Miniemulsion

Most emulsion polymers are produced by conventional emulsion polymerisation (202, 204, 215). In this process, monomer droplets are dispersed in a continuous aqueous phase and are kept colloidally stable against coalescence through the use of a surfactant. The surfactant also causes polymer particles to form by homogeneous or micellar nucleation upon initiation. For conventional emulsion polymerisation to be practical, the monomer must be at least slightly water-soluble, to allow the diffusion of monomer from the droplets to the site of polymerisation in the growing polymer particles. 

Relatively low amounts (typically 1 to 3 weight percent) of surfactant are required in emulsion polymerisation, although the particle size may be controlled to an extent by the amount (and type) of 

A broad range of monomers with relatively low water solubility have been polymerised by conventional emulsion polymerisation. Acrylics, methacrylics, styrene and vinyl acetate are the most common monomers used in preparing latexes for paints, textile binders, and adhesives. Acrylic, polyester, epoxy and urethane dispersions are used in industrial coatings, where higher strength is required. Butadiene is often copolymerised with styrene in producing synthetic rubber for tyre manufacture. 

Emulsion Type Conventional Emulsion Miniemulsion Microemulsion 

Duration of stability seconds to hours hours to months indefinitely 

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However, in the case of mini- and microemulsions, processing methods reduce the size of the monomer droplets close to the size of the micelle, leading to significant particle nucleation in the monomer droplets (17). Intense agitation, cosurfactant, and dilution are used to reduce monomer droplet size. Additives like cetyl alcohol are used to retard the diffusion of monomer from the droplets to the micelles, in order to further promote monomer droplet nucleation (18). The benefits of miniemulsions include faster reaction rates (19), improved shear stabiHty, and the control of particle size distributions to produce high soHds latices (20). 

J. Delgado, Miniemulsion Copolymerisation of Vinyl Acetate and n-ButylAcrylate, Ph.D. dissertation, 1986. 

Microemulsion and miniemulsion polymerization differ from emulsion polymerization in that the particle sizes are smaller (10-30 and 30-100 nm respectively vs 50-300 inn)4" and there is no monomer droplet phase. All monomer is in solution or in the particle phase. Initiation takes place by the same process as conventional emulsion polymerization. 

Microemulsion and miniemulsion polymerization processes differ from emulsion polymerization in that the particle sizes are smaller (10-30 and 30-100 nm respectively vs 50-300 ran)77 and there is no discrete monomer droplet phase. All monomer is in solution or in the particle phase. Initiation usually takes place by the same process as conventional emulsion polymerization. As particle sizes reduce, the probability of particle entry is lowered and so is the probability of radical-radical termination. This knowledge has been used to advantage in designing living polymerizations based on reversible chain transfer (e.g. RAFT, Section 9.5.2)." 2... 

Heterogeneous polymerization processes (emulsion, miniemulsion, non-aqueous dispersion) offer another possibility for reducing the rate of termination through what are known as compartmcntalization effects. In emulsion polymerization, it is believed that the mechanism for chain stoppage within the particles is not radical-radical termination but transfer to monomer (Section 5.2.1.5). These possibilities have provided impetus for the development ofliving heterogeneous polymerization (Sections 9.3.6.6, 9.4.3.2, 9.5.3.6). 

NMP of S in heterogeneous media is discussed in reviews by Qiu et at.,205 Cunningham,206 207 and Schork et a/.208 There have been several theoretical studies dealing with NMP and other living radical procedures in emulsion and miniemulsion."09 213 Butte et nr/.210 214 concluded that NMP (and ATRP) should be subject to marked retardation as a consequence of the persistent radical effect. Charlcux209 predicted enhanced polymerization rates for minicmulsion with small... 

NMP in miniemulsion has been more successful. In miniemulsion polymerization nuclealion lakes place directly in the monomer droplets that become the polymer particles. Particle sizes are small (<100 nm). Most w ork has used TEMPO and high reaction temperatures (120-140 °C) with S or BA as monomer. 

Various initiation strategies and surfactant/cosurfactant systems have been used. Early work involved in situ alkoxyamine formation with either oil soluble (BPO) or water soluble initiators (persulfate) and traditional surfactant and hydrophobic cosurfactants. Later work established that preformed polymer could perform the role of the cosurfactant and surfactant-free systems with persulfate initiation were also developed, l90 222,2i3 Oil soluble (PS capped with TEMPO,221 111,224 PBA capped with 89) and water soluble alkoxyamines (110, sodium salt""4) have also been used as initiators. Addition of ascorbic acid, which reduces the nitroxide which exits the particles to the corresponding hydroxylamine, gave enhanced rates and improved conversions in miniemulsion polymerization with TEMPO.225 Ascorbic acid is localized in the aqueous phase by solubility. 

In combination ATRP, the catalyst is again present in its more stable oxidized form. A slow decomposing conventional initiator e.g. AIBN) is used together with a normal ATRP initiator. Initiator concentrations and rate of radical generation arc chosen such that most chains arc initiated by the ATRP initiator so dispersities can be very narrow.290 The conventional initiator is responsible for generating the activator in situ and prevents build up of deactivator due to the persistent radical effect. Reverse or combination ATRP are the preferred modes of initiation for ATRP in emulsion or miniemulsion (Section 9.4.3.2).290 291... 

Much has been written on RAFT polymerization under emulsion and miniemulsion conditions. Most work has focused on S polymerization,409-520 521 although polymerizations of BA,461 522 methacrylates382-409 and VAc471-472 have also been reported. The first communication on RAFT polymerization briefly mentioned the successful semi-batch emulsion polymerization of BMA with cumyl dithiobenzoate (175) to provide a polymer with a narrow molecular weight distribution.382 Additional examples and discussion of some of the important factors for successful use of RAFT polymerization in emulsion and miniemulsion were provided in a subsequent paper.409 Much research has shown that the success in RAFT emulsion polymerization depends strongly on the choice of RAFT agent and polymerization conditions.214-409-520027... 

Miiiana-Perez, M., Gutron, C., Zundel, C., Anderez, J.M. and Salager, J.L. (1999) Miniemulsion formation by transitional inversion. Journal of Dispersion Science and Technology, 20, 893-905. 

Asua, J.M. (2002) Miniemulsion polymerization. Progress in Polymer Science, 27, 1283-1346. 

Tiarks, F., Landdfester, K. and Antonietti, M. (2001) Preparation of polymeric nanocapsules by miniemulsion polymerization. Langmuir, 17, 908-918. 

Galindo-Alvarez, J., Boyda, D., Marchal, Ph., Tribet, Ch., Perrin, P., Begue, E.M., Durand, A. and Sadder, V. (2011) Miniemulsion polymerization templates a systematic comparison between low energy emulsification (Near-PIT) and ultrasound emulsification methods. Colloids and Surfaces A Physicochemical and Engineering Aspects, 374 (1—3), 134—141. 

H.J. Barraza, F. Pompeo, E.A. O Rear, and D.E. Resasco, SWNT-filled thermoplastic and elastomeric composites prepared by miniemulsion polymerization. Nano Lett. 2, 797-802 (2002). 

Considering theoretically a copolymerization on the surface of a miniemulsion droplet, one should necessarily be aware of the fact that this process proceeds in the heterophase reaction system characterized by several spatial and time scales. Among the first ones are sizes of an individual block and macromolecules of the multiblock copolymer, the radius of a droplet of the miniemulsion and the reactor size. Taking into account the pronounced distinction in these scales, it is convenient examining the macrokinetics of interphase copolymerization to resort to the system approach, generally employed for the mathematical modeling of chemical reactions in heterophase systems [73]. 

This monomer concentration Ma in the formalism of the quasi-homogeneous approximation, unlike M a, refers to the whole volume of the two-phase system. The aforementioned quantities are connected by the simple relationship Ma = flM a where y01 stands for the volume fraction of the a-th phase in miniemulsion. An analogous relation, Ra = sdaR a, exists between the concentrations Ra of the a-th type active centers in the entire system and those R a in the surface layer of the a-th phase. This layer thickness da has the scale of average spatial size of the a-th type block, which hereafter is presumed to be small as compared to the average radius of miniemulsion drops. Apparently, in this case, the curvature of the interphase surface can be neg-... 

The values of this parameter are extensively reported in the literature for many monomers [80]. Using expressions (Eq. 80), it is easy to note that the overall number of moles of monomers being polymerized in unit time in a reaction system is proportional to the interphase surface of the miniemulsion. 

Taniguchi T, Takeuchi N, Kobaru S, Nakahira T (2008) Preparation of highly monodisperse fluorescent polymer particles by miniemulsion polymerization of styrene with a polymerizable surfactant. J Colloid Interface Sci 327 58-62... 

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