1,721,017 research outputs found
Critically evaluated propagation rate coefficients in free-radical polymerizations: Part III. Methacrylates with cyclic ester groups - (IUPAC Technical Report)
No full textPropagation rate coefficients, k(P), as a function of temperature, are reported for bulk free-radical homopolymerizations of oxiranylmethyl, cyclohexyl, and benzyl methacrylate at ambient pressure and low conversion. The data were obtained from experiments combining pulsed-laser initiated polymerization and size-exclusion chromatography. The data determined from experiments carried out in independent laboratories obey the consistency criteria established for this technique. The rate coefficients for the three monomers are well represented by a single Arrhenius relation
Requirements associated with studies into a chain-length dependence of propagation rate coefficients via PLP-SEC experiments
No full textExperiments directed toward the determination of free-radical propagation rate coefficients kappa(p) from pulsed laser initiated polymerizations (PLP) in conjunction with size-exclusion chromatography (SEC) were modeled using the program PREDICI. The simulations were carried out in order to investigate whether an experimentally observed variation of kappa(p) with the initiating laser pulse repetition rate is necessarily due to a chain-length dependence of kappa(p) The modeling was performed for laser pulse repetition rates ranging from 0.5 to 50 Hz and primary radical concentrations being varied by more than 2 orders of magnitude. The resulting molecular weight distributions were very different in shape. The kappa(p) data derived from the simulated molecular weight distributions showed a variation of about 30% with chain lengths, although the same propagation rate coefficient was introduced into all the simulations. The experimental finding of kappa(p) being dependent on chain length induced by using different laser pulse repetition rates is not necessarily due to a chain-length dependence of kappa(p). The modeling results show the importance of reporting all experimental details together with kappa(p) data derived from PLP-SEC experiments
Impact of hydrogen bonding on propagation kinetics in butyl methacrylate radical polymerizations
No full textPulsed laser initiated polymerizations of butyl methacrylate in butanol show a significant enhancement of the propagation rate coefficient k(p) compared to polymerizations in bulk. The activation energy E-A of k(p) is reduced due to the presence of butanol: at a monomer concentration of 0.8 mol.L-1 k(p) for polymerizations in butanol at 0degreesC is about 85% higher than in bulk, the difference reduces to 44% at 80degreesC. These findings are associated with a reduction in E-A of k(p) from 23.0 kJ.mol(-1) for bulk systems to 20.5 kJ.mol(-1) for polymerizations in butanol. The increase of k(p) in the presence of an alcohol is explained by the formation of hydrogen bonds between the OH group of the solvent and the carbonyl group of the monomer
Propagation kinetics in free-radical polymerizations
No full textThe combination of pulsed laser initiated polymerizations (PLP) with analysis of the generated polymer by size-exclusion chromatography (SEC) yields reliable individual rate coefficients for polymerizations of a large number of monomers in bulk and in solution. PLP-SEC experiments carried out in the presence of scCO(2) as a solvent show no unambiguous trend: while a significant reduction of k(p) is seen for some monomers, e.g. acrylates, k(p) for monomers such as vinyl acetate and styrene is not affected. It is suggested that the influence of CO2 on acrylate k(p) is not a true kinetic effect and that the experimental findings may be understood in terms of the occurrence of local monomer concentrations in the vicinity of the propagating radical. It is discussed that such local monomer concentrations may also contribute to a better understanding of why k(p) increases with ester size within the acrylate or within the methacrylate family, and why k(p) frequently is influenced by the initiating laser pulse repetition rate
The influence of hydrogen bonding on the propagation rate coefficient in free-radical polymerizations of hydroxypropyl methacrylate
No full textThe propagation rate coefficient k(P) was determined for hydroxypropyl methacrylate by applying pulsed laser initiated polymerizations and subsequent analysis of the polymer by size-exclusion chromatography. k(P) data were derived for polymerizations in bulk and in several solvents: toluene, tetrahydrofuran (THF), benzyl alcohol, and supercritical CO2. With the exception of THF, no solvent influence on k(p) was observed. For polymerizations in THF k(p) values 40% below the corresponding bulk data were obtained. In addition, the activation energy of kp for polymerizations in THF is higher than for the other systems. The results are explained by a complexation of the OH group contained in the ester group with THF. As a consequence, H bonds between OH groups and carbonyl O atoms, which occur in the other systems, are not formed in the presence of THF. This explanation is supported by Raman spectra, which show that. association of carbonyl groups does not occur for systems containing THF, whereas for all other systems the occurrence of two peaks at 1 703 cm(-1) and 1720 cm(-1) is indicative of the vibrations of two different - associated vs. not associated - types of carbonyl groups. Based on the change in activation energy it is suggested that a true kinetic solvent effect occurs
Rate coefficients of free-radical polymerization deduced from pulsed laser experiments
No full textPulsed laser techniques have enormously improved the quality by which rate coefficients of individual steps in free-radical polymerization may be measured. Pulsed laser initiated polymerization (PLP) in conjunction with size-exclusion chromatography (SEC) yields the propagation rate coefficient, k(p). The PLP-SEC-technique has been applied to a wide variety of homopolymerizations and copolymerizations, either in bulk or in solution. In addition to reporting kinetic data, experimental details of PLP, of SEC, and of the limitations associated with the accurate determination of the MWD are discussed. The single pulse (SP)-PLP method, which combines PLP with time-resolved NLR spectroscopy, allows for a very detailed insight into the termination rate coefficient, k(t), for homo- and copolymerizations. k(t), data are reported as a function of temperature, pressure, monomer conversion, solvent concentration, and partly also of chain length. This review considers literature up to December 2000. (C) 2002 Elsevier Science Ltd. All rights reserved
A novel approach to the understanding of the solvent effects in radical polymerization propagation kinetics
No full textPropagation kinetics of isobornyl methacrylate (iBoMA) and methyl methacrylate (MMA) free-radical homopolymerization were studied by the PLP-SEC method, which consists of pulsed laser initiated polymerizations and subsequent polymer analysis by size-exclusion chromatography. The reactions were carried out in bulk and in solution of several organic solvents: toluene, tetralin, tetrahydrofuran (THF), methyl isobutyrate, and phenethyl isobutyrate. In all polymerization systems, the activation energy of the observed propagation rate coefficient, k(p),(app), is not affected by either solvent type or solvent concentration. However, kp,app may be significantly different from the corresponding bulk values. In MMA polymerizations, kp,app values in solution were higher than in bulk, with the strongest enhancement of 50% observed for tetralin as solvent. In contrast, a solvent-induced lowering in propagation rate of up to 52% for THF as solvent was seen for iBoMA. The observed changes in propagation rate were assigned to the occurrence of a local monomer concentration in the vicinity of the free-radical chain end rather than to an intrinsic kinetic effect. The variation in local monomer concentrations is suggested to originate from differences in molar volume of the monomer and the solvent. Critical analysis of the results led to a linear correlation between solvent-induced changes in propagation rate and the difference in molar volumes of monomer and solvent. The correlation holds for both monomers
Aqueous phase size-exclusion-chromatography used for PLP-SEC studies into free-radical propagation rate of acrylic acid in aqueous solution
No full textPulsed laser polymerization (PLP) in conjunction with analysis of the resulting polymer by size-exclusion-chromatography (SEC) was used to measure propagation rate coefficients, kp, of acrylic acid (AA) in aqueous solution at concentrations of 20 and 40 wt % and temperatures between 2.6 and 28.5 degreesC. Under these conditions, more than 99% of acrylic acid exists in the nonionized form. The molecular weight distribution MWD) of poly(AA) is directly measured by aqueous SEC. The kp values at 20 wt 7( are about 60% higher than at 40 wt %. The activation energy of hp is close to 12.0 kJ . mol(-1) at both concentrations. At 25 degreesC and 20 wt %, k(p) of AA in aqueous solution exceeds 100 000 L . mol(-1).s(-1) which is the highest kp value determined by the PLP-SEC technique so far
PLP-SEC study into the free-radical propagation rate coefficients of partially and fully ionized acrylic acid in aqueous solution
No full textPropagation rate coefficients, kappa(p), for acrylic acid (AA) polymerization at 6 degreesC in aqueous solution were measured via pulsed laser polymerization (PLP) with the degree of ionization, alpha, varied over the entire range between 0 and 1. These measurements were carried out in conjunction with aqueous-phase size-exculsion chromatography (SEC). Strictly speaking, the reported kappa(p) 's are "apparent" propagation rate coefficients deduced from the PLP-SEC data under the assumption that the local monomer concentration at the radical site is identical to overall monomer concentration. At an AA concentration of 0.69 mol.L-1, the apparent kappa(p) decreaes from 111 000 L.mol(-1).s(-1) at alpha = 0 to 13 000 L.mol(-1).s(-1) at alpha = 1.0. The significant lowering of kappa(p) with higher alpha is attributed to the repulsion between both monomer molecules and macroradicals becoming negatively charged. Addition of up to 10 mol-% (with respect to AA) sodium hydroxide to the fully ionized aqueous AA solution leads to an enhancement of kappa(p) up to 57 000 L.mol(-1).s(-1)
Free-radical terpolymerization of styrene and two methacrylates in a homogeneous phase containing supercritical CO2
No full textStyrene-methyl methacrylate-glycidyl methacrylate terpolymers to be used as binders in powder coating materials were synthesized in a homogeneous fluid phase containing 20 wt % CO2. Reactions carried out at 120 degreesC and 350 bar yield almost complete monomer conversion. The molecular weights were around 3000 g.mol(-1) with a polydispersity index of 1.7. Molecular weight control was achieved by n-dodecyl mercaptan or by a catalytic chain-transfer agent. Simulations of the cumulative and differential copolymer composition are in excellent agreement with experimentally derived values
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