Radical Entry Research Articles

Articulating the Pueblo Cubano: Women’s Politicization and Productivity in Revolutionary Cuba, 1959 to 1969

How do political actors forge social solidarity across preexisting axes of social difference? This article investigates how political elites undertaking projects of political articulation—understood as linking together diverse constituencies to create integrated political blocs—contend with preexisting cultural constraints embedded in the social fabric. I do so by tracing how the post-1959 Cuban regime attempted to build a population-wide revolutionary identity despite persisting cultural understandings of women primarily as apolitical housewives. Through systematic analysis of a large corpus of state discourse in the form of speeches and women’s magazines, I show how regime leaders negotiated, with varying degrees of success over time, the cultural constraints that gender posed to their unifying project. Ultimately, the regime’s initiatives to politicize women through including them in mass campaigns and radicalizing their traditional household tasks were relatively successful, but cultural backlash against women’s increasing presence in the labor force prompted the institutionalization of a gendered division of labor in the economy that traditionalized their initially radical entry into the workplace. Analyzing how political elites confront and manage social differences within political blocs promises to contribute to a better understanding of the political production of social solidarity and its downstream effects on categorical inequalities.

Calculations of the Average Number of Radicals per Particle in Emulsion Polymerization

In emulsion polymerization, the free radicals enter the particles intermittently from the aqueous phase. The number of radicals per particleis given by the Smith-Ewart recursion relation which balances the rate of radical entry into, the rate of radical exit from and the rate of radical termination inside the particle. Models for emulsion polymerisation are based on the 0-1 kinetics or the pseudo-bulk kinetics. Small particles, low initiator concentrations and large number of particles favour the 0–1 kinetics, whereas the large particles, high initiator concentrations and small number of particles will favour pseudo-bulk kinetics. A given polymerization system may exhibit both these kinetic behaviours, initially following the 0-1 kinetics and during the later stages of polymerization following the pseudo-bulk kinetics. The aim of this work is to calculate the time dependent values of the average number of radicals per particle in emulsion polymerization for the pseudo-bulk kinetics.

Radical entry into Ni-catalyzed transfer hydrocyanation using cheap AIBN

In this issue of Chem Catalysis , Fan and Zhou describe the use of the widely available radical initiator azobis(isobutyronitrile) (AIBN) as an effective HCN surrogate for the Ni-catalyzed transfer hydrocyanation of unsaturated substrates. Different from the prior state of the art, the key intermediate is formed by a radical pathway. In this issue of Chem Catalysis , Fan and Zhou describe the use of the widely available radical initiator azobis(isobutyronitrile) (AIBN) as an effective HCN surrogate for the Ni-catalyzed transfer hydrocyanation of unsaturated substrates. Different from the prior state of the art, the key intermediate is formed by a radical pathway.

Ghost‐Mirror Approach for Accurate and Efficient Kinetic Monte Carlo Simulation of Seeded Emulsion Polymerization

AbstractThe kinetic Monte Carlo (kMC) method is well suited to the simulation of seeded emulsion polymerization reactions. Inadequate simulation volume results in an incorrect radical entry rate, and as such that inaccuracy propagates into the particle phase simulation. A novel kMC method is described here that has coined the Ghost‐Mirror approach (GM‐kMC) to guarantee sufficient aqueous phase simulation volume while minimizing the number of dispersed phase environments, and while maintaining accuracy and consistency with results produced by kMC simulations. To do so, a subset of a kMC method is replicated where only the aqueous phase reactions are treated in that extended volume; particle phases in those replicated volumes are untreated, as “ghosts”. In doing so, a critical condition for aqueous phase simulation volume is calculated to yield accurate results in the overall system. Important to emphasize though is that one does not require the equivalent number of simulated particles in the GM‐kMC approaches. The required number of particles to simulate accurate results can be even orders of magnitude smaller than the kMC approach; inherently offering significant gains in computational efficiency. This is extended further, reducing the required simulation time, by a hybrid extension to this GM‐kMC method.

Understanding the emulsion copolymerization kinetics of vinyl acetate and vinyl silanes

Radical entry affected by alkoxysilane monomers in free radical emulsion copolymerization with vinyl acetate.

Emulsifier-free emulsion polymerization of acrylonitrile-butadiene-carboxylic acid monomers: a kinetic study based on polymerization pressure profile

Emulsion polymerization of acrylonitrile-butadiene-carboxylic acid monomers (XNBR latexes) in the absence of emulsifier is associated with thermodynamic and kinetic complexities and is very sensitive to colloidal stability. In this paper, polymerization pressure profiles were utilized for kinetic studies of emulsifier-free emulsion terpolymerization of acrylonitrile, butadiene and two carboxylic acid monomers (i.e., acrylic acid and methacrylic acid). It was found that the type and amount of acidic monomer have a great impact on the nucleation rate as well as particle size distribution and carboxyl group distribution of XNBR latexes. Also, the type of the acidic monomer affects the rates of entry and exit of radicals to and from particles and consequently the average number of radicals per particle, where a higher rate of polymerization was observed for methacrylic acid latexes. The reaction temperature has profound effects on polymerization rate, particle size distribution and colloidal stability of the latex. Partitioning of pressure profiles into four distinct regions in accordance with well-established intervals of Harkins model and analyzing particle size distribution and carboxyl group distribution, established a conceptual framework for kinetic studies in emulsion polymerization of XNBR latexes.

A simple model for the semicontinuous heterophase polymerization: Ethyl methacrylate as a case example

A simple but comprehensive model considering homogeneous and micellar nucleation, coagulation, entry of radicals to particles and to micelles and radicals' exit from particles, is presented. The model is validated, in a starved semicontinuous heterophase polymerization of ethyl methacrylate, at three monomer addition rates. The model accurately describes the overall and instantaneous conversion, the average particle density and diameter, and the number and weight average molar masses evolutions over time. It is found that even though the average number of radicals is much smaller than 0.5, the system is not 0‐1. An empirical function was used to describe the gel effect. The homogeneous nucleation was the prevailing mechanism for particle formation and large exit rates of radicals were observed. POLYM. ENG. SCI., 60: 223–232, 2019. © 2019 Society of Plastics Engineers

Semibatch emulsion polymerisation modelling: polybutyl acrylate case study

ABSTRACTA comprehensive mathematical model is developed for emulsion polymerisation of butyl acrylate to polybutyl acrylate in a semi batch reactor. The mathematical framework is based on the population balance approach. The model includes such mechanistic details as: (i) particle generation from radicals entering micelles, (ii) coupling of the radical concentration in the aqueous phase and particle phase, (iii) determination of the particle phase radical concentration by radical entry into, exit from and termination inside the particle, (iv) thermodynamic equilibrium between the monomer concentration in the aqueous phase and the particle phase. The population balance equations are solved efficiently using orthogonal collocation. Dynamic simulations are compared to the experimental data taken from the literature. The predictions for the various average diameters of the particle size distribution, polydispersity index, standard deviation and full particle size distribution are also given.

Effect of Pickering stabilization on radical entry in emulsion polymerization

The production of latexes stabilized by solid particles, so‐called Pickering stabilizers, has attracted considerable attention due to its benefits, including the enhanced mechanical properties of the polymer films. Clays for instance were found to enhance particle stabilization in emulsion polymerization, in a comparable way to conventional surfactants. Their concentration thus determines the polymer particles size and number, and consequently the reaction rate. In this work, we investigate the impact of the presence of such rigid and big platelets at the polymer particle's surface on radical exchange between the aqueous phase and the polymer particles. It was found for the system underhand that the average number of radicals per particle ( ) was independent of the stabilizer layer. Therefore, a radical capture model independent of the clay concentration could be used to simulate reactions involving different clay concentrations and predict the evolution of the monomer conversion, particle size, and . © 2018 American Institute of Chemical Engineers AIChE J, 64: 2612–2624, 2018

Is Modeling the PSD in Emulsion Polymerization a Finished Problem? An Overview

AbstractSignificant progress has been made over the past 20–30 years in terms of the ability to develop and solve mechanistic models of emulsion polymerization processes, and in particular models for prediction of the particle size distribution. However, this does not imply that modeling of these economically important processes is by any means a “solved problem,” or that it is no longer necessary to perform fundamental research in this area. There are a number of areas where strong scientific work would increase the understanding of the process, including events in the aqueous phase, radical entry into growing particles, monomer partitioning, and especially the mechanisms and modeling of particle coagulation. image

Copper-Catalyzed Cascade Radical Addition–Cyclization Halogen Atom Transfer between Alkynes and Unsaturated α-Halogenocarbonyls

A Cu-catalyzed cascade radical addition/cyclization/halogen atom transfer between alkynes and α-halogeno-γ, δ-unsaturated carbonyl compounds for the synthesis of various substituted cyclopentenes is described. Since up to four Csp3–Csp2 bonds, two Csp3–Br bonds, and two carbocycles can be established in a single reaction, this 100% atom-efficient reaction exhibits the advantages of wide substrate scope, high functional group tolerance, and step-economics, and it offers an entry of the atom transfer radical addition/cyclization (tandem ATRA-ATRC) process to the synthesis of substituted cyclopentenes.

Modelling particle growth under saturated and starved conditions in emulsion polymerization

AbstractModelling of particle growth under saturated and starved conditions in emulsion polymerization was investigated in order to allow describing the broadening of the particle size distribution and account for the effects of restricted diffusion inside the monomer‐swollen polymer particles. In emulsion polymerization, if particle nucleation and coagulation are avoided, the reaction proceeds as a consequence of radical capture, desorption, and termination reactions inside the particles. Therefore, first, investigation of radical entry and desorption models was done under saturation of polymer particles with monomer, where diffusion parameters are constant. The discrimination criterion of the different models was based on fitting the reaction and the total particle size distribution. Indeed, the broadening of the particle size distribution was found to reveal a dependency of radical capture on the particle size; such an effect would be misestimated if only the mean particle size is considered. Second, the models for starved conditions that account for the variation of radical diffusion inside the polymer particles, such as the gel and glass effects, were investigated with comparison to a wide variety of experimental conditions spanning from weak to strong gelation. The dominating mechanisms as well as the interests of both intervals are highlighted.

Selective Adsorption of Aqueous Phase Co‐Oligomers on Latex Particles Part 1: Influence of Different Initiator Systems

For the first time a new analytical procedure to analyze small oligomers produced in a butyl acrylate‐methyl methacrylate emulsion copolymerization is applied. With this method, low molecular weight co‐oligomers are studied using MALDI‐ToF‐MS. Both the oligomers in the aqueous phase and the oligomers in the particle phase can be seen. Varying the initiator system, it is observed that the initiation step has an influence on the composition of these oligomers and hence on the absorption on to latex particles. Chain length dependent average compositions in the aqueous phase are observed. The chain length dependent oligomer composition can be described in terms of kinetic effects and preferential/selective adsorption of co‐oligomers on the particle surface. The kinetic effects and the extent of adsorption on the particle phase are both dependent on the various end groups. The preferential/selective adsorption of co‐oligomers on the particle phase is explained on the basis of solubility and interactions with the surface. The chain length dependent absolute oligomer amount decreases exponentially with chain length as confirmed by Monte Carlo simulations.This information is important to further understand the events of the aqueous phase of an emulsion copolymerization as well as the entry of radicals into the particles. image

Pickering emulsion polymerization kinetics of styrene: Comparison of bare and surface modified SiO2 nanoparticles

ABSTRACTA step towards the understanding of some mechanistic events occurring in the styrene Pickering emulsions, using a SiO2 dispersion, is presented. Polymerizations at 80°C with different levels of a water soluble initiator were performed. The emulsion polymer content was ca. 15% with conversions close to 90%. With conversion and particle size measurements, the particle density was estimated for bare and surface modified SiO2 particles. Then, the average number of radicals per particle was inferred, yielding a pseudo-bulk type polymerization. It was found that bare SiO2 nanoparticles do not participate in the nucleation mechanism; however, they, along with the initiator, promote an enhanced oligomer coagulation. On the other hand, the hexadecyltrimethylammonium bromide modified SiO2 nanoparticles do participate in the nucleation and coagulation mechanisms, yielding more stable and smaller poorly-covered polymer particles. This approach allowed untangling some events such as: particle nucleation, radical entry to particles, particle density, coagulation and vitreous and Trommsdorff effects.

Ultrafine nanolatexes made via monomer-starved semicontinuous emulsion polymerisation in the presence of a water-soluble chain transfer agent

Ideal chain transfer agents are usually water insoluble as they do not affect the kinetics of polymerisation reactions occurring in the water phase. Water-soluble chain transfer agents act non-ideally by easily crossing the water phase and affecting the kinetics of polymerisation including nucleation. In this research, a partially water-soluble chain transfer agent (CTA), 2-Butanethiol, was used in the monomer-starved semicontinuous emulsion polymerisation of styrene as a means to affect the kinetics of water phase and synthesise ultrafine nanolatexes. Batch emulsion polymerisations were also carried out for comparison. In the batch process, the termination of chain transferred radicals in the water phase was found to be quite dominant, resulting in the formation of large polymer particles with polydisperse molecular weights and slow rate of reaction. By contrast, for the semicontinuous process, the application of the CTA reduced the average size of particles, by enhancing the rate of nucleation via increasing the rate of radical entry into micelles, and provided a good controllability over molecular weight distribution as well as the rate of polymerisation.

The Search for a Hero Gene: Fact or Fiction

The radical entry of heroism research into scientific inquiry presents interesting challenges and possibilities for the study of heroism and the human condition more broadly. This ‘final frontier’ of the enduring phenomenon of heroism stands to offer remarkable, unprecedented, and controversial advances in our understanding of heroic and human behaviour. Is a genetic basis for heroism a real possibility? If so, what would its impacts be? Advances in genomics and increased interest in the fields of epigenetics and neuroplasticity might hold the key to its discovery. This article considers some of the leading emerging research in global health genomics and speculations in the scientific study of heroism, and its potential interrelationship with genetic and epigenetic well-being.

Suppressing the long‐chain branching in the synthesis of poly(styrene‐b‐butyl acrylate‐b‐styrene) in RAFT emulsion polymerization by tuning the interfacial properties

ABSTRACTReversible addition‐fragmentation chain transfer (RAFT) emulsion polymerization is becoming an important technique to synthesize the latex of block copolymers. A previous study showed that in the synthesis of polystyrene‐b‐poly(butyl acrylate)‐b‐polystyrene triblock copolymer via RAFT emulsion polymerization using amphiphilic oligo(acrylic acid‐styrene) macroRAFT as surfactant and mediator, the molecular weight distribution could be much broadened to PDI higher than 2. In this study, an in‐depth investigation was performed to decrease PDI. It was found that long‐chain branches could be formed in the synthesis of triblock block copolymer, leading to the appearance of a higher molecular weight shoulder in the GPC curve of the final product. The lower neutralization degree of acrylic acid (AA) units on the macroRAFT and shorter AA chains would help to suppress the formation the long‐chain branches, leading to PDI around 1.5. It is evidenced that the successful suppression is due to the promotion of radical entry as a result of decreased interfacial transport impedance. It is also evidenced that the presence of styrene during the polymerization of butyl acrylate could promote the formation of long chain branches. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015, 53, 1464–1473

Theoretical and experimental investigations of the inverse emulsion polymerization of acrylamide

ABSTRACTIn this study, the inverse emulsion polymerization modeling of polyacrylamide with population balance equations (PBEs) was performed. The PBEs were derived on the basis of the zero–one kinetic model. The effects of the surfactant steric barrier and surfactant reaction with radicals, including monomeric radicals, on the radical entry rate into the particle were taken into account. In the modified model, the coagulation phenomenon was included through consideration of the effects of forces not included in the Derjaguin, Landau, Verwey, and Overbeek (DLVO) theory; these include hydration and steric forces in addition to DLVO forces. The effects of the surfactant and initiator concentrations on the conversion, particle size, and average molecular weight (MW) were investigated by simulation and experimental studies. Increasing the surfactant concentration initially increased the conversion and decreased MW. A further increase in the surfactant concentration resulted in a decrease in the conversion and an increase in MW. The average particle size decreased with an increase in the surfactant concentration. An increase in the initiator concentration led to an increase in the monomer conversion and a decrease in the average MW. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 41916.

Mathematical Modeling and Simulation of Vinylidene Fluoride Emulsion Polymerization

In the present study a comprehensive model is employed to describe the dynamic behavior of an industrial scale reactor for the emulsion polymerization of vinylidene fluoride (VDF) under different initiator and chain transfer agent (CTA) addition policies. A comprehensive kinetic model combined with a thermodynamic and a particle population model were simultaneously solved to calculate the VDF addition rate, reactor pressure, molecular weight properties, and the particle size distribution of polyvinylidene fluoride produced in a semibatch emulsion polymerization reactor. The proposed emulsion polymerization model takes into account the gas–liquid equilibrium of VDF to determine the aqueous phase VDF concentration and the changes in operating pressure. The effects of particle crystallinity on radical entry and coagulation rate were also considered. The computational model results (cumulative monomer feed and feed rate, reactor pressure, molecular weights, and particle size distribution) are found to be in a...

Generation of radicals and antimalarial activity of dispiro-1,2,4-trioxolanes

The kinetic schemes of the intramolecular oxidation of radicals generated from substituted dispiro-1,2,4-trioxolanes (seven compounds) in the presence of Fe2+ and oxygen were built. Each radical reaction was defined in terms of enthalpy, activation energy, and rate constant. The kinetic characteristics were calculated by the intersecting parabolas method. The competition between the radical reactions was considered. The entry of radicals generated by each compound into the volume was calculated. High antimalarial activity was found for 1,2,4-trioxolanes, which generated hydroxyl radicals. The structural features of trioxolanes responsible for the generation of hydroxyl radicals were determined.