Radical Termination Research Articles

Radical Formation Initiates Solvent-Dependent Unfolding and β-sheet Formation in a Model Helical Peptide

We examined the effects of Cα-centered radical formation on the stability of a model helical peptide, N-Ac-KK(AL)10KK-NH2. Three, 100 ns molecular dynamics simulations using the OPLS-AA force field were carried out on each α-helical peptide in six distinct binary TIP4P water/2,2,2-trifluoroethanol (TFE) mixtures. The α-helicity was at a maximum in 20% TFE, which was inversely proportional to the number of H-bonds between water molecules and the peptide backbone. The radial distribution of TFE around the peptide backbone was highest in 20% TFE, which enhanced helix stability. The Cα-centered radical initiated the formation of a turn within 5 ns, which was a smaller kink at high TFE concentrations, and a loop at lower TFE concentrations. The highest helicity of the peptide radical was measured in 100% TFE. The formation of hydrogen bonds between the peptide backbone and water destabilized the helix, whereas the clustering of TFE molecules around the radical center stabilized the helix. Following radical termination, the once helical structure converted to a β-sheet rich state in 100% water only, and this transition did not occur in the nonradical control peptide. This study gives evidence on how the formation of peptide radicals can initiate α-helical to β-sheet transitions under oxidative stress conditions.

Radical Generation and Termination in SARA ATRP of Methyl Acrylate: Effect of Solvent, Ligand, and Chain Length

Supplemental activator and reducing agent (SARA) ATRP is an efficient way to diminish the amount of Cu catalyst. A kinetic study of radical generation and termination in SARA ATRP was conducted to better understand and improve the “livingness” of the process. Monomer conversions and CuII/L concentrations were concurrently determined by UV–vis–NIR spectroscopy during polymerization of methyl acrylate in dimethyl sulfoxide (DMSO) or in acetonitrile (MeCN) with tris[2-(dimethylamino)ethyl]amine (Me6TREN) or tris(2-pyridylmethyl)amine (TPMA) as ligands. The polymerization was well controlled in DMSO with either Me6TREN or TPMA and in MeCN with Me6TREN. However, when TPMA was used in MeCN, the rate of polymerization and the generation of CuII/L accelerated exponentially even using small surface area of Cu0, rendering control over polymerization under such conditions difficult, plausibly due to excessive comproportionation. The study revealed that the rates of both activation of (macro)alkyl halide by Cu0 and C...

Multivariate analysis of 13C NMR spectra of branched copolymers prepared by initiator-fragment incorporation radical copolymerization of ethylene glycol dimethacrylate and tert-butyl methacrylate

In this paper, we report chemometric approach for structural analysis of branched copolymers. To evaluate chemical compositions and degree of branching (DB) values in branched copolymers, multivariate analyses, such as principal component analysis (PCA) and partial least-squares (PLS) regression, were applied to the 13C nuclear magnetic resonance (NMR) spectra of the carbonyl carbons of the copolymers prepared by initiator-fragment incorporation radical copolymerization of ethylene glycol dimethacrylate (EGDMA) and tert-butyl methacrylate (TBMA) with dimethyl 2,2′-azobisisobutyrate (MAIB). PCA successfully extracted information on monomeric units, such as EGDMA units, TBMA units and MAIB fragments, the last of which were incorporated via initiation and primary radical termination. The chemical compositions and DB values of the copolymers were predicted by PLS regression. Proper selection of a training set was found to be important for the prediction: the training set has to contain branched copolymers along with poly(EGDMA) and poly(TBMA). PLS regression using the appropriate training set allowed us to predict quantitatively the chemical compositions and DB values, without any assignments of the individual peaks. Multivariate analysis of 13C nuclear magnetic resonance (NMR) spectra successfully evaluated chemical composition and degree of branching value of radically prepared branched copolymers without assigning individual signals, although signals became broadened with increase in branching unit.

Chemical amplification of magnetic field effects relevant to avian magnetoreception.

Magnetic fields as weak as the Earth's can change the yields of radical pair reactions even though the energies involved are orders of magnitude smaller than the thermal energy, kBT, at room temperature. Proposed as the source of the light-dependent magnetic compass in migratory birds, the radical pair mechanism is thought to operate in cryptochrome flavoproteins in the retina. Here we demonstrate that the primary magnetic field effect on flavin photoreactions can be amplified chemically by slow radical termination reactions under conditions of continuous photoexcitation. The nature and origin of the amplification are revealed by studies of the intermolecular flavin-tryptophan and flavin-ascorbic acid photocycles and the closely related intramolecular flavin-tryptophan radical pair in cryptochrome. Amplification factors of up to 5.6 were observed for magnetic fields weaker than 1 mT. Substantial chemical amplification could have a significant impact on the viability of a cryptochrome-based magnetic compass sensor.

Diffusion Controlled Oxidative Degradation of Un-stabilised Polyethylene

The chemical degradation processes of HDPE at 110 o C in air have been studied using light microscopy, FTIR analysis, micro-hardness and OsO4 staining. It is shown that carbonyl groups C=O, (as detected by FTIR) are formed only in the surface layer and the effect on mechanical properties is marginal. On the other hand the formation of isolated and conjugated C=C double bonds (as detected by OsO4 staining), show that cutting properties and micro-hardness are significantly altered. The progress of the degraded layer follows a diffusion process with sharp front and saturated concentration behind. An attempt is made to explain these observations by formation and mobility of free radicals. It is also shown that processing method, injection or compression moulding, can significantly influence the degradation process and the mechanical performance of the moulded component. based on initiation, propagation and termination of radicals, resulting in formation of more stable structures. Most reactions are dependent on the presence of oxygen and extraction of hydrogen. Solubility of oxygen in HDPE is sufficient for reactions, even at room temperature. Major functional groups formed are alkyl, alkoxy or peroxy radicals. Hydroperoxide are also formed and can decompose to hydroxy and peroxy radicals. Tertiary carbons are more reactive than secondary carbons and therefore have a shorter lifetime. Conjugated carbons are more stable than carbonyls and thus it can be expected that they dominate. Cross-linking also occurs and it will increase the strength but also the brittleness of the polymer. The number of possible reactions is very large and it is necessary to select those which are most common and result in more stable structures. The degradation process is most often studied using FTIR spectroscopy considering the formation of carbonyls (C=O) as the major chemical reaction. However the FTIR analysis indicate that most carbonyls are formed within the surface layer, exponentially decreasing with depth, leaving the major part of the polymer thickness un-degraded. This can be explained by photo-oxidation (17, 18). In the present work oxidation of HDPE is studied by comparing FTIR results with changes in mechanical properties, such as micro-indentation and cutting properties which, together with osmium tetroxide staining, reveal the concentration and progress of C=C double bonds. Injection moulded samples as well as compression moulded samples, (which have more homogeneous structure) are used. This shows the importance of polymer processing methods on the performance and durability of polymer components.

Cobalt(iii) and copper(ii) hydrides at the crossroad of catalysed chain transfer and catalysed radical termination: a DFT study

The article analyses the reactivity dichotomy leading different metal hydrides generated by H atom transfer in metal-mediated radical polymerization to preferentially yield catalysed chain transfer or radical termination by reaction with a monomer or a radical.

High‐performance thermoplastic elastomers/carbon nanotubes nanocomposites: Mechanical behavior, rheology, and durability

Advanced polystyrene‐b‐polybutadiene‐b‐polystyrene (SBS) based nanocomposites containing bare carbon nanotubes (CNTs) and CNTs bearing hydroxyl functional groups (CNTs‐OH) have been formulated and fully characterized. In particular, the reinforcement effect of CNTs and CNTs‐OH on SBS was deeply investigated through mechanical and dynamical thermo‐mechanical analysis and the rheological response of SBS‐based nanocomposites was investigated and discussed. The mechanical properties of SBS/CNTs and even more of SBS/CNTs‐OH nanocomposites are substantially superior to those of neat SBS, due to the uniformly dispersed nanofillers throughout the polymeric matrix. The presence of ‐OH groups onto CNTs promotes the stress transfer from polymer to CNTs, strengthening the overall performance of the nanocomposite. Moreover, the durability of SBS‐based nanocomposites was monitored through mechanical, dynamic mechanical, and rheological analysis. SBS‐based nanocomposites showed a significant improved photo‐stability with respect to the neat matrix, due to the presence of CNTs that are able to stabilize efficiently SBS because of their acceptor‐like electron properties. The presence of ‐OH groups onto CNTs enhance the protection ability of CNTs due to the larger amount of surface defects, that improves their radical scavenging activity. Furthermore, CNTs‐OH can act as chain‐breaking electron donor, becoming responsible for radical termination during SBS degradation. POLYM. COMPOS., 38:E381–E391, 2017. © 2015 Society of Plastics Engineers

A Molecular Weight Distribution Polydispersity Equation for the ATRP System: Quantifying the Effect of Radical Termination

Polydispersity quantifies the breadth of polymer molecular weight distribution, making it an important and frequently quoted chain microstructural property for characterization. An explicit expression of such an important variable is desirable for ease of calculation, correlation with experiment data, and/or parameter estimation. A review of published literatures shows that great efforts have been put forth by many researchers to derive these equations for various polymerization mechanisms. In atom transfer radical polymerization (ATRP), polydispersity depends on three factors: monomer conversion, number of monomer addition per activation/deactivation cycle, and amount of dead chains. The existing expressions available in the literature only account for, at most, two of these three factors, with the contribution from dead chains commonly neglected. This assumption results in polydispersity monotonically decreasing with conversion, which is often not observed in experiments. In this work, a new equation for polydispersity, which accounts for contributions of all the three aforementioned factors, is proposed. The validity of assumptions involved in the derivation is evaluated by comparing the polydispersity profiles to those simulated by the method of moments. In addition, this new equation is used to correlate several experiment data sets for verification, namely from ATRP of 2-hydroxyethyl methacrylate, methyl methacrylate, and N-isopropylacrylamide, showing better agreement than the existing equation. Although the equation derived here is strictly applicable to homogeneous (bulk and solution) normal ATRP, with further effort it may be extended to other types of ATRP as well as NMP and RAFT systems.

The effect of depletion of radicals on freely propagating hydrocarbon flames

In this work we investigate the properties and stability of deflagration waves in the model which include the two-step chain-branching reaction mechanism, first order reaction of conversion of radicals into products, and Newtonian cooling to the surroundings. The propagation velocity, the maximum temperature and concentration of radicals monotonically decay as the rate of the radical termination reaction is increased. The critical parameter values for the onset of instabilities and quenching of the travelling combustion wave are found. It is demonstrated that the depletion of radicals through the additional termination reaction enhances the onset of instabilities and extinction. The comparison of the efficiency of the radical and thermal quenching mechanisms is performed. The solutions emerging once the travelling deflagration wave becomes unstable are studied.

Expanding the Scope of RAFT Polymerization: Recent Advances and New Horizons

Reversible deactivation radical polymerization (RDRP) has revolutionized modern polymer chemistry over the past two decades, thus laying the groundwork for the synthesis of complex macromolecules and enabling the preparation of previously inaccessible materials. Reversible addition-fragmentation chain transfer (RAFT) polymerization has emerged as one of the most promising techniques because of its functional group tolerance, applicability to a wide range of vinyl monomers, and its nondemanding experimental conditions. However, despite the promise and clearly demonstrated utility of RAFT, limitations of the method sometimes still exist, including the occasional need for extended polymerization times, limited access to high molecular weight polymers, low “livingness” due to unavoidable radical termination events, etc. This Perspective focuses on recent advances that have been specifically designed to address many of these perceived limitations to reinforce the promise of RAFT for the synthesis of complex an...

Modeling the Influence of Diffusion-Controlled Reactions and Residual Termination and Deactivation on the Rate and Control of Bulk ATRP at High Conversions

In high-conversion atom transfer radical polymerization (ATRP), all the reactions, such as radical termination, radical deactivation, dormant chain activation, monomer propagation, etc. could become diffusion controlled sooner or later, depending on relative diffusivities of the involved reacting species. These diffusion-controlled reactions directly affect the rate of polymerization and the control of polymer molecular weight. A model is developed to investigate the influence of diffusion-controlled reactions on the high conversion ATRP kinetics. Model simulation reveals that diffusion-controlled termination slightly increases the rate, but it is the diffusion-controlled deactivation that causes auto-acceleration in the rate (“gel effect”) and loss of control. At high conversions, radical chains are “trapped” because of high molecular weight. However, radical centers can still migrate through (1) radical deactivation–activation cycles and (2) monomer propagation, which introduce “residual termination” reactions. It is found that the “residual termination” does not have much influence on the polymerization kinetics. The migration of radical centers through propagation can however facilitate catalytic deactivation of radicals, which improves the control of polymer molecular weight to some extent. Dormant chain activation and monomer propagation also become diffusion controlled and finally stop the polymerization when the system approaches its glass state.

Well-Defined High Molecular Weight Polystyrene with High Rates and High Livingness Synthesized via Two-Stage RAFT Emulsion Polymerization.

A highly living polymer with over 100 kg mol(-1) molecular weight is very difficult to achieve by controlled radical polymerization since the unavoidable side reactions of irreversible radical termination and radical chain transfer to monomer reaction become significant. It is reported that over 500 kg mol(-1) polystyrene with high livingness and low dispersity could be synthesized by a facile two-stage reversible addition-fragmentation transfer emulsion polymerization. The monomer conversion reaches 90% within 10 h. High livingness of the product is ascribed to the extremely low initiator concentration and the chain transfer constant for monomer unexpectedly much lower than the well-accepted values in the conventional radical polymerization. The two-stage monomer feeding policy much decreases the dispersity of the product.

New Phenomena in Organometallic-Mediated Radical Polymerization (OMRP) and Perspectives for Control of Less Active Monomers.

The impact of reversible bond formation between a growing radical chain and a metal complex (organometallic-mediated radical polymerization (OMRP) equilibrium) to generate an organometallic intermediate/dormant species is analyzed with emphasis on the interplay between this and other one-electron processes involving the metal complex, which include halogen transfer in atom transfer radical polymerization (ATRP), hydrogen-atom transfer in catalytic chain transfer (CCT), and catalytic radical termination (CRT). The challenges facing the controlled polymerization of "less active monomers" (LAMs) are outlined and, after reviewing the recent achievements of OMRP in this area, the perspectives of this technique are analyzed.

What Limits the Chain Growth from Flat Surfaces in Surface‐Initiated ATRP: Propagation, Termination or Both?

Surface‐initiated controlled radical polymerization, such as ATRP, has been proven to be a powerful method for preparation of well‐controlled functional grafted polymers. There are thousands of experimental works published, by varying polymers, surfaces, and applications. In comparison, theoretical developments are very lacking. Many fundamental questions still remain to be answered. These questions include, but not limited to: What determine the surface initiator efficiency? How many chains per square nanometer can be fully grown? What is the maximum thickness can polymer grow to? If chains are simultaneously grown from surface and solution, which has higher molecular weight? Answers to these questions are most helpful for innovation and further development in this important area. In this work, we employed recently developed theories to explain experimentally observed kinetic profiles of polymer layer thickness growth. What limits the growth of surface chains? Is it the monomer starvation in grafting layer that slow down propagation? Or is it the termination of radicals that stop the chain growth? It was found that no existing models could fully explain the often contradictory experimental observations. It is our hope that this work will provoke further discussions and inspire more effort in resolving the fundamental issues of this area.

Comment on “Aqueous SET-LRP catalyzed with in situ generated Cu(0) demonstrates surface mediated activation and bimolecular termination” by S. Samanta, et al., Polym. Chem., 2015, 6, 2084

A recent paper proposed a new polymerization mechanism for aqueous SET-LRP, but experimental results are consistent with conventional radical propagation and termination.

Effects of oxygen on light activation in covalent adaptable network polymers.

Light activated polymers are a novel group of active materials that deform when irradiated with light at specific wavelengths. This paper focuses on the understanding and evaluation of light activated covalent adaptable networks formed by radical polymerization reactions, which have potential applications as novel actuators, surface patterning, and light-induced bending and folding. In these polymer networks, free radicals are generated upon light irradiation and lead to evolution of the polymer network structure through bond exchange reactions. It is well known that oxygen is an important inhibitor in radical-based chemistry as oxygen reacts with free radicals and renders them as inactive species towards further propagation and reaction. However, it is unclear how radical depletion by oxygen may affect the light-induced actuation. This paper studies the effects of oxygen on both stress relaxation and bending actuation. Light induced stress relaxation experiments are conducted in an environmental chamber where the concentration of oxygen is controlled by the nitrogen flow. A constitutive model that considers oxygen diffusion, radical termination due to oxygen, and the polymer network evolution is developed and used to study the stress relaxation and bending, and the model predictions agree well with experiments. Parametric studies are conducted to identify the situations where the effects of oxygen are negligible and other conditions where they must be considered.

Microwave-enhanced persulfate oxidation to treat mature landfill leachate

Microwave oxidation process (MOP) was evaluated for treatment of landfill leachate. Kinetics of persulfate oxidation in MOP, effects of pH and persulfate doses on fates of derivative organic acids, and the energy cost of MOP were evaluated. The results showed that total organic carbon (TOC) removal of 79.4%, color removal of 88.4%, and UV254 removal of 77.1% were reached at MOP 550W/85°C within 30min. The kinetics of oxidation by MOP followed the first-order reaction. For a given persulfate dose, the reaction rate increased with the microwave power setting (775W>550W>325W>128W) with reaction rate constants ranging from 10−5 to 10−2min−1. The adverse effects on reaction rates under higher microwave power settings and high persulfate doses are plausibly caused by excessive persulfate oxidation and self-scavenging termination of free radicals. During the MOP treatment, TOC/COD ratio dropped with time and an 86.7% reduction in TOC/COD ratio after 120min at pH 7. Oxalic acid was the major derivative and its concentrations were higher under acidic conditions. Malic, lactic, and acetic acids were formed and soon degraded, and the solution pH has an insignificant effect on their fates. The energy cost of MOP (USD$6.03/m3) is essentially similar to that of conventional heating oxidation (CHO) (USD$6.10/m3).

SP–PLP–EPR Measurement of Iron-Mediated Radical Termination in ATRP

Radical termination was studied in the presence of iron(II) bromide, which is an attractive catalyst for atom-transfer radical polymerization (ATRP). Termination rate is measured in both the absence and presence of FeII via time-resolved EPR spectroscopy of propagating radicals produced by applying laser single pulses. This novel method quantifies the catalytic termination of two propagating radicals by FeII. The associated rate coefficient was measured in an extended temperature range for both acrylate and methacrylate polymerization. Detailed knowledge about the kinetics of FeII-catalyzed radical termination may be of general importance for iron-mediated ATRPs of acrylates.

Reaction model describing antioxidant depletion in polyethylene–clay nanocomposites under thermal aging

Antioxidants are typically added to polyethylene to extend its durability, and recently clay nanoparticles have been blended into polyethylene to improve mechanical properties. However, the clay nanoparticles also accelerate the rate of antioxidant depletion in polyethylene. This paper uses a mathematical model to describe the underlying mechanisms of antioxidant (hindered-phenol) depletion and to predict experimentally-measured antioxidant profiles in polyethylene–clay nanocomposites. The mathematical model uses a reaction kinetic scheme that includes free radical initiation and propagation reactions, antioxidant stabilization reactions and free radical termination reactions. In the model, alkyl free radicals oxidize rapidly. The role of antioxidants is to stabilize the oxidized free radicals to hydroperoxides, and interrupt propagation reactions. However, in nanocomposites, continuous depletion of antioxidant is caused by the clay acting as a catalyst to decompose hydroperoxides and regenerate alkyl free radicals. This cyclic hydroperoxide generation and decomposition leads to much faster antioxidant depletion in polyethylene nanocomposites. Phenoxyl radicals of antioxidants generated by stabilization reactions contribute to terminate polymeric free radicals and limit their accumulation. Predictions of antioxidant depletion are compared to experimental results for accelerated aging of polyethylene and nanocomposite samples.

Phosphorous antioxidants against polypropylene thermal degradation during rotational molding‐kinetic modeling

ABSTRACTPhosphorous antioxidants efficiency against molten polypropylene (PP) thermal oxidation was assessed during isothermal ageing and processing by rotational molding. During isothermal ageing, experimental data were compared to the ones calculated on the basis of a kinetic model. Phosphonite is more effective than phosphite. Both phosphite and phosphonite decompose hydroperoxide and prevent initiation of oxidation reactions. However, phosphonite hydrolysis product acts as a radical chain terminator and blocks propagation reactions. Kinetic constants of stabilization reactions were evaluated and discussed. Further, this kinetic modeling was coupled to a thermal software, able to predict polymer temperature evolution during rotational molding and the degradation critical temperature (DCT) of different stabilized PP. A DCT of 235°C was obtained for PP stabilized with phosphonite and hindered phenol against 215°C for PP stabilized with phosphite and the same phenol. This difference of 20°C, corresponding to 5 min more heating is significant to optimize rotational molding. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 41285.