Polymerization in inverse microemulsion

Barton J 1996 Free-radical polymerization in inverse microemulsions Prog. Polym. Sc/. 21 399-438... 

Vaskova V, Hlouskova Z, Barton J, Juranicova V (1992) Polymerization in inverse microemulsions, 4 locus of initiation by ammonium peroxodisulfate and 2,2-azoisobutyronitrile. Macromol Chem 193(3) 627-637... 

Vaskova V, Juranicova V, Barton J (1990) Polymerization in inverse microemulsions, 1. Homopolymerizations of water- and oil-soluble monomers in inverse microemulsions. Macromol Chem 191(3) 717-723... 

F. Candau, Polymerization in Inverse Microemulsions, in Comprehensive Polymer Science (eds. G. C. Eastmond, A. Ledwith, S. Russo and R Sigwalt), Vol. 4, Pergamon Press, Oxford, 1989, Chap. 13, p. 225. 

The polymerization of a water-soluble monomer such as AM, acrylic acid (AA), sodium acrylate (NaA), or 2-hydroxyethylmethacrylate (HEMA), can be carried out easily in inverse microemulsion or/and bicontinuous microemulsion. These water-soluble monomers also act as cosurfactants, increasing the flexibility and the fluidity of the interfaces, which enhances the solubilization of the monomer. A cosurfactant effect during the polymerization of vinyl acetate in anionic microemulsions has also been reported [12]. 

The polymerization of acrylamide (AM) and the copolymerization of acrylamide-sodium acrylate in inverse microemulsions have been studied extensively by Candau [10,11,13-15], Barton [16, 17], and Capek [18-20]. One of the major uses for these inverse microlatexes is in enhanced oil recovery processes [21]. Water-soluble polymers for high molecular weights are also used as flocculants in water treatments, as thickeners in paints, and retention aids in papermaking. 

Leong and Candau (18) obtained inverse latices of small size (<50nm) via photopolymerization of acrylamide in a microemulsion system of acrylamide, water, toluene and Aerosol OT. They observed that rapid polymerization and total conversion was achieved in less than 30 minutes. The microemulsions remained transparent and stable during polymerization. Candau et al. (19) also reported the results of a kinetic study of the polymerization of acrylamide in inverse microemulsions. Both oil soluble AIBN and water soluble potassium persulfate initiators were used. The rate was found to depend on the type of initiator, but in both cases neither autoacceleration nor dependence on initiator concentration was observed. An excellent review of microemulsion polymerization was published recently by Candau (20). 

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.)... 

Poly(amino acid)-based nanoparticles with different surface PEGylation were prepared. a,b-Poly(A-2-hydroxyethyl)-D,L-aspartamide (PHEAS) and PEG-modified PHEA (PHEAS-PEG) were functionalized with a methacrylate group and then polymerized by UV irradiation in inverse microemulsion. The resulting nanoparticles had a size of around 250 nm in diameter by TEM. The fluorescein-loaded PHEA-based nanoparticles were prepared in the presence of fluorescein sodium salts, and examined for cellular uptake using macrophage cells. ... 

Several methods have been reported in the scientific literature for the synthesis of microgel particles. These include emulsion polymerization (5), inverse microemulsion polymerization (6,7), and anionic copolymerization (8) (see Heterophase Polymerization). 

The photochemical UV radiation method was first employed by Leong and Candau [41] for the radical polymerization of acrylamide in inverse microemulsions stabilized by Aerosol OT. The polymerization was carried out using AIBN initiator and induced by UV irradiation. It was shown that the use of a microemulsion rather than an emulsion led to stable and clear microlatices d 50 nm) of uniform size, thus providing a way to overcome some of the problems of conventional inverse emulsion polymerization, such as instability of the latexes resulting in rapid flocculation and a broad particle size distribution. 

Candau F, Leong YS, Fitch RM (1985) Kinetic study of the polymerization of acrylamide in inverse microemulsion. J Polym Sci Polym Chem Ed 23 193-214... 

Graver MT, Hirsch E, Wittmann JC, Fitch RM, Candau F (1989) Percolation and particle nucleation in inverse microemulsion polymerization. J Phys Chem 93 4867-4873... 

Manufacturing processes have been improved by use of on-line computer control and statistical process control leading to more uniform final products. Production methods now include inverse (water-in-oil) suspension polymerization, inverse emulsion polymerization, and continuous aqueous solution polymerization on moving belts. Conventional azo, peroxy, redox, and gamma-ray initiators are used in batch and continuous processes. Recent patents describe processes for preparing transparent and stable microlatexes by inverse microemulsion polymerization. New methods have also been described for reducing residual acrylamide monomer in finished products. 

Microemulsion Polymerization. Polyacrylamide microemulsions are low viscosity, non settling, clear, thermodynamically stable water-in-od emulsions with particle sizes less than about 100 nm (98—100). They were developed to try to overcome the inherent settling problems of the larger particle size, conventional inverse emulsion polyacrylamides. To achieve the smaller microemulsion particle size, increased surfactant levels are required, making this system more expensive than inverse emulsions. Acrylamide microemulsions form spontaneously when the correct combinations and types of oils, surfactants, and aqueous monomer solutions are combined. Consequendy, no homogenization is required. Polymerization of acrylamide microemulsions is conducted similarly to conventional acrylamide inverse emulsions. To date, polyacrylamide microemulsions have not been commercialized, although work has continued in an effort to exploit the unique features of this technology (100). 

Microgels can be prepared by heterophase polymerization (free radical or controlled radical) of monomers in the presence of a crosslinking agent in aqueous phase. Heterophase polymerization techniques suitable for microgel synthesis are precipitation polymerization and inverse mini- and microemulsion. 

To synthesize water-soluble or swellable copolymers, inverse heterophase polymerization processes are of special interest. The inverse macroemulsion polymerization is only reported for the copolymerization of two hydrophilic monomers. Hernandez-Barajas and Hunkeler [62] investigated the copolymerization of AAm with quaternary ammonium cationic monomers in the presence of block copoly-meric surfactants by batch and semi-batch inverse emulsion copolymerization. Glukhikh et al. [63] reported the copolymerization of AAm and methacrylic acid using an inverse emulsion system. Amphiphilic copolymers from inverse systems are also successfully obtained in microemulsion polymerization. For example, Vaskova et al. [64-66] copolymerized the hydrophilic AAm with more hydrophobic methyl methacrylate (MMA) or styrene in a water-in-oil microemulsion initiated by radical initiators with different solubilities in water. However, not only copolymer, but also homopolymer was formed. The total conversion of MMA was rather limited (<10%) and the composition of the copolymer was almost independent of the comonomer ratio. This was probably due to a constant molar ratio of the monomers in the water phase or at the interface as the possible locus of polymerization. Also, in the case of styrene copolymerizing with AAm, the molar fraction of AAm in homopolymer compared to copolymer is about 45-55 wt% [67], which is still too high for a meaningful technical application. 

Candau and co-workers were the first to address the issue of particle nu-cleation for the polymerization of AM [13, 14] in an inverse microemulsion stabilized by AOT. They found that the particle size of the final microlatex (d 20-40 nm) was much larger than that of the initial monomer-swollen droplets (d 5-10 nm). Moreover, each latex particle formed contained only one polymer chain on average. It is believed that nucleation of the polymer particle occurs for only a small fraction of the final nucleated droplets. The non-nucleated droplets also serve as monomer for the growing particles either by diffusion through the continuous phase and/or by collisions between droplets. But the enormous number of non-nucleated droplets means that some of the primary free radicals continuously generated in the system will still be captured by non-nucleated droplets. This means that polymer particle nucleation is a continuous process [ 14]. Consequently, each latex particle receives only one free radical, resulting in the formation of only one polymer chain. This is in contrast to the large number of polymer chains formed in each latex particle in conventional emulsion polymerization, which needs a much smaller amount of surfactant compared to microemulsion polymerization. 

In this paper, we review the main results obtained so far in the field of inverse microemulsion polymerization. The role played by the monomers on the structural characteristics of the microemulsion prior to polymerization will be emphasized. The main properties of the final latices will be discussed in light of both basic and applied research. 

The main features of inverse microemulsion polymerization process have been reviewed with emphasis given to a search for an optimal formulation of the systems prior to polymerization. By using cohesive energy ratio and HLB concepts, simples rules of selection for a good chemical match between oils and surfactants have been established this allows one to predict the factors which control the stability of the resultant latices. The method leads to stable uniform inverse microlatices of water-soluble polymers with high molecular weights. These materials can be useful in many applications. 

These same workers described an inverse emulsion-type polymerization process.109 They used a conventional process in which microemulsions of water in hydrocarbon readily form in the presence of Aerosol-OT(AOT). The droplets are essentially swollen cells, where radius is controlled by the water/AOT ratio. As droplets collide, hydrophilic reagents contained in them are exchanged. When pyrrole and (NH4)2S208 dispersions were mixed, eventually a sediment appeared. However, if poly(vinyl pyrrolidone) (PVP) was added at different intervals, stable dispersions of small particles could be prepared. 

PPy nanotubes have also been synthesized by inducing polymerization in an inverse microemulsion.169 Nanotubes were synthesized by using bis(2-ethylhexyl)sul fosuccinate (20.3 mmol) in hexane (40 mL), which form tubelike or rodlike micelles. The oxidant (FeCl3(aq)) is effectively trapped in the core of the micelle. Addition of pyrrole then results in interfacial polymerization at the micelle surface, resulting in hollow nanotubes 95 nm in diameter and up to 5 pm in length. The electrical conductivity of these nanotubes was up to 30 S cm-1. 

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