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Nitroxide
Kinds of Nitroxide Terms modified by Nitroxide Selected AbstractsThe First Example of an Azaphenalene Profluorescent NitroxideEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 28 2007James P. Blinco Abstract The synthesis of the first example of an azaphenalene-based fused aromatic nitroxide TMAO, [1,1,3,3-tetramethyl-2,3-dihydro-2-azaphenalen-2-yloxyl, (5)], is described. This novel nitroxide possesses some of the structural rigidity of the isoindoline class of nitroxides, as well as some properties akin to TEMPO nitroxides. Additionally, the integral aromatic ring imparts fluorescence that is switched on by radical scavenging reactions of the nitroxide, which makes it a sensitive probe for polymer degradation.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007) [source] Towards a Better Understanding of Magnetic Interactions within m -Phenylene ,-Nitronyl Nitroxide and Imino Nitroxide Based Radicals, Part III: Magnetic Exchange in a Series of Triradicals and Tetraradicals Based on the Phenyl Acetylene and Biphenyl Coupling UnitsCHEMISTRY - A EUROPEAN JOURNAL, Issue 8 2005Laure Catala Dr. Abstract The present work completes and extends our previous reports1,,2 on the determination of the magnetic ground state and on the strength of the through bond exchange coupling within series of biradicals. This knowledge was subsequently exploited for the analysis of the magnetic interactions in their crystals. We report here the studies of series of triradicals incorporating ,-nitronyl nitroxides (NN) or ,-imino nitroxides (IN) as terminal radical fragments connected through a m -phenylene coupling unit in one case and a phenyl acetylene unit in other case. Tetraradical derivatives have also been studied. The studies of isolated molecules (EPR in solution and DFT calculations) allow the assessment of the magnetic interactions through the magnetic coupling unit. All triradical derivatives are found to exhibit a quartet ground state, whereas a singlet ground state is determined for the tetraradical. This last result reinforces previous findings that the singlet ground state is favoured in related biradicals involving similar m -phenylene couplers. Moreover, the through bond magnetic exchange coupling for the ortho,meta connectivity could be demonstrated as being ferromagnetic, thus ascertaining our previous hypotheses.1 The magnetic properties of the triradicals and tetraradicals in their solid state have been rationalized by using a previously proposed methodology,2 allowing to identify the most relevant magnetic pathways. Le présent travail complète et étend nos travaux précédents portant sur la détermination des couplages d'échange magnétique intramoléculaires de séries de biradicaux,1,,2 et partant de cette connaissance, de l'analyse des corrélations magnétostructurales dans l'état cristallin. Nous présentons ici l'étude de séries de triradicaux comprenant des radicaux , -nitronyl nitroxydes (NN) ou , -imino nitroxydes (IN) substitués en méta d'un groupement phénylène d'une part, et en para d'un groupement phénylène éthynylène d'autre part. Des dérivés tétraradicalaires ont aussi été synthétisés et étudiés. Les interactions d'échange magnétique à travers les liaisons sont estimées d'après l'étude de ces molécules isolées par RPE en solution ainsi que par des calculs effectués dans l'approximation de la fonctionnelle densité (DFT). Alors que l'ensemble des dérivés triradicalaires présente un état fondamental magnétique quartet de spin, un état singulet est proposé pour les tétraradicaux. Ce dernier résultat confirme nos travaux précédents1 ayant conclu à un état fondamental singulet pour des biradicaux basés sur le même coupleur. De plus, le couplage d'échange magnétique au travers du coupleur phénylène éthynylène pour des biradicaux substitués dans une topologie ortho,méta est ferromagnétique, confirmant ainsi les hypothèses antérieures.1 Suivant une méthodologie précédemment décrite,2 les propriétés magnétiques des triradicaux à l'état cristallin ont pu être analysées dans le détail et permettent de proposer des chemins d'interaction magnétique et des géométries de contacts intermoléculaires précises pour l'établissement d'interactions magnétiques bien identifiées. El presente trabajo completa y extiende nuestras anteriores estudios1,,2 sobre la determinacion del estado fundamental magnético y la fuerza del intercambio magnetico a través del enlace en una serie de biradicales. Esta información es luego usada en el análisis de las interacciones magneticas dentro de sus cristales. En este trabajo presentamos los estudios sobre una serie de triradicales que contienen , -nitroxidos nitronílicos (NN) o , -imino nitróxidos (IN) como fragmentos radicalarios terminales conectados a través de unidades acopladoras del tipo m -phenilénicas, en un caso, y fenil acetilénicas, en el otro. Se han estudiado también tetraradicales. Los estudios con moleculas aisladas (EPR en disolución y DFT) permiten evaluar las interacciones magneticas a través de las unidades acopladoras. Todos los triradicales se ve que presentan un estado fundamental del tipo cuadruplete, mientras que el cuadruplete se cree que tienen un estado fundamental singlete. Este último resultado refuerza las conclusiones de un estudio previo en el que se vió que el singlete es el estado fundamental en biradicales similares conectados por una unidad m -fenilénica. Además, el intercambio magnético a través del enlace para la conectividad orto y meta se demuestra que es ferromagnetica, confirmando hipótesis anteriores.1 Las propiedades magnéticas de los triradicales y tetraradicales en su estado sólido se han racionalizado empleando una matodología propuesta anteriormente,2 identificando los caminos magnéticos relevantes. [source] The First Example of an Azaphenalene Profluorescent NitroxideEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 28 2007James P. Blinco Abstract The synthesis of the first example of an azaphenalene-based fused aromatic nitroxide TMAO, [1,1,3,3-tetramethyl-2,3-dihydro-2-azaphenalen-2-yloxyl, (5)], is described. This novel nitroxide possesses some of the structural rigidity of the isoindoline class of nitroxides, as well as some properties akin to TEMPO nitroxides. Additionally, the integral aromatic ring imparts fluorescence that is switched on by radical scavenging reactions of the nitroxide, which makes it a sensitive probe for polymer degradation.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007) [source] Gas-phase radical,radical recombination reactions of nitroxides with substituted phenyl radicalsINTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 4 2004J. L. Heidbrink Fourier-transform ion cyclotron resonance mass spectrometry has been used to examine gas-phase reactions of four different nitroxide free radicals with eight positively charged pyridyl and phenyl radicals (some containing a Cl, F, or CF3 substituent). All the radicals reacted rapidly (near collision rate) with nitroxides by radical,radical recombination. However, some of the radicals were also able to abstract a hydrogen atom from the nitroxide. The results establish that the efficiency (kreaction/kcollision) of hydrogen atom abstraction varies with the electrophilicity of the radical, and hence is attributable to polar effects (a lowering of the transition-state energy by an increase in its polar character). The efficiency of the recombination reaction is not sensitive to substituents, presumably due to a very low reaction barrier. Even so, after radical,radical recombination has occurred, the nitroxide adduct was found to fragment in different ways depending on the structure of the radical. For example, a cationic fragment was eliminated from the adducts of the more electrophilic radicals via oxygen anion abstraction by the radical (i.e., the nitroxide adduct cleaves heterolytically), whereas adducts of the less electrophilic radicals predominantly fragmented via homolytic cleavage (oxygen atom abstraction). Therefore, differences in the product branching ratios were found to be attributable to polar factors. © 2004 Wiley Periodicals, Inc. Int J Chem Kinet 36: 216,229 2004 [source] Singlet Oxygen Detection in Skim Milk by Electron Spin Resonance SpectroscopyJOURNAL OF FOOD SCIENCE, Issue 2 2003D.G. Bradley ABSTRACT: 2,2,6,6-Tetramethyl-4-piperidone (TMPD) can react with singlet oxygen to produce the corresponding nitroxide, 2,2,6,6-tetramethyl-4-piperidone-1-oxyl (TAN), which can be detected by electron spin resonance (ESR) spectroscopy. ESR detected the formation of TAN in 5 ,M riboflavin phosphate buffer, or skim milk containing 20 mM TMPD during illumination. The effects of illumination, riboflavin, and oxygen on the formation of TAN indicated that the presence of each was required for singlet oxygen formation. The photo reduced riboflavin might reduce TAN to the hydroxylamine of TAN, thus decreasing the paramagnetic TAN signal. Maximum concentration of TAN was obtained at an added 10 mM riboflavin in buffer solution or in skim milk after 15 min of illumination in the presence of oxygen. [source] Electrospray mass spectrometry of stable iminyl nitroxide and nitronyl nitroxide free radicalsJOURNAL OF MASS SPECTROMETRY (INCORP BIOLOGICAL MASS SPECTROMETRY), Issue 9 2002Craig D. Smith Abstract Electrospray ionization (ESI) mass spectra have been recorded for a range of substituted nitronyl nitroxide and iminyl nitroxide monoradicals and biradicals. Secondary species formed in the ESI source were observed as the dominant ions in both the iminyl nitroxide and nitronyl nitroxide spectra. Daughter ion spectrometry was used to establish fragmentation mechanisms for the nitronyl nitroxide and iminyl nitroxide moieties as well as the secondary species under ESI conditions. Copyright © 2002 John Wiley & Sons, Ltd. [source] Free radical and nitroxide mediated polymerization of hydroxy,functional acrylates prepared via lipase,catalyzed transacylation reactionsJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 12 2010Dragos Popescu Abstract 3-Hydroxypropyl acrylate, 4-hydroxybutyl acrylate, 2-methyl-3-hydroxypropyl acrylate, 2-hydroxypropyl acrylate, neopentyl glycol acrylate, glyceryl acrylate, and dihydroxyhexyl acrylate were prepared via transacylation reaction of methyl acrylate with diols and triols catalyzed by Candida antarctica lipase B. After removal of the enzyme by filtration and the methyl acrylate by distillation, the monomers were polymerized via free radical polymerization (FRP) with azobisisobutyronitrile as initiator and nitroxide mediated polymerization (NMP) employing BlocbuilderÔ alkoxyamine initiator and SG-1 free nitroxide resulting in hydroxy functional poly(acrylates). The NMP kinetics are discussed in detail. In addition, the polymers obtained by FRP and NMP are compared and the results are related to the amount of bisacrylates that are present in the initial monomer mixtures resulting from the transacylation reactions. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 2610,2621, 2010 [source] Synthesis of bipolar charge transporting block copolymers and characterization for organic light-emitting diodeJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 7 2010Kousuke Tsuchiya Abstract A series of hole and electron transporting random and block copolymers consisting of triphenylamine moiety as a hole transporting unit and oxadiazole moiety as an electron transporting unit have been prepared via a nitroxide mediated radical polymerization. Oxadiazole monomers with t -butyl or trifluoromethyl groups, 2 and 7, respectively, were used for copolymerization. Photoluminescent measurements of polymers revealed that the formation of the exciplex between triphenylamine and oxadiazole units tends to occur in the order of random copolymers, block copolymers, and polymer blends, implying phase-separated morphologies in block or blend systems. The polymers were applied for OLED devices, and we found that the morphology in the polymer layer critically affected device performance. The block copolymer comprising hole and electron transporting units with the composition of 14/86 showed the highest external quantum efficiency over 10%. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 1461,1468, 2010 [source] Micellar behavior of well-defined polystyrene-based block copolymers with triethoxysilyl reactive groups and their hydrolysis,condensationJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 4 2010Cé Guinto Gamys Abstract Block copolymers of acryloxy propyl triethoxysilane and styrene were prepared through nitroxide-mediated polymerization using alkoxyamine initiators based on N - tert -butyl-1-diethylphosphono-2,2-dimethylpropyl nitroxide. The copolymers were characterized by 1H NMR, size exclusion chromatography and differential scanning calorimetry. Their micellar behavior in dioxane/methanol solutions was examined through static light scattering and transmission electron microscopy (TEM). TEM indicated the successful formation of spherical micelles which were subsequently frozen by the sol,gel process. Hydrolysis,condensation of the reactive ethoxysilyl side groups was followed by FTIR, 1H NMR, and 29Si NMR. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 784,793, 2010 [source] Controlled/living heterogeneous radical polymerization in supercritical carbon dioxideJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 15 2009Per B. Zetterlund Abstract Supercritical carbon dioxide (scCO2) is an inexpensive and environmentally friendly medium for radical polymerizations. ScCO2 is suited for heterogeneous controlled/living radical polymerizations (CLRPs), since the monomer, initiator, and control reagents (nitroxide, etc.) are soluble, but the polymer formed is insoluble beyond a critical degree of polymerization (Jcrit). The precipitated polymer can continue growing in (only) the particle phase giving living polymer of controlled well-defined microstructure. The addition of a colloidal stabilizer gives a dispersion polymerization with well-defined colloidal particles being formed. In recent years, nitroxide-mediated polymerization (NMP), atom transfer radical polymerization (ATRP), and reversible addition fragmentation chain transfer (RAFT) polymerization have all been conducted as heterogeneous polymerizations in scCO2. This Highlight reviews this recent body of work, and describes the unique characteristics of scCO2 that allows composite particle formation of unique morphology to be achieved. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 3711,3728, 2009 [source] Cyclic alkoxyamines for nitroxide-mediated radical polymerizationJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 24 2008Jean Ruehl Abstract A 5-membered cyclic alkoxyamine and a 17-membered cyclic alkoxyamine were synthesized and used in the polymerization of styrene. Polymerizations using the 5-membered cyclic alkoxyamine resulted in polymers with uncontrolled molecular weights and high polydispersities. Polymerizations using the 17-membered cyclic alkoxyamine produced oligomeric polymers in which multiple polymer chains are linked through NO-C bonds. EPR homolysis experiments revealed that the 5-membered cyclic alkoxyamine does not dissociate to form a nitroxide species, even at temperatures as high as 403 K. In contrast, the 17-membered cyclic alkoxyamine does dissociate to form nitroxide, but the rate of dissociation is slower than that of parent acyclic alkoxyamine 2,2,5-trimethyl-3-(1-phenylethoxy)-4-phenyl-3-azahexane. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 8049,8069, 2008 [source] Heterograft copolymers via double click reactions using one-pot techniqueJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 20 2008Aydan Dag Abstract The double click reactions (Cu catalyzed Huisgen and Diels,Alder reactions) were used as a new strategy for the preparation of well-defined heterograft copolymers in one-pot technique. The synthetic strategy to the various stages of this work is outlined: (i) preparing random copolymers of styrene (St) and p -chloromethylstyrene (CMS) (which is a functionalizable monomer) via nitroxide mediated radical polymerization (NMP); (ii) attachment of anthracene functionality to the preformed copolymer by the o -etherification procedure and then conversion of the remaining CH2Cl into azide functionality; (iii) by using double click reactions in one-pot technique, maleimide end-functionalized poly(methyl methacrylate) (PMMA-MI) via atom transfer radical polymerization (ATRP) of MMA and alkyne end-functionalized poly (ethylene glycol) (PEG-alkyne) were introduced onto the copolymer bearing pendant anthryl and azide moieties. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 6969,6977, 2008 [source] Dendrimer-like miktoarm star terpolymers: A3 -(B-C)3 via click reaction strategyJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 17 2008Ozcan Altintas Abstract Two samples of dendrimer-like miktoarm star terpolymers: (poly(tert -butyl acrylate))3 -(polystyrene-poly(,-caprolactone))3 (PtBA)3 -(PS-PCL)3, and (PS)3 -(PtBA-poly(ethylene glycol)3 were prepared using efficient Cu catalyzed Huisgen cycloaddition (click reaction). As a first step, azido-terminated 3-arm star polymers PtBA and PS as core (A) were synthesized by atom transfer radical polymerization (ATRP) of tBA and St, respectively, followed by the conversion of bromide end group to azide. Secondly, PS-PCL and PtBA-PEG block copolymers with alkyne group at the junction as peripheral arms (B-C) were obtained via multiple living polymerization mechanisms such as nitroxide mediated radical polymerization (NMP) of St, ring opening polymerization (ROP) of ,-CL, ATRP of tBA. Thus obtained core and peripheral arms were linked via click reaction to give target (A)3 -(B-C)3 dendrimer-like miktoarm star terpolymers. (PtBA)3 -(PS-PCL)3 and (PS)3 -(PEG-PtBA)3 have been characterized by GPC, DSC, AFM, and SAXS measurements. (PtBA)3 -(PS-PCL)3 did not show any self-organization with annealing due to the miscibility of the peripheral arm segments. In contrast, the micro-phase separation of the peripheral arm segments in (PS)3 -(PtBA-PEG)3 resulted in self-organized phase-separated morphology with a long period of , 13 nm. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 5916,5928, 2008 [source] One-pot preparation of 3-miktoarm star terpolymers via click [3 + 2] reactionJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 16 2007Ozcan Altintas Abstract The preparation of 3-miktoarm star terpolymers using nitroxide mediated radical polymerization (NMP), ring opening polymerization (ROP), and click reaction [3 + 2] are carried out by applying two types of one-pot technique. In the first one-pot technique, NMP of styrene (St), ROP of ,-caprolactone (,-CL), and [3 + 2] click reaction (between azide end-functionalized poly(ethylene glycol) (PEG-N3)/or azide end-functionalized poly(methyl methacrylate) (PMMA-N3) and alkyne) are carried out in the presence of 2-(hydroxymethyl)-2-methyl-3-oxo-3-(2-phenyl-2-(2,2,6,6-tetramethylpiperidin-1-yloxy)ethoxy) propyl pent-4-ynoate, 2, as an initiator for 48 h at 125 °C (one-pot/one-step). As a second technique, NMP of St and ROP of ,-CL were conducted using 2 as an initiator for 20 h at 125 °C, and subsequently PEG-N3 or azide end-functionalized poly(tert -butyl acrylate (PtBA-N3) was added to the polymerization mixture, followed by a click reaction [3 + 2] for 24 h at room temperature (one-pot/two-step). The 3-miktoarm star terpolymers, PEG-poly(,-caprolactone)(PCL)-PS, PtBA-PCL-PS and PMMA-PCL-PS, were recovered by a simple precipitation in methanol without further purification. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 3588,3598, 2007 [source] Deactivation reactions in the modeled 2,2,6,6-tetramethyl-1-piperidinyloxy-mediated free-radical polymerization of styrene: A comparative study with the 2,2,6,6-tetramethyl-1-piperidinyloxy/acrylonitrile systemJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 2 2007Andrzej Kaim Abstract The competitiveness of the combination and disproportionation reactions between a 1-phenylpropyl radical, standing for a growing polystyryl macroradical, and a 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) radical in the nitroxide-mediated free-radical polymerization of styrene was quantitatively evaluated by the study of the transition geometry and the potential energy profiles for the competing reactions with the use of quantum-mechanical calculations at the density functional theory (DFT) UB3-LYP/6-311+G(3df, 2p)//(unrestricted) Austin Model 1 level of theory. The search for transition geometries resulted in six and two transition structures for the radical combination and disproportionation reactions, respectively. The former transition structures, mainly differing in the out-of-plane angle of the NO bond in the transition structure TEMPO molecule, were correlated with the activation energy, which was determined to be in the range of 8.4,19.4 kcal mol,1 from a single-point calculation at the DFT UB3-LYP/6-311+G(3df, 2p)//unrestricted Austin Model 1 level. The calculated activation energy for the disproportionation reaction was less favorable by a value of more than 30 kcal mol,1 in comparison with that for the combination reaction. The approximate barrier difference for the TEMPO addition and disproportionation reaction was slightly smaller for the styrene polymerization system than for the acrylonitrile polymerization system, thus indicating that a ,-proton abstraction through a TEMPO radical from the polymer backbone could diminish control over the radical polymerization of styrene with the nitroxide even more than in the latter system. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 232,241, 2007 [source] N -(2-Hydroxypropyl)methacrylamide (HPMA) Copolymer-Linked Nitroxides: Potential Magnetic Resonance Contrast AgentsMACROMOLECULAR BIOSCIENCE, Issue 11 2003Yuan Huang Abstract N -(2-Hydroxypropyl)methacrylamide (HPMA) copolymer-linked nitroxides were synthesized as macromolecular contrast agents for MR imaging. Molar relaxivities of HPMA copolymer-linked nitroxides increased linearly in proportion to the number of nitroxides attached per gram of copolymer. HPMA copolymer-linked nitroxides with 15, 20 and 30 mol-% nitroxide exhibited higher relaxivities than gadolinium diethylenetriaminepentaacetic acid (Gd-DTPA). These results demonstrate the potential of HPMA copolymer-linked nitroxides as MR contrast agents for solid tumors. Structure of HPMA copolymer-linked nitroxides. [source] Synthesis of Polystyrene- block -Poly(methyl methacrylate) with Fluorene at the Junction: Sequential Anionic and Controlled Radical Polymerization from a Single CarbonMACROMOLECULAR CHEMISTRY AND PHYSICS, Issue 24 2009Nathan D. Contrella Abstract Polystyrene- block -poly(methyl methacrylate) (PS- b -PMMA) has been synthesized by sequential anionic and reverse atom transfer radical polymerization (ATRP) or a variation of nitroxide mediated polymerization (NMP) from a single initiating site, specifically the 9-carbon on 2,7-dibromofluorene or fluorene. The addition of the second arm (PS) relied on thermal decomposition of 2,2,-azoisobutyronitrile (AIBN) to generate radicals, abstracting the 9-H on the polymer-bound fluorene species to form the initiating radical. Styrene was not present in the reaction mixture when AIBN was decomposed, preventing competition between addition across the monomeric alkene and hydrogen abstraction from the fluorene. After 1,h, styrene was introduced and mediation of the subsequent radical polymerization was achieved by the presence of CuCl2/ligand or TEMPO. Characterization of the diblock copolymers by gel permeation chromatography (GPC) revealed substantial shifts in number average molecular weight () values compared to the anionically prepared PMMA macroinitiator, while polydispersity indices (PDI's) remained relatively low (typically,<,1.5). Characterization by UV detection with GPC (at 310,nm) verified that the diblock polymer is chromophore-bound, which was further verified by UV-vis spectroscopy of the isolated diblock. [source] Hybrid Aluminum Colored Pigments Based on Gradient Copolymers DesignMACROMOLECULAR CHEMISTRY AND PHYSICS, Issue 18 2009Mathieu Joubert Abstract A colored polymer/aluminium hybrid pigment was synthesized by nitroxide mediated polymerization initiated from an inorganic surface. This approach requires the preparation of a vinyl dye monomer able to copolymerize with n -butyl acrylate (n-BuA) and styrene (S) from the surface of aluminium flakes. The linearity of the ln([M]0/[M]t) and of the as a function of time and conversion constitute the criteria of control/"living" polymerization, i.e. linearity of respectively ln([M]0/[M]t),=,f(t) and the ,=,f(conversion) plots. Kinetic measurements reveal an upward deviation from the linearity for n -BuA polymerization for very high conversion. The introduction of S monomer restores the control of the polymerization. Both the length of the grafted chains and the dye/styrene molar ratio influence the color of the hybrid material. [source] Gelation and Hollow Particle Formation in Nitroxide-Mediated Radical Copolymerization of Styrene and Divinylbenzene in MiniemulsionMACROMOLECULAR CHEMISTRY AND PHYSICS, Issue 2 2009Per B. Zetterlund Abstract Network formation in the nitroxide-mediated cross-linking copolymerization of styrene and divinylbenzene (3 or 8.2 mol-% relative to total monomer) using the nitroxide 2,2,6,6-tetramethylpiperidinyl-1-oxy (TEMPO) at 125,°C can proceed markedly differently in aqueous miniemulsion compared to the corresponding solution polymerization depending on the organic-phase composition. When the organic phase comprises 54 vol-% of the hydrophobe tetradecane, gelation occurs at much lower conversion in miniemulsion than in solution, and at significantly lower conversion than predicted by Flory,Stockmayer gelation theory. This is proposed to be a result of an effect of the oil,water interface, whereby the concentration of polymer with pendant unsaturations is higher near the interface than in the particle interior. [source] Formation of Inorganic/Organic Nanocomposites by Nitroxide-Mediated Polymerization in Bulk Using a Bimolecular SystemMACROMOLECULAR CHEMISTRY AND PHYSICS, Issue 10 2004Julien Parvole Abstract Summary: A series of organic-inorganic nanoparticles were synthesized by nitroxide-mediated polymerization (NMP) of butyl acrylate initiated by a self-assembled monolayer of an azo initiator. The azo initiator was immobilized on silica particles in the presence of a stable nitroxide radical, SG1 (an acyclic , -phosphonylated nitroxide, N - tert -butyl- N -(1-diethylphosphono-2,2-dimethyl)propyl nitroxide). After preliminary qualitative characterization by X-ray spectroscopy (XPS) and Fourier-transform infrared (FTIR) measurements, the nanoparticles were studied by thermogravimetric analysis (TGA) to determine the polymer grafting density and to permit a comparison with corresponding values of the initiator monolayer. It was demonstrated that the grafting from polymerization exhibits a controlled character with a low polydispersity (,<,1.2) in a large range of molecular weights of the grafted chains (from 4,000 up to 145,000 g,·,mol,1) under the conditions when the stable radical SG1, acting as chain growth moderator tethered to the inorganic core, was used. [source] Novel copolymers via nitroxide mediated controlled free radical polymerization of vinyl chlorideMACROMOLECULAR SYMPOSIA, Issue 1 2003Thomas Wannemacher Abstract Controlled free radical polymerization (CFRP) of vinyl chloride (VCM) and copolymerization with several comonomers have been studied in aqueous suspension. Therefore di-tert-butylnitroxide and three novel nitroxyl radicals were used as mediating agents. Copolymerization of VCM with styrene, partly combined with acrylonitrile, maleic acid anhydride and maleic acid imide as well as methyl methacrylate, n-butyl methacrylate, butyl acrylate and butadiene have been achieved, demonstrating an efficient route for novel vinyl chloride copolymer architecture. [source] Kinetics of Stable Free Radical Mediated Polymerization inside Submicron ParticlesMACROMOLECULAR THEORY AND SIMULATIONS, Issue 9 2007Hidetaka Tobita Abstract Controlled/living radical polymerization systems in which the active period is extremely small, ,A,,,1, such as the cases of stable free radical mediated polymerization (or nitroxide mediated polymerization) and atom transfer radical polymerization, are considered theoretically. The polymerization rate, Rp, for such systems increases by lowering the trapping agent concentration [X]. When the polymerization is conducted inside small particles, Rp decreases with D below the diameter Dp,SMC at which a single molecule concentration (SMC) is equal to [X]bulk. On the other hand, when the average number of trapping agents in a particle is smaller than about 10, the fluctuation of nX among particles is significant, which leads to a larger Rp than in the cases where all particles contain the same nX. Because of the effects of SMC and fluctuation, Rp may show an acceleration window, Dp,SMC,<,Dp,<,Dp,Fluct where Rp is slightly larger than that in bulk. [source] ,Magic blue' , subtle reagent for EPR studies on H-abstraction from various substratesMAGNETIC RESONANCE IN CHEMISTRY, Issue 5 2006Hong-Yan Dou Abstract This paper is an EPR study on the generation of new ,magic blue' (MB) reagent containing bis{perfluoro[1-(2-fluorosulfonyl)ethoxy]ethyl}nitroxide 2 and perfluoro[1-nitroso-1-(2-fluorosulfonyl)ethoxy]ethane 3, both bearing chemically convertible terminal ,-fluorosulfonyl group SO2F, and its H-abstraction reactions from arylalkanes, alcohols, aldehydes and polymers, leading to a large number of new fluorinated nitroxides (FN). Copyright © 2006 John Wiley & Sons, Ltd. [source] Use of 3-acetoxymethoxycarbonyl-2,2,5,5-tetramethyl-1-pyrrolidinyloxyl as an EPR oximetry probe: Potential for in vivo measurement of tissue oxygenation in mouse brainMAGNETIC RESONANCE IN MEDICINE, Issue 6 2006Jiangang Shen Abstract Measurement of oxygen concentration and distribution in the brain is essential for understanding the pathophysiology of stroke. Low-frequency electron paramagnetic resonance (EPR) spectroscopy with a paramagnetic probe is an attractive imaging modality that potentially can be used to map O2 concentration in the brain. We examined two nitroxides, 3-methoxycarbonyl-2,2,5,5-tetramethyl-1-pyrrolidinyloxyl [2] and 3-acetoxymethoxycarbonyl-2,2,5,5-tetramethyl-1-pyrrolidinyloxyl [3], as pro-imaging agents to deliver 3-carboxy-2,2,5,5-tetramethyl-1-pyrrolidinyloxyl [1] across the blood,brain barrier (BBB). In primary cultured neurons, nitroxide [3] but not [2] was hydrolyzed by intracellular esterases to [1], which, being anionic at physiologic pH, was well retained intracellularly. In contrast, [2] was not well retained by neurons. In vivo pharmacokinetic and pharmacodynamic studies in mice suggested that esterase-labile nitroxide [3] crossed the BBB, and was converted to [1] and retained. Retention occurred in brain tissue and not in the extensive vasculature, as evidenced by the fact that removal of blood by whole-body saline perfusion did not eliminate the nitroxide EPR signal from the brain. The EPR linewidths of [1] and [3] were more O2 -sensitive than that of the commonly-used oximetry probe 4-oxo-2,2,6,6-tetramethylpiperidine-d16 -1- 15N-oxyl [4]. Moreover, we used [3] in vivo to estimate O2 concentration in mouse brains. These results indicate that nitroxide [3] could be useful for mapping O2 distribution in the brain following stroke. Magn Reson Med, 2006. © 2006 Wiley-Liss, Inc. [source] Cardiac applications of EPR imagingNMR IN BIOMEDICINE, Issue 5 2004Periannan Kuppusamy Abstract This review summarizes the development and application of a variety of EPR imaging modalities including spatial, spectral,spatial (spectroscopic), gated-imaging and oxygen mapping to cardiovascular studies. It has been hypothesized that free radical metabolism, oxygenation and nitric oxide generation in biological organs such as the heart may vary over the spatially defined tissue structure. We have developed instrumentation optimized for 3D spatial and 3D or 4D spectral,spatial imaging of free radicals at 1.2 GHz. Using this instrumentation high quality 3D spectral,spatial imaging of nitroxyl (nitroxide) metabolism was performed, as well as spatially localized measurements of oxygen concentrations, based on the oxygen-dependent line-broadening of the EPR spectrum. Both exogenously infused probes and endogenous radicals were used to obtain the images. It is demonstrated that the EPR imaging is a powerful tool which can provide unique information regarding the spatial localization of free radicals, oxygen and nitric oxide in biological organs and tissues. Copyright © 2004 John Wiley & Sons, Ltd. [source] Dual Chromophore-Nitroxides: Novel Molecular Probes, Photochemical and Photophysical Models and Magnetic MaterialsPHOTOCHEMISTRY & PHOTOBIOLOGY, Issue 4 2007Gertz I. Likhtenstein Over the last decades scientists have faced growing requirements in novel methods of fast and sensitive analysis of antioxidant status of biological systems, spin redox probing and spin trapping, investigation of molecular dynamics, and of convenient models for studies of photophysical and photochemical processes. In approaching this problem, methods based upon the use of dual chromophore-nitroxide (CN) compounds have been suggested and developed. A CN consists of two molecular sub-functionality (a chromophore and a stable nitroxide radical) tethered together by spacers. In the dual compound the nitroxide is a strong intramolecular quencher of the fluorescence from the chromophore fragment. Reduction to hydroxylamine, oxidation of the nitroxide fragment or addition of an active radical yield the fluorescence increase and the parallel decay of the fragment electron spin resonance (ESR) signal. At certain conditions the dual molecules undergo photomagnetic switching and form excited state multi-spin systems. These unique properties of CN were intensively exploited as the basis for several methodologies, which include molecular probing, modeling intramolecular photochemical and photophysical processes, and construction of new magnetic materials. [source] Zeolite Encapsulation Decreases TiO2 -photosensitized ROS Generation in Cultured Human Skin Fibroblasts,PHOTOCHEMISTRY & PHOTOBIOLOGY, Issue 1 2006Biao Shen ABSTRACT Sunscreens protect skin against sunburn. However, studies have demonstrated that UV-irradiated sunscreen components such as titanium dioxide (TiO2) promote the photogeneration of reactive oxygen species (ROS). Because encapsulation of TiO2 within zeolites alters its photocatalytic activity, supra-molecular composites based on NaY zeolite hosts containing TiO2 guests were prepared, and the effects on ROS formation in cells under UVA-irradiation evaluated. DCFH-DA (2,,7,-dichlorofluorescein diacetate) was used as a profluorescent probe to monitor intracellular ROS. The detection of in-tracellular 2,,7,-dichlorofluorescein (DCF) fluorescence by confocal microscopy revealed that DCFH-DA was taken up, hydrolyzed and oxidized by yeast cells and cultured human skin fibroblasts within 20 and 6 min, respectively. Higher DCF fluorescence was observed in fibroblasts following UVA irra-diation in the absence but not in the presence of the radical nitroxide, TEMPOL (4-hydroxy-2,2,6,6-tetramethylpipery-dine-1-oxyl), which exhibits superoxide dismutase-mimetic and catalase-mimetic activity. UVA-induced fluorescence increased by -50% in the presence of 32-nm anatase TiO2 particles and decreased by essentially an equal amount in the presence of TiO2 encapsulated within NaY zeolites (TiO2@NaY). Addition of the uncomplexed NaY host also decreased (by ,30%) the amount of UVA-induced fluorescence but, un-expectedly, the combination of the free guest and host (TiO2@NaY) caused a doubling of the fluorescence. Protection of cells against TiO2 -induced intracellular ROS by encapsulation suggests that supramolecular species may be beneficial in photoprotection of the skin. In contrast, the potentiation of TiO2 -induced ROS by uncomplexed NaY points to a critical role for formulation when free TiO2 is used as a sun screen ingredient. [source] Surface-initiated nitroxide-mediated polymerization: a tool for hybrid inorganic/organic nanocomposites ,in situ' synthesisPOLYMER INTERNATIONAL, Issue 10 2006Leïla Ghannam Abstract Hybrid silica particles comprised of an inorganic core and an organic polymer shell can be synthesized by surface-initiated nitroxide-mediated polymerization (SINMP) in the presence of a grafted alkoxyamine as initiator. The layer of initiator molecules attached or adsorbed to the particles surface is based on a nitroxide group. The synthesis and characterization of these alkoxyamine structures are briefly described. The grafted initiator particles are studied in order to determine the nature of the anchorage sites and the grafting density. With a stable radical or nitroxide as chain-growth moderator tethered to the inorganic core, it was demonstrated that the grafting from or to SINMP exhibits a control character with a very low polydispersity and good agreement between theoretical/experimental molecular weights. A comparative review of different alkoxyamines with a bimolecular system based on a grafted azoic initiator in the presence of an acyclic ,-phosphonylated nitroxide called SG1 is also described. Moreover, original methods for the synthesis of alkoxyamine grafted surface by in situ trapping of carbon radicals and some new developments for colored pigments will be reported. These topics could be one of the directions of the field in the next years. Copyright © 2006 Society of Chemical Industry [source] Resolution of ligand positions by site-directed tryptophan fluorescence in tear lipocalinPROTEIN SCIENCE, Issue 2 2000Oktay K. Gasymov Abstract The lipocalin superfamily of proteins functions in the binding and transport of a variety of important hydrophobic molecules. Tear lipocalin is a promiscuous lipid binding member of the family and serves as a paradigm to study the molecular determinants of ligand binding. Conserved regions in the lipocalins, such as the G strand and the F-G loop, may play an important role in ligand binding and delivery. We studied structural changes in the G strand of holo- and apo-tear lipocalin using spectroscopic methods including circular dichroism analysis and site-directed tryptophan fluorescence. Apo-tear lipocalin shows the same general structural characteristics as holo-tear lipocalin including alternating periodicity of a ,-strand, orientation of amino acid residues 105, 103, 101, and 99 facing the cavity, and progressive depth in the cavity from residues 105 to 99. For amino acid residues facing the internal aspect of cavity, the presence of a ligand is associated with blue shifted spectra. The collisional rate constants indicate that these residues are not less exposed to solvent in holo-tear lipocalin than in apo-tear lipocalin. Rather the spectral blue shifts may be accounted for by a ligand induced rigidity in holo-TL. Amino acid residues 94 and 95 are consistent with positions in the F-G loop and show greater exposure to solvent in the holo- than the apo-proteins. These findings are consistent with the general hypothesis that the F-G loop in the holo-proteins of the lipocalin family is available for receptor interactions and delivery of ligands to specific targets. Site-directed tryptophan fluorescence was used in combination with a nitroxide spin labeled fatty acid analog to elucidate dynamic ligand interactions with specific amino acid residues. Collisional quenching constants of the nitroxide spin label provide evidence that at least three amino acids of the G strand residues interact with the ligand. Stern-Volmer plots are inconsistent with a ligand that is held in a static position in the calyx, but rather suggest that the ligand is in motion. The combination of site-directed tryptophan fluorescence with quenching by nitroxide labeled species has broad applicability in probing specific interactions in the solution structure of proteins and provides dynamic information that is not attainable by X-ray crystallography. [source] The First Example of an Azaphenalene Profluorescent NitroxideEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 28 2007James P. Blinco Abstract The synthesis of the first example of an azaphenalene-based fused aromatic nitroxide TMAO, [1,1,3,3-tetramethyl-2,3-dihydro-2-azaphenalen-2-yloxyl, (5)], is described. This novel nitroxide possesses some of the structural rigidity of the isoindoline class of nitroxides, as well as some properties akin to TEMPO nitroxides. Additionally, the integral aromatic ring imparts fluorescence that is switched on by radical scavenging reactions of the nitroxide, which makes it a sensitive probe for polymer degradation.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007) [source] | ||||||||||||||||