The study of rotational mobility of stable nitroxyl radicals in polyvinylacetate

Analyses of the rotational diffusion characters of free and bonded nitroxyl radicals in polyvinylacetate were carried out. The radical rotational character essentially depends on the molecular sizes of the radicals. The movement of the “small” radical is matched by the arbitrary jump tumbling model. The rotation of the “large” radical (both probe and label) occurs by the Brownian rotational diffusion mechanism. The correlation times in the slow-motion region are calculated by taking into account the radical rotation mechanism. Comparison of the τ_c values for the free and bonded radicals with those obtained by the NMR technique shows that movements of the spin probes and labels depend not only on the short polymer segments but on other factors also.     

Which Factors Determine the Reactivity and Regioselectivity of Free Radical Substitution and Addition Reactions?

The relative importance of bond strength, steric effects, and polarity in determining the rate and orientation of free radical subsitution and free radical addition reaction is considered. The factors which control substitution reaction (radical transfer reaction) are gathered together as five “rules”, and a similar five “rules” are proposed for addition rections. These “rules” are shown to be special cases of two “laws” which govern all free radical reactions.     

梯度引发自由基聚合体系(Ⅰ)——基原法合成超高分子量聚合物

90年代在自由基聚合基础研究领域的一个重要成是“长 短终止”理论被进一步确认和接受[1 ,2 ] .按照该模型 ,聚合反应中的终止反应主要发生在长链自由基与短链自由基或初级自由基之间 ,即长链自由基之间很难进行终止反应 ,链终止常数随链长增加而急剧下降 .80年代初 ,Ｓｉｍｉｏｎｅｓｃｕ等[3] 曾报道了用等离子体照射封有单体的玻璃管 ,尔后放入暗处聚合的工作 ,发现不仅可得到分子量上千万的聚丙烯酸或聚丙烯酰胺 ,而且聚合活性可保持几十个小时 ;国内学者[4] 利用该法也得到了分子量接近千万的聚丙烯酰胺 .基于这种终止模型和实验结果 ,…     

A Missing Reaction Step in Dithiobenzoate-Mediated RAFT Polymerization

The debate on the mechanism of dithiobenzoate-mediated RAFT polymerization may be overcome by taking the so-called “missing step” reaction between a highly reactive propagating radical and the three-arm star-shaped product of the combination reaction of an intermediate RAFT radical and a propagating radical into account. The “missing step” reaction transforms a propagating radical and a not overly stable three-arm star species into a resonance-stabilized RAFT intermediate radical and a stable polymer molecule. The enormous driving force behind the “missing step” reaction is estimated via DFT calculations of reaction enthalpies and reaction free enthalpies.     

Water in polymer membranes. Part III : Water sorption and pore volume in cellulose acetate films

Changes in waveguide properties of several cellulose acetate membranes and one polyimide membrane were measured as a function of their exposure to varying levels of relative humidity. The volume fraction of water in the films and the occupied pore volumes were determined from refractive index and thickness changes. The dependence of the refractive index on water absorption is related to a competition between two processes: one of filling pores with no film expansion and one of “free expansion” where the film expands to completely accommodate the added water volume. The term “pore” is taken to mean a volume with molecular and not macroscopic dimensions. The hydration properties of these dense cellulose acetate membranes were affected by degree of acetylation, casting temperatures and annealing treatments. Annealing CA398 membranes at 180°C decreased film water concentration by reducing the amount of free expansion. Annealed CA398 membranes that were tested in a reverse osmosis cell were found to have high salt rejection compared to unannealed films. The hydration characteristics of a polyimide membrane are compared to cellulose acetate membranes.     

EPR study of photoinduced charge transfer in a poly(3-hexylthiophene)-fullerene composite

Magnetic, relaxation, and dynamic parameters of radical pairs of positively charged polarons and negatively charged anion radicals of fullerene that are induced by photons with an energy of 1.7–3.4 eV are studied by the methods of photoinduced electron paramagnetic resonance for the poly(3-hexylthiophene)fullerene composite. The above charge carriers show mutual independence, which is provided by a different interaction with their microenvironment. The paramagnetic susceptibility of spin pairs reflects the dipoledipole interaction and activation dynamics of paramagnetic sites in the polymer-fullerene system. The rate of recombination of radical pairs is controlled by the mutual space distribution of carriers of various charges and by the energy of excitation photons. Quasi-one-dimensional diffusion of polarons along polymer chains and rotational diffusion of fullerene molecules about the selected molecular axis are likewise controlled by the energy of photons and can be described in terms of the activation Elliott hopping model. The dependence of the main values of magnetic, relaxation, and dynamic parameters of charge carriers on the energy of photons is explained by the inhomogeneous distribution of molecular clusters in the polymer-fullerene composite. The annealing of this composite leads to an enhanced formation of polymer crystallites and fullerene clusters. Hence, the effective dimension of the system increases and its electron characteristics are improved.     

Transient free radicals in viscous solvents

The majority of free radicals are highly reactive species which participate in bimolecular reactions with each other. Validation of the theory of molecular diffusion and reactivity in the liquid state requires knowledge of rate constants of radical–radical reactions (recombination, disproportionation) and their viscosity dependencies. An accurate comparison of theory and experiment has become available due to experimentally measured diffusion coefficients of reactive radicals by transient grating technique. Initial distribution of radicals in solution can be not random but pair-wise as in photo- or thermoinitiation of free radical polymerization reactions. Probability of a radical escape of a partner (cage escape) characterizes the initiator efficiency. Despite decades of measurement of cage effect values, cage effect dynamics with free radicals have only been investigated quite recently. The present tutorial review considers the effect of viscosity of Newtonian liquid on two types of recombination—in the solvent bulk and in a cage. Further, since radicals are paramagnetic species, external magnetic field affects probability of their reactions in pairs. These effects are also observed in viscous liquids, and reasons for such observations are explained. The recently discovered low magnetic field effect is also observed on radical pairs in viscous liquids.     

Pyrolysis of CH2ClCH3: Considerations about its molecular nature

Numerical integrations of a hypothetical radical chain reaction model have been performed for the pyrolysis of CH₂ClCH₃ which is known to be molecular. Analyses of the modelling results have led to a better understanding of the participation (or nonparticipation) of “dead” radicals in the self-inhibition of the radical chain reaction. Attention is focused on the fact that apparently slow elementary reactions still may have to be taken into account in a pyrolysis mechanism when they produce “dead” radicals which can accumulate. © John Wiley & Sons, Inc.     

The nature of sites of absorption of low-molecular-mass compounds by butadiene–acrylonitrile copolymers

The rotational mobilities of the paramagnetic probes TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl) and BZONO (4-benzoate-2,2,6,6-tetramethylpiperidine-1-oxyl) are studied via EPR spectroscopy and the equilibrium swelling of random and multiblock butadiene–acrylonitrile copolymers of various compositions and stereostructures of butadiene units in n-heptane, methyl acetate, and toluene are studied. The nature of defective regions sorbing low-molecular-mass compounds is ascertained. The rotational mobilities of the probes in defects of various structures are estimated. The sites of sorption are found to be identical in crosslinked and noncrosslinked elastomers. It is shown that the sorption of n-heptane and methyl acetate appears in the same defective regions as those of the TEMPO radical, while the sorption of toluene emerges in sorption sites where the value of free volume is sufficient for the sorption of the bulky radical BZONO. A decay in local molecular mobility in structural defects (“holes”) does not hinder the absorption of lowmolecular- mass compounds if the free volume is sufficient.     

Kinetic modeling of vinyl acetate telomerization catalyzed by metal transition complexes under thermal and microwave heating

A kinetic model for the redox-initiated radical telomerization of vinyl acetate activated by microwave irradiation is presented. Four approaches based on the “microwave” or “thermal” effects were used to study the cases under microwave irradiation. One of the models assumes microwave-induced generation of radicals from monomer. The second model considers higher reaction temperatures than those reported in the experiments. The third model assumes that microwave radiation affects specifically catalyst-involved reactions. The fourth model is a combination of the second and third ones. The kinetic model captures well the effect of initiator, catalyst and solvent contents, as well as absence or presence of microwave irradiation, on polymerization rate and molecular weight development. Limiting monomer conversions and fairly constant values of number average molecular weight and molar mass dispersity were predicted by the model, which agrees with experimental observations.     

The study of network formation as a cluster-cluster diffusion-limited aggregation process: Modelling of the curing of an epoxy-resin using the Monte Carlo method

The study of network formation during the curing of an epoxy-resin with anhydride and tertiary amine has been carried out by the Monte Carlo method. Both dynamical and structural properties are studied. The model resembles the well known cluster-cluster aggregation: the reaction is modelled as a “geometrical” phenomenon and neglects attractive and repulsive energies except for an excluded volume condition. A reaction occurs always when two active sites are in the vicinity to each other, meaning that the “chemical” processes are taken to be much faster than diffusion (as in diffusion-limited aggregation). Results are presented of two-dimensional square lattice simulations with periodic boundary conditions and are discussed with respect to the proposed mechanism of the curing reaction and to the assumptions of the diffusion-limited nature of the processes. The scope and limitations of such two-dimensional simulations are discussed.     

“Living” free radical copolymerization of styrene and N-vinylcarbazole

Poly[styrene-co-(N-vinylcarbazole)] copolymers with controlled molecular weights and narrow polydispersities were synthesized by nitroxide-mediated “living” free radical copolymerization using an initiator/capping agent system consisting of benzoyl peroxide (BPO) and the stable nitroxyl radical 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO). The copolymerization behaves in a “living” fashion and allows the synthesis of poly[styrene-co-(N-vinylcarbazole)]/polystyrene block copolymers via a controlled chain-extension reaction of the prepared copolymers with styrene.     

“Metal‐Free” Catalytic Oxygen Reduction Reaction on Heteroatom‐Doped Graphene is Caused by Trace Metal Impurities

The oxygen reduction reaction (ORR) is of high industrial importance. There is a large body of literature showing that metal‐based catalytic nanoparticles (e.g. Co, Mn, Fe or hybrid Mn/Co‐based nanoparticles) supported on graphene act as efficient catalysts for the ORR. A significant research effort is also directed to the so‐called “metal‐free” oxygen reduction reaction on heteroatom‐doped graphene surfaces. While such studies of the ORR on nonmetallic heteroatom‐doped graphene are advertised as “metal‐free” there is typically no sufficient effort to characterize the doped materials to verify that they are indeed free of any trace metal. Here we argue that the claimed “metal‐free” electrocatalysis of the oxygen reduction reaction on heteroatom‐doped graphene is caused by metallic impurities present within the graphene materials.     

Measurement of rate processes of free radicals in homogeneous and micellar solutions by cidnp

CIDNP is used to study rate processes of free radicals in both homogeneous and micellar solution. An estimate of the lifetime of the phenyl-acetyl radical at ambient temperature (τ__co?10^?7 sec) produced during photolysis of dibenzyl ketone is made based on quantitative CIDNP measurements and computer simulations. Observation of CIDNP in micellar solution is shown to be consistent with an isotropic medium which restricts diffusion on a short time scale, allowing for an increased tendency toward cage reaction. In the case of t-butyl/pivaloyl radical pairs, escape of the radical fragments from the micelle is shown to be competitive with decarbonylation of the pivaloyl radical Likewise, CIDNP is consistent with product yield results which show the enhanced tendency of triplet born benzyl radical pairs to undergo cage reaction when they are sequestered in a micelle.     

Geometrical aspects of self diffusion in liquid water

In the late 1950s Cohen and Turnbull proposed a model for the self diffusion process in high density fluids, which considers the diffusion process as driven by the free volume fluctuations. An explicit expression was derived for the diffusion coefficient on hard sphere systems, and the results were consistent with molecular dynamic simulations. However the model is not able to reproduce experimental or simulation results for associated fluids, particularly in liquid water, where there is a huge variation of the diffusion coefficient without a significant change in the density. This implies that other properties, besides the free volume, have to be considered in modelling the diffusion process. The model we propose here takes into account both the volume and the shape of the “cage” surrounding the molecules. The “cage” is defined through the Voronoi polyhedra which are calculated from molecular dynamics simulations. The resulting diffusion coefficients, which have been calculated making use only of structural properties, are in good agreement with those evaluated directly from the long time behaviour of the molecular mean square displacements during the molecular dynamics runs.     

Molecular Recognition Ability of Molecularly Imprinted Polymer Nano- and Micro-Particles by Reversible Addition-Fragmentation Chain Transfer Polymerization

The role of molecularly imprinted polymers (MIPs) is changing from academic to applied researches. Challenging problems about MIP will be more highlighted in applicable uses and solving these problems is vital. The controlled/“living” radical polymerization (CLRP) techniques are applicable to solve the challenging problems in MIPs. The “living” nature of CLRP helps to improve the heterogeneity of binding sites in MIPs as a main challenge where precise control over sizes, compositions, and surface functionalities is achieved. Among different techniques of CLRP, reversible addition-fragmentation chain transfer (RAFT) technique presents distinguished benefits such as compatibility and tolerance to a wide range of functional monomers and mild reaction conditions rather than other CLRP techniques. In this review, in order to obtain more insights into the potential benefits of RAFT polymerization in fabrication of nano and micro MIP networks, recent research in advanced MIP materials for different templates with improved morphology, efficiency, and binding capacities with respect to traditional free radical polymerization (FRP) will be discussed. MIPs prepared via RAFT method have advantages of MIPs as high performance molecular recognition devices and CLRP as controllable polymerization mechanism, simultaneously.     

Regenerable‐Catalyst‐Aided,Opened to Air and Sunlight‐Driven “CuAAC&ATRP” Concurrent Reaction for Sequence‐Controlled Copolymer

An ideal stimuli‐responsive controlled/living radical polymerization should have the ability to manipulate the reaction through spatiotemporal “on/off” controls, achieving the polymerization under fully open conditions and allowing for precise control over macromolecular architecture with defined molecular weights and monomer sequence. In this contribution, the photo (sunlight)‐induced electron transfer atom transfer radical‐polymerization (PET‐ATRP) can be realized to be reversibly activated and deactivated under fully open conditions utilizing one‐component copper(II) thioxanthone carboxylate as multifunctional photocatalyst and oxygen scavenger. The polymerization behaviors are investigated, presenting controlled features with first‐order kinetics and linear relationships between molecular weights and monomer conversions. More importantly, “CuAAC&ATRP” concurrent reaction combining PET‐ATRP, photodriven deoxygenation, and photoactivated CuAAC click reaction is successfully employed to synthesize the sequence‐defined multiblock functional copolymers, in which the iterative monomer additions can be easily manipulated under fully open conditions.     

Dependence of growth of the phases of multiphase binary systems on the diffusion parameters

A mathematical model of the diffusion interaction of a binary system with several phases on the equilibrium phase diagram is presented. The theoretical and calculated dependences of the layer thickness of each phase in the multiphase diffusion zone on the isothermal annealing time and the ratio of the diffusion parameters in the neighboring phases with an unlimited supply of both components were constructed. The phase formation and growth in the diffusion zone during “reactive” diffusion corresponds to the equilibrium state diagram for two components, and the order of their appearance in the diffusion zone depends only on the ratio of the diffusion parameters in the phases themselves and on the duration of the incubation periods. The dependence of phase appearance on the incubation periods, annealing time, and difference in the movement rates of the components across the interface boundaries was obtained. An example of the application of the model for processing the experimental data on phase growth in a two-component three-phase system was given.     

High-conversion polymerization. IV. A definition of the onset of the gel effect

The gel effect in free radical polymerization of vinyl monomers has been recognized as the result of the increased viscosity of the reaction solution of polymer in monomer, which causes a decrease in the rate of the termination reaction. This effect manifests itself as an increase in the rate of polymerization over that rate to be expected in its absence. Definition of the onset of the gel effect has become necessary for several purposes. Previously, it has been common to define the onset phenomenologically, i.e., in terms of the increase in the rate of polymerization. It is proposed here that the onset of the gel effect is best defined on a fundamental basis, i.e., as occurring at that conversion at which the rate of segmental diffusion of the polymeric radicals equals the rate of their translational diffusion. Experimental evidence is presented that shows that the small minima predicted by this definition do exist for both rates and degrees of polymerization. Measurements of the viscosities of solutions of polymers in their monomers suggest that the polymer concentrations at which the “chain-entanglement” phenomena are observed are the same as those for the onset of the gel effect for styrene, methyl methacrylate, and butyl methacrylate.     

Application of free‐volume theory to self diffusion of solvents in polymers below the glass transition temperature: A review

Theories based on free‐volume concepts have been developed to characterize the self and mutual‐diffusion coefficients of low molecular weight penetrants in rubbery and glassy polymer‐solvent systems. These theories are applicable over wide ranges of temperature and concentration. The capability of free‐volume theory to describe solvent diffusion in glassy polymers is reviewed in this article. Two alternative free‐volume based approaches used to evaluate solvent self‐diffusion coefficients in glassy polymer‐solvent systems are compared in terms of their differences and applicability. The models can correlate/predict temperature and concentration dependencies of the solvent diffusion coefficient. With the appropriate accompanying thermodynamic factors they can be used to model concentration profiles in mutual diffusion processes that are Fickian such as drying of coatings. The free‐volume methodology has been found to be consistent with molecular dynamics simulations. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011