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Termination Rate Coefficients (termination + rate_coefficient)

Distribution by Scientific Domains

Polymers and Materials Science	100%

Selected Abstracts

Termination Rate Coefficients for Radical Homopolymerization of Methyl Methacrylate and Styrene at Low Conversion,

MACROMOLECULAR CHEMISTRY AND PHYSICS, Issue 5 2010
David R. Taylor
Abstract A comprehensive and systematic study of overall termination rate coefficients, kt, in low-conversion radical (homo)polymerization of methyl methacrylate and styrene is presented. Values of kt were determined by gravimetric analysis of steady-state experiments, employing 2,2,-azoisobutyronitrile as initiator. The values delivered by this simple method were found to be in qualitative and quantitative agreement with those from more modern and sophisticated techniques for measuring kt. Accordingly, correlations for bulk, low-conversion kt as a function of temperature are given for each monomer. The effects of initiator concentration, cI, and temperature on bulk kt were studied in a controlled way for both monomers. Additionally, ethyl benzene was used as solvent in order to investigate rigorously the effect of monomer concentration, cM, on styrene kt. The trends found by these systematic studies were considered in the light of what is known about the chain-length dependence of termination. Styrene's behavior was always found to be qualitatively in accord with expectation, although the variations of kt with cI and cM were not as strong as should be the case. However its activation energy, 15,kJ,·,mol,1, is shown to be almost perfectly in agreement with theory. Methyl methacrylate, on the other hand, is recalcitrant in that its overall kt does not make manifest the chain-length dependent termination that has been directly measured by other techniques. Possible reasons for these discrepancies are discussed, as are reasons for the difference in values between kt for the two monomers. On the latter topic it is concluded likely that the chain-length dependence of termination at short chain lengths is primarily responsible for styrene having kt that is higher by a factor of about 3, with there also being a contribution that arises from styrene's slower propagation. [source]

Design Criteria for Accurate Measurement of Bimolecular Radical Termination Rate Coefficients via the RAFT-CLD-T Method

MACROMOLECULAR THEORY AND SIMULATIONS, Issue 9 2008
Geoffrey Johnston-Hall
Abstract The reversible addition-fragmentation chain transfer chain length dependent termination (RAFT-CLD-T) technique allows a simple experimental approach to obtain chain-length-dependent termination rate coefficients as a function of conversion, k(x). This work provides a set of criteria by which accurate k(x) can be obtained using the RAFT-CLD-T method. Visualization of three-dimensional plots varying all kinetic rate parameters and starting concentrations demonstrates that only certain combinations give an accurate extraction of k(x). The current study provides hands-on guidelines for experimentalists applying the RAFT-CLD-T method. [source]

High-Temperature Propagation and Termination Kinetics of Styrene to High Conversion Investigated by Electron Paramagnetic Resonance Spectroscopy

MACROMOLECULAR CHEMISTRY AND PHYSICS, Issue 6 2004
Per B. Zetterlund
Abstract Summary: The free radical bulk polymerization of styrene at 120,°C has been investigated over almost the entire conversion range using electron paramagnetic resonance spectroscopy, Fourier-transform near-infrared spectroscopy and gel permeation chromatography. The free radical concentration went through a sharp maximum that coincided with the peak in the rate of polymerization during the gel effect, and shifted to higher conversion with increasing initiator concentration. The termination rate coefficient (kt), decoupled from the initiator efficiency (f) by use of the instantaneous degree of polymerization, remained close to constant up to as high as approximately 80% conversion, at which a dramatic decrease occurred. Both the propagation rate coefficient (kp) and f fell orders of magnitude near 80% conversion in spite of the temperature being above the glass transition temperature of the system. The value of kp increased with the initiator concentration at a given conversion in the highest (diffusion-controlled) conversion range. Termination rate coefficient (kt) versus conversion for bulk free radical polymerization of St initiated by TBP at 120,°C. [TBP],=,0.15 (,), 0.10 (,) and 0.05 M (). [source]

Free Radical Bulk Polymerization of Styrene: Simulation of Molecular Weight Distributions to High Conversion Using Experimentally Obtained Rate Coefficients

MACROMOLECULAR THEORY AND SIMULATIONS, Issue 6 2003
Per B. Zetterlund
Abstract Previously obtained experimental conversion-dependences of the propagation rate coefficient (kp), the termination rate coefficient (kt) and the initiator efficiency (f) for the free-radical bulk polymerization of styrene at 70,°C have been used to simulate the full molecular weight distributions (MWD) to high conversion using the software package PREDICI, providing a robust test of the kinetic model adopted. Satisfactory agreement with the experimental MWD's (GPC) was obtained up to approximately 70% conversion. Beyond 70% conversion, the high MW shoulder that appears was correctly predicted, although the amount of such polymer was somewhat underestimated. This discrepancy is believed to probably have its origin in experimental error in the conversion-dependences of kp, kt and f, in particular kt, that were employed in the simulations, rather than indicate a more fundamental short-coming of the model employed. [source]

Termination in Dilute-Solution Free-Radical Polymerization: A Composite Model

MACROMOLECULAR THEORY AND SIMULATIONS, Issue 5 2003
Gregory B. Smith
Abstract Literature data are summarized for the chain-length-dependence of the termination rate coefficient in dilute solution free-radical polymerizations. In essence such experiments have yielded two parameter values: the rate coefficient for termination between monomeric free radicals, k, and a power-law exponent e quantifying how kt values decrease with increasing chain length. All indications are that the value e,,,0.16 in good solvent is accurate, however the values of k which have been deduced are considerably lower than well-established values for small molecule radicals. This seeming impasse is resolved by putting forward a ,composite' model of termination: it is proposed that the value e,,,0.16 holds only for long chains, with e being higher for small chains , the value 0.5 is used in this paper, although it is not held to dogmatically. It is then investigated whether this model is consistent with experimental data. This is a non-trivial task, because although the experiments themselves and the ways in which they are analyzed are elegant and not too complicated, the underlying theory is sophisticated, as is outlined. Simulations of steady-state polymerization experiments are first of all carried out, and it is shown that the composite model of termination both recovers the e values which have been found and beautifully explains why these experiments considerably underestimate the true value of k. Simulations of pulsed-laser polymerizations find the same, although not quite so strikingly. It is therefore concluded that our new termination model, which retains the virtue of simplicity and in which all parameter values are physically reasonable, is consistent with experimental data. Taking a wider view, it seems likely that the situation of the exponent e varying with chain length will not just be the case in dilute solution, but will be the norm for all conditions, which would give our model and our work a general relevance. Normalized chain length distributions from PLP simulations. [source]