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Induction Period (induction + period)
Selected AbstractsNucleation kinetics and growth aspects of semi organic non-linear optical bis thiourea cadmium acetate single crystalsCRYSTAL RESEARCH AND TECHNOLOGY, Issue 9 2006R. Sankar Abstract Nucleation parameters such as metastable zone width, induction period and interfacial energy have been determined for the aqueous solution growth of bis thiourea cadmium acetate (BTCA) single crystals. Solubility of BTCA has been determined for various temperatures. Metastable zone width and induction period values have been estimated in order to optimize the growth parameters. The interfacial tension values derived from experimentally determined induction period are found to be comparable with theoretical values. Bulk crystals of BTCA have been grown using the optimized growth parameters. The grown crystals have been subjected to structural, optical and mechanical property studies. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Influence of filling fat type on praline products with nougat fillingEUROPEAN JOURNAL OF LIPID SCIENCE AND TECHNOLOGY, Issue 12 2007Biljana Pajin Abstract The aim of this research was to examine the composition and functional characteristics of three samples of commercial filling fat intended for the making of nougat fillings for confectionary products. The crystallization rate of the fats was measured under static conditions using the NMR technique and under dynamic conditions as well as using a rotational rheometer. The functional characteristics of the fats were estimated by means of sensory values of the prepared praline product. Under the static conditions, all investigated filling fats showed a fast crystallization rate (,2,4%/min). The sample with the highest lauric acid contents started to crystallize immediately (no induction period). The longest induction period was found for the sample with the lowest saturated fatty acid and trans fatty acid contents. During the crystallization under dynamic conditions, all investigated samples started to crystallize more rapidly (rapid viscosity increase) at practically the same temperature (,17,°C). Measuring solid fat content and viscosity are two approaches of determining the suitability of a filling fat to be used in pralines. Both measurements provide valuable information, and particularly viscosity build-up is very important from the view of production on factory scale. [source] Evaluation of the stability of blends of sunflower and rice bran oilEUROPEAN JOURNAL OF LIPID SCIENCE AND TECHNOLOGY, Issue 5 2007Samia Mezouari Abstract Blends of sunflower oil (SFO) and rice bran oil (RBO) were evaluated for their stability. Additionally, known amounts of natural antioxidants extracted from RBO were added to SFO, and their protective effect was compared to that of the blends. The results found indicate that by raising the amount of RBO, from 10 to 50%, an increase of OLO, OLP, PPL, OOO, PPO, OPO, 18:1 and 16:0 occurred, followed by a decrease of LLL, LLO, and 18:2. These changes in fatty acid and triacylglycerol (TAG) composition led to an increase of the oil stability index at 120,°C and a reduction of polymer TAG formation in the heated blends at 180,°C during 8,h. A comparable protective effect of natural antioxidants to that of blending was observed in a 50,:,50 blend, by remarkably increasing the induction period. [source] Kinetics and mechanism of the oxidation of 4-methyl-3-thiosemicarbazide by acidic bromate,INTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 4 2002Sreekantha B. Jonnalagadda The oxidation of 4-methyl-3-thiosemicarbazide (MTSC) by bromate and bromine was studied in acidic medium. The stoichiometry of the reaction is extremely complex, and is dependent on the ratio of the initial concentrations of the oxidant to reductant. In excess MTSC and after prolonged standing, the stoichiometry was determined to be H3CN(H)CSN(H)NH2 + 3BrO3, , 2CO2 + NH4+ + SO42, + N2 + 3Br, + H+ (A). An interim stoichiometry is also obtained in which one of the CO2 molecules is replaced by HCOOH with an overall stoichiometry of 3H3CN(H)CSN(H)NH2 + 8BrO3, , CO2 + NH4+ + SO42, + HCOOH + N2 + 3Br, + 3H+ (B). Stoichiometry A and B are not very different, and so mixtures of the two were obtained. Compared to other oxidations of thiourea-based compounds, this reaction is moderately fast and is first order in both bromate and substrate. It is autocatalytic in HOBr. The reaction is characterized by an autocatalytic sigmoidal decay in the consumption of MTSC, while in excess bromate conditions the reaction shows an induction period before autocatalytic formation of bromine. In both cases, oxybromine chemistry, which involves the initial formation of the reactive species HOBr and Br2, is dominant. The reactions of MTSC with both HOBr and Br2 are fast, and so the overall rate of oxidation is dependent upon the rates of formation of these reactive species from bromate. Our proposed mechanism involves the initial cleavage of the CN bond on the azo-side of the molecule to release nitrogen and an activated sulfur species that quickly and rapidly rearranges to give a series of thiourea acids. These thiourea acids are then oxidized to the sulfonic acid before cleavage of the CS bond to give SO42,, CO2, and NH4+. © 2002 Wiley Periodicals, Inc. Int J Chem Kinet 34: 237,247, 2002 [source] Production of zero trans Iranian vanaspati using chemical transesterification and blending techniques from palm olein, rapeseed and sunflower oilsINTERNATIONAL JOURNAL OF FOOD SCIENCE & TECHNOLOGY, Issue 3 2008Jamshid Farmani Summary Chemical transesterification and blending techniques were used for producing zero trans fats suitable for use as Iranian vanaspati. Triple blends of palm olein (POo), rapeseed (RSO) and sunflower oil (SFO) were subjected to two different treatments: (i) blending and then transesterification (BT) and (ii) transesterification of pure POo before blending with RSO and SFO (TB). The changes in slip melting point (SMP), solid fat content (SFC), carbon number (CN) triacylglycerol (TAG) composition, induction period (IP) of oxidation at 120 °C and IP of crystallisation at 20 °C of blends before and after treatments were investigated. Both BT and TB methods resulted in an increase in the CN48 TAG molecules, SMP and SFC, and a decrease in the IP of oxidation and crystallisation of initial blends. Samples made by TB method had higher CN48 TAG content, SMP, SFC and IP of oxidation, and lower IP of crystallisation than those made by BT method. Correlation between SFC at 20 °C and saturated fatty acid (SFA) content of the treated blends indicated that the SFA must be higher than 33.1% and 26.8% for BT and TB methods, respectively, to obtain fats suitable for use as vanaspati. [source] Selective Hydrogenation of Amides using Ruthenium/ Molybdenum CatalystsADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 5 2010Graham Beamson Abstract Recyclable, heterogeneous bimetallic ruthenium/molybdenum catalysts, formed in situ from triruthenium dodecacarbonyl [Ru3(CO)12] and molybdenum hexacarbonyl [Mo(CO)6], are effective for the selective liquid phase hydrogenation of cyclohexylcarboxamide (CyCONH2) to cyclohexanemethylamine (CyCH2NH2), with no secondary or tertiary amine by-product formation. Variation of Mo:Ru composition reveals both synergistic and poisoning effects, with the optimum combination of conversion and selectivity at ca. 0.5, and total inhibition of catalysis evident at ,1. Good amide conversions are noted within the reaction condition regimes 20,100,bar hydrogen and 145,160,°C. The order of reactivity of these catalysts towards reduction of different amide functional groups is primary>tertiary,secondary. In situ HP-FT-IR spectroscopy confirms that catalyst genesis occurs during an induction period associated with decomposition of the organometallic precursors. Ex situ characterisation, using XRD, XPS and EDX-STEM, for active Mo:Ru compositions, has provided evidence for intimately mixed ca. 2.5,4,nm particles that contain metallic ruthenium, and molybdenum (in several oxidation states, including zero). [source] Re-induction of obese body weight occurs more rapidly and at lower caloric intake in beaglesJOURNAL OF ANIMAL PHYSIOLOGY AND NUTRITION, Issue 3 2010D. Nagaoka Summary For the purpose of investigating the mechanism of obesity-induction/re-induction including weight-cycling in beagles, a study was conducted using commercially available dog food combined with human food to mimic at home-snacking and diet-supplementation behaviours. Adult female beagles, which had free access to water and exercise, were used (n = 9). All dogs were initially offered two times their daily calculated number of calories using a dry extruded diet plus blend of canola and soybean oils and allowed to eat ad libitum. After 3 weeks, Pecan shortbread cookies were added to the diet mixture. Obesity was induced during a 19-week period with 1875,2250 kcal/day consumed, on average, during this period. The dogs were then subjected to a weight-loss regimen while consuming 490,730 kcal/day. After weight loss, a similar degree of obesity was re-induced for 17 weeks even though dogs consumed only 1125,1250 kcal/day. Body weight, body condition scores, kcal consumption and food efficiency were recorded. Results indicated that less time and fewer kcal were required to re-induce the same degree of obesity compared with the initial obesity induction. Human snack foods appeared to stimulate appetite and thus contribute to the obese state. Food efficiency was also increased during the obesity-reinduction period compared with the induction period. This information may help pet owners better understand the need to limit table scraps and human-type food snacks in dogs prone to obesity as well as weight maintenance after weight loss. [source] Preparation of polyacrylonitrile with improved isotacticity and low polydispersityJOURNAL OF APPLIED POLYMER SCIENCE, Issue 5 2010Jianguo Jiang Abstract The preparation of a polymer with both low polydispersity and high tacticity is one current challenge we face and warrants thorough investigation from both the theoretical and experimental standpoints. In this study, we synthesized polyacrylonitrile (PAN) with simultaneously controlled molecular weight and tacticity on the basis of the strategy of the atom transfer radical polymerization (ATRP) of acrylonitrile (AN) in the presence of Lewis acids. A new combined initiation system of 3-bromopropionitrile (3-BPN)/Cu2O/N,N,N,,N,-tetramethylethylenediamine (TMEDA) was used for the ATRP of AN for the first time. When the polymerization was performed with the ratio [AN]0/[Initiator]0/[Cu2O]0/[TMEDA]0 = 190/1/0.5/1.5 (where the subscript 0 indicates the initial conditions) in ethylene carbonate at 60°C for 48 h, the polydispersity of the obtained PAN was 1.13, and the molecular weight was up to 13,710. The polymerization kinetics results show that the polymerizations proceeded with a living/controlled nature except that an induction period existed during the polymerization process because of the lower initiating activity of 3-BPN. Also, two kinds of Lewis acid, AlCl3 and yttrium trifluororomethanesulflnate, were used in the ATRP system of AN for the tacticity control. The addition of 0.01 equiv (relative to AN) of the Lewis acid AlCl3 in the polymerization afforded PAN with an improved isotacticity [meso/meso triad (mm) = 0.32] and a very narrow polydispersity (1.06). © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010 [source] Kinetic and heat transfer modeling of rubber blends' sulfur vulcanization with N - t -butylbenzothiazole-sulfenamide and N,N -di- t -butylbenzothiazole-sulfenamideJOURNAL OF APPLIED POLYMER SCIENCE, Issue 1 2007Likozar Abstract Vulcanization kinetics and heat transfer for various blends of natural (NR) and polybutadiene (BR) rubber were studied simultaneously using a mechanistic approach when developing vulcanization model kinetics. Rubber process analyzer (RPA), dynamic scanning calorimetry (DSC), and Fourier transform infrared spectroscopy (FTIR) methods were used for the study. The model reaction scheme was based on one of the best possible proposed individual reaction mechanisms. Molecular modeling was applied to distinguish between the reactivity of chemically similar species. The kinetics of N - t -butylbenzothiazole-sulfenamide (TBBS) and N,N -di- t -butylbenzothiazole-sulfenamide (TBSI) were treated separately using FTIR experiment data, and then incorporated in a model suitable for two-accelerator vulcanization. The proposed model quite well describes the thermal equilibration during the induction period despite a few simplifications. During cure and over-cure periods the course of vulcanization was described using a rigorous kinetic model. Physical and chemical model parameters were calculated from experimental data. Average heat transfer coefficient minimum during induction period was found to be at a weight ratio of BR and NR 1 : 1. The activation energy of significant reactions between rubber and other species was found to vary linearly with vulcanization compound composition. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 293,307, 2007 [source] Evaluation of the Self-Heating Tendency of Vegetable Oils by Differential Scanning CalorimetryJOURNAL OF FORENSIC SCIENCES, Issue 6 2008Amélie Baylon M.S. Abstract:, The evaluation of the self-heating propensity of a vegetable (or animal) oil may be of significant importance during the investigation of a fire. Unfortunately, iodine value and gas chromatographic-mass spectrometric analysis do not lead to meaningful results in this regard. To the contrary, differential scanning calorimetry (DSC), which does not measure the chemical composition of the oil, but rather its thermodynamic behavior, produces valuable results. After a thorough literature review on the autooxidation of vegetable oils, several oils with different self-heating tendencies were analyzed using a Mettler-Toledo differential scanning calorimeter DSC 25 between 40°C and 500°C. Analyses were carried out both under air and nitrogen atmosphere to identify the phenomena due to autooxidation reactions. Using DSC, it was possible to observe the induction period of the oil (when available), the three different exothermic events, and the autoignition temperature (relatively independent of the oil type). [source] Chemical-looping combustion process: Kinetics and mathematical modelingAICHE JOURNAL, Issue 4 2010Ion Iliuta Abstract Chemical Looping Combustion technology involves circulating a metal oxide between a fuel zone where methane reacts under anaerobic conditions to produce a concentrated stream of CO2 and water and an oxygen rich environment where the metal is reoxidized. Although the needs for electrical power generation drive the process to high temperatures, lower temperatures (600,800°C) are sufficient for industrial processes such as refineries. In this paper, we investigate the transient kinetics of NiO carriers in the temperature range of 600 to 900°C in both a fixed bed microreactor (WHSV = 2-4 g CH4/h/g oxygen carrier) and a fluid bed reactor (WHSV = 0.014-0.14 g CH4/h per g oxygen carrier). Complete methane conversion is achieved in the fluid bed for several minutes. In the microreactor, the methane conversion reaches a maximum after an initial induction period of less than 10 s. Both CO2 and H2O yields are highest during this induction period. As the oxygen is consumed, methane conversion drops and both CO and H2 yields increase, whereas the CO2 and H2O concentrations decrease. The kinetics parameter of the gas,solids reactions (reduction of NiO with CH4, H2, and CO) together with catalytic reactions (methane reforming, methanation, shift, and gasification) were estimated using experimental data obtained on the fixed bed microreactor. Then, the kinetic expressions were combined with a detailed hydrodynamic model to successfully simulate the comportment of the fluidized bed reactor. © 2010 American Institute of Chemical Engineers AIChE J, 2010 [source] Incubation period in the 2,2,4,4-tetramethyl-1-piperidinyloxy-mediated thermal autopolymerization of styrene: Kinetics and simulationsJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 24 2006Enrique Saldívar-Guerra Abstract Mechanisms and simulations of the induction period and the initial polymerization stages in the nitroxide-mediated autopolymerization of styrene are discussed. At 120,125 °C and moderate 2,2,4,4-tetramethyl-1-piperidinyloxy (TEMPO) concentrations (0.02,0.08 M), the main source of radicals is the hydrogen abstraction of the Mayo dimer by TEMPO [with the kinetic constant of hydrogen abstraction (kh)]. At higher TEMPO concentrations ([N,] > 0.1 M), this reaction is still dominant, but radical generation by the direct attack against styrene by TEMPO, with kinetic constant of addition kad, also becomes relevant. From previous experimental data and simulations, initial estimates of kh , 1 and kad , 6 × 10,7 L mol,1 s,1 are obtained at 125 °C. From the induction period to the polymerization regime, there is an abrupt change in the dominant mechanism generating radicals because of the sudden decrease in the nitroxide radicals. Under induction-period conditions, the simulations confirm the validity of the quasi-steady-state assumption (QSSA) for the Mayo dimer in this regime; however, after the induction period, the QSSA for the dimer is not valid, and this brings into question the scientific basis of the well-known expression kth[M]3 (where [M] is the monomer concentration and kth is the kinetic constant of autoinitiation) for the autoinitiation rate in styrene polymerization. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 6962-6979, 2006 [source] Design and synthesis of multifunctional glycidyl ethers that undergo frontal polymerizationJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 21 2006James V. Crivello Abstract An investigation of the photoactivated cationic ring-opening frontal polymerizations of a series of alkyl glycidyl ethers has been carried out with the aid of a novel technique, optical pyrometry. With this technique, the effects of the monomer structure on the frontal behavior of these monomers have been examined. Upon irradiation with UV light, the photopolymerizations of many alkyl glycidyl ethers display a prolonged induction period at room temperature as the result of the formation of long-lived, relatively stable secondary oxonium ions. The input of only a small amount of thermal activation energy is required to induce the further reaction of these species with a consequent autoaccelerated exothermic ring-opening polymerization. Photoactivated frontal polymerizations have been observed for both mono- and polyfunctional alkyl glycidyl ether monomers. The ability of monomers to exhibit frontal behavior has been found to be related to their ability to stabilize the secondary oxonium ion intermediates through multiple hydrogen-bonding effects to the ether groups present in the molecule. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 6435,6448, 2006 [source] Effects of Saccharide Set Retarders on the Hydration of Ordinary Portland Cement and Pure Tricalcium SilicateJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 1 2010Linghong Zhang The effects of aliphatic sugar alcohols (e.g., threitol, xylitol, sorbitol) on the hydration of tricalcium silicate (C3S) and ordinary portland cement (OPC) were investigated and compared with those of sucrose, a well-established cement set retarder. Only sugar alcohols which contain threo diol functionality retarded the setting of C3S and OPC, their efficacy increasing with the number of threo hydroxy pairs and, to a smaller extent, with the overall population of hydroxy groups. None, however, were as effective as sucrose. The initial and final setting times increased exponentially with the concentration of saccharide, although the hydration of OPC was less inhibited than that of C3S. Saccharides function as "delayed accelerators," that is, cement hydration is first inhibited and then proceeds faster than in saccharide-free cement. This behavior is consistent with the theory that the induction period is controlled by slow formation and/or poisoning of the stable calcium silicate hydrate (CSH) nuclei. The early inhibiting influence of saccharides on CSH precipitation is apparently stronger than on the growth of crystalline calcium hydroxide. Saccharides did not negatively affect the degree of hydration and compressive strength of fully set OPC paste; on the contrary, sorbitol yielded modest increases. [source] Effect of cyanoguanidine-metal and urea-metal complexes on the thermal degradation of poly(vinyl chloride)JOURNAL OF VINYL & ADDITIVE TECHNOLOGY, Issue 1 2010Abir S. Abdel-Naby Tin, nickel, cobalt, zinc, and copper complexes of cyanoguanidine and urea were synthesized and investigated as thermal stabilizers for rigid poly(vinyl chloride) at 180°C in air. Their stabilizing efficiencies were evaluated by measuring the induction period (the period during which no evolved hydrogen chloride could be detected) and the rate of dehydrochlorination as determined by continuous potentiometric measurements, in addition to the extent of discoloration. The results clearly revealed the greater efficiency of all of the investigated metal complexes as compared to those of well-recognized reference stabilizers. The tin complex always exhibited the highest efficiency irrespective of the type of ligand used. The nickel and cobalt complexes also possessed high stabilizing efficiencies. The order of the stabilizing potency of the various metal complexes was Sn , Co, Ni , Zn, Cu. Combining the ligand itself with dimethyltin- s,s,-bis (isooctyl thioglycolate), as a reference stabilizer containing a tin atom, led to a true synergism. This synergistic effect might be attributed to the formation in situ of a complex between the ligand and the tin atom. J. VINYL ADDIT. TECHNOL., 2010. © 2010 Society of Plastics Engineers [source] Photostabilizing effect of some thermal stabilizers for poly(vinyl chloride) in the presence of fillerJOURNAL OF VINYL & ADDITIVE TECHNOLOGY, Issue 4 2009D. Braun The commercial stabilizers Ca-Zn stearate and dibutyltin- S,S,-di(isooctyl thioglycolate) have been investigated as photostabilizers for rigid poly(vinyl chloride) (PVC) in the presence of two types of CaCO3 as filler (natural and modified). The results reveal the costabilizing efficiency of CaCO3, especially the natural type, when it is used in concentrations of 1,30% by weight. The stabilizing efficiency of CaCO3 is indicated by the longer induction period and the lower rate of dehydrochlorination. The stabilizing effect of CaCO3 may be attributed because of its opacity and action as a screening agent by reflecting ultraviolet light, thereby hindering the penetration of the light into the polymer matrix. In addition, CaCO3 has the ability to absorb the HCl evolved during processing. The slight difference in the stabilizing effect of natural CaCO3 compared to that of the modified one can be attributed to the modification process, that may decrease the number of active sites on the surface of CaCO3, that in turn are responsible for the absorption of the evolved HCl. VINYL ADDIT. TECHNOL., 2009. © 2009 Society of Plastics Engineers [source] Synergistic effect of metal soaps and natural zeolite on poly(vinyl chloride) thermal stabilityJOURNAL OF VINYL & ADDITIVE TECHNOLOGY, Issue 2 2005Sevdiye Atakul The synergistic effect of metal soaps (zinc stearate and calcium stearate) and/or natural zeolite (clinoptilolite) on PVC thermal stability was investigated. For this purpose, PVC plastisol was prepared by mixing poly(vinyl chloride) (PVC) and dioctyl phthalate (DOP) and stabilized with different amounts of metal soaps and zeolite. Kinetic studies of dehydrochlorination at 140 and 160°C were done for unstabilized and stabilized PVC plastigels using 763 PVC Thermomat equipment. The stabilizing effect of zeolite on the increase in the induction period of the sample was considered to result from the absorption of HCl, a property that was thought to reduce the autocatalytic effect of HCl evolved at the initial stages of dehydrochlorination. Since the induction time of the sample having 0.53% of zinc stearate and 0.53% of zeolite was higher than those of the PVC plastigels having only zinc stearate or zeolite, the synergistic effect on thermal stability was observed at low levels of these additives. J. VINYL. ADDIT. TECHNOL., 11:47,56, 2005. © 2005 Society of Plastics Engineers [source] Video Microscopy for the Investigation of Gas Phase CopolymerizationMACROMOLECULAR MATERIALS & ENGINEERING, Issue 11 2005Daniela Ferrari Abstract Summary: Video microscopy as a tool for investigating olefin gas phase copolymerization is presented for the first time in this paper. The central theme of this work is the study of the comonomer effect shown by an unbridged metallocene catalyst supported on silica. By using video microscopy, it is possible to observe the increase in catalytic activity in terms of particle growth as well as monomer consumption. The observation that a more pronounced induction period in the particle growth profile is shown with increasing propylene concentration led us to investigate the copolymers obtained at different polymerization times using 13C NMR analysis and single particle energy dispersive X-ray (EDX mapping). This allowed us to adapt the "polymer growth and particle expansion model" to the copolymerization. Besides physical causes for the comonomer effect, we wanted to determine whether the catalyst structure plays an important role in the comonomer effect. To this end we investigated two metallocenes bearing the same long bridging unit but differing in the ligand bound to the zirconium center. One metallocene bears a cyclopentadienyl ring, while the other bears an indenyl group. From a close analysis of the 13C NMR, it is clear that both catalysts insert ethylene more easily then propylene, probably due to the long bridging unit that results in a narrower aperture angle of the ligand. In addition to this, the indenyl ligand does not allow the formation of propylene blocks even at high propylene concentration. Snapshot of the polymer particles taken after 165 min of ethylene-1-butene copolymerization with catalyst 1. [source] Ziegler-Natta/Metallocene Hybrid Catalyst for Ethylene PolymerizationMACROMOLECULAR REACTION ENGINEERING, Issue 6 2007Mostafa Ahmadi Abstract A Ziegler-Natta/metallocene hybrid catalyst was produced and utilized in the polymerization of ethylene with the aim of producing bimodal polyethylene. The MgCl2 adduct was prepared by a melt quenching method and Cp2ZrCl2 and TiCl4 catalysts were loaded, respectively, after treating the surface with TiBAl. The polymerization kinetics involved an induction period, followed by fragmentation and expansion of particles. SEM micrographs revealed that the spherical morphology was retained through the initial mild reaction conditions of induction period. The polymers produced showed bimodal molecular weight distribution patterns, suggesting that both components of the hybrid catalyst were active over the support. [source] Becoming a new doctor: a learning or survival exercise?MEDICAL EDUCATION, Issue 7 2007Jeremy Brown Objective, This study set out to establish why some new doctors view their training as a valuable period in their professional development, whereas others see it as a year to be endured and survived. Methods, This multi-method case study focused on the interaction of key participants within 1 deanery, sampling the 237 pre-registration house officers (PRHOs) and 166 educational supervisors populating the associated 12 National Health Service trusts at the time (2001). The design of the case study was predicated on gathering the views of both teachers and learners in a way that allowed each stage of the data collection process to inform and influence the next phase. Results, Lack of formal guidance and support were common characteristics associated with the first few days in post. The first day in post as a doctor is, for most, an experience that is hard to prepare for, even after a useful induction period. Those PRHOs who felt they were not guided or advised on how to undertake their new professional responsibilities tended to feel undervalued and under-recognised as individuals. Conclusions, Without the support of senior colleagues who can help the new doctor reflect on quite difficult and uncertain situations, new doctors will almost certainly perceive the first year of the new Foundation Programme as a survival exercise. If new doctors are working in an environment where their learning is properly facilitated, they are more likely to recognise their progress in their professional development and be more proactive in addressing concerns about professional expectations. [source] Mechanism of the ClO2 generation from the H2O2 -HClO3 reactionTHE CANADIAN JOURNAL OF CHEMICAL ENGINEERING, Issue 4 2000Guohong Yin Abstract The development of chlorine containing species during the hydrogen peroxide-based chlorine dioxide generation process has been determined. Accordingly, two distinct phases, namely the induction period and the steady-state phase, were identified. In the induction period, it was observed that chloride and chlorous acid are generated, while chlorine, a byproduct from some methanol-based processes, is not detectable. The absence of chlorine is explained by the fast reaction kinetics between hydrogen peroxide and chlorine, which results in the formation of chloride. In the steady-state phase, due to the accumulation of chloride and chlorous acid during the induction period, the reaction between chloric acid and chlorous acid, which is responsible for the generation of chlorine dioxide in the hydrogen peroxide-based ClO2 process, becomes possible. Chloride is a catalyst in such a reaction. On a mis en évidence la formation d'espéces chlorées pendant le procédé de production de dioxyde de chlore basé sur le peroxyde d'hydrogéne. Ainsi, deux phases, soient la période d'induction et la phase à l'état stationnaire, ont éte identifiées. Dans la période d'induction, on a observé la production de chlorure et d'acide chloreux, tandis que le chlore, un sous-produit de certains precédes basés sur le méthanol, n'est pas décelable. L'absence de chlore s'explique par la cinétique de réaction rapide entre le peroxyde d'hydrogène et le chlore, ce qui aboutit à la formation de chlorure. Dans la phase stationnaire, du fait de l'accumulation du chlorure et de l'acide chloreux pendant la période d'induction, la réaction entre l'acide chlorique et l'acide chloreux, qui est à l'origine de la production du dioxyde de chlore dans le procéde au ClO2 basé sur le peroxyde d'hydrogéne, devient possible. Le chlorure est un catalyseur dans ce type de réaction. [source] The rate of transport through a phosphate translocator affects delayed luminescence induction: an experiment and a theoretical modelANNALS OF APPLIED BIOLOGY, Issue 1 2001S KHUZNETSOVA Summary Delayed luminescence (DL) induction curves were studied in leaves from a mutant pea line containing mutations at both the r and rb loci, compared with leaves from wild type plants. Genes at the r and rb loci encode starch branching enzyme and ADP - glucose pyrophosphorylase, respectively. The presence of mutations at both loci, previously known to reduce the starch content in the dry mature seed by 75%, have been shown to lower the starch level in leaves by at least 20%. During induction, the half-time for the DL intensity decrease from maximum to steady state in the mutant was 1.5 ± 0.2 times longer than for the wild type. It is proposed that the prolongation of the induction period in leaves from the mutant plants is caused by a lack of inorganic phosphate (Pi) restricting the rate of ATP synthesis at the beginning of induction. The reduced Pi would be compensated by triose flow from the chloroplast, via the triose phosphate translocator, being exchanged for Pi from the cytosol. Analysis of our theoretical photosynthesis model confirmed that a decrease in the rate of Pi released from the Calvin cycle could lead to a prolongation of the induction period. [source] Aqueous phase carbon dioxide and bicarbonate hydrogenation catalyzed by cyclopentadienyl ruthenium complexesAPPLIED ORGANOMETALLIC CHEMISTRY, Issue 11 2007Sylvain S. Bosquain Abstract The water-soluble ruthenium(II) complexes [Cp,RuX(PTA)2]Y and [CpRuCl(PPh3)(mPTA)]OTf (Cp, = Cp, Cp*, X = Cl and Y = nil; or X = MeCN and Y = PF6; PTA = 1,3,5-triaza-7-phosphaadamantane; mPTA = 1-methyl-1,3,5-triaza-7-phosphaadamantane) were used as catalyst precursors for the hydrogenation of CO2 and bicarbonate in aqueous solutions, in the absence of amines or other additives, under relatively mild conditions (100 bar H2, 30,80 °C), with moderate activities. Kinetic studies showed that the hydrogenation of HCO3, proceeds without an induction period, and that the rate strongly depends on the pH of the reaction medium. High-pressure multinuclear NMR spectroscopy revealed that the ruthenium(II) chloride precursors are quantitatively converted into the corresponding hydrides under H2 pressure. Copyright © 2007 John Wiley & Sons, Ltd. [source] Preliminary evidence for in vitro methylation of tributyltin in a marine sedimentAPPLIED ORGANOMETALLIC CHEMISTRY, Issue 11 2001Alfred J. Vella Abstract Recent reports from our laboratory on the occurrence of methylbutyltins in marine sediments and seawater suggest that these compounds are formed in the environment by the methylation of both tributyltin (TBT) and thatof its degradation products, i.e. dibutyltin and monobutyltin, to give MenBu(4,n)Sn for which n,=,1, 2 and 3 respectively. We investigated the possibility of inducing methylation of TBT in seawater,sediment mixtures in experiments carried out in vitro using environmental materials collected from a yacht marina in Msida, Malta. Three water,sediment mixtures, which were shown to contain TBT, dibutyltin and monobutyltin but no other organotins, were spiked with tributyltin chloride (90,mg in 100,ml sea-water/100,ml sediment); to one mixture was added sodium acetate and to another methanol, to act as possible additional carbon sources, and all mixtures were allowed to stand at 25,°C in stoppered clear-glass bottles in diffused light for a maximum of 315 days. Speciation and quantification of organotins was performed using aqueous phase boroethylation with simultaneous solvent extraction followed by gas chromatography with flame photometric detection. The atmosphere inside the bottles quickly became reducing with abundant presence of H2S, and after an induction period of about 112 days, and only in the reaction mixture containing methanol, methyltributyltin (MeBu3Sn) was observed in both sediment (maximum concentration 0.87 µgSn g,1) and overlying water (maximum concentration 6.0 µgSn l,1). The minimum conversion yield of TBT into MeBu3Sn was estimated to be 0.3%. MeBu3Sn has a significantly lower affinity for sediment than TBT and, therefore, is more mobile in the marine environment, possibly also migrating into the atmosphere to generate a hitherto unsuspected flux of organotin into that phase. Copyright © 2001 John Wiley & Sons, Ltd. [source] Effect of hydrogen on the synthesis of carbon nanofibers by CO disproportionation on ultrafine Fe3O4ASIA-PACIFIC JOURNAL OF CHEMICAL ENGINEERING, Issue 5 2009Wenxin Lu Abstract Carbon nanofibers (CNFs) are grown by catalytic CO disproportionation over ultrafine Fe3O4 catalyst at a hydrogen concentration of 0,29.17%, and the time-depending rates of CNFs growth are continuously monitored and the morphologies of the as-synthesized CNFs are analyzed. Increasing H2 concentration will lower CO dissociation energy and assist catalyst reconstruction so as to shorten the induction period and increase the growth rate of CNFs, but it will also increase the rate of catalyst deactivation because carbon hydrogasification is not possible and carbon diffusion in the catalyst particle is rate limiting. As a result of H2 -induced catalyst reconstruction and carbon deposition, the morphology of the CNFs changes from twisty to helical and to straight and becomes less entangled when the H2 concentration is increased. Copyright © 2009 Curtin University of Technology and John Wiley & Sons, Ltd. [source] Transient Effects of Overexpressing Anthranilate Synthase , and , Subunits in Catharanthusroseus Hairy RootsBIOTECHNOLOGY PROGRESS, Issue 5 2005Christie A. M. Peebles Catharanthus roseus produces two economically valuable anticancer drugs, vinblastine and vincristine. These drugs are members of the terpenoid indole alkaloids and accumulate in small quantities within the plant; thus these two drugs are expensive to produce. Metabolic engineering efforts have focused on increasing the alkaloids in this pathway through various means such as elicitation, precursor feeding, and gene overexpression. Recently we successfully expressed Arabidopsis genes encoding a feedback-insensitive anthranilate synthase , subunit under the control of the glucocorticoid-inducible promoter system and the anthranilate synthase , subunit under the control of a constitutive promoter in C. roseus hairy roots. In this work we look at the transient behaviors of terpenoid indole alkaloids over a 72 h induction period in late exponential growth phase cultures. Upon induction, the tryptophan, tryptamine, and ajmalicine pools accumulated over 72 h. In contrast, the lochnericine, hörhammericine, and tabersonine pools decreased and leveled out over the 72 h induction period. Visible changes within the individual compounds usually took from 4 to 12 h. [source] A Cyclical Semicontinuous Process for Production of Human ,1 -Antitrypsin Using Metabolically Induced Plant Cell Suspension CulturesBIOTECHNOLOGY PROGRESS, Issue 2 2005Melody M. Trexler Transgenic rice suspension cultures were utilized to produce a human therapeutic protein, recombinant ,1 -antitrypsin (rAAT), in a cyclical, semicontinuous operation. Recombinant protein production was induced by removing the carbon source from the cell culture medium. The transgenic rice cells secreted the rAAT into the medium, and therefore medium exchanges could be performed for consecutive growth and protein expression phases. The process consisted of three cycles over a 25,28 day period, with growth phases lasting 4,6 days each and protein expression phases lasting 2.5,5 days each. Biomass and sugar concentrations, oxygen uptake rate, cell viability, culture pH, total extracellular protein, and active rAAT were measured throughout the cyclical process. The data profiles were reproducible between separate cyclical runs where, following each induction period, cell growth and viability could be reestablished once sucrose was added back to the culture. Volumetric productivities ranged from 3 to 12 mg active rAAT/(L day) for individual cycles with overall volumetric productivities of 4.5 and 7.7 mg active rAAT/(L day). [source] High-Speed Living Polymerization of Polar Vinyl Monomers by Self-Healing Silylium CatalystsCHEMISTRY - A EUROPEAN JOURNAL, Issue 34 2010Dr. Yuetao Zhang Abstract This contribution describes the development and demonstration of the ambient-temperature, high-speed living polymerization of polar vinyl monomers (M) with a low silylium catalyst loading (, 0.05,mol,% relative to M). The catalyst is generated in situ by protonation of a trialkylsilyl ketene acetal (RSKA) initiator (I) with a strong Brønsted acid. The living character of the polymerization system has been demonstrated by several key lines of evidence, including the observed linear growth of the chain length as a function of monomer conversion at a given [M]/[I] ratio, near-precise polymer number-average molecular weight (Mn, controlled by the [M]/[I] ratio) with narrow molecular weight distributions (MWD), absence of an induction period and chain-termination reactions (as revealed by kinetics), readily achievable chain extension, and the successful synthesis of well-defined block copolymers. Fundamental steps of activation, initiation, propagation, and catalyst "self-repair" involved in this living polymerization system have been elucidated, chiefly featuring a propagation "catalysis" cycle consisting of a rate-limiting CC bond formation step and fast release of the silylium catalyst to the incoming monomer. Effects of acid activator, catalyst and monomer structure, and reaction temperature on polymerization characteristics have also been examined. Among the three strong acids incorporating a weakly coordinating borate or a chiral disulfonimide anion, the oxonium acid [H(Et2O)2]+[B(C6F5)4], is the most effective activator, which spontaneously delivers the most active R3Si+, reaching a high catalyst turn-over frequency (TOF) of 6.0×103,h,1 for methyl methacrylate polymerization by Me3Si+ or an exceptionally high TOF of 2.4×105,h,1 for n -butyl acrylate polymerization by iBu3Si+, in addition to its high (>90,%) to quantitative efficiencies and a high degree of control over Mn and MWD (1.07,1.12). An intriguing catalyst "self-repair" feature has also been demonstrated for the current living polymerization system. [source] Identification of a Highly Efficient Alkylated Pincer Thioimido,Palladium(II) Complex as the Active Catalyst in Negishi CouplingCHEMISTRY - A EUROPEAN JOURNAL, Issue 17 2009Jing Liu Dr. Abstract PdIIate complex: A novel alkylated pincer thioimido,Pd complex generated from a catalyst precursor and basic organometallic reagents (RM) was observed by in situ IR, 1H,NMR, and 13C,NMR spectroscopies for the first time and proved to be the active catalyst in stoichiometric and catalytic reactions of aryl iodides with RM (see scheme). The catalyst, as an electron-rich PdII species, promoted the Negishi coupling of aryl iodides and alkylzinc reagents with high efficiency, even at low temperatures (0 or ,20,°C). The induction period of Negishi coupling catalyzed by pincer thioamide,palladium complex 1 was investigated. A heterogeneous mechanism was excluded by kinetic studies and comparison with Negishi coupling reactions promoted by Pd(OAc)2/Bu4NBr (a palladium,nanoparticle system). Tetramer 2 was isolated from the reaction of 1 and organozinc reagents. Dissociation of complex 2 by PPh3 was achieved, and the structure of resultant complex 8 was confirmed by X-ray diffraction analysis. A novel alkylated pincer thioimido,PdII complex, 7, generated from catalyst precursor 1 and basic organometallic reagents (RM), was observed by in situ IR, 1H,NMR, and 13C,NMR spectroscopy for the first time. The reaction of 7 with methyl 2-iodobenzoate afforded 74,% of the cross-coupled product, methyl 2-methylbenzoate, together with 60,% of PdII complex 2. Furthermore, the catalyst, as an electron-rich PdII species, efficiently promoted the Negishi coupling of aryl iodides and alkylzinc reagents without an induction period, even at low temperatures (0,°C or ,20,°C). To evaluate the influence of the catalyst structure upon the induction period, complex 9 was prepared, in which the nBu groups of 1 were displaced by more bulky 1,3,5-trimethylphenyl groups. Trimer 10 was isolated from the reaction of complex 9 and basic organometallic reagents such as CyZnCl or CyMgCl (Cy: cyclohexyl); this is consistent with the result obtained with complex 1. The rate in the induction period of the model reaction catalyzed by 9 was faster than that with 1. Plausible catalytic cycles for the reaction, based upon the experimental results, are discussed. [source] Olefin Epoxidation with Hydrogen Peroxide Catalyzed by Lacunary Polyoxometalate [,-SiW10O34(H2O)2]4,CHEMISTRY - A EUROPEAN JOURNAL, Issue 2 2007Keigo Kamata Abstract The tetra- n -butylammonium (TBA) salt of the divacant Keggin-type polyoxometalate [TBA]4[,-SiW10O34(H2O)2] (I) catalyzes the oxygen-transfer reactions of olefins, allylic alcohols, and sulfides with 30,% aqueous hydrogen peroxide. The negative Hammett ,+ (,0.99) for the competitive oxidation of p -substituted styrenes and the low value of (nucleophilic oxidation)/(total oxidation), XSO=0.04, for I -catalyzed oxidation of thianthrene 5-oxide (SSO) reveals that a strongly electrophilic oxidant species is formed on I. The preferential formation of trans -epoxide during epoxidation of 3-methyl-1-cyclohexene demonstrates the steric constraints of the active site of I. The I -catalyzed epoxidation proceeds with an induction period that disappears upon treatment of I with hydrogen peroxide. 29Si and 183W,NMR spectroscopy and CSI mass spectrometry show that reaction of I with excess hydrogen peroxide leads to fast formation of a diperoxo species, [TBA]4[,-SiW10O32(O2)2] (II), with retention of a ,-Keggin type structure. Whereas the isolated compound II is inactive for stoichiometric epoxidation of cyclooctene, epoxidation with II does proceed in the presence of hydrogen peroxide. The reaction of II with hydrogen peroxide would form a reactive species (III), and this step corresponds to the induction period observed in the catalytic epoxidation. The steric and electronic characters of III are the same as those for the catalytic epoxidation by I. Kinetic, spectroscopic, and mechanistic investigations show that the present epoxidation proceeds via III. [source] | |||||||||||||||||||||||||||||||