Nanoreactors polymer-stabilized

Graf, A., Winterhalter, M., and Meier, W. (2001). Nanoreactors from polymer-stabilized liposomes. Langmuir, 17, 919-23. 

Fig. 20 Diagram of polymer-stabilized nanoreactor with encapsulated enzyme. Reprinted with permission from [21]. Copyright 2001, American Chemical Society...

The reaction described in this example is carried out in miniemulsion.Miniemulsions are dispersions of critically stabilized oil droplets with a size between 50 and 500 nm prepared by shearing a system containing oil, water,a surfactant and a hydrophobe. In contrast to the classical emulsion polymerization (see 5ect. 2.2.4.2), here the polymerization starts and proceeds directly within the preformed micellar "nanoreactors" (= monomer droplets).This means that the droplets have to become the primary locus of the nucleation of the polymer reaction. With the concept of "nanoreactors" one can take advantage of a potential thermodynamic control for the design of nanoparticles. Polymerizations in such miniemulsions, when carefully prepared, result in latex particles which have about the same size as the initial droplets.The polymerization of miniemulsions extends the possibilities of the widely applied emulsion polymerization and provides advantages with respect to copolymerization reactions of monomers with different polarity, incorporation of hydrophobic materials, or with respect to the stability of the formed latexes. 

Miniemulsion is a special class of emulsion that is stabilized against coalescence by a surfactant and Ostwald ripening by an osmotic pressure agent, or costabilizer. Compared with conventional emulsion polymerization process, the miniemulsion polymerization process allows all types of monomers to be used in the formation of nanoparticles or nanocapsules, including those not miscible with the continuous phase. Each miniemulsion droplet can indeed be treated as a nanoreactor, and the colloidal stability of the miniemulsion ensures a perfect copy from the droplets to the final product. The versatility of polymerization process makes it possible to prepare nanocapsules with various types of core materials, such as hydrophilic or hydrophobic, liquid or solid, organic or inorganic materials. Different techniques can be used to initiate the capsule wall formation, such as radical, ionic polymerization, polyaddition, polycondensation, or phase separation from preformed polymers. 

Figure 84 Schematic representation of mechanisms and basic reagents of a mini emulsion polymerization process. The snrfactant and co-stabilizer stabilize the nanometric droplets, which act as polymerization nanoreactors, consequently producing polymer particles with approximately an equal size to the initial monomer droplets (50-500nm). The polymer particles are obtained from the nanoreactors formed from a dispersion procedure, in which micrometric droplets were ruptured by the use of high-energy processes.

The typical nucleation mechanism coupled with high colloidal stability is achieved by the use of the stabilizing substances. The result is polymer particles with size comparable to nanoreactor precursors of the polymerization process, as shown in Fig. 8.4. Bearing this characteristic in mind, there is a need for optimal conditions of operation, mechanisms, and dispersion stability. These characteristics play a vital role on the particle morphology of the polymers obtained at the end of the process [26,34]. 

A dendrimer has a better-defined shape than ordinary polymers or surfactants and possesses dynamic inner cavities. That makes them capable of encapsulating of guests and acting as nanoreactors and nanoporous stabilizers [2,17],... 

Encapsulation of enzymes in various polymer superstructures (dendrimers, polymersomes, PICsomes, or LbL capsules) provides significant advantages, such as higher stability and preserved catalytic activity over longer periods of time than bulk conditions. The process of encapsulation/entrapment of biomolecules in various 3D supramolecular assemblies should not affect the stability of the final nanoreactor structure (which is driven by the chemical nature of the copolymer used for their formation). If necessary cross-linking procedures can be used to increase the stability. However, it is also necessary to determine if... 

The advantageous properties of polymer nanoreactors in terms of payload protection and stability make them feasible for lowering the toxicity of contrast agents, the side effects of drugs, and their uncontrolled systemic distribution. These have led to novel developments of nanoreactor applications either in the blood stream or by mimicking cell organelles [4,8,9,46,57],... 

The synthesis of palladium nanoparticles on montmorillonite layer silicates was studied. The Pd particles were prepared in situ in the interlamellar space of montmorillonite dispersed in an aqueous medium. Macromolecules were adsorbed on the support from an aqueous solution, followed by adsorption and reduction of Pd ions. The Pd° nanoparticles appear and grow in the internal, interlamellar space as well as on the external surfaces of the lamellae. Well-crystallized kaolinite clay can be disaggregated by the intercalation of DMSO to individual lamellae, which may serve as excellent supports for metal nanoparticles. After the adsorption of palladium precursor, metal nanocrystals were reduced by hydrazine or sodium borohydride between the kaolinite lamellae, i.e., in the interfacial layer acting as a nanoreactor. The incorporation of nanoparticles between the lamellae was shown hy XRD measinements. This procedure makes possible the steric control and restriction of nanoparticle growth. The stability of nanoparticles can be further enhanced hy the addition of polymers (PVP) and surfactants (alkyl-ammonium salts) that are also adsorbed between the kaolinite lamellae. The presence of the particles was also verified and their sizes were quantified by TEM measurements. 

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