31,950 research outputs found
"Assessing the RAFT Equilibrium Constant via Model Systems: An EPR Study" - Response to a Comment
No full textWe have presented an EPR-based approach for deducing the RAFT equilibrium constant, Keq, of a dithiobenzoate-mediated system [Meiser, W. and Buback M. Macromol. Rapid Commun. 2011, 32, 1490]. Our value is by four orders of magnitude below Keq from ab initio calculations for the identical monomer-free system. Junkers et al. [Macromol. Rapid Commun. 2011, 32, 1891] claim that our EPR approach would be model dependent and our data could be equally well fitted by assuming slow addition of radicals to the RAFT agent and slow fragmentation of the so-obtained intermediate radical as well as high cross-termination rate. By identification of all side products, our EPR-based method is shown to be model independent and to provide reliable Keq values, which demonstrate the validity of the intermediate radical termination model
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
POLY 2-Propagation and termination of radicals: Detailed analysis of polymerization kinetics via pulsed-laser-assisted techniques and high-pressure experiments
No full text
POLY 2-Propagation and termination of radicals: Detailed analysis of polymerization kinetics via pulsed-laser-assisted techniques and high-pressure experiments
No full textSP-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
Termination, Propagation, and Transfer Kinetics of Midchain Radicals in Methyl Acrylate and Dodecyl Acrylate Homopolymerization
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