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"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
EPR Investigations into the Kinetics of Trithiocarbonate-Mediated RAFT-Polymerization of Butyl Acrylate
No full textElectron paramagnetic resonance (EPR) spectroscopy is used for measuring rate coefficients of addition, k(ad), and fragmentation, k, together with the associated equilibrium constants, K-eq, for butyl acrylate polymerizations mediated by S-ethyl propan-2-ylonate-S'-propyl trithiocarbonate (EPPT) and by S-S'-bis(methyl-2-propionate) trithiocarbonate (BMPT). Experiments at -40 degrees C yield k(ad) = (3.4 +/- 0.3) x 10(6) L mol(-1) s(-1), k = (1.4 +/- 0.4) x 10(2) s(-1), and K-eq = (2.6 +/- 0.8) x 10(4) L mol(-1) for EPPT and k(ad) = (4.1 +/- 0.9) x 10(6) L mol(-1) s(-1), k = (4.5 +/- 0.5) x 10(1) s(-1), and K-eq = (8 +/- 4) x 10(4) L mol(-1) for BMPT. The K-eq values are in satisfactory agreement with data from ab initio calculations
Mechanism of CPDB-Mediated RAFT Polymerization of Methyl Methacrylate: Influence of Pressure and RAFT Agent Concentration
No full textReversible addition-fragmentation chain transfer (RAFT) polymerizations of methyl methacrylate (MMA) in bulk at 60 degrees C were performed at five pressures up to 200 MPa using 2-(2'-cyanopropyl)dithiobenzoate (CPDB) as RAFT agent at concentrations between 1.5 x 10(-3) and 2.0 x 10(-2) mol L(-1). Applying high pressure during polymerization increases the rate of polymerization, but no effect on polydispersity was observed. Molecular weight distributions and average molecular weights of the final polymer indicated the successful control of MMA polymerization even at low CPDB concentrations. The slight retardation observed is adequately described by the dependence the termination rate coefficient, k(t), on the chain-length
SP-PLP-EPR Study into Termination and Transfer Kinetics of Non-Ionized Acrylic Acid Polymerized in Aqueous Solution
No full textPolymerization of nonionized acrylic acid (AA) in aqueous solution has been studied via single pulse-pulsed laser polymerization in conjunction with electron paramagnetic resonance spectroscopy. Termination of two types of radicals, secondary chain-end radicals (SPRs) and midchain radicals (MCRs), as well as intramolecular chain transfer (backbiting) of SPRs have been studied between 5 and 40 degrees C at initial AA concentrations of 10 and 50 wt % in the presence of 15 wt % poly(AA). Predici modeling of the measured SPR and MCR concentration vs time traces after single-pulse initiation at t = 0 yields rate coefficients for backbiting, k(bb), and for propagation from an MCR, k(p)(t). These rate coefficients increase toward lower AA-in-water concentration. Estimates for termination of two SPRs, k(t)(s,s), and of an SPR and an MCR, k(t)(s,t) have been obtained by assuming the reported composite-model behavior for acrylates to also hold for acrylic acid polymerization.Fonds der Chemischen Industrie; BASF SE, Ludwigshafen, German
Kinetics of Dithiobenzoate-Mediated Methyl Methacrylate Polymerization
No full textThe equilibrium constant, K-eq, of the reversible addition-fragmentation chain transfer (RAFT) model system methyl methacryl dithiobenzoate (MMADB) and of methyl methacrylate (MMA) polymerization mediated by MMADB is studied via electron paramagnetic resonance (EPR) spectroscopy in the range 70 to 110 degrees C and at -40 degrees C, respectively. The measured difference in activation energy of the addition and fragmentation steps is: E-a(k(ad)) - E-a(k(frag)) approximate to -36.6 kJ mol(-1). Significant amounts of "missing step" products from reaction of the cross-termination product with a methyl methacrylyl radical are found. The fast "missing step" reaction and low K-eq, due to slow k(ad), are responsible for rate retardation being absent in dithiobenzoate-mediated MMA polymerization
Assessing the RAFT Equilibrium Constant via Model Systems: An EPR Study
No full textReversible addition-fragmentation chain transfer (RAFT) equilibrium constants, Keq, for the model system cyano-iso-propyl dithiobenzoate (CPDB) - cyano-iso-propyl radical (CIP) have been deduced via electron paramagnetic resonance (EPR) spectroscopy. The CIP species is produced by thermal decomposition of azobis-iso-butyronitrile (AIBN). In solution of toluene at 70 degrees C, Keq has been determined to be (9 +/- 1) L.mol(-1). Measurement of Keq kad/kb between 60 and 100 degrees C yields Delta E(a) = (-28 +/- 4) kJ.mol(-1) as the difference in the activation energies of k(ad) and k(beta). The data measured on the model system are indicative of fast fragmentation of the intermediate radical produced by addition of CIP to CPDB
EPR Study into Cross-Termination and Fragmentation with the PhenylethylPhenylethyl Dithiobenzoate RAFT Model System
No full textThe reversible additionfragmentation chain transfer (RAFT) equilibrium constant of the phenylethyl1-phenylethyl dithiobenzoate model system is deduced via EPR spectroscopy to be 31 +/- 4 L mol1 at 110 degrees C. The difference in activation energies of addition and fragmentation, Ea(kad) Ea(k) approximate to 45.6 kJ mol1, is close to that of the polystyrylpolystyryl dithiobenzoate system. Significant amounts of products from cross-termination and from missing step reaction of the cross-termination product with the phenylethyl radical demonstrate that intermediate radical termination, rather than slow fragmentation of the RAFT intermediate radical, is responsible for the rate retardation in the dithiobenzoate-mediated styrene polymerization
Easy Access to the RAFT Equilibrium Constant
No full textA method is presented for determination of reliable values of the equilibrium constant K(eq) between propagating radicals (P(center dot)) and intermediate radicals (INT(center dot)) in reversible addition-fragmentation chain transfer (RAFT) polymerization. The technique is based oil EPR measurement of the ratio of concentrations of the two radical species Under quasi-equilibrium conditions ill low RAFT agent contents. The method requires no calibration of the EPR Setup for absolute radical concentration measurement. [Data for butyl acrylate polymerization mediated by benzyl propyl trithiocarbonate are presented. The value Of K(eq) at - 40 degrees C was determined to be (1.0 +/- 0.1) x 10(4) L mol(-1).Fonds der Chemischen Industri
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