Miniemulsion and Microemulsion Polymerization

From a synthetic point of view, emuision poiymerization is not suitabie for all monomers. For monomers that are highly water-soluble or, on the other hand, almost insoluble in water, the standard emulsion polymerization technique is not suitable. For water-soluble monomers, besides emulsion polymerization, aqueous phase polymerization can also occur, in which case one could resort to inverse emulsion polymerization, whereby water droplets containing the monomer are polymerized in an oil phase. 

In the case of monomers with low water solubility, another problem arises. In emulsion polymerizations, transport of monomer from monomer droplets to the growing polymer particles is needed, which demands a minimum water solubility of the monomer. For example dodecylmethacrylate (water solubility of 0.065 mmol/L) cannot be polymerized by conventional emulsion polymerization. Another reason for hydrophobic monomers to polymerize slowly in emulsion polymerization could be that entry of radicals is slow because the oligomers do not grow to their critical chain length [32]. Another solution to this problem is to directly polymerize within the monomer droplets, which have to be very small in order to keep the benefits of producing polymer in the form of latex. In contrast to emulsion polymerization, where the droplets are of the same size as those in suspension polymerization (10-100pm), in mini- and microemulsion polymerization the droplets are very much smaller and enable the polymerization to take place directly within the monomer droplets. 

In miniemulsion polymerization, the droplets are in the range from 50 to 500 nm. A combination of a surfactant (e.g. SDS) and a hydrophobe or costabilizer (for example, a long chain alkane or alcohol) is used. The droplets are formed using devices like ultrasonifiers, homogenizers or even static mixers. The miniemulsions are thermodynamically unstable and therefore are only stable for a limited period of time, ranging from hours to days. 

In principle, polymerization proceeds in the monomer droplets and the final particle number is close to the initial number of monomer droplets. However, in many cases not all droplets are initiated to become polymer particles, but only a fraction ( 20%) of the initial number of monomer droplets. This effect is related to Ostwald ripening and often a hydrophobe is added in the recipes to prevent this from happening. 

The miniemulsion process is also very suitable for the preparation of hybrid latex particles. One can have the inorganic particles already present in the droplets, then the polymerization reaction results in encapsulated nanoparticles (see [33]). 

---

In the last few years, many efforts have been given to the preparation of magnetic latexes in dispersed media using suspension, precipitation, dispersion, emulsion, miniemulsion and microemulsion polymerizations. In this review chapter, the synthesis and functionalization of magnetic core-shell polymer particles in dispersed media have been reviewed with the main focus on emulsion polymerization. 

In the second chapter (Preparation of polymer-based nanomaterials), we summarize and discuss the literature data concerning of polymer and polymer particle preparations. This includes the description of mechanism of the radical polymerization of unsaturated monomers by which polymer (latexes) dispersions are generated. The mechanism of polymer particles (latexes) formation is both a science and an art. A science is expressed by the kinetic processes of the free radical-initiated polymerization of unsaturated monomers in the multiphase systems. It is an art in that way that the recipes containing monomer, water, emulsifier, initiator and additives give rise to the polymer particles with the different shapes, sizes and composition. The spherical shape of polymer particles and the uniformity of their size distribution are reviewed. The reaction mechanisms of polymer particle preparation in the micellar systems such as emulsion, miniemulsion and microemulsion polymerizations are described. The short section on radical polymerization mechanism is included. Furthermore, the formation of larger sized monodisperse polymer particles by the dispersion polymerization is reviewed as well as the assembling phenomena of polymer nanoparticles. 

Finally, miniemulsion and microemulsion polymerization are two subcategories of emulsion polymerization with monomer droplets in water with much smaller droplets than in emulsion polymerization (about 50-1,000 nm in miniemulsion polymerization and 10-100 nm in microemulsion process, compared to 1-100 mm in diameter) [19, 20]. Water-insoluble costabilizers such as hexadecane and cetyl alcohol are present along with the surfactant to stabilize the monomer droplets against diffusional degradation [16]. 

FIGURE 54.23 A schematic representation of inverse miniemulsion or microemulsion polymerization for the preparation of nanometer-sized particles of water-soluble and water-swellable polymers as well as cross-linked particles in the presence of cross-linkers. (Reprinted from Polymer, 50(19), Oh, J.K., Bencherif, S.A., and Matyjaszewski, K., Atom transfer radical polymerization in inverse miniemulsion A versatile route toward preparation and functionalization of microgels/nanogels for targeted drug delivery applications, 4407-4423. Copyright 2009, with permission from Elsevier.)... 

Dispersed polymers are also produced by inverse emulsion polymerization, miniemulsion polymerization, dispersion polymerization and microemulsion polymerization. 

From a mathematical point of view, limiting cases of macroemulsion polymerization are mini- and microemulsion polymerizations. In miniemulsion polymerization, only small monomer droplets are present and these are also the main reaction locus. In microemulsion polymerization, the monomer droplets are also small and, in principle, reaction can take place in the monomer droplets as well as in the micelles and polymer particles. An important feature of a microemulsion is that it is thermodynamically stable, whereas the other emulsion types are only kinetically stable. However, if monomer is added very slowly and a small amount of surfactant is present, polymer particles gradually swell starting from a micelle population only. Thus, the emulsion polymerizations differ with respect to the populations present, but in all cases the latex obtained consists of segregated entities with a size at least one order of magnitude smaller than in suspension polymerization. In the most... 

A broad range of polymers are produced by polymerization in heterogeneous media, including polyolefins manufactured by slurry (high density polyethylene and isotactic polypropylene) and gas phase (linear low density polyethylene and high density polyethylene) polymerization coatings and adhesives produced by emulsion and miniemulsion polymerization flocculants obtained by inverse emulsion and microemulsion polymerization poly(vinyl chloride) (PVC) and polystyrene produced by suspension polymerization and toners synthesized by dispersion polymerization. As a whole, they represent more than 50% of the polymer produced worldwide [1]. 

FIGURE 4.1 Schematic representation of microemulsion, emulsion, miniemulsion, and suspension polymerization. 

A typical styrene miniemulsion polymerization process using cetyl alcohol or hexadecane as the costabilizer does not show a constant reaction rate period and it can be divided into four major regions based on the polymerization rate versus monomer conversion curve [11, 47], as shown schematically in Figure 5.4b. For comparison, the polymerization rate versus conversion profiles for conventional emulsion polymerization (Figure 5.4a) and microemulsion polymerization (Figure 5.4c) are also included in this figure. First, the rate of miniemulsion polymerization increases rapidly to a primary maximum and then decreases with increasing monomer conversion. This is followed by the increase of polymerization rate to a secondary maximum. After the secondary maximum is achieved, the rate of polymerization then decreases rapidly toward the end of polymerization [47]. 

The radical polymerization in disperse systems may be divided into several types according to the nature of continuous phase and the polymerization loci the dispersion, emulsion, miniemulsion, microemulsion, suspension, etc. 

There are four main types of liquid-phase heterogeneous free-radical polymerization microemulsion polymerization, emulsion polymerization, miniemulsion polymerization and dispersion polymerization, all of which can produce nano- to micron-sized polymeric particles. Emulsion polymerization is sometimes called macroemulsion polymerization. In recent years, these heterophase polymerization reactions have become more and more important... 

Synthesis of polymer microspheres in the presence of magnetic nanoparticles, such as suspension polymerization or its modified versions, dispersion polymerization, surface-initiated radical polymerization, acid-catalyzed condensation polymerization, emulsion polymerization, mini-/microemulsion polymerization, in situ oxidative polymerization, inverse emulsion cross-linking, emulsion/double emulsion-solvent evaporation, and supercritical fluid extraction of o/w miniemulsion... 

However, in the case of miniemulsion, processing methods reduce the size of the monomer droplets closer to the size of the micelle, leading to significant particle nucleation in the monomer droplets (73) and therefore lack of dependence on monomer tranpsort across the aqueous phase. Intense agitation, cosurfactant, and dilution are used to reduce monomer droplet size. Additives such as cetyl alcohol are used to retard the diffusion of monomer from the droplets to the micelles, in order to further promote monomer droplet nucleation (74). The benefits of miniemulsions include inclusion of hydrophobic moieties in latex particles (75), faster reaction rates (76), improved shear stability, and the control of particle-size distributions to produce high solids latices (77). Microemulsion Polymerization (qv) (78) employs very high surfactant levels to make very small latex particles. 

Upon addition of the initiator, the polymerization reaction proceeds, the characteristics of the products depending on the initial monomer dispersion. Thus, in the case of the microemulsion polymerization, small particles (20-60nm) are formed emulsion and miniemulsion polymerizations lead to polymer dispersions of similar size (most often 80-3(X)nm), but miniemulsion polymerization allows the production of composite particles not attainable otherwise. Suspension polymerization yields relatively large particles (50-1000pm). 

Figure 1.4. Initial conditions for (a) the conventional emulsion polymerization, (b) miniemulsion polymerization with the surfactant concentration lower than its critical micelle concentration, and (c) microemulsion polymerization. O (>10 nm in diameter for the conventional emulsion polymerization and <10 nm in diameter for miniemulsion polymerization) and ( 10 nm in diameter) represent emulsified monomer droplets and monomer-swollen micelles, respectively.

In summary, there has been a tremendous effort devoted to the fundamental aspects of emulsion polymerization mechanisms, kinetics and processes since the early twentieth century. Representative review or journal articles concerning the conventional emulsion polymerization can be found in literature [20-25, 48-60]. The research areas related to both miniemulsion [42,61-64] and microemulsion [44-47, 65] polymerizations have received increasing interest recently. 

---