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Low Conversions (low + conversion)
Selected AbstractsTermination Rate Coefficients for Radical Homopolymerization of Methyl Methacrylate and Styrene at Low Conversion,MACROMOLECULAR CHEMISTRY AND PHYSICS, Issue 5 2010David 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] Homogeneous Hydrogenation of Tri- and Tetrasubstituted Olefins: Comparison of Iridium-Phospinooxazoline [Ir-PHOX] Complexes and Crabtree Catalysts with Hexafluorophosphate (PF6) and Tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (BArF) as CounterionsADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 1 2008Bettina Wüstenberg Abstract Four iridium complexes with achiral phosphino-oxazoline (PHOX) ligands were readily prepared in two steps starting from commercially available phenyloxazolines. The air-stable complexes with tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (BArF) as counterion showed high reactivity in the hydrogenation of a range of tri- and tetrasubstituted olefins. The best results were obtained with an iridium complex (11) derived from a dicyclohexylphosphino-oxazoline ligand bearing no additional substituents in the oxazoline ring. With several substrates, which gave only low conversion with the Crabtree catalyst, [Ir(Py)(PCy3)(COD)]PF6, full conversion was observed. The productivity of the Crabtree catalyst could be strongly increased by replacing the hexafluorophosphate anion with tetrakis[3,5-bis(trifluoromethyl)phenyl]borate. In one case, in the hydrogenation of a tetraalkyl-substituted CC bond, [Ir(Py)(PCy3)(COD)]BArF gave higher conversion than catalyst 11. However, with several other substrates complex 11 proved to be superior. [source] High temperature copolymerization of styrene/ethyl acrylate: Reactivity ratio estimation in bulk and solutionADVANCES IN POLYMER TECHNOLOGY, Issue 3 2004Nahla Sahloul Abstract Styrene/ethyl acrylate (Sty/EA) free-radical copolymerizations have been conducted in bulk with and without initiator and in solution using p -xylene and m -xylene (30 wt% and 60 wt% solvent level) at 100°C and 130°C. The monomer reactivity ratio values and their temperature dependence have been determined from low conversion copolymer composition data using the computer software package RREVM, which is based on the error in variables model (EVM) method. Copolymer composition data at low conversion confirmed the Mayo,Lewis model at both temperatures. The reactivity ratio values of the Sty/EA system do not seem to change with dilution by nonpolar solvents at low concentration from the reactivity ratio values obtained in bulk with and without initiator. However, xylene isomers were found to have a slight effect on the reactivity ratio value of ethyl acrylate when high solvent concentration was utilized. © 2004 Wiley Periodicals, Inc. Adv Polym Techn 23: 186,195, 2004; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/adv.20009 [source] Compartmentalization in Atom Transfer Radical Polymerization (ATRP) in Dispersed Systems,MACROMOLECULAR THEORY AND SIMULATIONS, Issue 8 2006Yasuyuki Kagawa Abstract Summary: Compartmentalization in atom transfer radical polymerization (ATRP) in dispersed systems at low conversion (<10%) has been investigated by means of a modified Smith,Ewart equation focusing on the system n -butyl acrylate/CuBr/4,4,-dinonyl-2,2,-dipyridyl at 110,°C. Compartmentalization of both propagating radicals and deactivator was accounted for in the simulations. As the particle diameter (d) decreases below 70 nm, the polymerization rate (Rp) at 10% conversion increases relative to the corresponding bulk system, goes through a maximum at 60 nm, and thereafter decreases dramatically as d decreases further. This behavior is caused by the separate effects of compartmentalization (segregation and confined space effects) on bimolecular termination and deactivation. The very low Rp for small particles (d,<,30 nm) is due to the pseudo first-order deactivation rate coefficient being proportional to d,3. Simulated propagating radical concentration ([P,]) as a function of particle diameter (d) at 10% conversion for ATRP of n -butyl acrylate ([nBA]0,=,7.1 M, [PBr]0,=,[CuBr/dNbpy]0,=,35.5 mM) in a dispersed system at 110,°C. The dotted line indicates the simulated [P,] in bulk at 10% conversion. [source] Positron emission tomography of [18F]-big endothelin-1 reveals renal excretion but tissue-specific conversion to [18F]-endothelin-1 in lung and liverBRITISH JOURNAL OF PHARMACOLOGY, Issue 4 2010Peter Johnström Background and purpose:, Big endothelin-1 (ET-1) circulates in plasma but does not bind to ET receptors until converted to ET-1 by smooth muscle converting enzymes. We hypothesized that tissue-specific conversion of [18F]-big ET-1 to [18F]-ET-1 could be imaged dynamically in vivo within target organs as binding to ET receptors. Methods:, [18F]-big ET-1 conversion imaged in vivo following infusion into rats using positron emission tomography (PET). Key results:, [18F]-big ET-1 was rapidly cleared from the circulation (t1/2= 2.9 ± 0.1 min). Whole body microPET images showed highest uptake of radioactivity in three major organs. In lungs and liver, time activity curves peaked within 2.5 min, then plateaued reaching equilibrium after 10 min, with no further decrease after 120 min. Phosphoramidon did not alter half life of [18F]-big ET-1 but uptake was reduced in lung (42%) and liver (45%) after 120 min, consistent with inhibition of enzyme conversion and reduction of ET-1 receptor binding. The ETA antagonist, FR139317 did not alter half-life of [18F]-big ET-1 (t1/2= 2.5 min) but radioactivity was reduced in all tissues except for kidney consistent with reduction in binding to ETA receptors. In kidney, however, the peak in radioactivity was higher but time to maximum accumulation was slower (,30 min), which was increased by phosphoramidon, reflecting renal excretion with low conversion and binding to ET receptors. Conclusions and implications:, A major site for conversion was within the vasculature of the lung and liver, whereas uptake in kidney was more complex, reflecting excretion of [18F]-big ET-1 without conversion to ET-1. This article is part of a themed section on Imaging in Pharmacology. To view the editorial for this themed section visit http://dx.doi.org/10.1111/j.1476-5381.2010.00685.x [source] The classical kinetic model for radical chain oxidation of hydrocarbon substrates initiated by bimolecular hydroperoxide decompositionINTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 11 2006X. Colin Kinetic modeling of the low-temperature (typically T , 200°C) thermal aging of polymers is a problem of great technological importance, owing to the continuously increasing needs of industry in terms of reliable methods for lifetime prediction. In the temperature domain under consideration, for most hydrocarbon substrates, oxidation proceeds by a radical chain reaction initiated by bimolecular hydroperoxide decomposition. In other words, the reaction generates its own initiator, which explains its strong autoaccelerated character. The most pertinent model is, to our opinion, the model elaborated by Tobolsky et al. (J Am Chem Soc 1950, 72, 1942) in the early 1950s. This model is, however, based on three questionable assumptions: the existence of a stationary state for radical concentrations (hypothesis S), the presence of oxygen in excess (hypothesis E), and the fact that the onset of steady state can be observed in the domain of low conversions, where the substrate consumption can be neglected (hypothesis L). One hypothesis (S) lacks consistency. A sounder alternative, which does not modify significantly the mathematical expressions of the model, will be proposed. The other hypotheses (E and L) can be justified in certain cases, but the limits of their domain of validity were never established to our knowledge. It is tried, here, to express these limits in function of fundamental parameters: rate constants and concentrations of reactants. © 2006 Wiley Periodicals, Inc. Int J Chem Kinet 38:666,676, 2006 [source] Millisecond catalytic reforming of monoaromatics over noble metalsAICHE JOURNAL, Issue 4 2010C. M. Balonek Abstract The millisecond autothermal reforming of benzene, toluene, ethylbenzene, cumene, and styrene were independently studied over five noble metal-based catalysts: Pt, Rh, Rh/,-Al2O3, Rh,Ce, and Rh,Ce/,-Al2O3, as a function of carbon-to-oxygen feed ratio. The Rh,Ce/,-Al2O3 catalyst exhibited the highest feedstock conversion as well as selectivities to both synthesis gas and hydrocarbon products (lowest selectivities to H2O and CO2). Experimental results demonstrate a high stability of aromatic rings within the reactor system. Benzene and toluene seem to react primarily heterogeneously, producing only syngas and combustion products. Ethylbenzene and cumene behaved similarly, with higher conversions than benzene and toluene, and high product selectivity to styrene, likely due to homogeneous reactions involving their alkyl groups. Styrene exhibited low conversions over Rh,Ce/,-Al2O3, emphasizing the stability of styrene in the reactor system. © 2009 American Institute of Chemical Engineers AIChE J, 2009 [source] Kinetic investigation of the RAFT polymerization of p -acetoxystyreneJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 12 2010Solène I. Cauët Abstract The kinetics of the RAFT polymerization of p -acetoxystyrene using a trithiocarbonate chain transfer agent, S -1-dodecyl- S,-(,,,,-dimethyl-,,-acetic acid)trithiocarbonate, DDMAT, was investigated. Parameters including temperature, percentage initiator, concentration, monomer-to-chain transfer agent ratio, and solvent were varied and their impact on the rate of polymerization and quality of the final polymer examined. Linear kinetic plots, linear increase of Mn with monomer conversion, and low final molecular weight dispersities were used as criteria for the selection of optimized polymerization conditions, which included a temperature of 70 or 80 °C with 10 mol % AIBN initiator in bulk for low conversions or in 1,4-dioxane at a monomer-to-solvent volume ratio of 1:1 for higher conversions This study opens the way for the use of DDMAT as a chain transfer agent for RAFT polymerization to incorporate p -acetoxystyrene together with other functional monomers into well-defined copolymers, block copolymers, and nanostructures. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 2517,2524, 2010 [source] The kinetics of enhanced spin capturing polymerization: Influence of the nitrone structureJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 4 2009Edgar H. H. Wong Abstract Several nitrones and one nitroso compound have been evaluated for their ability to control the molecular weight of polystyrene via the recently introduced radical polymerization method of enhanced spin capturing polymerization (ESCP). In this technique, molecular weight control is achieved (at ambient or slightly elevated temperatures) via the reaction of a growing radical chain with a nitrone forming a macronitroxide. These nitroxides subsequently react rapidly and irreversibly with propagating macroradicals forming polymer of a certain chain length, which depends on the nitrone concentration in the system. Via evaluation of the resulting number-average molecular weight, Mn, at low conversions, the addition rate coefficient of the growing radicals onto the different nitrones is determined and activation energies are obtained. For the nitrones N - tert -butyl-,-phenylnitrone (PBN), N -methyl-,-phenylnitrone (PMN), and N -methyl-,-(4-bromo-phenyl) nitrone (pB-PMN), addition rate coefficients, kad,macro, in a similar magnitude to the styrene propagation rate coefficient, kp, are found with spin capturing constants CSC (with CSC = kad,macro/kp) ranging from 1 to 13 depending on the nitrone and on temperature. Activation energies between 23.6 and 27.7 kJ mol,1 were deduced for kad,macro, congruent with a decreasing CSC with increasing temperature. Almost constant Mn over up to high monomer to polymer conversions is found when CSC is close to unity, while increasing molecular weights can be observed when the CSC is large. From temperatures of 100 °C onward, reversible cleavage of the alkoxyamine group can occur, superimposing a reversible activation/deactivation mechanism onto the ESCP system. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 1098,1107, 2009 [source] Chemorheological analysis of an epoxy-novolac molding compoundPOLYMER ENGINEERING & SCIENCE, Issue 2 2000T. H. Hsieh The chemorheological behavior of an epoxy-novolac molding compound was studied by a combination of differential scanning calorimetry and dynamic rheological measurements. Based on a modified version of Kamal and Sourour's kinetic expression, a procedure aiming at the phenomenological description of cure kinetics was developed. In combination with our kinetic study, an empirical Arrhenius-type expression was adopted for the description of the dependence of complex viscosity on temperature, frequency, and conversion by allowing the pre-exponential factor and the flow activation energy to be frequency- and conversion-dependent. At low conversions (, < ,0.05), the system behaves approximately as a thermoplastic material; at higher conversions, the rheological behavior of the system was dominated by the extent of cure reaction. [source] Ligand-Activated Lithium-Mediated Zincation of N -PhenylpyrroleCHEMISTRY - A EUROPEAN JOURNAL, Issue 35 2007Anne Seggio Abstract Metalation of N -phenylpyrrole by using an in situ mixture of ZnCl2,TMEDA (0.5,equiv; TMEDA=N,N,N,,N,-tetramethylethylenediamine) and LiTMP (1.5,equiv; TMP=2,2,6,6-tetramethylpiperidino) was optimized. The reaction carried out at room temperature in THF resulted in incomplete metalation (56,% conversion) and selectivity (mixture of 2-iodo and 2,2,-diiodo derivatives in an 86:14 ratio after trapping with iodine). By using diethyl ether (DEE), toluene, or hexane instead of THF, low conversions of 17, 38, or 23,% were observed, respectively, but the formation of the diiodide was avoided. When hexane was used as solvent, strong lithium-complexing ligands such as [12]crown-4 and N,N,-dimethylpropylideneurea (DMPU) inhibited the reaction whereas more (hemi)labile ligands (TMEDA>THF,DME) favored it. This result shows that a temporary accessibility of lithium to interact with the rest of the base and/or the substrate is a prerequisite for an efficient metalation. A 75,% yield of 2-iodo- N -phenylpyrrole was obtained after reaction with the base in the presence of five equivalents of TMEDA for two hours at room temperature, and subsequent trapping with iodine. We were able to successfully replace the spare TMP with a less expensive butyl group. [source] | |||||||||||||