Miniemulsion polycondensation

In contrast to the process of creating a secondary dispersion as was used for the preparation of, e.g., polyurethanes and epoxide resins, it was shown that the miniemulsion polymerization process allows one to mix monomeric components together, and polyaddition and polycondensation reactions can be performed after miniemulsification in the miniemulsified state [125]. 

Crespy D, Stark M, Hoffmann-Richter C, Ziener U, Landfester K (2007) Polymeric nanoreactors for hydrophilic reagents synthesized by interfacial polycondensation on miniemulsion droplets. Macromolecules 40(9) 3122-3135... 

Barrere and Landfester [184] prepared a hybrid miniemulsion in which isophorone diisocyanate was condensation polymerized with dodecanediol to form polyurethane at the same time that the polystyrene or polyBA was free radical polymerized. Unlike previous work, the polyurethane was not prepared in organic solvent in advance. Therefore, in this one-pot synthesis, polyaddition and free radical polymerization both take place in the same particle. HD was used as the costabihzer. After miniemulsification, the polycondensation was allowed to take place, and then a free radical initiator was added to polymerize the styrenic or acrylic monomer. Molecular weight distributions were bimodal the PU had a substantially lower molecular weight than the polyacrylate. Neither intra- nor interparticle phase separation could be detected by TEM the particles appeared to be homogeneous. No measurements of grafting were made, but since there was no unsaturation in the PU, none was expected. 

A. D. Crespy, M. Stark, C. Hoffmann-Richter, U. Ziener, K. Landfester, Polymeric nanoreactors for hydrophilic reagents synthesized by interfacial polycondensation on miniemulsion droplets. Macromolecules, 2007, 40, 3122 b) O. Gazit, R. Khalfln, Y. Cohen, R. Tannenbaum, Self-assembled diblock copolymer nanoreactors as catalysts for metal nanoparticle synthesis, J. Phys. Chem. C, 2009, 113, 576. 

Torini L, AigiUier JF, Zydowicz N (2005) Interfacial polycondensation encapsulation in miniemulsion. Macromolecules 38 3225-3236... 

When an oil-soluble monomer and a water-soluble monomer reside in their respective oil and aqueous phase, respectively, they react at the miniemulsion droplet surface, thereby forming the nanocapsules. Torini et al. have shown that interfacial polycondensation of isophorone diisocyanate (IPDI) and 1,6-hexanediol in cyclohexane-water mini ulsions stabilized with SDS resulted in stable polyurethane nanocapsules with a diameter of around 200 nm and narrow size distribution. A critical SDS concentration higher than 1 wt%, relative to the organic dispersed phase, is required to ensure the stability of the miniemulsion and nanocapsules. Hexadecane was used as costabilizer, and the surfactant-to-costabilizer ratio, varied between 1 1 and 1 3, has no influence on the size and size distribution of the miniemulsion and nanocapsules. The nanocapsules obtained in such conditions are stable over 18 months, without any increase in the z-average diameter and the zcto-potential. 

Hydrophilic materials can be encapsulated with the inverse minianulsions by using interfacial polymerization such as polyaddition and polycondensation, radical, or anionic polymerization. Crespy et al. reported that silver nitrate was encapsulated and subsequently reduced to give silver nanoparticles inside the nanocapsules. The miniemulsions were prepared by anulsilying a solution of amines or alcohols in a polar solvent with cyclohexane as the nonpolar continuous phase. The addition of suitable hydrophobic diisocyanate or diisothiocyanate monomers to the continuous phase allows the polycondensation or the cross-linking reactions to occur at the interface of the droplets. By using different monomers, polyurea, polythiourea, or polyurethane nanocapsules can be formed. The waU thickness of the capsules can be directly tuned by the quantity of the reactants. The nature of the monomers and the continuous phase are the critical factors for the formation of the hollow capsules, which is explained by the interfacial properties of the systan. The resulting polymer nanocapsules could be subsequently dispersed in water. 

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. 

Fig. 13 Formation of polymer shells by interfacial reactions, shown here with an inverse miniemulsion. Left Aqueous dispersion containing a Upophobe (which can also be a functional molecule, e.g., Gd(DTPA) [78]) and monomer A (e.g., diamine, diol) in an organic solvent (e.g., cyclohextme). Middle Solution of monomer B (e.g., diisocyanate) in the same organic solvent used for preparation of the miniemulsion is added. Right Polymeric shell is generated by interfa-cial polycondensation of monomers A and B...

The aim of this chapter is to describe the possibilities offered by the miniemulsion process for performing chain polymerization, polyaddition, polycondensation, and modifications of polymers, and to outline the current trends in this field of research. Whilst the different polymerization types performed in miniemulsion are discussed in detail, descriptions of so-called secondary or artificial miniemulsions (i.e., miniemulsions with a preformed polymer) will not be included at this point... 

Polyaddition and polycondensation reactions usually lead to functional polymers, since the polymers produced are terminated with reactive functional groups. A higher degree of functionality is easily affordable if monomers with additional reactive groups are used that do not participate in the step-growth polymerization. In emulsion polymerizations, neither polyaddition nor polycondensation reactions can be carried out consequently, the miniemulsion technique is of special interest as no diffusion of the monomers takes place. The first polyaddition in miniemulsion were performed in 2000, with the reaction of polyepoxides and hydrophobic diamines, bisphenols, and dimercaptanes [105]. Stable latexes of epoxy resins could be obtained, and apparent molecular weights of up to 20 000 g mol were measured. 

Polyaddition or polycondensation at the droplet/continuous phase interface leads to the formation of nanocapsules. Polystyrene-polyurea core-shell partides were prepared by miniemulsifying the styrene and hydrophobic diisocyanate monomers, followed by the addition of a diamine to the miniemulsion and the radical polymerization of styrene [122]. The presence of a polyurea shell was shown to prevent the migration of an encapsulated dye. Torini et al. carried out the reaction between a diisocyanate dissolved in oil and a diol dissolved in water, added following miniemulsification of the first monomer in the aqueous continuous phase [123]. 

PU nanoparticles have also been synthesized using several techniques such as suspension-polycondensation [77], interfacial polycondensation and concomitant anulsification [78], suspension polyaddition [79], and dispersion in organic solvent using supercritical carbon dioxide [80,81]. The preparation of PU nanoparticles via miniemulsion techniques was also reported. 

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