1,721,204 research outputs found
Influences of the structural design of RAFT agents on living radical polymerization kinetics
No full textThe influence of a p-substituent on the Z-group of cumyl dithiobenzoate (CDB) was investigated with respect to the rate retardation of CDB mediated RAFT polymerizations. The rate retardation is significantly decreased indicating the possible involvement of the p-position in a reversible termination reaction. CDB mediated copolymerizations of styrene and 2-hydroxyethyl methacrylate (HEMA) may indicate a change of the reactivity ratios when compared to conventional copolymerizations. The usage of macromolecular RAFT agents showed unexpected kinetic behavior, such as acceleration. © 2003 American Chemical Society
Why Do We Need More Active ATRP Catalysts?
No full textAtom transfer radical polymerization (ATRP) is a staple technique for the preparation of polymers with well-defined architecture. In ATRP, the catalyst governs the equilibrium between propagating radicals and dormant species, thus affecting the polymerization control for a range of monomers and transferable atoms employed in the process. The design and the use of highly active catalysts could diminish the amount of transition metal complexes, extend ATRP to less active monomers and give access to new chain-end functionalities. At the same time, very active catalysts can be involved in formation of organometallic species. Herein, the role of the catalyst on the ATRP equilibrium is carefully elucidated, together with recent observations on the impact of the catalyst nature on formation of organometallic species and relevant side reactions. Based on this knowledge, a perspective on the benefits and challenges that derive from the use of highly active ATRP catalysts is presented
Externally controlled atom transfer radical polymerization
No full textSpatial and temporal regulations of ATRP by external stimuli are presented. Various ATRP techniques, eATRP, photoATRP, and mechanoATRP, are controlled by electrical current, light, and mechanical forces, respectively. Conversely, ARGET and SARA ATRP are controlled by chemical reducing agents. ICAR ATRP is a thermally regulated process through decomposition of a radical initiator. The aim of this review is to highlight the use of external regulations in ATRP and to summarize the state-of-the-art and future perspectives, focusing on mechanistic aspects, synthetic procedures, preparation of polymers with complex architectures and functional materials, and their applications
Obtaining chain length dependent termination rate coefficients via thermally initiated reversible addition fragmentation chain transfer experiments current status and future challenges
No full textThe reversible addition fragmentation chain transfer (RAFT) process can be utilized in conjunction with rate of polymerization measurements to accurately map the chain length dependence of the termination rate coefficient. This novel approach was originally applied to styrene polymerization and has been termed the RAFT chain length dependent termination (RAFT-CLD-T) method. The RAFT-CLD-T technique is discussed in the context of the prerequisite analysis parameters as well as the choice of RAFT agent. In the present contribution we critically compare the data obtained via RAFT-CLD-T thus far for the monomers styrene (Sty), methyl methacrylate (MMA), methyl acrylate (MA), butyl acrylate (BA), dodecyl acrylate (DA), and vinyl acetate (VAc). For monomers with relatively low reactivity propagating radicals (MMA), a strong chain length dependence of kt in the small chain length regime was observed, indicated by a relatively high α value (in the frequently used expression k t i,i = kt 0·l -α). With increasing chain length, the α value is continuously decreasing, caused by a slow transition from translational diffusion to segmental diffusion as predicted by the composite model of chain length dependent termination. For monomers with higher reactivity propagating radicals (MA, VAc), a linear dependence of kt with chain length was observed (α = 0.36 for MA and 0.09 for VAc). Within the acrylate class, an interesting influence of the side chain was found. In the small chain length regime, α is increasing with increasing length of the side chain from 0.36 in case of MA to 1.2 in DA, which may be attributed to an increased shielding of the polymeric radical. At longer chain lengths, the α value of MA is significantly higher than those for BA and DA, where a is strongly decreasing with increasing chain length. This may indicate a different flexibility and coil structure of MA compared to BA and DA. In general, the acrylates display significantly higher a values in the long chain region than MMA and VAc, which we assign to the presence of mid-chain radicals. The data obtained via three dimensional simultaneous mapping of the chain length and conversion dependence of kt (3D-RAFT-CLD-T) for MA and VAc are also highlighted. © 2006 American Chemical Society
The Synthesis, Self-Assembly and Self-Organisation of Polysilane Block Copolymers
Full text linkBlock copolymers containing polysilane blocks are unique in that the polysilane components possess electro-active properties and are readily photodegradable. This review will discuss and assess the two major approaches to the synthesis of polysilane block copolymers via pre-formed polymer chain coupling and living polymerisation techniques. The self-organisation of polysilane block copolymers and the morphologies adopted in thin films are reviewed. Amphiphilic polysilane-containing block copolymers self-assemble in solvents selective for one block and a number of examples are highlighted. The versatility of these materials is highlighted by recent significant applications including the preparation of hollow crosslinked micellar aggregates in aqueous solutions and in patterned thin film generation subsequently employed as templates for the growth of cell cultures and CaCO (3.
Atom Transfer Radical Polymerization: Billion Times More Active Catalysts and New Initiation Systems
No full textApproaching 25 years since its invention, atom transfer radical polymerization (ATRP) is established as a powerful technique to prepare precisely defined polymeric materials. This perspective focuses on the relation between structure and activity of ATRP catalysts, and the consequent choice of the initiating system, which are paramount aspects to well-controlled polymerizations. The ATRP mechanism is discussed, including the effect of kinetic and thermodynamic parameters and side reactions affecting the catalyst. The coordination chemistry and activity of copper complexes used in ATRP are reviewed in chronological order, while emphasizing the structure–activity correlation. ATRP-initiating systems are described, from normal ATRP to low ppm Cu systems. Most recent advancements regarding dispersed media and oxygen-tolerant techniques are presented, as well as future opportunities that arise from progressively more active catalysts and deeper mechanistic understanding
Ab initio evaluation of the thermodynamic and electrochemical properties of alkyl halides and radicals and their mechanistic implications for atom transfer radical polymerization
No full textHigh-level ab initio molecular orbital calculations are used to study the thermodynamics and electrochemistry relevant to the mechanism of atom transfer radical polymerization (ATRP). Homolytic bond dissociation energies (BDEs) and standard reduction potentials (SRPs) are reported for a series of alkyl halides (R-X; R = CH2CN, CH(CH3)CN, C(CH3)2CN, CH2COOC2H5, CH(CH3)COOCH3, C(CH3)2COOCH3,
C(CH3)2COOC2H5, CH2Ph, CH(CH3)Ph, CH(CH3)Cl, CH(CH3)OCOCH3, CH(Ph)COOCH3, SO2Ph, Ph; X = Cl, Br, I) both in the gas phase and in two common organic solvents, acetonitrile and dimethylformamide.
The SRPs of the corresponding alkyl radicals, R•, are also examined. The computational results are in a very good agreement with the experimental data. For all alkyl halides examined, it is found that, in the solution phase, one-electron reduction results in the fragmentation of the R-X bond to the corresponding alkyl radical and halide anion; hence it may be concluded that a hypothetical outer-sphere electron transfer
(OSET) in ATRP should occur via concerted dissociative electron transfer rather than a two-step process with radical anion intermediates. Both the homolytic and heterolytic reactions are favored by electronwithdrawing substituents and/or those that stabilize the product alkyl radical, which explains why monomers such as acrylonitrile and styrene require less active ATRP catalysts than vinyl chloride and vinyl acetate.
The rate constant of the hypothetical OSET reaction between bromoacetonitrile and CuI/TPMA complex was estimated using Marcus theory for the electron-transfer processes. The estimated rate constant kOSET = 10-11 M-1 s-1 is significantly smaller than the experimentally measured activation rate constant (kISET = 82 M-1 s-1 at 25 °C in acetonitrile) for the concerted atom transfer mechanism (inner-sphere electron transfer, ISET), implying that the ISET mechanism is preferred. For monomers bearing electron-withdrawing groups, the one-electron reduction of the propagating alkyl radical to the carbanion is thermodynamically
and kinetically favored over the one-electron reduction of the corresponding alkyl halide unless the monomer bears strong radical-stabilizing groups. Thus, for monomers such as acrylates, catalysts favoring ISET over OSET are required in order to avoid chain-breaking side reactions
Thermodynamic Components of the Atom Transfer Radical Polymerization Equilibrium: Quantifying Solvent Effects
No full textA thermodynamic scheme representing the atom transfer radical polymerization (ATRP) equilibrium as the formal sum of equilibria involving carbon-halogen bond homolysis and three additional distinct thermodynamic contributions related to the catalyst is rigorously evaluated. The reduction/oxidation
of both the metal complex and the halogen atom, and the affinity of the higher oxidation state of the catalyst for halide anions (or “halidophilicity”), are measured. The validity and self-consistency of the model are verified by independently measuring, computing, or calculating the overall ATRP equilibrium constant and all four contributing equilibrium constants for one catalyst/alkyl halide combination in acetonitrile. As a thorough demonstration of the value and effectiveness of the scheme, the different equilibrium constants were measured or calculated in 11 different organic solvents, and a comparison of their values was used to both
understand and predict catalyst activity in ATRP with high accuracy. The scheme explains quite well, for example, why the ATRP equilibrium constant is greater in dimethyl sulfoxide than in acetone by a factor of about 80 and why in acetonitrile and three different alcohols it is nearly identical. The solvent effects are also quantitatively analyzed in terms of Kamlet-Taft parameters, and linear solvation energy relationships are employed to extrapolate catalyst activity over 7 orders of magnitude in 17 more organic solvents and water
Redox-switchable atom transfer radical polymerization
No full textTemporal control in atom transfer radical polymerization (ATRP) relies on modulating the oxidation state of a copper catalyst, as polymer chains are activated by L/CuI and deactivated by L/CuII. (Re)generation of L/CuI activator has been achieved by applying a multitude of external stimuli. However, switching the Cu catalyst off by oxidizing to L/CuII through external chemical stimuli has not yet been investigated. A redox switchable ATRP was developed in which an oxidizing agent was used to oxidize L/CuI activator to L/CuII, thus halting the polymerization. A ferrocenium salt or oxygen were used to switch off the Cu catalyst, whereas ascorbic acid was used to switch the catalyst on by (re)generating L/CuI. The redox switches efficiently modulated the oxidation state of the catalyst without sacrificing control over polymerization
- …
