLY 2-Propagation and termination of radicals: Detailed analysis of polymerization kinetics via pulsed-laser-assisted techniques and high-pressure experiments
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POLY 2-Propagation and termination of radicals: Detailed analysis of polymerization kinetics via pulsed-laser-assisted techniques and high-pressure experiments
No full text
SP-PLP-EPR Measurement of Iron-Mediated Radical Termination in ATRP
No full textRadical termination was studied in the presence of iron(II) bromide, which is an attractive catalyst for atom-transfer radical polymerization (ATRP). Termination rate is measured in both the absence and presence of Fe-II via time-resolved EPR spectroscopy of propagating radicals produced by applying laser single pulses. This novel method quantifies the catalytic termination of two propagating radicals by Fell. The associated rate coefficient was measured in an extended temperature range for both acrylate and methacrylate polymerization. Detailed knowledge about the kinetics of Fe-II-catalyzed radical termination may be of general importance for iron-mediated ATRPs of acrylates.Fonds der Chemischen Industrie; DFG [CHE 10-26060
SP-PLP-EPR Investigations into the Chain-Length-Dependent Termination of Methyl Methacrylate Bulk Polymerization
No full textTermination kinetics of methyl methacrylate (MMA) bulk. polymerization has been studied via the single pulsed laser polymerization-electron paramagnetic resonance method. MMA-d(8) has been investigated to enhance the signal-to-noise quality of microsecond time-resolved measurement of radical concentration. Chain-length-dependent termination rate coefficients of radicals of identical size k(t)(i,i), are reported for 5-70 degrees C and up to i = 100. k(t)(i,i) decreases according to the power-law expression k(t)(i,i) = k(t)(1,1).i(-alpha). At 5 degrees C, k(t) for two MMA radicals of chain-length unity is k(t)(1,1) = (5.8 +/- 1.3) . 10(8)L . mol(-1) . s(-1). The associated activation energy and power-law exponent are: E(A)(k(t)(1,1)) approximate to 9 +/- 2 kJ . mol(-1) and alpha approximate to 0.63 +/- 0.15, respectively
SP-PLP-EPR - A Novel Method for Detailed Studies into the Termination Kinetics of Radical Polymerization
No full textSingle pulse pulsed laser polymerization electron paramagnetic resonance (SP-PLP-EPR) has been introduced as a powerful method for the very detailed analysis of termination kinetics. During polymerization an intense laser pulse is applied in order to almost instantaneously produce a burst of radicals. The decay of radical concentration is measured by highly time-resolved EPR and is analyzed with respect to the rate coefficients for the termination of two radicals of identical size. SP-PLP-EPR experiments have been carried out for an itaconate monomer, for several methacrylates in bulk and in a solution of ionic liquids, for methacrylic acid in aqueous solution, and for the solution polymerization of butyl acrylate in toluene at low temperature. The data fully support the composite model, which assumes a stronger chain-length dependence of termination for radicals of smaller size and a weaker one for large radicals. The SP-PLP-EPR technique is also applicable in systems with more than one type of growing radicals, as is the case with butyl acrylate polymerization at higher temperature and with RAFT polymerizations, where the novel method may be used for a comprehensive kinetic analysis
in Organic Media: A Response
No full textRadical propagation rate is adequately understood in the light of fundamental kinetic theory. Differences in bulk and solution k(p) are primarily of entropic origin, with the effects depending on the differences in polarity and size of monomer and solvent molecules, respectively. Experimental data for the propagation rate coefficient of secondary acrylate radicals demonstrate that bulk and solution-in-toluene k(p) exhibit distinctly different behavior. Bulk k(p) is clearly enhanced in passing from methyl acrylate (MA) to dodecyl acrylate (DA), whereas solution-in-toluene k(p) is approximately constant with a slight tendency to decrease from MA to DA. This k(p) behavior has also been found in a recent study, in which it has, however, been concluded that k(p) in solution of toluene (and of butyl acetate) displays a similar behavior to bulk. This surprising and inadequate conclusion requires the present comment and rectification to be made
Mechanism of dithiobenzoate-mediated RAFT polymerization: A missing reaction step
No full textThe debate on the mechanism of dithiobenzoate-mediated RAFT polymerization may be resolved by including the reaction between a propagating radical and the star-shaped combination product from irreversible termination into the kinetic scheme. By this step, a highly reactive propagating radical and a not overly stable three-arm star species are transformed into the resonance-stabilized RAFT intermediate radical and a very stable polymer molecule. The time evolution of concentration is discussed for the main-equilibrium range of CDB-mediated methyl acrylate polymerization
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