Prog. Polym. Sci. 27 :2002) 1283±1346 www.elsevier.com/locate/ppolysci Miniemulsion polymerization Jose M. Asua* Institute for Polymer Materials POLYMAT) and Grupo de IngenierõÂa QuõÂmica,Facultad de Ciencias QuõÂmicas, The University of the Basque Country,Apdo 1072,20080 Donostia-San Sebastia Ân,Spain Received 10 September 2001; revised 2 January 2002; accepted 12 January 2002 Abstract Miniemulsion polymerization has recently exploded in terms of publications and the development of a wide range of useful polymer materials only accessible through this polymerization technique. In the ®rst part of this article, the fundamental aspects involved in the preparation and polymerization of monomer miniemulsions are reviewed. The second part deals with the application of miniemulsion polymerization for the production of high solids low viscosity latexes, dispersion of polymers of well de®ned microstructure through controlled radical polymerization, use of catalytic polymerization in aqueous media, encapsulation of inorganic solids, incorporation of hydrophobic monomers, preparation of hybrid polymer particles, implementation of anionic and step polymer- ization in aqueous dispersed media, and process intensi®cation by using continuous reactors. q 2002 Elsevier Science Ltd. All rights reserved. Keywords: Miniemulsion polymerization; Costabilizer; Ostwald ripening; Droplet nucleation; High solids low viscosity latexes; Controlled radical polymerization; Catalytic polymerization; Encapsulation; Hydrophobic latexes; Hybrid polymer particles; Continuous reactors Contents 1. Introduction ..................................................................1284 2. Early work ...................................................................1285 3. Preparation of monomer miniemulsions ..............................................1287 3.1. Formulations ..............................................................1287 3.2. Methods of preparation ......................................................1289 3.3. Homogenization devices .....................................................1290 3.4. Measuring droplet size distribution .............................................1292 3.5. Processes controlling droplet size distribution ......................................1294 3.5.1. Homogenization stage .................................................1294 3.5.2. Storage stage ........................................................1299 * Fax: 134-943-212-236. E-mail address: [email protected] :J.M. Asua). 0079-6700/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved. PII: S0079-6700:02)00010-2 1284 J.M. Asua / Prog. Polym. Sci. 27 2002) 1283±1346 3.6. Effect of the preparation variables on the droplet size distribution and droplet stability . .1303 3.6.1. Homogenization .....................................................1303 3.6.2. Aging .............................................................1307 4. Polymerization of monomer miniemulsions ...........................................1309 4.1. Particle nucleation .........................................................1309 4.2. Particle growth ............................................................1317 4.3. Mathematical modeling ......................................................1319 4.4. Comparison between latexes produced by miniemulsion polymerization and by conventional emulsion polymerization .....................................................1319 5. Applications ..................................................................1320 5.1. Production of high solids low viscosity latexes .....................................1320 5.2. Continuous polymerization reactors . ............................................1321 5.3. Controlled radical polymerization in dispersed media ................................1323 5.3.1. Effect of compartmentalization ..........................................1323 5.3.2. Stable free radical polymerization ........................................1324 5.3.3. Atom transfer radical polymerization ......................................1326 5.3.4. Reversible addition±fragmentation chain transfer .............................1327 5.3.5. Stability and nucleation of monomer droplets in CRP ..........................1328 5.4. Catalytic polymerization .....................................................1329 5.4.1. Catalytic polymerization of ethylene ......................................1329 5.4.2. Ring-opening metathesis polymerization ...................................1329 5.5. Encapsulation of inorganic solids . ............................................1330 5.6. Incorporation of hydrophobic monomers .........................................1331 5.7. Hybrid polymer particles .....................................................1331 5.8. Miniemulsion polymerization in non-aqueous systems ...............................1333 5.9. Anionic polymerization ......................................................1333 5.10. Step polymerization in aqueous dispersed media ...................................1334 5.11. Production of low-molecular weight polymers in dispersed media .......................1335 5.12. Latexes with special particle morphology .........................................1335 6. Summary ....................................................................1336 Acknowledgements ................................................................1337 References ......................................................................1337 1. Introduction Polymeric dispersions are used in a wide variety of applications such as synthetic rubber, paints, adhesives, binders for non-woven fabrics, additives in paper and textiles, leather treatment, impact modi®ers for plastic matrices, additives for construction materials and ¯occulants [1,2]. They are also used in biomedical and pharmaceutical applications such as diagnostic tests and drug delivery systems. The rapid increase of this industry is due to environmental concerns and governmental regulations to substitute solvent-based systems by water borne products, as well as to the fact that polymeric disper- sions have unique properties that meet a wide range of market needs. Commonly, these products are produced by means of conventional emulsion polymerization. In this process, monomer is dispersed in an aqueous solution of surfactant with a concentration exceeding the critical micelle concentration :CMC) and polymerization is started by means of an :most often water-soluble) initiator system. In principle, polymer particles can be formed by entry of radicals into the micelles :heterogeneous nucleation), precipitation of growing oligomers in the aqueous phase J.M. Asua / Prog. Polym. Sci. 27 2002) 1283±1346 1285 :homogeneous nucleation), and radical entry in monomer droplets. However, monomer droplets are relatively large :1±10 mm) compared to the size of monomer-swollen micelles :10±20 nm), and hence the surface area of the micelles is orders of magnitude greater than that of the monomer droplets. Consequently, the probability for a radical to enter into the monomer droplets is very low, and most particles are formed by either homogeneous or heterogeneous nucleation. Once they are nucleated, the polymer particles undergo substantial growth by polymerization. The monomer required for the polymerization must be transported from the monomer droplets by diffusion through the aqueous phase. In some cases, this represents a severe limitation of the conventional emulsion polymerization. Thus, water resistance of coatings prepared from dispersed polymers is signi®cantly improved if very hydrophobic monomers, e.g. lauryl and stearyl methacrylates are incor- porated into the polymer backbone. However, mass transfer of these monomers from monomer droplets to polymer particles through the aqueous phase is diffusionally controlled, and hence they cannot be readily incorporated into the polymer in conventional emulsion polymerization. The need of mass transport of monomer through the aqueous phase would be greatly diminished if all :or at least a large fraction) of the monomer droplets were nucleated. Prevalent droplet nucleation can only occur if the surface area of the monomer droplets is large compared with that of the micelles, and this requires submicron droplet size. The word miniemulsion was coined [3] to describe submicron oil-in-water dispersions that are stable for a period ranging from hours to months. Review articles by El-Aasser et al. [4,5] summarized the work done up to 1995. The kinetics of miniemulsion polymeriza- tion has been recently reviewed by Capek and Chern [6]. This paper reviews the main aspects concerning the preparation and polymerization of monomer miniemulsions. 2. Early work Ugelstad et al. [7] were the ®rst to demonstrate that under conditions in which the droplet size is small enough, nucleation of monomer droplets could account for an important part of the particles formed. The method used to produce the small droplet size was inspired in previous reports showing that the presence of long chain fatty alcohols drastically increased the capacity of anionic surfactants to disperse and stabilize oil-in-water emulsions [8±11]. In those works, it was reported that the presence of fatty alcohols led to the reduction of the interfacial energy and to the formation of ordered structures far exceeding the molecular size at the oil±water interface. Ugelstad et al. [7] stirred cetyl alcohol :CA) with water and sodium lauryl sulfate :SLS) at 60 8C and then styrene was added