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Radical Mechanism (radical + mechanism)
Selected AbstractsInsight into the Role of Oxidation in the Thermally Induced Green Band in Fluorene-Based Systems,ADVANCED FUNCTIONAL MATERIALS, Issue 4 2007R. Grisorio Abstract The causes of the spectral instability of poly[9,9-dioctylfluoren-2,7-diyl-co-2,,7,-spiro(cyclohexane-1,9,-fluorene)] during thermal annealing in air, which leads to a green photoluminescence (PL) emission band, are investigated. The Igreen/Iblue ratio evolution (I,=,intensity) is found to be independent of the amount of monoalkylfluorene defects, despite the fact that their presence might be regarded as a trigger for the radical process leading to polymer degradation in the presence of a trace amount of metal catalyst. Furthermore, the absence of a correlation between the degree of oxidation of the material and the Igreen/Iblue ratio indicates that the spatial disposition of fluorenones formed during the thermal degradation of the material, rather than their amount, is to be strictly related to the Igreen/Iblue ratio. The evidenced formation of fluorenone agglomerates, which could be considered the cause for the consistent increase in the Igreen/Iblue ratio during a thermal oxidation of a polyfluorene, confirms that the radical mechanism can also involve dialkylfluorene systems. Finally, the higher resistance to thermal degradation shown by spirocyclohexane fluorene units with respect to dioctylfluorene ones allows the synthesis of new, spectrally stable, fluorene-based copolymers. [source] Ab initio Emulsion Polymerization by RAFT (Reversible Addition,Fragmentation Chain Transfer) through the Addition of CyclodextrinsHELVETICA CHIMICA ACTA, Issue 8 2006Bojana Apostolovic Abstract A novel process to produce homo- and copolymers by RAFT polymerization in emulsion is presented. It is known that RAFT-controlled radical polymerization can be conducted in emulsion polymerization without disturbing the radical segregation characteristic of this process, thus leading to polymerization rates identical to those encountered in the corresponding nonliving systems. However, RAFT agents are often characterized by very low water solubility and, therefore, they diffuse very slowly from the monomer droplets, where they are initially solubilized, to the reaction loci, i.e., the polymer particles. Accordingly, when used in emulsion polymerization, they are practically excluded from the reaction. In this work, we show that cyclodextrins, well-known for their ability to form water-soluble complexes with hydrophobic molecules, facilitate the transport across the H2O phase of the RAFT agent to the polymer particles. Accordingly, chains grow through the entire process in a controlled way. This leads to the production of low-polydispersity polymers with well-defined structure and end functionalities as well as to the possibility of synthesizing block copolymers by a radical mechanism. [source] Role of phenoxy radicals in PCDD/F formationINTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 9 2002Sukh Sidhu In this work, the role of phenoxy radicals in polychlorinated dibenzo- p -dioxins and polychlorinated dibenzofurans (PCDD/F) formation was investigated by studying the slow oxidation of 2-chlorophenol (2-CP) and 2-chloroanisole (2-CA) at a gas-phase concentration of 4 ppm (,2.1 × 104 ,g/m3) over a temperature range of 400,800°C. Residence times were maintained at 2.0 ± 0.10 s. PCDD/F reaction products were dibenzofuran, dibenzo- p -dioxin, 4-chlorodibenzofuran, 1-chlorodibenzo- p -dioxin, 4,6-dichlorodibenzofuran, and 1,6-dichlorodibenzo- p -dioxin (1,6-DCDD). Major products observed in these experiments were 2,6-dichlorophenol, 3-phenyl-2-propenal, 1-indanone, 1,3-isobenzofurandione, and 3-phenyl-2-propenoyl chloride. The 2-CP and 2-CA experiments, along with the variable concentration 2-CA experiments, showed that the concentration of radicals present in the oxidation system has a significant effect on the PCDD/F product distribution and ultimately the PCDD/PCDF ratio. Also, the observation of dichlorinated phenoxy phenol and dichlorinated dihydroxybiphenyl, the proposed intermediate species in the radical,radical mechanism, suggests that radical,radical mechanism dominates gas-phase PCDD/F formation. This information will be helpful in constructing a detailed kinetic mechanism of PCDD/F formation/destruction in combustor postcombustion zone. © 2002 Wiley Periodicals, Inc. Int J Chem Kinet 34: 531,541, 2002 [source] Metal-Free and Copper-Promoted Single-Pot Hydrocarboxylation of Cycloalkanes to Carboxylic Acids in Aqueous MediumADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 17 2009Marina Abstract A simple and effective method for the transformation, under mild conditions and in aqueous medium, of various cycloalkanes (cyclopentane, cyclohexane, methylcyclohexane, cis - and trans -1,2-dimethylcyclohexane, cycloheptane, cyclooctane and adamantane) into the corresponding cycloalkanecarboxylic acids bearing one more carbon atom, is achieved. This method is characterized by a single-pot, low-temperature hydrocarboxylation reaction of the cycloalkane with carbon monoxide, water and potassium peroxodisulfate in water/acetonitrile medium, proceeding either in the absence or in the presence of a metal promoter. The influence of various reaction parameters, such as type and amount of metal promoter, solvent composition, temperature, time, carbon monoxide pressure, oxidant and cycloalkane, is investigated, leading to an optimization of the cyclohexane and cyclopentane carboxylations. The highest efficiency is observed in the systems promoted by a tetracopper(II) triethanolaminate-derived complex, which also shows different bond and stereoselectivity parameters (compared to the metal-free systems) in the carboxylations of methylcyclohexane and stereoisomeric 1,2-dimethylcyclohexanes. A free radical mechanism is proposed for the carboxylation of cyclohexane as a model substrate, involving the formation of an acyl radical, its oxidation and consequent hydroxylation by water. Relevant features of the present hydrocarboxylation method, besides the operation in aqueous medium, include the exceptional metal-free and acid-solvent-free reaction conditions, a rare hydroxylating role of water, substrate versatility, low temperatures (ca. 50,°C) and a rather high efficiency (up to 72% carboxylic acid yields based on cycloalkane). [source] Highly Efficient Direct Carboxylation of Propane into Butyric Acids Catalyzed by Vanadium ComplexesADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 10 2007Marina Abstract A direct and highly efficient carboxylation of propane by carbon monoxide into butyric acids (mainly isobutyric and, in a smaller amount, n -butyric), in the presence of potassium peroxodisulphate (K2S2O8) and in trifluoroacetic acid solution, has been achieved by using a vanadium catalytic system based on Ca[V{ON(CH(CH3)COO)2}2] (synthetic amavadine), its model compounds Ca[V{ON(CH2COO)2}2] or [VO{N(CH2CH2O)3}] , other simpler vanadium compounds, such as [VO(acac)2] or VOSO4, are less active. Overall yields (based on propane) of carboxylic acids up to 70,% and TON values up to 18.4×103 have been reached. The effects of various factors such as the propane and carbon monoxide pressures, temperature, time, catalyst amount and radical traps have been investigated, the reactions are shown to proceed via both C - and O -centred radicals, with K2S2O8 playing the role of an oxidant via a free radical mechanism. [source] Thermal degradation behavior of poly(vinyl chloride) in the presence of poly(glycidyl methacrylate)JOURNAL OF APPLIED POLYMER SCIENCE, Issue 4 2008Magdy W. Sabaa Abstract The thermal degradation behavior of poly(vinyl chloride) (PVC) in presence of poly(glycidyl methacrylate) (PGMA) has been studied using continuous potentiometric determination of the evolved HCl gas from the degradation process from one hand and by evaluating the extent of discoloration of the degraded samples from the other. The efficiency of blending PGMA with dibasic lead carbonate (DBLC) conventional thermal stabilizer has also been investigated. A probable radical mechanism for the effect of PGMA on the thermal stabilization of PVC has been suggested based on data reported by FTIR and elemental analyses. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008 [source] Ozonation of 1,3,6-naphthalenetrisulfonic acid in presence of heavy metalsJOURNAL OF CHEMICAL TECHNOLOGY & BIOTECHNOLOGY, Issue 8 2004M Sánchez-Polo Abstract A study was conducted of the mechanisms by which heavy metals, commonly present in industrial effluents, increase the purification effectiveness of ozone in the removal of organic contaminants of low biodegradability. For this purpose, the ozonation of 1,3,6-naphthalenetrisulfonic acid (NTS) in the presence of Ni(II), Fe(II), Mn(II), Zn(II), Sr(II), Cr(III), Cd(II), Hg(II), and Cu(II) was examined. The presence of small amounts of Mn(II), Fe(II), Ni(II), Zn(II), and Cr(III) was observed in the system, increasing the degradation rate of the NTS and transforming the dissolved organic matter into CO2. The mineralization of the organic matter was highly favored, especially in the first minutes of treatment. The results obtained appear to indicate that the activity of the metals in the NTS ozonation process is related to their reduction potential. Thus, metals susceptible to oxidation by ozone are potential promoters of NTS ozonation. The presence of Fe(II) or Mn(II) during NTS ozonation increased its degradation rate by 79% and 72% respectively. Moreover, the reaction kinetics of metal oxidation with ozone controls the increase in the purification effectiveness of these systems. The presence of radical scavengers (tert -butanol or bicarbonate) in the medium during the promoted ozonation of NTS showed a negative effect on this process, and the NTS degradation rate decreased with an increasing concentration of these inhibitors in the system. These results confirm that the degradation of NTS by ozone in the presence of heavy metals occurs by a radical mechanism. O3/Zn(II) and O3/Fe(II) systems were applied to the decontamination of urban waste waters. The presence of Zn(II) or Fe(II) during the ozonation produced a reduction during the first 5 min of treatment of 20% or 44%, respectively, in the concentration of dissolved organic matter present in the system. These results show that ozonation in the presence of heavy metals is a highly promising system for the purification of waste waters and industrial effluents. Copyright © 2004 Society of Chemical Industry [source] Direct-temperature mass spectrometric detection of volatile terpenoids and natural terpenoid polymersin fresh and artificially aged resinsJOURNAL OF MASS SPECTROMETRY (INCORP BIOLOGICAL MASS SPECTROMETRY), Issue 6 2003Dominique Scalarone Abstract Electron impact (EI) ionization and ammonia chemical ionization (NH3/CI) direct-temperature mass spectrometry (DTMS) was used to characterize five natural terpenoid resins: dammar, mastic, colophony, Manila copal and sandarac. Compositional differences were highlighted by the identification of low molecular mass compounds, ranging from di- to triterpenoids, and polymeric components, based on polycadinene and polycommunic acid. Photo-ageing processes occurring under accelerated indoor and outdoor exposure conditions were also investigated. NH3/CI and tetramethylammonium hydroxide EI were applied to increase the sensitivity towards highly oxidized molecules. Oxidation and cross-linking reactions were found to affect mostly triterpenoid resins and diterpenoid abietane and pimarane molecules. Oxidation proceeds through a radical mechanism, generally starting from conjugated double bonds. Oxygen atoms are incorporated in the terpenoid structures in the form of alcohols, ketones and carboxylic acids. Oxidized cadinene oligomers released by pyrolytic degradation of the polycadinene fraction of dammar were detected even in unaged samples. Evidence is given indicating the occurrence of cleavages in the cross-linked polycommunic structure of aged sandarac and Manila copal. Bond scissions produce oligomeric fragments based on the communic acid structure and sufficiently volatile to be desorbed at low temperature in DTMS measurements. Copyright © 2003 John Wiley & Sons, Ltd. [source] Architecture of Polymeric Superstructures Constructed by Mesoscopically Ordered Cubic LatticesMACROMOLECULAR RAPID COMMUNICATIONS, Issue 4 2003Koji Ishizu Abstract Highly monodisperse crosslinked core-shell polymer microspheres could be prepared easily by introducing special crosslinking reagents into the segregated core in block copolymer assembly films. The crosslinked core was stabilized sterically by highly branched shell chains in solution. These microspheres moved like pseudo-latex. The microspheres formed a lattice with a body-centered cubic (BCC) structure near the overlap threshold (C*). This structure changed to a face-centered cubic (FCC) lattice in the bulk region of the films. Photofunctionalized core-shell microspheres were prepared by introducing dithiocarbamate (DC) groups into shell parts by means of polymer reactions, where DC groups could be propagated using vinyl monomers such as methyl methacrylate (MMA) with living radical mechanism. Polymeric superstructure (three microphase-separated structure) films were constructed by graft copolymerization of MMA initiated with photofunctionalized microspheres such as macroinitiators under UV irradiation, exhibiting self-coloring due to Bragg diffraction. These materials can be used for the construction of optical devices such as for the fabrication of light modulators. Photograph of a solution of the microsphere in MMA. [source] Photo-CIDNP Study of the Interaction of Tyrosine with Nifedipine.PHOTOCHEMISTRY & PHOTOBIOLOGY, Issue 3 2004An Attempt to Model the Binding Between Calcium Receptor, Calcium Antagonist Nifedipine This article proposes a new approach to the modeling of the molecular-level mechanism of ligand-receptor interaction for Ca2+ receptor binding site. Chemically induced dynamic nuclear polarization (CIDNP) technique has been used to unravel fine details of the reaction in the model system composed of one of the known Ca2+ antagonist drugs, nifedipine (NF), and isolated amino acid residuals (e.g. tyrosine [Tyr]) of Ca2+ receptor binding site. It has been conclusively demonstrated that the reaction between NF and Tyr resulting in the oxidation product,nitroso form of NF,obeys the radical mechanism. CIDNP data in combination with the results of mathematical modeling of the structures of ligandreceptor complexes have allowed to propose the mechanism of the interaction of NF with Ca2+ receptor binding site. [source] Fullerene-functionalized polycarbonate: Synthesis under microwave irradiation and nonlinear optical propertyPOLYMER ENGINEERING & SCIENCE, Issue 4 2006Huixia Wu Fullerenation of polycarbonate (PC), a commercially important optical polymer, was achieved by direct reaction of C60 and PC in the presence of azo-bis-isobutyronitrile (AIBN), using 1,1,2,2-tetrachloroethane as the solvent under microwave irradiation (MI). Compared with conventional heating process, MI could significantly enhance the rate of the fullerenation under identical reaction conditions. The C60 content of the fullerene-functionalized polycarbonate (C60 -PC) could be controlled via varying the C60/PC feed ratio and the reaction time. The C60 -PCs are soluble in common organic solvents such as THF and chloroform. The products were characterized by gel permeation chromatography, UV,vis, FTIR, TGA, DSC, 1H NMR, and 13C NMR. The reaction of C60 with PC under MI was monitored by electron spin resonance spectra, the fullerene radicals were detected in reaction solutions and also in the solid product polymers, indicating the radical mechanism of the reaction. The nonlinear optical property of C60 -PCs in THF was investigated by the open-aperture z -scan technique at 527 nm, and its nonlinear absorption coefficient was found to be in the same order as that of C60. POLYM. ENG. SCI., 46:399,405, 2006. © 2006 Society of Plastics Engineers [source] Copolymerization of bromophenylmaleimide with ethyl or butyl methacrylatePOLYMER INTERNATIONAL, Issue 7 2003HF Naguib Abstract N - p -Bromophenylmaleimide (BrPMI) does not polymerize in solution by conventional free radical mechanism. However, it readily polymerized in bulk when mixed with a free radical initiator and heated in a microwave oven for 7,8 min. Copolymerization of ethyl methacrylate or butyl methacrylate with BrPMI was conducted in dioxane. The copolymers were characterized by IR and 1H NMR spectroscopy and gel permeation chromatography. The monomer reactivity ratios were calculated by a non-linear least-square analysis. Thermal analysis indicated a great improvement in thermal stability of the copolymers compared with the methacrylate homopolymers. BrPMI was also polymerized in bulk in the DSC pan, which allowed the calculation of the activation energy of its polymerization. Copyright © 2003 Society of Chemical Industry [source] Free radical polymerization of methyl methacrylate initiated by the diphosphine Mo(0) complexesAPPLIED ORGANOMETALLIC CHEMISTRY, Issue 11 2006Ayfer Mente Abstract The polymerization of methyl methacrylate MMA catalyzed by [Mo(CO)4L2] [L2 = diphenylphosphinomethane (dppm), diphenylphosphinoethane (dppe) or diphenylphosphinopropane (dppp)] has been studied. The activity of these single-component catalysts depends on the length of the (CH2)n bridge of diphosphine ligand. Thus, the dppm derivative displays higher activity than dppe or dppp ligands. These complexes, as free radical initiators, afforded the methyl methacrylate polymerization in chlorinated solvents. The mechanism of the polymerization was discussed and a radical mechanism was proposed. Copyright © 2006 John Wiley & Sons, Ltd. [source] Unique Ligand-Based Oxidative DNA Cleavage by Zinc(II) Complexes of Hpyramol and HpyrimolCHEMISTRY - A EUROPEAN JOURNAL, Issue 18 2007Palanisamy, Uma Maheswari Dr. Abstract The zinc(II) complexes reported here have been synthesised from the ligand 4-methyl-2- N -(2-pyridylmethyl)aminophenol (Hpyramol) with chloride or acetate counterions. All the five complexes have been structurally characterised, and the crystal structures reveal that the ligand Hpyramol gradually undergoes an oxidative dehydrogenation to form the ligand 4-methyl-2- N -(2-pyridylmethylene)aminophenol (Hpyrimol), upon coordination to ZnII. All the five complexes cleave the ,X174 phage DNA oxidatively and the complexes with fully dehydrogenated pyrimol ligands were found to be more efficient than the complexes with non-dehydrogenated Hpyramol ligands. The DNA cleavage is suggested to be ligand-based, whereas the pure ligands alone do not cleave DNA. The DNA cleavage is strongly suggested to be oxidative, possibly due to the involvement of a non-diffusible phenoxyl radical mechanism. The enzymatic religation experiments and DNA cleavage in the presence of different radical scavengers further support the oxidative DNA cleavage by the zinc(II) complexes. [source] Photophysical and Phototoxic Properties of the Antibacterial Fluoroquinolones Levofloxacin and MoxifloxacinCHEMISTRY & BIODIVERSITY, Issue 5 2004Giampietro Viola Two antibacterial fluoroquinolones, levofloxacin and moxifloxacin, were investigated to evaluate their photophysical properties and to explore the mechanism of their phototoxicity. Photophysical experiments were carried out in aqueous solution by stationary and time-resolved fluorimetry, and by laser flash photolysis, to obtain information on the various decay pathways of the excited states of the drugs and on transient species formed upon irradiation. The results obtained show that levofloxacin is able to photosensitize red blood cell lysis in an oxygen-independent way and induce a high decrease in cell viability after UVA irradiation, although to a lesser degree than the racemic mixture ofloxacin. Moxifloxacin, which is an 8-MeO-substituted fluoroquinolone, is less phototoxic than the other compounds. Cellular phototoxicity was inhibited by the addition of superoxide dismutase, catalase, and free radical and hydroxyl radical scavengers (BHA, GSH, mannitol, and DMTU), indicating the involvement of superoxide anion and/or a radical mechanism in their cytotoxicity. A good correlation was observed between lipid peroxidation, protein photodamage, and cellular phototoxicity, indicating that test compounds exert their toxic effects mainly in the cellular membrane. Experiments carried out on pBR322 DNA show that these derivatives do not significantly photocleave DNA directly, but single-strand breaks were evidenced after treatment of photosensitized DNA by two base-excision-repair enzymes, and Endo III. [source] Oxidation of 1,4-Dioxane over Ti-MWW in the Presence of H2O2CHEMSUSCHEM CHEMISTRY AND SUSTAINABILITY, ENERGY & MATERIALS, Issue 3 2008Weibin Fan Dr. Ti time for dioxane: Oxidation of 1,4-dioxane with aqueous H2O2 over various titanosilicates was investigated. Use of Ti-MWW as catalyst leads to much higher conversions than with TS-1 and Ti-Beta under solvent-free conditions and is accounted for by a radical mechanism. The number of active intermediate Ti species is highly dependent on the substrate, solvent, and titanosilicate used. [source] | |||||||||||||