Reversible addition fragmentation transfer (RAFT) polymerization of styrene in a miniemulsion: A mechanistic investigation

The RAFT polymerization of styrene in miniemulsion using 1-phenylethyl phenyl-dithioacetate (PEPDTA) as a RAFT agent was investigated, in attempt to reveal the mechanism for the often observed inferior performance such as low polymerization rate, broad molecular weight distribution and particle size distribution in the RAFT miniemulsion polymerization with regular levels of surfactant and co-stabilizer (1 wt% sodium dodecyl sulfate and 2 wt% hexadecane). It is strongly evident that a few of large oligomer particles consisting of oligomer, RAFT agent (RAFT agent refers to the original RAFT agent), and monomer would be formed in the early stage of the polymerization due to the superswelling of the first nucleated droplets. With the regular levels of surfactant and co-stabilizer, the observed low polymerization rate, broadened molecular weight distribution, slow conversion of the RAFT agent, lower N_p, and broadened particle size distribution could be well explained by the formation of these large oligomer particles and their prolonged existence. When the formation of the oligomer particles was suppressed by increasing surfactant and co-stabilizer levels and wise selection of types of RAFT agent, the molecular weight distribution could be narrowed to around 1.3 and particle size distribution could be close to that of the conventional non-living miniemulsion polymerization.     

RAFT miniemulsion polymerization of methyl methacrylate

It has been well documented that RAFT miniemulsion polymerization has broader molecular weight distribution, compared with its bulk polymerization counterpart. Interestingly, it was found that the PDI value of RAFT miniemulsion polymerization of methyl methacrylate (MMA) mediated by 2-cyranoprop-2-yl dithiobenzoate (CPDB) was still as low as its corresponding bulk polymerization did. PDI could be as low as 1.13 even with typical sodium dodecyl sulfate (SDS, 1 wt%, surfactant) and n-hexadecane (HD, 2 wt%, costablizer) concentrations. When the polymerization was carried out at 60 °C, a dramatic increase in PDI (>1.4) was observed after 80% monomer conversion since RAFT addition reaction became diffusion-controlled. Increasing the polymerization temperature to 80 °C could reduce the PDI to 1.2 even at 100% monomer conversion. The compartmentalization effect of radicals was surprisingly absence before 30% monomer conversion but became pronounced afterwards in the miniemulsion polymerization. Thus, it still took less time to finish the miniemulsion polymerization with the increase of the surfactant levels.     

Comparison of RAFT polymerization of methyl methacrylate in conventional emulsion and miniemulsion systems

In this study, we have conducted the reversible addition-fragmentation chain transfer (RAFT) polymerization of methyl methacrylate (MMA) in two heterogeneous systems, i.e. conventional emulsion and miniemulsion, with identical reaction conditions. The main objective is to compare the living character in both systems according to the nucleation mechanism, the latex stability, the particle sizes and particle size distributions of latexes, the molecular weights and molecular weight distributions (or polydispersity index, PDI) of PMMA, and the kinetics of the RAFT polymerization. The RAFT agent used in both systems was 2-cyanoprop-2-yl dithiobenzoate (CPDB). The effects of an oil-soluble initiator 2,2′-azobisisobutyronitrile (AIBN) and a water-soluble initiator kalium persulfate (KPS) on the RAFT/emulsion and RAFT/miniemulsion polymerizations were investigated. Methyl-β-cyclodextrin (Me-β-CD) was used as a solubilizer. The average molecular weights and molecular weight distributions (PDIs) of dried PMMA samples were characterized by gel permeation chromatography (GPC). The experimental results showed that the RAFT/miniemulsion polymerization of MMA exhibited better living character than that of RAFT/emulsion polymerization under the conditions of our experiment. The PDI of PMMA in RAFT/miniemulsion polymerization was decreased with the addition of Me-β-CD. However, Me-β-CD did not have influence on the PDI of PMMA prepared in RAFT/emulsion polymerization.     

Synthesis of structured nanoparticles of styrene/butadiene block copolymers via RAFT seeded emulsion polymerization

The structured nanoparticles of styrene (St) and butadiene (Bd) block copolymers were prepared by RAFT seeded emulsion polymerization of butadiene. It was confirmed that the block copolymers of PSt-b-P(St-co-Bd) was formed with controlled molecular weight and rather low PDI at low composition of the P(St-co-Bd) segment. With more incorporation of butadiene, the branching reaction of polybutadiene became obvious, leading to higher PDI and positive deviation of M_n from the theoretical predication. At the gel point, the composition of the P(St-co-Bd) segment was estimated to be 0.72. After this, the gel fraction increased quickly. The morphology of structured nanoparticles could be largely tuned simply by the copolymer composition. With the composition of the P(St-co-Bd) segment increased from 0.37 to 0.92, the morphology within the structured particles changed from the polybutadiene domains-in-polystyrene matrix, perforated concentric-spherical layer, concentric-spherical multi-layers, bi-continuous, to broken layers of polystyrene in polybutadiene matrix. It was found that the morphology of the block copolymer within nanoparticles was dependent on d/L values, which was in excellent agreement with the theoretical prediction.     

TEMPO-mediated radical polymerization of styrene in aqueous miniemulsion: Macroinitiator concentration effects

The control/livingness in nitroxide-mediated polymerization of styrene (S) in aqueous miniemulsion at 125 °C employing a poly(S)-2,2,6,6-tetramethylpiperidinyl-1-oxy (PS-TEMPO) macroinitiator and the surfactant sodium dodecylbenzenesulfonate has been shown to depend strongly on the macroinitiator concentration for particles of approximate number-average diameter 65 nm. The control/livingness was relatively poor at [PS-TEMPO]₀ ≤ 0.02 M due to the combined effect of enhanced spontaneous initiation and the interface effect (whereby deactivation is suppressed due to interfacial activity of TEMPO). Satisfactory control/livingness was obtained at higher [PS-TEMPO]₀ as a result of the interface effect and enhanced spontaneous initiation exerting less pronounced influence per chain than at lower [PS-TEMPO]₀. Polymerizations using the sulfonate surfactant DOWFAX 8390 gave similar results, indicating that the present macroinitiator concentration effects are not specific to SDBS-based systems. The results also demonstrate that TEMPO-mediated polymerization of S in miniemulsion can proceed at a higher rate than in bulk with good control/livingness.     

Quantification of spontaneous initiation in radical polymerization of styrene in aqueous miniemulsion at high temperature

The spontaneous (thermal) initiation rate (R_i,th; no added initiator) in radical polymerization of styrene in aqueous miniemulsion at 110 and 125 °C with sodium dodecylbenzenesulfonate or poly(vinyl alcohol) as surfactants (colloidal stabilizers) has been estimated using a novel approach based on the total number of chains. In qualitative agreement with previous work at lower temperatures, R_i,th was found to be 3.1-15.1 times greater than that in bulk. According to the activation energy and conversion dependence of R_i,th, the radical generation mechanism differs from that in bulk. The experimental evidence is consistent with the enhanced R_i,th in miniemulsion being related to the oil-water interface, with radical generation in the aqueous phase playing a negligible role. The implications with regards to nitroxide-mediated radical polymerization in aqueous dispersed systems are discussed.     

Synthesis of carboxylic acid functionalized nanoparticles by reversible addition-fragmentation chain transfer (RAFT) miniemulsion polymerization of styrene

In this study, an addition-fragmentation chain transfer agent bearing carboxylic acid, 4-toluic acid dithiobenzoate (TADB), was used to synthesize carboxylic acid functionalized PS nanospheres via the miniemulsion polymerization. In addition, non-functionalized RAFT agent, benzyl dithiobenzoate (BDB), was also used to compare the surface properties of the PS nanoparticles. For the TADB system, the rate of polymerization was approximately two-fold faster than the BDB system, while the molecular weights and PDI of PS remain intact.With increasing the molar ratio of [TADB]/[AIBN] from 0 to 3.0, the average particle diameter is substantially increased from 90 to 126 nm. The absolute value of zeta potential and conductivity also correspondingly increase from 49.1 mV and 3.47 mS/cm to 53.9 mV and 4.21 mS/cm, respectively. The results indicate that the surface of PS nanospheres could be functionalized by means of a carboxylic acid group on the RAFT agent and the stability of the PS miniemulsion latex could be significantly improved.     

Synthesis of polystyrene encapsulated nanosaponite composite latex via miniemulsion polymerization

The synthesis and characterization of polystyrene encapsulated nanosaponite composite suspension via miniemulsion polymerization are reported in this study. The particle size of nanoclay and its pre-modification are critical to successfully producing a stable complex suspension. The final products were characterized by X-ray diffraction spectra, transmission electron microscopy (TEM), scanning electron microscopy (SEM), thin window energy dispersive spectroscopy (EDS), and light scattering. The results show that ar-vinylbenzyltrimethylammonium chloride (VBTAC) modified nanosaponite could be fully exfoliated and encapsulated inside the polystyrene latex via in situ miniemulsion polymerization. When the concentration of hexadecane (a co-stabilizer used in the miniemulsion polymerization) was high, the final composite particles are composed mainly of spherical particles with size less than 100 nm, and a small number of hemispherical or bowl-structured particles of size ∼100 nm to 1000 nm. The phase separation due to the existence of large amounts of hexadecane accounted for the formation of a variety of morphologies.     

Thermal storage nanocapsules by miniemulsion polymerization

Paraffin wax was nanoencapsulated by miniemulsion polymerization. Low solid content batch polymerizations were carried out and the influence of paraffin wax/methyl methacrylate ratio on polymeriztion kinetics, phase change properties, and thermal stability were investigated. Paraffin concentration controls the nucleation mechanism and nanocapsules particle size. The latent heat capacity increases as the content of encapsulated paraffin wax increases. The maximum amount of encapsulated paraffin wax achieved under the experimental conditions was 60 wt %, presenting a latent heat capacity of 140.3 J/g. Moreover, the encapsulated systems present in all cases better thermal stability than pure paraffin wax. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Nitroxide-mediated radical polymerization of styrene in miniemulsion: model studies of alkoxyamine-initiated systems

A mathematical model has been developed to describe the behavior of the nitroxide-mediated miniemulsion polymerization (NMMP) of styrene initiated by alkoxyamine initiators. 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. The influence of nitroxide partitioning on the polymerization kinetics was examined by modeling systems initiated by the alkoxyamines BST and hydroxyl-BST; BST and hydroxyl-BST are benzoylstyryl radicals terminated by the nitroxides TEMPO and 4-hydroxyl-TEMPO, respectively.Predicted monomer conversions, number average molecular weights and polydispersities were in agreement with experimentally measured values. Simulations and mathematical analysis showed that the rate of styrene NMMP is not strongly influenced by the partitioning properties of TEMPO and 4-hydroxyl-TEMPO because of the complex interaction between reaction equilibrium, phase equilibrium, termination and thermal initiation. However, in the absence of styrene thermal initiation, nitroxide partitioning was found to have a significant influence on the polymerization kinetics. The model was also used to make quantitative estimates of: the population of active and dormant polymer radicals derived from both alkoxyamine initiators and thermal initiation; the population of dead polymer chains; and the number molecular weight distributions of living and dead polymer chains.     

通过细乳液聚合方法制备自交联型有机硅乳液

苯乙烯与甲基丙烯酸-3-三甲氧基硅丙酯(M PS)经细乳液聚合,制得了稳定的自交联型的水分散涂料,用透射电子显微镜(TEM)、动态光散射仪(DLS)和红外光谱(IR)表征了乳胶粒的形态和结构。发现与传统乳液聚合相比,细乳液聚合法可显著提高乳液的稳定性,且M PS加入量的增加会导致乳液稳定性降低。     

Polydimethylsiloxane latexes and copolymers by polymerization and polyaddition in miniemulsion

Different synthetic pathways have been explored to synthesize polysiloxane-acrylate and polysiloxane-urethane latexes with small and narrowly distributed particle sizes, using the specific advantages of the miniemulsion approach. Besides cross-linked latexes with high siloxane content, also hybrid particles were generated.     

甲基丙烯酸甲酯-甲基丙烯酸三氟乙酯共聚物细乳液的合成及其表征

以甲基丙烯酸甲酯(MMA)和甲基丙烯酸三氟乙酯(TFEMA)为反应单体进行细乳液聚合,制得了MMA-co-TFEMA共聚物乳液。利用FTIR和GPC表征了共聚物的结构和分子量,采用激光光散射法及透射电镜对乳胶粒子的粒径大小及形貌进行了表征,并通过接触角法对共聚物膜的表面性能进行了研究。结果表明,与常规乳液相比,以细乳液方法制得的MMA-co-TFEMA共聚物乳液稳定性好,粒径分布窄,聚合物的分子量分布窄,并且共聚物膜随着氟含量的增加疏水疏油性增强,表面能降低。     

Simulation studies on the origin of the limiting conversion phenomenon in hybrid miniemulsion polymerization

Hybrid systems of acrylic monomers are studied as they undergo miniemulsion free radical polymerization in the presence of unsaturated alkyd resins. The goal of such polymerizations is to induce grafting between the acrylic and alkyd components, which would retard microdomain phase separation, and produce materials with the crosslinking capability of alkyd coatings (normally applied from a solvent system) with the environmental and cost benefits of an aqueous system. In this paper, the observed limiting monomer conversion phenomenon will be explored via simulation studies.Two mathematical models describing the kinetics of bulk hybrid polymerization of an alkyd-acrylate system were developed. The first was a homogenous model in which the kinetics of retardive chain transfer was used to attempt to simulate the observed phenomenon of limiting conversion. The second model was a core-shell model in which polymerization takes place in an acrylic-rich shell, while the alkyd-rich core serves as a reservoir for acrylic monomer and alkyd. Based on the results from these models, the cause of limiting conversion was attributed to the combined role of the glass effect and the partitioning effect of the monomer into a core-shell system and its subsequent entrapment. Retardive chain transfer was not capable of producing the observed limiting conversion.     

Synthesis, characterization, and energy transfer studies of dye-labeled poly(butyl methacrylate) latex particles prepared by miniemulsion polymerization

Poly(butyl methacrylate) (PBMA) latex particles have been copolymerized with new fluorescent naphthalimide dyes by miniemulsion polymerization. A new pair of naphthalimide dye monomers was synthesized and copolymerized with butyl methacrylate (BMA) via miniemulsion polymerization, producing approximately 80 nm diameter particles with a narrow size distribution. We were able to prepare polymers with molecular weights in excess of 100,000 g/mol. We also prepared 30,000 g/mol polymers using 1-dodecanethiol as a chain transfer agent. GPC and UV characterization suggest that nearly all of the dye monomers were incorporated into the PBMA polymer chains. The polymerized naphthalimide dyes can be used as a donor-acceptor pair for fluorescence resonance energy transfer (FRET) experiments. The analysis of FRET experiments is complicated by the slightly non-exponential decay of the donor naphthalimide dye. We propose a simple method to deal with this non-exponential behavior in the data analysis. Using our approach, we find that the Förster radius (R_o) between the donor and the acceptor dyes incorporated in the PBMA latex is 3.8 nm. This value is similar to the 3.6 nm Förster radius of a comparable model dye pair in ethyl acetate obtained by a different method.     

Homogeneously distributed magnetite in the polystyrene spherical particles using the miniemulsion polymerization

The polystyrene spherical particles with homogeneously distributed magnetites were prepared using the conventional miniemulsion polymerization. In the first, the magnetite nanoparticles were coated with oleic acid in aqueous Fe³⁺/Fe²⁺ solution using excess ammonium hydroxide via co-precipitation method. In the second, the miniemulsion polymerization of styrene was carried out using various concentrations of potassium persulfate (KPS) as an initiator, H-08E as an emulsifier, hexadecane as a co-emulsifier and acrylic acid as a dispersing agent in the presence of oleic acid coated magnetite at 70 °C for 24 h. The particle size and its distribution of the homogeneously embedded magnetites were influenced by the concentration of the initiator (KPS) and acrylic acid (AAc). In addition, the emulsifier, H-08E, affects the size and the shape of the PS particles. The optimum conditions for the homogeneously distributed magnetite in the spherical PS particles with the narrow distribution were 5 wt.% styrene, 0.2 g KPS, 0.2 g AAc, and 0.12 g H-08E by inducing 364 nm in diameter, 12.04% in the coefficient of variation (C_v) and 22.1% of the maximum magnetite content.     

Challenges for industrialization of miniemulsion polymerization

Miniemulsion polymerization facilitates the synthesis complex materials that cannot be produced otherwise. These materials have a broad range of potential applications including among others adhesives, coatings, anticounterfeiting, textile pigments, bio-based polymer dispersions, gene and drug delivery, anti-viral therapy, tissue engineering, catalyst supports, polymeric photoresists, energy storage and self-healing agents. However, 40 years after the pioneering work of Ugelstad, El-Aasser and Vanderhoff the promises have not been fulfilled and the presence of miniemulsion polymerization in commercial products is scarce. This article reviews the advances in the field, discusses the reasons for this delay and analyzes the challenges that have to be overcome in order to fully use this process in commercial practice.     

Polymerization strategies to overcome limiting monomer conversion in silicone-acrylic miniemulsion polymerization

The limiting conversion phenomenon observed in high solid content silicone-modified acrylic miniemulsion polymerizations was investigated. It was found that the limiting conversion was mainly due to the formation of inactive radicals upon propagation of butyl acrylate radicals with the vinyl end groups of the polydimethylsiloxane. Polymerization strategies that allowed overcoming this problem and achieving high monomer conversion were implemented.     

Network formation in nitroxide-mediated radical copolymerization of styrene and divinylbenzene in miniemulsion: Effect of macroinitiator hydrophilicity

Nitroxide-mediated crosslinking radical copolymerizations of styrene and divinylbenzene have been carried out in aqueous miniemulsion with Dowfax 8390 as surfactant at 125 °C employing TEMPO-based macroinitiators of various hydrophilicities (random copolymers comprising styrene and methyl acrylate). The pendant conversion (i.e. the degree of crosslinking) increased with increasing macroinitiator hydrophilicity and with decreasing particle size. It is proposed that a concentration gradient is generated within the particle such that the macroinitiator concentration tends to be higher near the oil-water interface than in the particle interior. The macroinitiators are surface active and are thus preferentially adsorbed at/located near the oil-water interface. This in turn leads to an increase in the ratio [pendant unsaturation]/[monomer] in the vicinity of propagating radicals, which are generated from the macroinitiators, and thus an increase in pendant conversion. This effect is enhanced by an increase in macroinitiator hydrophilicity and a decrease in particle size. The present approach offers novel means of controlling network formation in dispersed systems.