Micelle polymerization

Flammouda A, Gulik T and Piieni M-P 1995 Synthesis of nanosize latexes by reverse micelle polymerization Langmuir 3656-9... 

Water-soluble initiator is added to the reaction mass, and radicals are generated which enter the micelles. Polymerization starts in the micelle, making it a growing polymer particle. As monomer within the particle converts to polymer, it is replenished by diffusion from the monomer droplets. The concentration of monomer in the particle remains as high as 5—7 molar. The growing polymer particles require more surfactant to remain stable, getting this from the uninitiated micelles. Stage I is complete once the micelles have disappeared, usually at or before 10% monomer conversion. 

In suspension polymerization, the monomer is agitated in a solvent to form droplets, and then stabilized through the use of surfactants to form micelles. The added initiator is soluble in the solvent such that the reaction is initiated at the skin of the micelle. Polymerization starts at the interface and proceeds towards the center of the droplet. Polystyrene and polyvinyl chloride are often produced via suspension polymerization processes. 

Hirai T, Watanabe T, Komasawa I (2000) Preparation of semiconductor nanoparticle-polymer composites by direct reverse micelle polymerization using polymerizable surfactants. J Phys Chem B 104 8962-8966... 

The comonomer distribution can be alternated by controlling the synthesis conditions, such as the copolymerization at different reaction temperatures at which the thermally sensitive chain backbone has different conformations (extended coil or collapsed globule). In this way, hydrophilic comonomers can be incorporated into the thermally sensitive chain backbone in a more random or more segmented (protein-like) fashion. On the other hand, short segments made of hydrophobic comonomers can be inserted into a hydrophilic chain backbone by micelle polymerization. One of the most convenient ways to control and alternate the degree of amphiphilicity of a copolymer chain, i.e., the solubility difference of different comonomers in a selective solvent, is to use a thermally sensitive polymer as the chain backbone, such as poly(N-isopropylacrylamidc) (PNIPAM) and Poly(N,N-diethylacrylamide) (PDEA). In this way, the incorporation of a hydrophilic or hydrophobic comonomer into a thermally sensitive chain backbone allows us to adjust the degree of amphiphilicity by a temperature variation. 

If the surfactant concentration in a macroemulsion is greatly increased, or if the monomer concentration is greatly reduced, a microemulsion results. Microemulsions are thermodynamically stable systems in which all of the monomer resides within the micelles. At high surfactant concentration, the micelles may form a bicontinuous network, rather than discrete micelles. Polymerization (with water- or oil-soluble initiator) of the monomer within a microemulsion is referred to as microemulsion polymerization. The particles produced in this way are extremely small, ranging from 10 to 100 nm. 

Nanoparticles, 10-1000 nm polymeric particles, are prepared from the same natural and synthetic biodegradable polymers as microspheres. ° Albumin nanoparticles are prepared by the cross-linking processes mentioned previously. For the preparation of particles from synthetic polymers, heterogeneous bulk polymerization techniques of suspension, emulsion, and micelle polymerization are often used. 

Micelle polymerization differs from emulsion polymerization in that all of the monomer and the drug is contained within micelles composed of surfactant. 

Micelles—Polymeric chains that can carry poorly soluble drugs to the target area. 

Figure 5. Clusters generated by irradiation in various hard (oxides, semiconductors, metals, zeolites, electrodes, carbon nanotubes) or soft matrices (liquids, micelles, polymeric membranes, mesophases).

Potassium dodecanoate micelles solubilize about 40 styrene molecules [SI], If only the monomer in a single micelle polymerized die molar mass of the polystyrene formed could not be significantly in excess of 4160 g mol, whereas the average molar mass determined visctmietrically was 2.25 x 10 g mol (511. Polymerization was initiated in cmly 1 in 700 micelles. After the micelles have completely dispersed the onulsion may be destabilized by loss of emulsifier from monomo- droplets although the optimum emulsifier concentration is sufficient to keep the emulsion stable until all the residual mcaiomer is absorbed into the latex particles at the end of Interval n. 

As a result, a vast number of nano-carriers have been used for the tumour-targeted delivery of active principles they include liposomes," " micelles," " polymeric nanoparticles," " " solid lipid nanoparticles (SLN)," dendrimers, nanoshells, and magnetic nanoparticles. " Examples of nano-carriers with proven clinical efficacy are reported in Table 12.2. 

Effects of sodium dodecyl sulfate (SDS) on the electromagnetic properties of PANI/y-Fe Oj nanocomposites, prepared by using the reverse micelle polymerization where aniline, ferrous/ferric salts, and SDS act as monomer, precursor of y-Fe O and surfactant, respectively, were investigated by Hsieh et al. [188] (Figure 2.18). It was showed that the y-Fe O content and particle size in the PANI/y-Fe Oj nanocomposites decreased, crystallinity, conductivity, and dielectric properties (i.e., permittivity and... 

The emulsion process allows the production of particles with a diameter in the nanometer range (10-10 nm). The monomer is dispersed in water with a water-soluble initiator and a surfactant which forms uniform micelles. Polymerization occurs inside micelles, in which the monomer is able to diffuse, because the initiator is not miscible in the dispersed monomer phase. 

Initiator decomposes in the aqueous phase, and the radicals diffuse into monomer micelles. Polymerization begins, increasing the size of the particles as new monomer diffuses in. Additional surfactant diffuses to the surface as the particles grow. 

Keywords Block copolymers Polymer-drug conjugates Polymeric micelles Polymeric vesicles... 

The monomer droplets and the micelles swollen with monomer compete for the free radicals generated in the aqueous phase, but since there are many more micelles than droplets in the system most of the free radicals enter micelles. Polymerization is initiated within individual micelles. The monomer consumed during the resulting polymerization is replenished by diffusion of new monomer molecules from the aqueous phase, which in turn, is kept saturated with monomer from the droplets of monomer. Polymerization continues within a given micelle until a second free radical enters the micelle, in which case termination quickly occurs because of the small volume of the reaction locus. The micelle then remains inactive until a third free radical enters, and so on. As reaction proceeds the micelles become larger and are disrupted to form particles of polymer swollen with monomer which are stabilized by soap molecules... 

Compared to surfactant micelles, polymeric micelles have a much higher thermodynamic stability. It is well-known that micelles have a critical micelle concentration (CMC) below which only unimers exist but above which both micelles and unimers are present. It was reported that polymeric micelles have a CMC around 10 -10 M [58,59], which is 1000-fold lower than that of surfactant micelles (10 -10 ). The CMC of polymeric micelles is affected by many factors, including the properties of the core-forming blocks, such as hydrophobicity, the glass transition temperature (Tg), the degree of crystallinity, and the length and ratio of the hydrophilic and hydrophobic... 

Scholz, C., lijima, M., Nagasaki, Y., Kataoka, K., 1995, A novel reactive polymeric micelle. Polymeric micelle with aldehyde groups on its surface. Macromolecules 28 7295-7297. 

Emulsion polymerization is similar to suspension polymerization in the sense that the reaction also takes place in the presence of a water phase and the applied monomer forms a second liquid phase. However, in this case the added radical initiator is not soluble in the monomer droplets but in the water phase. To allow the monomer to come into contact with the initiator an emulsifier is added to the reaction mixture that creates micelles in the systems. By diffusion processes both monomer molecules and initiator molecules reach the micelle. Polymerization takes places and a polymer particle suspended in the water phase forms that is much smaller than the original monomer droplet (see Figure 5.3.12 for a graphical illustration of these steps). At the end of the overall emulsion polymerization process, all monomer droplets have been consumed by the polymerization reaction in the micelles. Typical emulsifiers for emulsion polymerization are natural or synthetic detergents, such as, for example, sodium palmitate or sodium alkyl sulfonates. Emulsion polymerization is very versatile and is applied for many polymers [e.g., PVC, styrene copolymers, poly(methacryl esters)] in batch, semi-continuous, and continuous processes. In some cases, the obtained polymer particles in water are directly applied as technical products for coatings, lacquer applications, or as adhesives. In other cases the formed product is further treated to obtain the dry polymer. Note that the aqueous phase in emulsion polymerization always contains some isolated emulsifier and also some monomer. Moreover, the formed polymer contains the emulsifier as impurity. 

The preparation of waterborne latices by emulsion polymerization usually employs polymerization in aqueous micellar surfactant solutions. The simplest manifestation of the process involves the presence in an aqueous medium of emulsified monomer drops, micellar surfactant, and a water-soluble polymerization (free-radical) initiator. A combination of monomers is often used, exemplified by combinations of compounds such as methyl methacrylate, butyl acrylate, and styrene. Typically monomers have only slight water solubility. The classic qualitative picture of this process was described by Harkins [2] more than 60 years ago. A schematic of that process is shown in Figure 9.1. The key step is solubilization of the monomers in the micelles where polymerization is initiated. Both the low solubility of the monomer and the relatively low surface area of the monomer emulsion drops means that initiation of polymerization is essentially confined to the micelles. Polymerization now proceeds as more monomer is transported to the swelling micelles from the... 

To produce monodisperse nanoparticles vsdth very small sizes, high surfactant concentrations (up to 10-15 wt%) are usually required. However, colloidal crystal assemblies require a surfactant-free polymer surface. Surfactant-free emulsion polymerization processes have been developed for PS and a number of acrylate-based polymers [15-21]. In this process a poorly water-soluble monomer and water-soluble initiator, usually peroxodisulfate, are dispersed in water. Polymerization is initiated by the sulfate radical and charged oligomers are formed, which, when above the critical micelle concentration, aggregate into micelles. Polymerization further takes place in these micelles, resulting in monodisperse polymer nanopartides on the scale of the original micelles. 

---