Behavior of free radical transfer between aqueous phase and latex particles in emulsion polymerization

The free radical transfer between aqueous phase and latex particles was investigated in the seeded emulsion polymerization using potassium persulfate (KPS) as an initiator. The effects of seed particle size, initiator concentration and polymerization temperature on the radical entry into and/or exit from the particles were emphatically examined. The results suggest that the aqueous-phase radical entry into the particles should be a competitive process, i.e. the competition between radical diffusion from the bulk aqueous phase to the interface of particles and its reactions in the aqueous phase determines whether it may be adsorbed into the particles or not. This implies that the reactions and properties of radicals in the aqueous phase play an essential role in particle nucleation in the emulsion polymerization.     

RADICAL DESORPTION IN EMULSION POLYMERIZATION OF VINYL ACETATE

In the present paper a model equation for calculating the radical desorption rate constant,k_0,in the emulsion polymerization of vinyl acetate was suggested and the various parameters forevaluating k_0 were determined.Effects of reaction temperature,emulsifier concentration,initiator con-centration,monomer conversion and phase ratio on k_0 were studied.It indicates that the desorptionof radicals from latex particles to aqueous phase must be taken into account in the modelling ofemulsion polymerization for the monomers with higher hydrophilicity such as vinyl acetate.     

Winsor I-like (micro)emulsion polymerization of styrene initiated by oil-soluble initiator

Batch emulsion polymerization of styrene initiated by an oil-soluble initiator and stabilized by non-ionic emulsifier (Tween 20) has been investigated. The rate of polymerization vs. conversion curve shows the two non-stationary rate intervals typical for the non-stationary-state polymerization. This behavior is a result of the continuous particle nucleation and the decrease of monomer concentration at the reaction loci with increasing conversion. The initial increase of the polymerization rate is attributed to the increase of particle number and the polymerization proceeding under the monomer-saturated condition—the Winsor I-like (micro)emulsion polymerization. The decrease of the polymerization rate is the result of the depressed transfer of monomer from the monomer reservoir to the reaction loci. Above 50 °C the monomer emulsion separates into two phases: the upper transparent monomer phase and the lower blue colored (microemulsion) phase. The polymerization mixture consists of the microdroplets (act as the reaction loci) and large degradable monomer droplets (act as the reservoir monomer and emulsifier). The continuous release of emulsifier from the monomer phase and the microdroplets induce the continuous particle nucleation up to high conversion. The initial formation of large particles results from the agglomeration of unstable growing particles and monomer droplets. The size of large (highly monomer-swollen) particles decreases with conversion and they merge with the growing particles at ca. 40-50% conversion. The coarse initial emulsion transformed during polymerization to the fine (semitransparent) polymer emulsion as a result of the continuous particle nucleation, the shrinking of highly monomer-swollen polymer particles and the depletion of monomer droplets. The low overall activation energy of polymerization is mainly ascribed to the decreased barrier for entering radicals into the latex particles with increasing temperature.     

Modelling particle size distributions and secondary particle formation in emulsion polymerisation

An extensive model is given for the particle size distribution (PSD), particle number, particle size and amount of secondary nucleation in emulsion polymerisations. This incorporates what are thought to be all of the complex competing processes: aqueous phase kinetics for all radical species arising from both initiator and from exit (desorption), radical balance inside the particles, particle formation by both micellar and homogeneous nucleation mechanisms, and coagulation (the rate of which is obtained using the Healy–Hogg extension of DLVO theory). The predictions of the model are compared to extensive experimental results on rates, time evolution of the particle size distribution, and relative amounts of secondary nucleation, for styrene initiated by persulfate with sodium dodecyl sulfate and with sodium dihexyl sulfosuccinate as surfactants. For this system values of almost all of the many parameters needed for the model are available from independent measurements, and thus no significant parameter adjustment is plausible. Accord with experiment is imperfect but quite acceptable, supporting the validity of the various mechanisms in the model. Effects such as the experimental variation of particle number with ionic strength, as well as calculated coagulation rate coefficients as functions of particle size, suggest that coagulation of precursor (i.e., newly-formed) particles is a significant effect, even above the cmc. The modelling also suggests why secondary nucleation occurs readily in systems stabilised with polymeric surfactant.     

Experimental study of emulsion polymerization with crosslinking

Herein is reported the results of an extensive experimental investigation of the kinetics of emulsion polymerization as affected by crosslinking in the polymer particles. The model monomer system, methyl methacrylate (MMA) and ethylene glycol dimethacrylate (EGDMA), was chosen for this study due to its earlier comprehensive investigation in bulk polymerization. Standard recipes with sodium dodecylsulfate (SDS) as anionic emulsifier and potassium persulfate (KPS) as initiator were used for the batch emulsion polymerizations. Results, which clearly show the effect of crosslinking on the kinetics, are discussed in detail. These include swellability of polymer particles by monomer; polymer particle nucleation rates, below and above the critical micelle concentration (CMC); average number of radicals per particle; and gel-sol levels. It was found advantageous to use electron spin resonance (ESR) to follow radical concentrations during crosslinking in polymer particles. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 66: 935–957, 1997     

Semibatch emulsion polymerization of butyl acrylate. II. Effects of emulsifier distribution

The effects of emulsifier distribution ratio between the initial charge and the feed on particle formation and kinetics of butyl acrylate emulsion polymerization, using sodium lauryl sulfate as emulsifier and potassium persulfate as initiator, were investigated. The number of particles increased with initial emulsifier concentration in the reactor charge. It was shown that traditional ranking, in terms of number of particles produced, of semibatch emulsion polymerization with monomer emulsion feed is not always justified and a semibatch emulsion polymerization can produce far more particles than a conventional batch emulsion polymerization. The number of polymer particles was found to be practically independent of the emulsifier distribution ratio between the charge and the feed for a high overall emulsifier concentration, while for a low overall emulsifier concentration, the number of particles increased with initial loading of the emulsifier. The polydispersity index (PDI) of the final latexes showed a minimum with emulsifier distribution. A bimodal particle size distribution, and a latex with a large PDI, was obtained when there was no emulsifier in the charge. As the initial emulsifier charge increased, a unimodal PSD with a smaller PDI was obtained. With higher proportions of emulsifier in the initial charge, the PDI rose again due to particle nucleation at monomer‐starved conditions, and a skewed unimodal PSD was obtained. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 79: 582–597, 2001     

乳液聚合成核阶段的模拟与分析

建立了乳液聚合成核阶段的Monte Carlo模型，并用计算机对一个体积为10^-17m^3的微型反应器中苯乙烯的乳液聚合进行了模拟。以计算机生成随机数作为自由基被胶束和乳胶粒捕获的几率，模拟了在微型反应器中每一个自由基的生成、被胶束或乳胶粒捕获的过程以及每一个乳胶粒的生成及增长过程。通过对每一个乳胶粒在增长过程中各参数的统计计算，研究了乳液聚合成核阶段诸参数(乳胶粒数目、乳胶粒直径与粒径分布、单体转化率、聚合反应速率等)与乳化剂浓度[S]及引发剂浓度[I]的关系。结果表明，苯乙烯的乳液聚合体系中乳胶粒数目与[S]^0.5996[I]^0.4016成正比：在成核阶段乳胶粒直径分布先变宽后变窄，乳液聚合过程中乳胶粒直径分布有自动变窄的趋势；成核阶段持续时间t12与[S]^0.60[I]^0.60成正比，成核阶段结束时的单体转化率X12与[S]^1.20[I]^0.20成正比。     

Semibatch emulsion polymerization of butyl acrylate. I. Effect of monomer distribution

The effects of initial monomer charge on the particle formation and on the rate of polymerization were investigated for semibatch emulsion polymerization of butyl acrylate using sodium lauryl sulfate (SLS) as a surfactant and potassium persulfate (KPS) as an initiator. For the semibatch process with monomer (M) feed, it was found that by varying the monomer distribution ratio between the initial reactor charge and the feed it is possible to alter the contribution of monomer‐flooded and monomer‐starved nucleation mechanisms to the whole nucleation process. The number of particles increases as the initial monomer charge decreases, if the monomer concentration is below a critical value for any fixed system. The increase in number of particles is associated with a broad particle‐size distribution which might depict an emerging second peak on the particle‐size distribution curve. For low emulsifier concentration systems, a larger number of particles was obtained for a lower amount of monomer charge. Particle coagulation and emulsifier adsorption on the monomer droplets were counted as the main reasons for such behavior. For a semibatch process with monomer emulsion (ME) feed, the larger number of particles was formed at a lower initial monomer charge, similar to an M‐add semibatch process. However, the application of monomer charge to an ME‐add process was found to increase the possibility of secondary nucleation and led to the occurrence of a bimodal particle‐size distribution. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 3094–3110, 1999     

Monte Carlo模拟在丁二烯/苯乙烯乳液聚合中的应用研究

以概率论中的大数法则和中心极限定理为基础,通过对成核阶段条件的简化,建立了丁二烯/苯乙烯乳液聚合成核阶段的Monte Carlo模型,模拟考察了乳化剂、引发剂浓度对丁苯乳液聚合反应速率的影响,对丁苯乳聚成核阶段模拟结果进行回归表明,乳胶粒数目N p与乳化剂浓度[S]和引发剂浓度[I]的关系为：N p=k[S]0.606 2[I]0.405 5,与Smith-Ewart经典理论相吻合。改善乳液聚合生成的乳胶粒径分布以及乳胶粒数目是提高乳液聚合中单体转化率最为有效的方法。模拟计算结果表明,将单体转化率从62%提高到70%,引发剂浓度的增加量应在12.90%~34.89%（摩尔分数）之间,乳化剂浓度的增加量应在12.6%~22.16%（摩尔分数）之间。与实验结果进行对比,数据吻合,验证了模拟计算的可靠性和有效性。     

On the fine emulsion polymerization of styrene with non-ionic emulsifier

Summary The fine emulsion polymerizations of styrene initiated by a redox system ammonium peroxodisulfate/sodium thiosulfite stabilized by a non-ionic emulsifier were kinetically investigated. The dependence of the rate of polymerization on conversion or the emulsifier concentration was described by a curve with maximum at medium conversion. The maximum rate of polymerization is proportional to the − 0.45^th and 1.5^th power of initiator and emulsifier concentration, the number of particles to the 0.32^nd and 1.3^rd power of initiator and emulsifier concentration and the molecular weight to the − 0.62^th and −0.97^th power of initiator and emulsifier concentration, respectively. The results show a strong decrease in turbidity at around 20% conversion when emulsion turns into translucent latex. Deviation from the micellar nucleation model was attributed to the solubility of emulsifier in monomer, high level of nonmicellar aggregates, thick interfacial layer and transfer emulsion to microemulsion. The strong decrease of molecular weight with increasing emulsifier concentration is attributed to chain transfer events promoted by the high level of emulsifier at the reaction loci. Received: 16 November 1998/Revised version: 25 May, 10 October 1999/Accepted: 10 October 1999     

Particle Nucleation in Semi-batch Surfactant-free Emulsion Polymerization of Acrylic Monomers

Based on the coagulative particle nucleation mechanism, a two-step model has been developed for the semi-batch surfactant-free emulsion polymerization of butyl acrylate, or butyl acrylate in the presence of a small amount of acrylic acid. During Stage 1, precursor particles are first formed by phase separation of the oligomeric radicals in the aqueous phase. These precursor particles are extremely unstable and tend to aggregate until a stable (primary) particle size is achieved. The size of the newly formed particles is controlled mainly by limited coagulation among the precursor particles in this stage. During the early part of Stage 2, the rate of aggregation of these precursor particles to form a primary particle may be slower than the rate of coagulation of the primary particles. This factor would lead to a decrease in the particle concentration with time. Later on, the particle concentration starts to level off and finally reaches a steady value due to the effect of limited coagulation. In this stage, the particle growth is achieved mainly by polymerization of the feed monomer inside the particles. This model has been assessed by a number of experiments. The reaction variables selected for this purpose are the concentrations of initiator, acrylic acid and NaHCO₃, and the agitation speed. The proposed model predicts the experimental data of the particle concentration and particle size reasonably well. © of SCI.     

Mass transfer and radical flux effects in dispersed‐phase polymerization of isooctyl acrylate

The kinetics of dispersed phase polymerization of a highly water‐insoluble monomer (isooctyl acrylate) were explored in emulsion, miniemulsion, and microsuspension polymerization. The effects of monomer water solubility and choice of initiator (oil‐ vs. water‐soluble) strongly impact the final product (particle size and molecular weight distribution). For emulsion polymerization, as the surfactant concentration was increased, there was a transition from homogenous to micellar nucleation near the CMC, then a drop in nucleation rate at high surfactant concentration due to insufficient radical flux to support more nucleation. For miniemulsion polymerization, a slow rate of growth of (droplet) nucleation with surfactant concentration was found, followed (at the CMC) by an increase in the rate of nucleation with added surfactant as the mode of nucleation switched to micellar. The conversion‐time kinetics of microsuspensions could be modeled with a bulk polymerization model. IOA is sufficiently insoluble in the aqueous phase that emulsion polymerization may or may not be reaction limited. The presence of a stabilizer such a PAA, the use of an oil‐soluble initiator such as BPO, and the insolubility of IOA in the aqueous phase all push the polymerization locus toward droplet (microsuspension) nucleation and bulk kinetics.© 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 5649–5666, 2006     

A combinatorial approach to evaluation of monomer conversion and particle size distribution in vinyl chloride emulsion polymerization

This work is targeted to study emulsion polymerization of vinyl chloride monomer (VCM) using experimental and mathematical methods. To fulfill this goal, a computer code was developed on the basis of zero–one population balance by which the effects of initiator and emulsifier concentration on the evolution of VCM conversion were investigated in the course of polymerization. The model was also trained to capture the coagulation of the particles. This enabled to adopt a reliable way of evaluating the particle size distribution (PSD). In particular, the rates of homogeneous and micellar nucleation mechanisms were simulated and reasonably predicted alterations in the PSD and the number of polymer particles under the influence of aforementioned parameters. The results from modeling were satisfactorily consistent with the experimental outputs and obviously visualized the impact of initiator and surfactant concentration on the PSD of the prepared PVC latexes.     

Continuous emulsion polymerization of vinyl acetate. I. Operation in a single continuous stirred tank reactor using sodium lauryl sulfate as emulsifier

Continuous emulsion polymerizations of vinyl acetate were carried out at 50 °C in a single continuous stirred‐tank reactor using sodium lauryl sulfate as emulsifier and potassium persulfate as initiator. It was found that (1) the so‐called limit cycles could take place in monomer conversion, the number of polymer particles and the molecular weight of polymers produced under certain operating conditions, (2) the time‐average steady‐state monomer conversion was proportional to the 0.31 power of the emulsifier concentration in the feed, to the 0.50 power of the initiator concentration, to the ?1.0 power of the monomer concentration, and to the 0.90 power of the mean residence time, and (3) the time‐average steady‐state number of polymer particles produced was proportional to the 2.1 power of the emulsifier concentration in the feed, to the ?0.80 power of the initiator concentration, to the 0 power of the monomer concentration, and to the ?0.92 power of mean residence time. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 2748–2754, 2002     

Dispersion Polymerization of Styrene by Initiation Within Monomer Phase with Lauroyl Peroxide

Dispersion polymerization kinetics of styrene was investigated in this study. A monomer phase soluble initiator, namely lauroyl peroxide, and a water phase soluble emulsifier, namely sodium dodecyl sulfate, were used. the polymerizations were carried out in a magnetic drive, sealed, cylindrical polymerization reactor, in nitrogen atmosphere. Two different fatty alcohols, cetyl and lauryl alcohols, were tried as particle size increasing agents. the effects of initiator, emulsifier concentrations, monomer/water ratio, and the weight ratio of particle size increasing agent to emulsifier on the total monomer conversion, average size, and size distribution of the large latex particles were studied. the agglomeration conditions were tested by changing the polymerization temperature, initiator, and emulsifier concentrations. the results indicated that the polymerization rate and the average size of the latex increased with increasing emulsifier concentration. the average size of the latex was also increased with increasing initiator concentration. the use of cetyl alcohol in the presence of emulsifier resulted in an appreciable increase in the polymerization rate and in the average size of the latex. the effect of fatty alcohols on the polymerization rate in the dispersion polymerization system was completely different than that in the swollen emulsion polymerization. the agglomeration of the large latex particles was increased with decreasing emulsifier and increasing initiator concentrations.     

The role of aqueous-phase kinetics in emulsion polymerizations

The kinetics of species in the aqueous phase control many events in emulsion polymerization: the rate of entry of free radicals into particles (equivalent to initiator efficiency), the rate of exit (desorption) of free radicals from particles, the fate of desorbed free radicals and of free-radical species derived directly from aqueous-phase initiator. Aqueous-phase kinetics also dominate particle nucleation and re-seeding (secondary nucleation), and the in situ formation of surfactant. The mechanisms of each of these events are discussed, and it is shown how general methods can be constructed to deduce the ratedetermining events for each of these. The methodology is then applied extensively to styrene, which leads to the following conclusions. (a) The aqueous-phase events which govern entry (initiator efficiency) are propagation and termination, with entry occurring irreversibly when a critical degree of propagation z is reached so that the resulting species (a di- or tri-styrenesulfonate species in the case of styrene with persulfate initiator) is sufficiently surface-active that, once adsorbed onto the particle it does not desorb before it propagates; the actual adsorption event is sufficiently rapid so as not to be rate-determining except during nucleation. (b) Exit of free radicals is governed by transfer inside the particle to form a monomeric radical which may desorb and diffuse irreversibly away from the parent particle before it propagates therein. (c) The fate of desorbed free radicals in the wide range of styrene systems examined is to re-enter another particle and remain therein, rather than the other possible fates (aqueous-phase termination or re-exit until intra-particle termination eventually occurs). (d) Below the cmc, nucleation is by the homogeneous-coagulative mechanism, while above the cmc, nucleation is through a process which combines the essential features of both homogeneous-coagulative and micellar-entry models. (e) Analysis of the aqueous-phase products produced in an emulsion polymerization shows that the species involved in termination, entry and exit also undergo subsequent reactions: hydrolysis and reaction with persulfate.     

Kinetics and colloidal parameters of miniemulsion polymerization of butyl acrylate

BACKGROUND: The miniemulsion polymerization of butyl acrylate initiated by a macromonomeric azoinitiator (macroinimer, MIM) and stabilized by the non‐ionic emulsifier Tween 60 (TW‐60) was investigated. RESULTS: The monomer conversion and the polymerization rate increase with the amount of MIM and then decrease. The desorption rate constants were estimated using the Ugelstad/O'Toole, Gilbert and Nomura models. The Ugelstad/O'Toole and Gilbert models suggest an increase in the k′_des value with increasing emulsifier concentration at the highest MIM concentration while the Nomura model proposes no variation in k′_des with an increase in both TW‐60 and MIM concentrations. The polymerization rate increases in the following order with regard to initiator: MIM < ammonium persulfate < dibenzoyl peroxide < 2, 2′‐azobisisobutyronitrile. CONCLUSION: The increase in the rate of polymerization can be discussed in terms of both increased particle concentration and the gel effect. The size of the polymer particles decreases and the number of polymer particles increases with both TW‐60 and MIM concentrations. This behaviour is attributed to the formation of a larger number of smaller monomer and/or polymer particles and higher particle nucleation rate. The observed long nucleation period for the MIM‐initiated polymerization is attributed to the creation of a crosslinked structure and the immobilization of MIM chains. Copyright © 2009 Society of Chemical Industry     

Monomer Effects on Emulsion Polymerization with ASR as the Surfactant

In this paper, alkali soluble resin (ASR) was evaluated as a surfactant in the emulsion polymerizations of methyl methacrylate (MMA), ethyl methacrylate (EMA), and butyl methacrylate (BMA). It was found that the number of particles formed was proportional to the hydrophilicity of monomer and the concentration of initiator. Kinetic analysis indicated that the monomer concentration within the latex particle influences the average number of radicals per latex particle. The increase of monomer concentration within the particle enhances radical desorption from the particle and reduces the average number of radicals per particle. Experimental results show that the grafting reaction of ASR is proportional to the concentration of initiator or ASR. But the hydrophilicity of monomer is the major factor which influences the grafting reaction. This phenomenon is due to the fact that particle nucleation is different between the hydrophilic and hydrophobic monomer.     

丙烯酸丁酯细乳液单电子转移-蜕化链转移聚合反应

单电子转移-蜕化链转移(SET-DT)聚合是一种单体适用性广、对聚合环境要求不苛刻的活性自由基聚合方法。以Na_2S_2O_4为催化剂,CHI3为引发剂,采用水相细乳液聚合法进行丙烯酸丁酯(BA)的SET-DT活性自由基聚合,考察了聚合温度、引发剂/催化剂浓度、催化剂滴加方式和乳化剂浓度对聚合动力学、聚丙烯酸丁酯(PBA)数均分子量和分子量分布的影响。结果表明,细乳液聚合速率明显大于悬浮聚合,可在较低温度(30℃以下)、较低引发剂和催化剂浓度(BA,CHI_3和Na_2S_2O_4的初始摩尔浓度比为1 600:1:8)下实现BA的快速聚合;通过聚合过程滴加Na_2S_2O_4催化剂和增加十二烷基硫酸钠主乳化剂浓度,可提高聚合速率;采用低引发剂浓度和催化剂逐步滴加聚合得到的PBA的平均分子量较大,分子量分布较窄。     

Nitroxide mediated living radical polymerization of styrene in miniemulsion—modelling persulfate-initiated systems

Recently we have constructed a mechanistic model describing the nitroxide mediated miniemulsion polymerization (NMMP) of styrene at 135°C, using alkoxyamine initiators to control polymer growth (Nitroxide-Mediated Polymerization of Styrene in Miniemulsion. Modeling Studies of Alkoxyamine-Initiated Systems, 2001b). The model has since been expanded to describe styrene NMMP at 135°C using TEMPO and the free radical initiator, potassium persulfate (KPS). The model includes mechanisms describing reactions in the aqueous and organic phases, particle nucleation, the entry and exit of oligomeric radicals, and the partitioning of nitroxide and styrene between the aqueous and organic phases. Predicted monomer conversions, number average molecular weights and polydispersities were in agreement with experimentally measured values. Model simulations revealed that for systems employing high ratios of TEMPO:KPS, the consumption of TEMPO by polymer radicals derived from KPS decomposition and styrene thermal initiation (using the accepted literature kinetic rates) is not sufficient to lower TEMPO concentrations to levels where polymer growth can occur. By accounting for the consumption of TEMPO by acid-catalyzed disproportionation, TEMPO concentrations are significantly reduced, allowing for accurate model predictions of monomer conversion, number average molecular weight and polydispersity at every experimental condition considered.