Miniemulsion approach

Kim N, Sudol ED, Friedel P, El-Aasser MS (2003) Poly (vinyl alcohol) stabilization of acrylic emulsion polymers using the miniemulsion approach. Macromolecules 36 5573-5579... 

Living free-radical polymerization represents a promising technique to produce polymers with highly controlled structures. Different possible systems known from bulk polymerizations have been used in miniemulsions. The living free radical polymerization of, e.g., styrene via the miniemulsion approach allows one to eliminate the drawback of the bulk system where an increase in polydis-persity was found at high conversions due to the very high viscosity of the reaction medium [90]. 

The miniemulsion approach is extensively discussed in the contribution of Weiss and Landfester [33]. 

In the case of miniemidsion polymerization, aqueous droplets are generated by sonication of the two-phase mixture and become stabilized in organic solvent by oil-soluble surfactants. The kinetically stable emulsion is formed usually at surfaaant concentration below or near its CMC. The inverse miniemulsion approach has been successfully used for the preparation of hoUow PNIPAM miaosphaes or PNIPAM miaogels functionalized by AAc. The synthesis of miaogels based on AAc, AAm, and hydroxy ethyl methacrylate (HEMA) was reported. The size of the miaogels prepared in invase miniemulsion systems typically varies between 150 and 300 nm. 

Living polymerization in water also led to polymers with a relatively narrow molecular mass distribution (1.1-1.3) and molecular masses, which showed linear increase with conversion, indicating the living character of this polymerization [320]. Recently, Matyjaszewski et al. reported both reverse and direct ATRP of n-butyl methacrylate in an aqueous dispersed system via the miniemulsion approach, characterized by a linear increase of the molecular mass with conversion and a narrow distribution of molecular masses [321]. The suspension-type process of living polymerization of MMA in water not only led to well controlled and high molecular masses and low PDIs, but also the polymerization proceeded without the addition of Al(0-i-Pr)3 and clearly faster than ATRP in organic solvents [322]. 

Matyjaszewski, K., et al. (2000). Atom transfer radical polymerization of n-butyl methacrylate in an aqueous dispersed system a miniemulsion approach. J. Polym. ScL, Part A Polym. Chem., 35(Suppl.) 4724 734. 

One of the main disadvantages of the MMT Pickering miniemulsions approach is the low stability of the dispersion for long storage times and the low solids content achieved so far. ... 

Considering theoretically a copolymerization on the surface of a miniemulsion droplet, one should necessarily be aware of the fact that this process proceeds in the heterophase reaction system characterized by several spatial and time scales. Among the first ones are sizes of an individual block and macromolecules of the multiblock copolymer, the radius of a droplet of the miniemulsion and the reactor size. Taking into account the pronounced distinction in these scales, it is convenient examining the macrokinetics of interphase copolymerization to resort to the system approach, generally employed for the mathematical modeling of chemical reactions in heterophase systems [73]. 

Four different approaches for controlled radical polymerization have been adapted to the miniemulsion polymerization process ... 

Any colloidal material provides an intrinsically favorable accessibility to its surface when compared to bulk material. Therefore, the availability of receptor binding sites should be facilitated by using colloidal MIPs. Submicron scale MIPs were prepared by precipitation polymerization, emulsion polymerization, and miniemulsion polymerization. Precipitation polymerization uses the insolubility of the formed polymer microgel in a certain solvent, whereas emulsion and miniemulsion polymerization employ two solvent phases for the preparation of the colloidal polymer. The latter methods offer the opportunity for tailoring the surface of the colloids exclusively, thereby enhancing the accessibility of the binding sites. Each of the three approaches has their own characteristics and will be described in the following sections. 

As a new approach to the preparation of water-soluble polymers in inverse miniemulsions, a redox initiation system consisting of ceric ions and carbohydrate-based surfactant Span 60 as a reducing agent has been successfully used for the... 

As a different approach, preformed hydrophilic polymers in aqueous solution can also be used for the miniemulsification process. In this case, the formulation process should be carried out in an inverse miniemulsion with a hydrophobic continuous phase. In order to obtain microgel nanoparticles, the polymer chains have to be crosslinked in the inverse miniemulsion prior to the transfer to an aqueous continuous phase. As a nice example, gelatin has been used for the formation of microgel nanoparticles [4],... 

The synthesis of nanocapsules can best be obtained in miniemulsion using different approaches [107], One possibility is based on the phase separation process within a droplet during the polymerization [108], Here, vinyl monomers were polymerized in the presence of a hydrophobic oil. During the polymerization, the polymer becomes insoluble in the oil, leading to a phase separation. With properly chosen physicochemical properties of monomer and encapsulated material, a polymeric shell surrounding the liquid core can be formed. 

In another approach, the polymer is precipitated from the continuous phase onto on stable nanodroplets in an inverse miniemulsion [109], In this case, a miniemulsion with the liquid core material is formed in a continuous phase that consists of a mixture of a solvent and a nonsolvent for the polymer. That way, PMMA nanocapsules encapsulating an antiseptic agent could be produced. 

Abstract The subject of miniemulsion polymerization is reviewed. The approach taken is one that combines a review of the technology with historical and tutorial aspects. Rather than developing an absolutely exhaustive review, a tutorial approach has been taken, emphasizing the critical features and advantages of miniemulsion polymerization. In keeping with this tutorial approach, a discussion of conventional emulsion polymerization is included in order to be able to compare and contrast miniemulsion polymerization and conventional emulsion polymerization later in the review. Areas where miniemulsion polymerization has been adopted commercially, or where it is likely to be adopted are highlighted. 

The effects of the (water-soluble) initiator concentration on the polymerization of polymer-stabilized miniemulsion are shown in Table 2. An increase in the initiator concentration does not change the number of particles, but does increase the rate of polymerization. This is due to an increase in the number of radicals per particle. However, the number of radicals per particle ranged from just 0.5 to 0.8, indicating that the kinetics (after nucleation) are still essentially Smith Ewart Case II. The number of particles was found to be proportional to the initiator concentration raised to the power of 0.002 0.001. Macroemulsion polymerizations, in contrast, show a dependence of 0.2 and 0.4 for methyl methacrylate and styrene, respectively [141]. The fact that the exponent approaches zero indicates that all or nearly all of the droplets are being nucleated. 

The shear stabihties of mini- and macroemulsion latexes were compared and quantitatively evaluated with respect to their particle size distributions by Rodrigues and Schork [146]. Although miniemulsion latexes exhibit many of the properties of macroemulsion latexes, there may be subtle differences in particle size distribution and surface characteristics due to differences in their polymerization mechanisms. To study the effects of these differences on the shear stabihties of the miniemulsions, a quantitative approach was developed where changes in the average diameter and total number of particles have been related to the particle size distribution before and after shearing. 

Samer [104] carried out similar copolymerizations with similar results. An example of his data is given in Fig. 16. Here 2-ethylhexyl acrylate (EHA) was copolymerized with MMA in batch. The miniemulsion polymerizations (two are shown) follow the copolymer equation, while the macroemulsion polymerization gives EHA incorporation that is lower than predicted by the copolymer equation, presumably due to the low concentration of EHA at the locus of polymerization. The dotted hne in Fig. 16 is for a model derived by Samer that accurately predicts the copolymer composition. Samer derived this model by adapting the work of Schuller [149]. Schuller modified the reactivity ratios for the macroemulsion polymerization of water-soluble monomers to take into accoimt that the comonomer concentration at the locus of polymerization is different from the comonomer composition in the reactor due to the water solubilities of the monomers. Samer used the same approach to account for the fact that the comonomer concentration at the locus of polymerization might be different from that of the reactor due to transport limitations of water insoluble comonomers. 

Several approaches have recently been developed that directly apply natural architectures for artificial chanical reactions, some of which are detailed in different chapters of this book. Although not classified as homogeneous catalysis, the reduction of metal salts inside nanoreactors could be the first step on the way to reactivity with the corresponding metal coUoids or nanoparticles in e.g. hydrogenation reactions. A variety of carrier systems have been studied lately, including virus capsids, polymeric micelles, miniemulsions and hollow core-shell particles, as nanoreactors and hosts for the synthesis and encapsnlation of well-defined, stable nanoparticles. ... 

Besides the layer-by-layer technique, which can be applied with or without the use of sacrificial cores [165,166] and usually requires polyelectrolytes, the miniemulsion technique is a highly suitable and versatile method for the formation of capsule formation with sizes down to 100 nm. Even the formation of inorganic capsules (e.g., [167]) by the miniemulsion polymerization is possible. For the formation of polymeric nanocapsules, three general approaches (see Figs. 16, 17, and 23) can be distinguished ... 

---