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Growing Radicals (growing + radical)
Selected AbstractsComparative study of kinetics and reactivity indices of free radical polymerization reactions,INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 4 2005K. Van Cauter Abstract Density functional theory calculations are used to determine the kinetics and reactivity indices of the first propagation steps of the polyethylene and poly(vinyl chloride) polymerization. Transition state theory is applied to evaluate the rate coefficient from the microscopically determined energies and partition functions. A comparison with the experimental Arrhenius plots validates the level of theory. The ability of reactivity indices to predict certain aspects of the studied propagation reactions is tested. Global softnesses of the reactants give an indication of the relative energy barriers of subsequent monomer additions. The correlation between energy and hardness profiles along the reaction path confirm the principle of maximum hardness. Local indices predict the regioselectivity of the attack of the growing radical to vinyl chloride. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005 [source] The kinetics of enhanced spin capturing polymerization: Influence of the nitrone structureJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 4 2009Edgar H. H. Wong Abstract Several nitrones and one nitroso compound have been evaluated for their ability to control the molecular weight of polystyrene via the recently introduced radical polymerization method of enhanced spin capturing polymerization (ESCP). In this technique, molecular weight control is achieved (at ambient or slightly elevated temperatures) via the reaction of a growing radical chain with a nitrone forming a macronitroxide. These nitroxides subsequently react rapidly and irreversibly with propagating macroradicals forming polymer of a certain chain length, which depends on the nitrone concentration in the system. Via evaluation of the resulting number-average molecular weight, Mn, at low conversions, the addition rate coefficient of the growing radicals onto the different nitrones is determined and activation energies are obtained. For the nitrones N - tert -butyl-,-phenylnitrone (PBN), N -methyl-,-phenylnitrone (PMN), and N -methyl-,-(4-bromo-phenyl) nitrone (pB-PMN), addition rate coefficients, kad,macro, in a similar magnitude to the styrene propagation rate coefficient, kp, are found with spin capturing constants CSC (with CSC = kad,macro/kp) ranging from 1 to 13 depending on the nitrone and on temperature. Activation energies between 23.6 and 27.7 kJ mol,1 were deduced for kad,macro, congruent with a decreasing CSC with increasing temperature. Almost constant Mn over up to high monomer to polymer conversions is found when CSC is close to unity, while increasing molecular weights can be observed when the CSC is large. From temperatures of 100 °C onward, reversible cleavage of the alkoxyamine group can occur, superimposing a reversible activation/deactivation mechanism onto the ESCP system. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 1098,1107, 2009 [source] Magnetic field effects on the copolymerization of water-soluble and ionic monomersJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 2 2009Ignacio Rintoul Abstract The effect of magnetic field (MF) on the radical copolymerization of a series of water-soluble and ionic monomers is presented including acrylamide (AM), acrylic acid (AA), its ionized form acrylate (A,), and diallyldimethylammonium chloride (DADMAC). The following combinations have been studied: AM/AA, AM/A,, AM/DADMAC, and AA/DADMAC. In addition to the MF, strong electrostatic interactions are present for the majority of monomer combinations and conditions. Although the monomer consumption rate (Rp) increased up to 65% applying a MF of 0.1 Tesla, the composition of the resulting copolymers was not affected under such conditions. Despite this increase of Rp by MF, the electrostatic repulsion between ionic monomers and charged growing radicals dominates Rp and governs the copolymer composition with and without MF. The order of the experimentally obtained reactivity ratios reflects the extent of electrostatic interaction: rAM/AA (1.41) < r (3.10) < rAA/DADMAC (4.25) < rAM/DADMAC (6.95) and rAA/AM (2.20) > rDADMAC/AA (0.25) > r (0.17) > rDADMAC/AM (0.03). Overall, weak MF offers to reduce the production time without modifying the product composition. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 373,383, 2009 [source] A new tetradentate ligand for atom transfer radical polymerizationJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 14 2004Shijie Ding Abstract The properties of a ligand, including molecular structure and substituents, strongly affect the catalyst activity and control of the polymerization in atom transfer radical polymerization (ATRP). A new tetradentate ligand, N,N,-bis(pyridin-2-ylmethyl-3-hexoxo-3-oxopropyl)ethane-1,2-diamine (BPED) was synthesized and examined as the ligand of copper halide for ATRP of styrene (St), methyl acrylate (MA), and methyl methacrylate (MMA), and compared with other analogous linear tetrdendate ligands. The BPED ligand was found to significantly promote the activation reaction: the CuBr/BPED complex reacted with the initiators so fast that a large amount of Cu(II)Br2/BPED was produced and thus the polymerizations were slow for all the monomers. The reaction of CuCl/BPED with the initiator was also fast, but by reducing the catalyst concentration or adding CuCl2, the activation reaction could be slowed to establish the equilibrium of ATRP for a well-controlled living polymerization of MA. CuCl/BPED was found very active for the polymerization of MA. For example, 10 mol% of the catalyst relatively to the initiator was sufficient to mediate a living polymerization of MA. The CuCl/BPED, however, could not catalyze a living polymerization of MMA because the resulting CuCl2/BPED could not deactivate the growing radicals. The effects of the ligand structures on the catalysis of ATRP are also discussed. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 3553,3562, 2004 [source] Synthesis and Characterization of Poly(N -vinylimidazole- co -acrylonitrile) and Determination of Monomer Reactivity RatiosMACROMOLECULAR CHEMISTRY AND PHYSICS, Issue 8 2004Nursel Pekel Abstract Summary: Radical-initiated solution copolymerization of N -vinylimidazole (VIM) and acrylonitrile (AN) was carried out with 2,2, -azobisisobutyronitrile (AIBN) as an initiator in benzene at 70,°C in nitrogen atmosphere. The structure and composition of synthesized copolymers for a wide range of monomer feeds were determined by FTIR, 1H and 13C NMR spectroscopy with the aid of recorded analytical absorption bands for VIM (667 cm,1, CN of imidazole ring) and AN (2,242 cm,1, CN group), as well as by using the areas of proton and carbon atom signals from corresponding functional groups of monomer units. Monomer reactivity ratios for VIM (M1)-AN (M2) pair were determined by nonlinear regression (NLR), Kelen,Tüdös (KT) and Fineman,Ross (FR) methods. They were found to be r1,=,0.24 and r2,=,0.15 for the NLR method, r1,=,0.22 and r2,=,0.094 for the KT method, and r1,=,0.24 and r2,=,0.12 for the FR method, respectively. The relatively high activity observed of VIM growing macroradical and the results of FTIR and 1H NMR structural analysis of copolymers suggest the formation of complexed linkages between monomers and growing radicals in chain propagation reactions. Similar complexation between monomer,comonomer units in the structure of formed macromolecules showed an increase in isotactic triad fractions in the copolymer. Complex formation between the imidazole ring and nitrile group in both the monomer mixture and chain growing reactions. [source] | ||||||||||