Home About us Contact
|
Home About us Contact | ||
|
|
||
Reversible Addition (reversible + addition)
Terms modified by Reversible Addition Selected AbstractsAb 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] Reversible addition,fragmentation chain-transfer graft polymerization of styrene: Solid phases for organic and peptide synthesisJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 23 2002Leonie Barner Abstract The ,-initiated reversible addition,fragmentation chain-transfer (RAFT)-agent-mediated free-radical graft polymerization of styrene onto a polypropylene solid phase has been performed with cumyl phenyldithioacetate (CPDA). The initial CPDA concentrations range between 1 × 10,2 and 2 × 10,3 mol L,1 with dose rates of 0.18, 0.08, 0.07, 0.05, and 0.03 kGy h,1. The RAFT graft polymerization is compared with the conventional free-radical graft polymerization of styrene onto polypropylene. Both processes show two distinct regimes of grafting: (1) the grafting layer regime, in which the surface is not yet totally covered with polymer chains, and (2) a regime in which a second polymer layer is formed. Here, we hypothesize that the surface is totally covered with polymer chains and that new polymer chains are started by polystyrene radicals from already grafted chains. The grafting ratio of the RAFT-agent-mediated process is controlled via the initial CPDA concentration. The molecular weight of the polystyrene from the solution (PSfree) shows a linear behavior with conversion and has a low polydispersity index. Furthermore, the loading of the grafted solid phase shows a linear relationship with the molecular weight of PSfree for both regimes. Regime 2 has a higher loading capacity per molecular weight than regime 1. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 4180,4192, 2002 [source] Reversible, Fine Performance Tuning of an Organometallic Molecular Wire by Addition, Ligand Replacement and Removal of Dicobalt FragmentsEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 23 2010Yuya Tanaka Abstract Communication between the two iron centres in (dithienylethyne)diyl complex 1 can be finely tuned by reversible addition to, ligand replacement at and removal from the C,C moiety in 1 of dicobalt fragments Co2(CO)n(PR3)6,n. Performance analysis reveals that disparate mechanisms are in operation for the two systems. In the case of the dicobalt adducts, indirect communication via the dicobalt steppingstone can be finely tuned by controlling the electronic structure of the dicobalt unit. [source] The Detection of PHIP Effects Allows New Insights into the Mechanism of Olefin Isomerisation during Catalytic HydrogenationEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 28 2008Alessandra Viale Abstract PHIP (parahydrogen-induced polarisation) effects in the 1H NMR spectra of the products of Rh-complex-catalysed alkyne hydrogenation brings to light the fact that the cis,trans isomerisation of the formed olefin occurs through the formation of a ,-bonded intermediate stabilised by the reversible addition of a hydrogen molecule at the metal centre. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008) [source] Temperature Responsive Solution Partition of Organic,Inorganic Hybrid Poly(N -isopropylacrylamide)-Coated Mesoporous Silica Nanospheres,ADVANCED FUNCTIONAL MATERIALS, Issue 9 2008Po-Wen Chung Abstract A series of poly(N -isopropylacrylamide)-coated mesoporous silica nanoparticle materials (PNiPAm-MSNs) has been synthesized by a surface-initiated living radical polymerization with a reversible addition,fragmentation chain transfer (RAFT) reaction. The structure and the degree of polymerization of the PNiPAm-MSNs has been characterized by a variety of techniques, including nitrogen sorption analysis, 29Si and 13C solid-state NMR spectroscopy, transmission electron microscopy (TEM), and powder X-ray diffraction (XRD). The thermally induced changes of the surface properties of these polymer-coated core,shell nanoparticles have been determined by examining their partition activities in a biphasic solution (water/toluene) at different temperatures. [source] Block copolymers of poly(ethylene oxide) and poly(vinyl alcohol) synthesized by the RAFT methodologyJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 7 2009Yin-Yin Tong Abstract A methodology for the synthesis of well-defined poly(ethylene oxide)- block -poly(vinyl alcohol) (PEO- b -PVA) and PVA- b -PEO- b -PVA polymers was reported. Novel xanthate end-functionalized PEOs were synthesized by a series of end-group transformations. They were then used to mediate the reversible addition,fragmentation chain transfer polymerization of vinyl acetate to obtain well-defined poly(ethylene oxide)- b -poly(vinyl acetate) (PEO- b -PVAc) and PVAc- b -PEO- b -PVAc. When these block copolymers were directly hydrolyzed in methanol solution of sodium hydroxide, polymers with brown color were obtained, which was due to the formation of conjugated unsaturated aldehyde structures. To circumvent these side reactions, the xanthate groups were removed by adding a primary amine before hydrolysis and the products thus obtained were white powders. The polymers were characterized by gel permeation chromatography, 1H NMR spectroscopy and FT-IR. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 1901,1910, 2009 [source] Original approach to multiblock copolymers via reversible addition,fragmentation chain transfer polymerizationJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 11 2007Hesna Gemici ,,,-Thiol-terminated (co)polymers synthesized via reversible addition,fragmentation chain transfer polymerization can be oxidized to yield multiblock copolymers via the formation of disulfide bridges. [source] Kinetic analysis of the cross reaction between dithioester and alkoxyamine by a Monte Carlo simulation,JOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 3 2007Yong Ao Abstract A model reaction of dithioester and alkoxyamine is proposed to probe the reversible addition,fragmentation chain transfer (RAFT) process. The kinetics of the model reaction is analyzed and compared with that of pure alkoxyamine homolysis with a Monte Carlo simulation. Although the pure alkoxyamine obeys the law of persistent radical effect, the model reaction results in higher concentration of the persistent radical during the main period of the reaction. However, for a very fast RAFT process or a very low addition rate constant, the time dependence of the persistent radical concentration is quite close to that of pure alkoxyamine. Furthermore, the cross termination between the intermediate and alkyl radicals causes a retardation effect for the model reaction when the intermediate is relatively long-lived. The Monte Carlo simulation indicates that it is feasible to measure the individual rate constants of the RAFT process, such as the rate constant of addition, with a large excess of alkoxyamine. In addition, the special feature of the system with different leaving groups in the alkoxyamine and dithioester is also discussed. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 374,387, 2007 [source] Latices of poly(fluoroalkyl mathacrylate)- b -poly(butyl methacrylate) copolymers prepared via reversible addition,fragmentation chain transfer polymerizationJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 3 2007Xiaodong Zhou Abstract Poly(fluoroalkyl mathacrylate)- block -poly(butyl methacrylate) diblock copolymer latices were synthesized by a two-step process. In the first step, a homopolymer end-capped with a dithiobenzoyl group [poly(fluoroalkyl mathacrylate) (PFAMA) or poly(butyl methacrylate) (PBMA)] was prepared in bulk via reversible addition,fragmentation chain transfer (RAFT) polymerization with 2-cyanoprop-2-yl dithiobenzoate as a RAFT agent. In the second step, the homopolymer chain-transfer agent (macro-CTA) was dissolved in the second monomer, mixed with a water phase containing a surfactant, and then ultrasonicated to form a miniemulsion. Subsequently, the RAFT-mediated miniemulsion polymerization of the second monomer (butyl methacrylate or fluoroalkyl mathacrylate) was carried out in the presence of the first block macro-CTA. The influence of the polymerization sequence of the two kinds of monomers on the colloidal stability and molecular weight distribution was investigated. Gel permeation chromatography analyses and particle size results indicated that using the PFAMA macro-CTA as the first block was better than using the PBMA RAFT agent with respect to the colloidal stability and the narrow molecular weight distribution of the F-copolymer latices. The F-copolymers were characterized with 1H NMR, 19F NMR, and Fourier transform infrared spectroscopy. Comparing the contact angle of a water droplet on a thin film formed by the fluorinated copolymer with that of PBMA, we found that for the diblock copolymers containing a fluorinated block, the surface energy decreased greatly, and the hydrophobicity increased. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 471,484, 2007 [source] Polymer having a trithiocarbonate moiety in the main chain: Application to reversible addition,fragmentation chain transfer controlled thermal and photoinduced monomer insertion polymerizationsJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 21 2006Suguru Motokucho Abstract A polymer having a trithiocarbonate moiety in its main chain was synthesized with a cyclic, five-membered dithiocarbonate as a building block. The trithiocarbonate in the polymer acted as a reversible addition,fragmentation chain transfer reagent to mediate a controlled insertion polymerization of styrene into the polymer main chain, giving the corresponding sequence-ordered polymer having a well-defined polystyrene segment in the main chain. During the polymerization, the polystyrene segment in the main chain gained its molecular weight, which maintained a linear relationship with the conversion of styrene. The insertion polymerization of styrene was induced not only thermally but also by ultraviolet irradiation. This photoinduced polymerization was well controlled by the trithiocarbonate moiety to give the corresponding polymer, whose structure was virtually the same as that obtained by the thermal polymerization. © 2006 Wiley Periodicals, Inc. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 6324,6331, 2006 [source] Synthesis and characterization of core,shell-type polymeric micelles from diblock copolymers via reversible addition,fragmentation chain transferJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 10 2006Ping Zhang Abstract A method was developed to enable the formation of nanoparticles by reversible addition,fragmentation chain transfer polymerization. The thermoresponsive behavior of polymeric micelles was modified by means of micellar inner cores and an outer shell. Polymeric micelles comprising AB block copolymers of poly(N -isopropylacrylamide) (PIPAAm) and poly(2-hydroxyethylacrylate) (PHEA) or polystyrene (PSt) were prepared. PIPAAm- b -PHEA and PIPAAm- b -PSt block copolymers formed a core,shell micellar structure after the dialysis of the block copolymer solutions in organic solvents against water at 20 °C. Upon heating above the lower critical solution temperature (LCST), PIPAAm- b -PHEA micelles exhibited an abrupt increase in polarity and an abrupt decrease in rigidity sensed by pyrene. In contrast, PIPAAm- b -PSt micelles maintained constant values with lower polarity and higher rigidity than those of PIPAAm- b -PHEA micelles over the temperature range of 20,40 °C. Structural deformations produced by the change in the outer polymer shell with temperature cycles through the LCST were proposed for the PHEA core, which possessed a lower glass-transition temperature (ca. 20 °C) than the LCST of the PIPAAm outer shell (ca. 32.5 °C), whereas the PSt core with a much higher glass-transition temperature (ca. 100 °C) retained its structure. The nature of the hydrophobic segments composing the micelle inner core offered an important control point for thermoresponsive drug release and the drug activity of the thermoresponsive polymeric micelles. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 3312,3320, 2006 [source] Synthesis of comb-like polystyrene with poly(N -phenyl maleimide- alt-p -chloromethyl styrene) as macroinitiatorJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 6 2006Yan Shi Abstract The copolymerization of N -phenyl maleimide and p -chloromethyl styrene via reversible addition,fragmentation chain transfer (RAFT) process with AIBN as initiator and 2-(ethoxycarbonyl)prop-2-yl dithiobenzoate as RAFT agent produced copolymers with alternating structure, controlled molecular weights, and narrow molecular weight distributions. Using poly(N -phenyl maleimide- alt - p -chloromethyl styrene) as the macroinitiator for atom transfer radical polymerization of styrene in the presence of CuCl/2,2,-bipyridine, well-defined comb-like polymers with one graft chain for every two monomer units of backbone polymer were obtained. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 2069,2075, 2006 [source] Macromolecular design via reversible addition,fragmentation chain transfer (RAFT)/xanthates (MADIX) polymerizationJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 22 2005Sébastien Perrier Abstract Among the living radical polymerization techniques, reversible addition,fragmentation chain transfer (RAFT) and macromolecular design via the interchange of xanthates (MADIX) polymerizations appear to be the most versatile processes in terms of the reaction conditions, the variety of monomers for which polymerization can be controlled, tolerance to functionalities, and the range of polymeric architectures that can be produced. This review highlights the progress made in RAFT/MADIX polymerization since the first report in 1998. It addresses, in turn, the mechanism and kinetics of the process, examines the various components of the system, including the synthesis paths of the thiocarbonyl-thio compounds used as chain-transfer agents, and the conditions of polymerization, and gives an account of the wide range of monomers that have been successfully polymerized to date, as well as the various polymeric architectures that have been produced. In the last section, this review describes the future challenges that the process will face and shows its opening to a wider scientific community as a synthetic tool for the production of functional macromolecules and materials. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43:5347,5393, 2005 [source] Synthesis of well-defined three-armed polystyrene having thiourethane,isocyanurate as the core structure derived from trifunctional five-membered cyclic dithiocarbonateJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 22 2005Akane Suzuki Abstract The synthesis of a three-armed polymer with an isocyanurate,thiourethane core structure is described. Monofunctional reversible addition,fragmentation chain transfer (RAFT) agent 2 and trifunctional RAFT agent 5 were prepared from mercapto-thiourethane and tris(mercapto-thiourethane), which were obtained from the aminolysis of mono- and trifunctional five-membered cyclic dithiocarbonates, respectively. The radical polymerization of styrene in the presence of 2,2,-azobis(isobutyronitrile) and RAFT agent 2 in bulk at 60 °C proceeded in a controlled fashion to afford the corresponding polystyrene with desired molecular weights (number-average molecular weight = 3000,10,100) and narrow molecular weight distributions (weight-average molecular weight/number-average molecular weight < 1.13). On the basis of the successful results with the monofunctional RAFT agents, three-armed polystyrene with thiourethane,isocyanurate as the core structure could be obtained with trifunctional RAFT agent 5 in a similar manner. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 5498,5505, 2005 [source] Controlled radical polymerization of a trialkylsilyl methacrylate by reversible addition,fragmentation chain transfer polymerizationJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 22 2005M. N. Nguyen Abstract The reversible addition,fragmentation chain transfer (RAFT) polymerization of a hydrolyzable monomer (tert -butyldimethylsilyl methacrylate) with cumyl dithiobenzoate and 2-cyanoprop-2-yl dithiobenzoate as chain-transfer agents was studied in toluene solutions at 70 °C. The resulting homopolymers had low polydispersity (polydispersity index < 1.3) up to 96% monomer conversion with molecular weights at high conversions close to the theoretical prediction. The profiles of the number-average molecular weight versus the conversion revealed controlled polymerization features with chain-transfer constants expected between 1.0 and 10. A series of poly(tert -butyldimethylsilyl methacrylate)s were synthesized over the molecular weight range of 1.0 × 104 to 3.0 × 104, as determined by size exclusion chromatography. As strong differences of hydrodynamic volumes in tetrahydrofuran between poly(methyl methacrylate), polystyrene standards, and poly(tert -butyldimethylsilyl methacrylate) were observed, true molecular weights were obtained from a light scattering detector equipped in a triple-detector size exclusion chromatograph. The Mark,Houwink,Sakurada parameters for poly(tert -butyldimethylsilyl methacrylate) were assessed to obtain directly true molecular weight values from size exclusion chromatography with universal calibration. In addition, a RAFT agent efficiency above 94% was confirmed at high conversions by both light scattering detection and 1H NMR spectroscopy. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 5680,5689, 2005 [source] Design strategies for controlling the molecular weight and rate using reversible addition,fragmentation chain transfer mediated living radical polymerizationJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 15 2005Michael J. Monteiro Abstract Living radical polymerization has allowed complex polymer architectures to be synthesized in bulk, solution, and water. The most versatile of these techniques is reversible addition,fragmentation chain transfer (RAFT), which allows a wide range of functional and nonfunctional polymers to be made with predictable molecular weight distributions (MWDs), ranging from very narrow to quite broad. The great complexity of the RAFT mechanism and how the kinetic parameters affect the rate of polymerization and MWD are not obvious. Therefore, the aim of this article is to provide useful insights into the important kinetic parameters that control the rate of polymerization and the evolution of the MWD with conversion. We discuss how a change in the chain-transfer constant can affect the evolution of the MWD. It is shown how we can, in principle, use only one RAFT agent to obtain a polymer with any MWD. Retardation and inhibition are discussed in terms of (1) the leaving R group reactivity and (2) the intermediate radical termination model versus the slow fragmentation model. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 3189,3204, 2005 [source] Living/controlled copolymerization of acrylates with nonactivated alkenesJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 24 2004Shengsheng Liu Abstract The living/controlled copolymerization of methyl acrylate with 1-alkenes and norbornene derivatives through several radical polymerization techniques has been achieved. These techniques include atom transfer radical polymerization, reversible addition,fragmentation transfer polymerization, nitroxide-mediated polymerization, and degenerative transfer polymerization. These systems display many of the characteristics of a living polymerization process: the molecular weight increases linearly with the overall conversion, but the polydispersity remains low. Novel block copolymers have been synthesized through the sequential addition of monomers or chain extension. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 6175,6192, 2004 [source] Dendrimers as scaffolds for multifunctional reversible addition,fragmentation chain transfer agents: Syntheses and polymerizationJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 23 2004Xiaojuan Hao Abstract The synthesis and characterization of novel first- and second-generation true dendritic reversible addition,fragmentation chain transfer (RAFT) agents carrying 6 or 12 pendant 3-benzylsulfanylthiocarbonylsulfanylpropionic acid RAFT end groups with Z-group architecture based on 1,1,1-hydroxyphenyl ethane and trimethylolpropane cores are described in detail. The multifunctional dendritic RAFT agents have been used to prepare star polymers of poly(butyl acrylate) (PBA) and polystyrene (PS) of narrow polydispersities (1.4 < polydispersity index < 1.1 for PBA and 1.5 < polydispersity index < 1.3 for PS) via bulk free-radical polymerization at 60 °C. The novel dendrimer-based multifunctional RAFT agents effect an efficient living polymerization process, as evidenced by the linear evolution of the number-average molecular weight (Mn) with the monomer,polymer conversion, yielding star polymers with molecular weights of up to Mn = 160,000 g mol,1 for PBA (based on a linear PBA calibration) and up to Mn = 70,000 g mol,1 for PS (based on a linear PS calibration). A structural change in the chemical nature of the dendritic core (i.e., 1,1,1-hydroxyphenyl ethane vs trimethylolpropane) has no influence on the observed molecular weight distributions. The star-shaped structure of the generated polymers has been confirmed through the cleavage of the pendant arms off the core of the star-shaped polymeric materials. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 5877,5890, 2004 [source] Synthesis of hydrophilic/CO2 -philic poly(ethylene oxide)- b -poly(1,1,2,2-tetrahydroperfluorodecyl acrylate) block copolymers via controlled/living radical polymerizations and their properties in liquid and supercritical CO2JOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 10 2004Zhi Ma Abstract Hydrophilic/CO2 -philic poly(ethylene oxide)- b -poly(1,1,2,2-tetrahydroperfluorodecyl acrylate) block copolymers were synthesized via reversible addition,fragmentation chain transfer (RAFT) polymerization, iodine transfer polymerization (ITP), and atom transfer radical polymerization (ATRP) in the presence of either degenerative transfer agents or a macroinitiator based on poly(ethylene oxide). In this work, both RAFT and ATRP showed higher efficiency than ITP for the preparation of the expected copolymers. More detailed research was carried out on RAFT, and the living character of the polymerization was confirmed by an ultraviolet (UV) analysis of the SC(S)Ph or SC(S)SC12H25 end groups in the polymer chains. The quantitative UV analysis of the copolymers indicated a number-average molecular weight in good agreement with the value determined by 1H NMR analysis. The properties of the macromolecular surfactants were investigated through the determination of the cloud points in neat liquid and supercritical CO2 and through the formation of water-in-CO2 emulsions. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 2405,2415, 2004 [source] Controlled, radical reversible addition,fragmentation chain-transfer polymerization in high-surfactant-concentration ionic miniemulsionsJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 4 2004J. B. McLeary Abstract Living free-radical polymerization of methacrylate and styrenic monomers with ionic surfactants was carried out with reversible addition,fragmentation chain transfer in miniemulsion with different surfactant types and concentrations. The previously reported problem of phase separation was found to be insignificant at higher surfactant concentrations, and control of the molar mass and polydispersity index was superior to that of published miniemulsion systems. Cationic and anionic surfactants were used to examine the validity of the argument that ionic surfactants interfere with transfer agents. Ionic surfactants were suitable for miniemulsion polymerization under certain conditions. The colloidal stability of the miniemulsions was consistent with the predictions of a specific model. The living character of the polymer that comprised the latex material was shown by its transformation into block copolymers. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 960,974, 2004 [source] Living free-radical polymerization (reversible addition,fragmentation chain transfer) of 6-[4-(4,-methoxyphenyl)phenoxy]hexyl methacrylate: A route to architectural control of side-chain liquid-crystalline polymersJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 19 2003Xiaojuan Hao Abstract Side-chain liquid-crystalline polymers of 6-[4-(4,-methoxyphenyl)phenoxy]hexyl methacrylate with controlled molecular weights and narrow polydispersities were prepared via reversible addition,fragmentation chain transfer (RAFT) polymerization with 2-(2-cyanopropyl) dithiobenzoate as the RAFT agent. Differential scanning calorimetry studies showed that the polymers produced via the RAFT process had a narrower thermal stability range of the liquid-crystalline mesophase than the polymers formed via conventional free-radical polymerization. In addition, a chain length dependence of this stability range was found. The generated RAFT polymers displayed optical textures similar to those of polymers produced via conventional free-radical polymerization. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 2949,2963, 2003 [source] Modeling the reversible addition,fragmentation transfer polymerization processJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 11 2003Aileen R. Wang Abstract A kinetic model has been developed for reversible addition,fragmentation transfer (RAFT) polymerization with the method of moments. The model predicts the monomer conversion, number-average molecular weight, and polydispersity of the molecular weight distribution. It also provides detailed information about the development of various types of chain species during polymerization, including propagating radical chains, adduct radical chains, dormant chains, and three types of dead chains. The effects of the RAFT agent concentration and the rate constants of the initiator decomposition, radical addition, fragmentation, disproportionation, and recombination termination of propagating radicals and cross-termination between propagating and adduct radicals on the kinetics and polymer chain properties are examined with the model. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 1553,1566, 2003 [source] RAFTing down under: Tales of missing radicals, fancy architectures, and mysterious holesJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 3 2003Christopher Barner-Kowollik Abstract This highlight describes recent developments in reversible addition,fragmentation transfer (RAFT) polymerization. Succinct coverage of the RAFT mechanism is supplemented by details of synthetic methodologies for making a wide range of architectures ranging from stars to combs, microgels, and blocks. In addition, RAFT reactions in different media such as emulsion and ionic liquids receive attention. Finally, a specific example of a novel material design is briefly introduced, whereas polymers prepared via RAFT are adopted for microporous/honeycomb membrane design. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 365,375, 2003 [source] Characterization and rheological properties of model alkali-soluble rheology modifiers synthesized by reversible addition,fragmentation chain-transfer polymerizationJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 2 2003E. Sprong Abstract Model alkali-soluble rheology modifiers of different molar masses were synthesized by the reversible addition,fragmentation chain-transfer polymerization of methyl methacrylate, methacrylic acid, and two different associative macromonomers. The polymerization kinetics showed good living character including well-controlled molar mass, molar mass linearly increasing with conversion, and the ability to chain-extend by forming an AB block copolymer. The steady-shear and dynamic properties of a core-shell emulsion, thickened with the different model alkali-soluble rheology modifiers, were measured at constant pH and temperature. The steady-shear data for latex solutions with conventional rheology modifiers exhibited the expected thickening, whereas the associative rheology modifiers showed contrasting rheology behavior. The dynamic measurements revealed that the latex solutions thickened with the conventional rheology modifiers exhibit solid-like (dominant G,) behavior as compared with the associative rheology modifiers that give the latex solution a liquid-like (dominant G,) character. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 223,235, 2003 [source] Reversible addition,fragmentation chain-transfer graft polymerization of styrene: Solid phases for organic and peptide synthesisJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 23 2002Leonie Barner Abstract The ,-initiated reversible addition,fragmentation chain-transfer (RAFT)-agent-mediated free-radical graft polymerization of styrene onto a polypropylene solid phase has been performed with cumyl phenyldithioacetate (CPDA). The initial CPDA concentrations range between 1 × 10,2 and 2 × 10,3 mol L,1 with dose rates of 0.18, 0.08, 0.07, 0.05, and 0.03 kGy h,1. The RAFT graft polymerization is compared with the conventional free-radical graft polymerization of styrene onto polypropylene. Both processes show two distinct regimes of grafting: (1) the grafting layer regime, in which the surface is not yet totally covered with polymer chains, and (2) a regime in which a second polymer layer is formed. Here, we hypothesize that the surface is totally covered with polymer chains and that new polymer chains are started by polystyrene radicals from already grafted chains. The grafting ratio of the RAFT-agent-mediated process is controlled via the initial CPDA concentration. The molecular weight of the polystyrene from the solution (PSfree) shows a linear behavior with conversion and has a low polydispersity index. Furthermore, the loading of the grafted solid phase shows a linear relationship with the molecular weight of PSfree for both regimes. Regime 2 has a higher loading capacity per molecular weight than regime 1. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 4180,4192, 2002 [source] Long-lived intermediates in reversible addition,fragmentation chain-transfer (RAFT) polymerization generated by , radiationJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 8 2002Christopher Barner-Kowollik Abstract A novel experimental procedure is presented that allowed probing of reversible addition,fragmentation chain-transfer (RAFT) free-radical polymerizations for long-lived species. The new experimental sequence consisted of gamma irradiation of a mixture of initial RAFT agent (cumyl dithiobenzoate) and monomer at ambient temperature, a subsequent predetermined waiting period without initiation source also at ambient temperature, and then heating of the reaction mixture to a significantly higher temperature. After each sequence step, the monomer conversion and molecular weight distribution were determined, indicating that controlled polymer formation occurs only during the heating period. The results indicated that stable intermediates (either radical or nonradical in nature) are present in such experiments because thermal self-initiation of the monomer can be excluded as the reason for polymer formation. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 1058,1063, 2002 [source] The biomolecule ubiquinone exerts a variety of biological functions,BIOFACTORS, Issue 1-4 2003Hans Nohl Abstract The chemistry of ubiquinone allows reversible addition of single electrons and protons. This unique property is used in nature for aerobic energy gain, for unilateral proton accumulation, for the generation of reactive oxygen species involved in physiological signaling and a variety of pathophysiological events. Since several years ubiquinone is also considered to play a major role in the control of lipid peroxidation, since this lipophilic biomolecule was recognized to recycle ,-tocopherol radicals back to the chain-breaking form, vitamin E. Ubiquinone is therefore a biomolecule which has increasingly focused the interest of many research groups due to its alternative pro- and antioxidant activity. We have intensively investigated the role of ubiquinone as prooxidant in mitochondria and will present experimental evidences on conditions required for this function, we will also show that lysosomal ubiquinone has a double function as proton translocator and radical source under certain metabolic conditions. Furthermore, we have addressed the antioxidant role of ubiquinone and found that the efficiency of this activity is widely dependent on the type of biomembrane where ubiquinone exerts its chain-breaking activity. [source] | |||||||||||||