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Radical Coupling (radical + coupling)
Selected AbstractsQuenching of Singlet Oxygen by Tertiary Aliphatic Amines.HELVETICA CHIMICA ACTA, Issue 10 2006Products, Structural Effects on Rates Abstract A kinetic and product study of the reaction of a series of , -methyl-substituted N -methylpiperidines with thermally generated 1O2 in MeCN was carried out. It was found that as the number of , -methyl groups (Me in , -position relative to the N-atom) increases, the rate of 1O2 quenching (physical plus chemical) slightly decreases. This finding shows that, with respect to the reaction rate, steric effects are much more important than electronic effects as the latter should have produced the opposite result. The opposite outcome was instead found for the chemical quenching that leads to the N -demethylation products and N -formyl derivatives. The same trend was observed for the ratio between N -demethylation and formation of the N -formyl derivatives (NH/NCHO ratio). All these results are consistent with the mechanism reported in Scheme,1 where an exciplex is first formed that by a H-atom transfer process produces an , -amino-substituted C-radical. The latter forms the product of N -demethylation by one electron oxidation, or affords the N -formyl derivative by radical coupling (Scheme,1). Similar results were obtained with N,N -dimethylcyclohexanamine. However, this ,acyclic' amine exhibited behaviors quite distinct from those of the N -methylpiperidines series, with respect to reaction rate, extent of chemical quenching, and NH/NCHO ratio. [source] Preparation of H-shaped ABCAB terpolymers by atom transfer radical couplingJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 1 2009Xiaolan Luo Abstract H-shaped ABCAB terpolymers composed of polystyrene (PS) (A), poly(ethylene oxide) (PEO) (B), and poly(tert -butyl acrylate) (PtBA) (C) were prepared by atom transfer radical coupling reaction using ABC star terpolymers as precursors, CuBr and N,N,N,,N,,N,-pentamethyldiethylenetriamine (PMDETA) as catalysts, and nanosize copper as the reducing agent. The synthesis of 3-miktoarm star terpolymer PS-PEO-(PtBA-Br) involved following steps: (1) the preparation of PS with an active and an ethoxyethyl-ptotected hydroxyl group at the same end; (2) the preparation of diblock copolymer PS- b -PEO with ethoxyethyl-protected group at the junction point through the ring-opening polymerization (ROP) of EO; (3) after de-protection of ethoxyethyl group and further modification of hydroxyl group, tBA was polymerized by atom transfer radical polymerization using PS- b -PEO with 2-bromoisobutyryl functional group as macroinitiator. The H-shaped terpolymer could be successfully formed by atom transfer radical coupling reaction in the presence of small quantity of styrene, CuBr/PMDETA, and Cu at 90 °C. The copolymers were characterized by SEC, 1H NMR, and FTIR in detail. The optimized coupling temperature is 90 °C. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 59,68, 2009 [source] One-pot synthesis of heterograft copolymers via "graft onto" by atom transfer nitroxide radical coupling chemistryJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 20 2008Qiang Fu Abstract Heterograft copolymers poly(4-glycidyloxy-2,2,6,6-tetramethylpiperidine-1-oxyl- co - ethylene oxide)- graft -polystyrene and poly(tert -butyl acrylate) (poly (GTEMPO- co -EO)- g -PS/PtBA) were synthesized in one-pot by atom transfer nitroxide radical coupling (ATNRC) reaction via "graft onto." The main chain was prepared by the anionic ring-opening copolymerization of ethylene oxide (EO) and 4-glycidyloxy-2,2,6,6-tetramethylpiperidine-1-oxyl (GTEMPO) first, then the polystyrene and poly (tert -butyl acrylate) with bromine end (PS-Br, PtBA-Br) were prepared by atom transfer radical polymerization (ATRP). When three of them were mixed each other in the presence of CuBr/N,N,N,,N,,N,-pentamethyldiethylenetriamine (PMDETA) at 90 °C, the formed secondary carbon radicals at the PS and PtBA chain ends were quickly trapped by nitroxide radicals on poly(GTEMPO- co -EO). The heterograft copolymers were well defined by 1H NMR, size exclusion chromatography, fourier transform infrared, and differential scanning calorimetry in detail. It was found that the density of GTEMPO groups on main chain poly(GTEMPO- co -EO), the molecular weights of PS/PtBA side chains, and the structure of macroradicals can exert the great effects on the graft efficiency. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 6770,6779, 2008 [source] Photoinitiated polymerization of methacrylic monomers in a polystyrene matrix: Kinetic, mechanistic, and structural aspectsJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 12 2001J. L. Mateo Abstract The kinetics and mechanism of the photoinitiated polymerization of tetrafunctional and difunctional methacrylic monomers [1,6-hexanediol dimethacrylate (HDDMA) and 2-ethylhexyl methacrylate (EHMA)] in a polystyrene (PS) matrix were studied. The aggregation state, vitreous or rubbery, of the monomer/matrix system and the intermolecular strength of attraction in the monomer/matrix and growing macroradical/matrix systems are the principal factors influencing the kinetics and mechanism. For the PS/HDDMA system, where a relatively high intermolecular force of attraction between monomer and matrix and between growing macroradical and matrix occurs, a reaction-diffusion mechanism takes place at low monomer concentrations (<30,40%) from the beginning of the polymerization. For the PS/EHMA system, which presents low intermolecular attraction between monomer and matrix and between growing macroradical and matrix, the reaction-diffusion termination is not clear, and a combination of reaction-diffusion and diffusion-controlled mechanisms explains better the polymerization for monomer concentrations below 30,40%. For both systems, for which a change from a vitreous state to a rubbery state occurs when the monomer concentration changes from 10 to 20%, the intrinsic reactivity and kp/kt1/2 ratio (where kp is the propagation kinetic constant and kt is the termination kinetic constant) increase as a result of a greater mobility of the monomer in the matrix (a greater kp value). The PS matrix participates in the polymerization process through the formation of benzylic radical, which is bonded to some extent by radical,radical coupling with the growing methacrylic radica, producing grafting on the PS matrix. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 2049,2057, 2001 [source] Lignin Chemistry: Biosynthetic Study and Structural Characterisation of Coniferyl Alcohol Oligomers Formed In Vitro in a Micellar EnvironmentCHEMISTRY - A EUROPEAN JOURNAL, Issue 20 2010Samantha Reale Dr. Abstract Model coniferyl alcohol lignin (the so-called dehydrogenative polymerisate, DHP) was produced in water under homogeneous conditions guaranteed by the presence of a micellised cationic surfactant. A complete study of the activity of the enzymatic system peroxidase/H2O2 under our reaction conditions was reported and all the reaction products up to the pentamer were characterised by 1H,NMR spectroscopy and ESI mass spectrometry. Our system, and the molecules that have been generated in it, represent a closer mimicry of the natural microenvironment since an enzyme, under micellar conditions, reproduces the cell system better than in buffer alone. On the basis of the oligomers structures a new biosynthetic perspective was proposed that focused attention on a coniferyl alcohol dimeric quinone methide as the key intermediate of the reaction. A formal, strictly alternate sequence of a radical and an ionic step underlines the reaction, thus generating ordered oligolignols structures. Alternatively to other model lignins, our olignols present a lower degree of radical coupling between oligomeric units. This offers a closer biosynthetic situation to the observation of a low rate of radical generation in the cell wall. [source] | ||||||||||