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Living/Controlled Radical Polymerizations in Dispersed Phase Systems Prog. Polym. Sci. 27 *2002) 1039±1067 www.elsevier.com/locate/ppolysci Living/controlled radical polymerizations in dispersed phase systems Michael F. Cunningham* Department of Chemical Engineering, Queen's University, Kingston, Ont., Canada K7L 3N6 Received 5 November 2001; revised 25 January 2002; accepted 28 January 2002 Abstract Living/controlled radical polymerization provides a route to synthesizing materials with designed microstruc- ture and narrow molecular weight distributions. A variety of living radical systems have been developed in recent years, and are based on either reversible termination *SFRP, ATRP) or reversible transfer mechanisms *RAFT, degenerative transfer). Application of living radical polymerization to heterogeneous systems such as emulsion and miniemulsion polymerization may provide process and economic advantages over the traditional homoge- neous bulk and solution polymerizations. However, adaptation of living radical chemistry to aqueous dispersions poses several challenges relating to maintaining effective control over the growth of living chains. These chal- lenges originate from having two or even three phases in the reaction mixture, which can lead to issues related to phase partitioning of the controlling agent, transport of the controlling agent between phases, the role of aqueous phase kinetics, and the phenomena of particle nucleation and colloidal stability. This review examines recent progress in this area, with an emphasis on unresolved issues and future opportunities. q 2002 Elsevier Science Ltd. All rights reserved. Keywords: Living radical polymerization; Controlled radical polymerization; Stable free radical polymerization; Atom transfer radical polymerization; Reversible addition±fragmentation transfer; Degenerative transfer; Emulsion polymerization; Mini- emulsion polymerization Contents 1. Introduction ..................................................................1040 1.1. Scope of the review ........................................................1040 1.2. Aqueous dispersed phase polymerizations ........................................1040 1.2.1. Emulsion polymerization ...............................................1040 1.2.2. Miniemulsion polymerization ............................................1042 2. Living radical polymerizations in dispersed aqueous systems ...............................1043 * Tel.: 11-613-533-2782; fax: 11-613-533-6637. E-mail address: [email protected] *M.F. Cunningham). 0079-6700/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved. PII: S0079-6700*02)00008-4 1040 M.F. Cunningham / Prog. Polym. Sci. 27 02002) 1039±1067 2.1. Overview of living radical polymerization ........................................1043 2.2. Aqueous dispersed systems with reversible termination ...............................1046 2.2.1. Stable free radical polymerization ........................................1046 2.2.2. Atom transfer radical polymerization ......................................1053 2.3. Aqueous dispersed systems with reversible transfer .................................1057 2.3.1. Overview ..........................................................1057 2.3.2. Reversible addition±fragmentation transfer ..................................1058 2.3.3. Degenerative transfer using iodine exchange ................................1061 3. Block copolymer synthesis ........................................................1063 4. Concluding remarks .............................................................1063 Acknowledgements ................................................................1064 References ......................................................................1064 1. Introduction `Living' or `controlled' radical polymerizations provide a novel and potentially inexpensive route to designing polymers with controlled microstructure. Extensive research has been conducted into homo- geneous bulk and solution living radical polymerizations, but investigations into aqueous dispersed phase systems *emulsion and miniemulsion polymerization) have only recently appeared. There are a number of incentives to use emulsion/miniemulsion polymerization on a commercial scale, including ease of mixing and good heat transfer. Furthermore, there currently exists substantial investment in emulsion polymerization facilities throughout the world, and therefore there is considerable interest in adapting living radical polymerizations to emulsion-based polymerization systems. 1.1. Scope of the review This review summarizes recent progress in living/controlled radical polymerizations conducted in dispersed aqueous systems *emulsion and miniemulsion polymerization), with the emphasis on those aspects of operating in a heterogeneous environment that in¯uence the polymerization rate, the molecular weight distribution and the livingness of the system. The important living radical chemistries are brie¯y reviewed but it is not within the scope of this review to provide a comprehensive summary of recent activity in those ®elds. More details of the various living radical systems can be found in the references of the papers referred to in this review. Related reviews of interest have been written by Qiu et al. [1] and Claverie and Kanagasabapathy [2]. 1.2. Aqueous dispersed phase polymerizations 1.2.1. Emulsion polymerization Emulsion polymerization yields polymer particles in the 50±500 nm range, starting from an oil-in- water dispersion of monomer droplets in an aqueous surfactant solution. It is a widely used industrial process employed to make coatings, paints, adhesives and resins. Monomers used in emulsion poly- merization are typically only sparingly soluble in water *e.g. styrene, acrylates, methacrylates) although a few percent of water-soluble comonomers such as acrylic/methacrylic acid are often added to enhance stability. Surfactants can be either ionic or nonionic. Anionic surfactants such as sodium dodecyl sulfate M.F. Cunningham / Prog. Polym. Sci. 27 02002) 1039±1067 1041 Nomenclature AB degenerative transfer agent ARi polymer molecule of length i terminated by a degenerative transfer fragment ARj polymer molecule of length j terminated by a degenerative transfer fragment Bz radical generated from degenerative transfer agent Di dead polymer molecule of chain length i Di1j dead polymer molecule of chain length i 1 j FRj dithioester as a reversible addition±fragmentation chain transfer *RAFT) agent carrying a polymer molecule of chain length j I initiator M monomer M1z 1-mer radical generated by thermal initiation M2z dimer radical generated by thermal initiation Mtn/ligand transition metal complex for atom transfer reaction, without the halide R0z initiator radical Riz polymer radical of chain length i Ri11z polymer radical of chain length i 1 1 RiF z Rj RAFT agent in radical from, carrying two polymer molecules of chain length i and j, respectively Rjz polymer radical of chain length j Tz nitroxide radical TH hydroxylamine TRi polymer molecule of chain length i capped by a nitroxide radical XRi alkyl halide of chain length i X-Mtn11/ligand transition metal complex for atom transfer reaction, with the halide *SDS) or sodium dodecyl benzene sulfonate *SDBS) are most commonly used in research studies but nonionics are usually added to commercial formulations to provide greater colloidal stability. A typical emulsion polymer formulation contains an aqueous phase, consisting of surfactant above its critical micelle concentration *CMC), usually a water-soluble initiator such as potassium persulfate *KPS) although monomer-soluble initiators can also be used, and an organic phase consisting of mono- mers dispersed in 1±20 mm droplets. The aqueous phase may be buffered. Because the surfactant concentration is above the CMC, a high concentration of monomer-swollen micelles is also present in the aqueous phase. Upon heating, initiator decomposes to give aqueous phase radicals that propagate with small amounts of monomer dissolved in the aqueous phase *the aqueous phase is saturated with monomer). After adding a few monomer units, the aqueous oligoradicals become suf®ciently hydro- phobic to enter micelles, thereby initiating particles. *For styrene, the aqueous phase radicals need to be 2±3 units long to enter micelles or particles [3].) Particle nucleation continues until all micelles have either been nucleated to form particles, or dispersed to stabilize the growing particle surface area. Particles can continue to be nucleated, albeit at a lower rate, by homogeneous nucleation as long as monomer droplets exist in the system [4±6]. Once the micelles have been depleted, the particles continue to grow. Monomer droplets function as reservoirs, with monomer diffusing through the 1042 M.F. Cunningham / Prog. Polym. Sci. 27 02002) 1039±1067 aqueous phase to the particles. As long as monomer droplets are present, equilibrium swelling of the particles with monomer is maintained. Particles continue to grow until the monomer droplets are depleted, after which the remaining monomer in the particles is polymerized. During polymerization, small radicals can exit from the particles thereby lowering the number of radicals per particle and therefore the reaction rate. The probability of exit of a small radical depends on its water solubility and how it partitions between the particles and aqueous phase. Radicals formed from transfer to monomer can often exit but the probability of exit decreases rapidly as monomer units are added by propagation. A distinguishing
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