Recent developments in miniemulsion polymerization

Some developments in miniemulsion polymerization aiming at taking advantage of its unique mechanisms minimizing the drawbacks of this technique are discussed. The discussion includes preparation of highly concentrated latexes, miniemulsion polymerization in continuous stirred tank reactors (CSTRs), and elimination of the low-molecular-weight hydrophobe.     

Recent advances in olefin polymerization

Various metallocenes were tested for syndiotactic polymerization of propylene. Reduction in the amount of costly methylalumoxane (MAO) or replacement with the other components was accomplished. Small modifications in the metallocene ligands brought about remarkable differences in the resulting catalysts concerning their storage stability, H₂ response as well as stereo-specificity. Syndiotactic polypropylene (SPP) in the melt state behaves very differently compared to isotactic polypropylene (IPP). The notion that these polymers have different distributions of molecular entanglements are pursued, and verified for the first time. Excellent dispersibility of other olefin polymers in SPP matrix contributed to improve the usefulness of SPP.     

Photoinduced miniemulsion polymerization

This study describes the radical photopolymerization of acrylate monomers in miniemulsion. Starting from nanosized acrylate droplets (<100 nm) which encapsulate a type I radical photoinitiator (BAPO), UV irradiation led after a few minutes to the formation of polymer nanoparticles of similar size. The present study deals with the kinetics aspects of this reaction and the colloidal properties of the resulting polymer dispersions. Real-time Fourier transform near-infrared spectroscopy in transmission was implemented to follow continuously the fast photopolymerization process. In addition, the spatial resolution of the photoinduced process allowed the online monitoring of the evolution of the miniemulsion size during the UV irradiation through dynamic light scattering.     

"活性"/可控自由基聚合新进展

概述了当前“活性”／可控自由基聚合(CPR)的三种主要方法，硝基氧调介聚合(NMP)、原子转移自由基聚合(ATRP)、可逆加成-断裂链转移聚合(RAFT)，特别是近年来的进展情况。     

Group-transfer polymerization: Recent advances and applications

Abstract: Catalysis of GTP of MMA with nucleophilic anions on cross-linked polystyrene supports was studied. With anion-bound supported catalyst, evidence is presented for formation in solution of ester enolates as reaction intermediates. Study of the cyanide-catalyzed initiation of GTP of MMA by TMSCN, has provided quantitative data for the association constant K_a for the complexation of cyanide by TMSCN and, by inference, an upper limit for the K_a for the association of this nucleophilic anion with silyl ketene acetals. The effects of i-propyl- and t-butoxy-silyl analogs of TMSCN on anion-complexation and on initiation and propagation of GTP are discussed. Coordination by hydrogen-bonding of nucleophilic anions to acetonitrile is shown to be the mechanism for “livingness-enhancement” of anion-catalyzed GTP at low concentrations of acetonitrile. GTP was used to prepare an ABC triblock dispersant, poly(methacrylic acid)-block-poly(2-phenylethyl methacrylate)-block-poly(ethoxytriethylene glycol methacrylate), and the surface activity of an aqueous solution of the potassum salt was compared with that of other polymer architectures.     

Butyl acrylate RAFT polymerization in miniemulsion

Few successes about butyl acrylate (BA) RAFT miniemulsion homopolymerization were reported, even though styrene, methyl methacrylate, and vinyl acetate had been successfully applied in reversible addition fragmentation transfer (RAFT) miniemulsion polymerization. In this article, four types of RAFT agent with various designed R and Z groups [benzyl dithioisobutyrate (BDIB), 1-phenylethyl phenyldithioacetate (PEPDTA), cumyl dithioisobutyrate (CDIB), benzyl dithiobenzoate] were used to mediate BA miniemulsion polymerization using the conditions (5 wt % hexadance and sodium dodecyl sulfate) effective for styrene and methyl methacrylate systems. All four types of the RAFT agents effectively control over the bulk polymerization. In contrast, only BDIB resulted in a rather narrow molecular weight distribution in the miniemulsion polymerization. A pronounced inhibition and rate retardation were observed in both bulk and miniemulsion polymerizations mediated by CDIB and benzyl dithiobenzoate. When compared with the bulk polymerization, a much longer inhibition period (over eight times) was observed in the CDIB-mediated miniemulsion polymerization. It was concluded that only the RAFT agent with the primary R group and Z group with less stabilizing ability to the intermediate radicals is effective to mediate BA miniemulsion polymerization in terms of achieving a narrow molecular weight distribution and short inhibition period. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 2304–2315, 2007     

Stable free radical miniemulsion polymerization

Stable emulsions of polystyrene have been prepared by nitroxide-mediated living-radical miniemulsion polymerization. Optimization of the ratios and amounts of surfactant, cosurfactant, and nitroxide provided both good latex properties and good living-radical chemistry. Results were superior to those obtained in comparable bulk polymerizations. Conversions of 87% were obtained in 6 hours at 135°C without the use of a rate accelerant and the molecular weight distributions were typically in the 1.1 range.     

Macromolecular surfactants for miniemulsion polymerization

The use of polymeric surfactants as stabilizers in miniemulsion polymerization was reviewed. The structural characteristics of reported polymeric surfactants were detailed and compared. The concept of multi-functional polymeric surfactants was evidenced. The specificities brought by polymeric surfactants in the process of miniemulsion polymerization in comparison to molecular surfactants were analysed for the stability of the initial monomer emulsion, polymerization kinetics and characteristics of the obtained latexes. The contribution of polymeric surfactants to the control of the characteristics of the obtained nanoparticles was detailed with regard to the nature of the core material and to the surface coverage. Polymeric surfactants can be seen as powerful tools for the design of original nanoparticles. On the basis of the available data, possible research topics are suggested.     

Recent advances in propylene polymerization with MgCl2 supported catalysts

The role of Lewis bases in MgCl₂ supported catalysts for olefin polymerization is a subject of continuous interest and discussion in order to obtain more and more active and stereospecific catalysts and to explain their stereoregulating mechanism. Through molecular calculation and conformational analysis it was possible to identify chelating diethers that have the correct oxygen-oxygen distance necessary to tightly coordinate with the Mg ions of the support, even in the presence of other strong Lewis acids, and unable to give secondary reactions with TiCl₄, AlR₃, Ti-C and Ti-H bonds. The use of these donors has allowed the synthesis of catalytic systems that are both highly active and stereospecific even in the absence of external donors. Kinetic data of propylene polymerization with these catalyst systems are reported. The importance of the distance between the donor atoms in bifunctional Lewis bases has been proved also in the case of new classes of internal donors. Molecular modelling studies have enabled us to formulate models of active sites, located on some corners of MgCl₂ crystallites, whose chirality is induced by the presence of a donor molecule in their environment. These models could explain, at least in part, the exceptional increase of isotactic polymer productivity observed for stereospecific catalyst systems, containing only the internal donor, with respect to catalysts lacking the Lewis base and could account for the influence of the donor on the molecular properties of the obtained polymers.     

Nitroxide mediated living radical polymerization in miniemulsion

The introduction of the aqueous phase into a living radical polymerization increases the complexity of the kinetics by creating the possibility of species partitioning between the aqueous and organic phases, and introducing aqueous phase reactions which could play a significant role particularly in chain initiation and/or particle nucleation. We have conducted a series of styrene miniemulsion polymerizations in which the solubility of initiator and nitroxide have been systematically varied. Experiments were run using either water-soluble (potassium persulphate) or oil-soluble (benzoyl peroxide) initiator, and either TEMPO or 4-hydroxy-TEMPO. These two nitroxides vary considerably in their water solubility. The effects of initiator and nitroxide solubility in water on conversion-time behaviour, molecular weight and initiator efficiency are presented.     

Microwave-assisted nitroxide-mediated miniemulsion polymerization of styrene

Nitroxide-mediated free-radical miniemulsion polymerizations (NMRPs) of styrene were successfully performed under microwave irradiation at 135 °C. The polymerizations proceeded in a controlled manner, yielding polymers that showed an incremental increase in molecular weight with conversion and had narrow molecular weight distributions. The resulting latexes were colloidally stable. The polymerization behavior, molecular weights of polymers and Z-average size of latex particles were also investigated under two different heating methods, microwave irradiation and conventional heating.     

Recent progress in nitroxide-mediated polymerizations in miniemulsion

Recent developments in nitroxide-mediated polymerizations conducted in emulsion and miniemulsion have advanced the field across a range of both experimental and theoretical fronts. This article reviews progress in bicomponent initiating systems (including use of camphorsulfonic acid to enhance rate), unimolecular initiating systems, miniemulsions not requiring the use of volatile costabilizers, polymerization of acrylates, mathematical modeling and simulation, and theoretical understanding with regards to issues such as compartmentalization, preservation of polymer chain livingness, the role of aqueous phase kinetics and phase partitioning. These topics are discussed and analyzed to present an integrated portrait of the current status of nitroxide-mediated polymerizations in emulsion/miniemulsion and to identify the most pressing concerns, issues, and opportunities. To cite this article: M.F. Cunningham, C. R. Chimie 6 (2003).     

Recent advances in controlled/living radical polymerization in emulsion and dispersion

Effective ways to conduct controlled/living radical polymerization (CRP) in emulsion systems are necessary for commercial latex production without significant modification of current industrial facilities. Conducting CRP in emulsion media is more complicated and more challenging than its application in homogeneous bulk. These challenges come from the intrinsic kinetics of emulsion polymerization. They include mass transport, slow chain growth mechanism, and exit of short radicals from polymeric particles. This review describes the recent developments of CRP in heterogeneous dispersion, including miniemulsion, microemulsion, dispersion, and especially emulsion. Various approaches for conducting emulsion CRP are detailed, including controlled seeded emulsion polymerization, nanoprecipitation, use of short oligomers as macroinitiators for in situ block copolymerization, and RAFT‐mediated self‐assembly. In addition many remaining challenges of the current methods barring wide spread industrial application of emulsion CRP are also suggested. © Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 6983–7001, 2008     

High-performance fluorescent particles prepared via miniemulsion polymerization

Highly fluorescent polymer particles were prepared with Eu beta-diketonates complex as a fluorophore by miniemulsion polymerization technique. Eu beta-diketonates complex has a long decay time, a large Stokes shift, and very narrow emission bands in comparison with other organic fluorescent compounds. Aqueous miniemulsion was prepared by mixing monomer, crosslinker, hydrophobe, and Eu beta-diketonates complex and then putting the mixture into an aqueous solution of surfactant, followed by ultrasonication. An aqueous solution of initiator was added to the miniemulsion to obtain fluorescent polymer particles, which were monodispersed without aggregation. Particle size was decreased to deca-nano scale by increasing the amount of surfactant. Fluorescent intensity was increased by using Eu beta-complex coordinated with additional ligand. Further fluorescence quantum yields and fluorescent properties in the presence of DNA were investigated to the confirm superiority of Eu beta-diketonates complexes in polymer particles.     

Encapsulation of inorganic particles via miniemulsion polymerization

The encapsulation of TiO₂ particles via miniemulsion polymerization is strongly dependent on the size and stability of the inorganic particles in the monomer medium in which they are initially dispersed. It was found from XPS and FT‐IR studies that both the hydrophilic and hydrophobic TiO₂ particles, which were studied, have hydroxyl groups present on their surfaces, which can strongly interact with the amine end‐groups of the polymeric stabilizer, OLOA370 (polybutene‐succinimide diethyl triamine). It was found from the dispersion and adsorption studies that the amount of OLOA370 retained on the TiO₂ particles is strongly dependent on the area exposed by the sonification that is applied to break up the aggregates in the dispersion process. The TiO₂ dispersions in styrene monomer were themselves dispersed as miniemulsion droplets and subsequently polymerized. It was concluded from the density gradient column (DGC) analysis of the latexes obtained from the encapsulation polymerizations, that the stability of the inorganic particles in the monomer, as well as their particle size, significantly influence the encapsulation efficiencies. The use of the hydrophilic titanium dioxide particles in combination with the stabilizer, OLOA370, resulted in a good dispersibility, dispersion stability, and small TiO₂ particle size. This lead to better encapsulation efficiencies compared to the hydrophobic particles. The poorer results obtained with the hydrophobic TiO₂ particles were attributed to their larger particle size, which resulted from the reduced adsorption of the OLOA370. Fewer hydroxyls and the presence of the trimethoxy octyl silane (TMOS) groups, which themselves are unable to provide sufficient steric stability, are proposed to explain these findings.     

Encapsulation of silica nanoparticles by miniemulsion polymerization

Hybrid silica/polystyrene nanoparticles were synthesized by miniemulsion polymerization. With the objective to prepare core‐shell hybrid nanoparticles having narrow particle size distributions (PSDs) as well as a high degree of silica encapsulation, the effect of adding surface modifiers, the size of silica nanoparticles, the ratio styrene/silica, the surfactant concentration, and the presence of ethanol in the reaction mixture were studied. A synergistic effect was observed using oleic acid (OA) together with 3‐(trimethoxysilyl)propyl methacrylate (TPM) in the compatibilization step between the organic phase (monomer) and inorganic nanoparticles (silica). Mono and multinuclear eccentric core‐shell hybrid nanoparticles were obtained. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 935–948, 2009     

Preparation of polyurethane dispersions by miniemulsion polymerization

With a two‐step miniemulsion polymerization, hydrophobic polyurethane (PU) dispersions were prepared with a cosurfactant, the costabilizer hexadecane (HD) in the oil phase, and sodium dodecyl sulfate (SDS) in the water phase. The first step involved the formation of NCO‐terminated prepolymers between isophorone diisocyanate and poly(propylene glycol) oligomer in toluene. Next, PU dispersions were produced by a miniemulsion method in which an oil phase containing NCO‐terminated prepolymers, HD, the chain extender 1,4‐butanediol (BD), the crosslinking agent trimethylol propane (TMP), and the catalyst dibutyltin dilaurate was dispersed in the water phase containing SDS. The influence of experimental parameters, such as the ultrasonication time, concentrations of SDS and HD, and TMP/BD and NCO/OH equivalent ratios, on the sizes of the miniemulsion droplets and polymer particles, as well as the molecular weights and thermal properties of the PU polymer, was examined. The chemical structure of the produced PU polymer was identified with a Fourier transform infrared spectrometer. The molecular weight distribution and average particle size were measured through gel permeation chromatography and dynamic light scattering, respectively. The thermal stability of the PU polymer was characterized with thermogravimetric analysis. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 4870–4881, 2005     

Recent advances in the field of diolefin polymerization with transition metal catalysts

Recent advances in the field of catalysis for 1,3-diene polymerization and in the interpretation of the polymerization mechanism are examined. Catalysts prepared from methylaluminoxanes and soluble transition metal compounds are in general more active than the analogous systems prepared from AlR₃. With some catalysts, however, (e.g. lanthanide systems) a high activity is obtained only when transition metal compounds containing preformed metal-carbon bonds are used. Methylaluminoxanes affect also the stereospecificity of the polymerization. Active and stereospecific systems are obtained from monocyclopentadienyl derivatives of Ti and aluminoxanes. Recent views on the factors that determine stereospecificity are examined. Schemes are presented for the formation of iso- or syndiotactic polymers, with 1,2, cis-1,4 or trans-1,4 structure, from various dienes.