Controlled Dispersion Polymerization of Methyl Methacrylate in Supercritical Carbon Dioxide via RAFT Andrew M. Gregory, Kristofer J. Thurecht,* and Steven M. Howdle* School of Chemistry, UniVersity of Nottingham, UniVersity Park, Nottingham NG7 2RD, UK ReceiVed September 7, 2007; ReVised Manuscript ReceiVed NoVember 28, 2007 ABSTRACT: We present a comprehensive investigation of the controlled dispersion polymerization of methyl methacrylate (MMA) in supercritical carbon dioxide (scCO 2 ) utilizing reversible addition fragmentation chain transfer (RAFT) polymerization to produce living microparticles. These microparticles show controlled molecular weight evolution, low polydispersities (PDI < 1.20), and well-defined spherical morphology (diameter ca. 1.40 μm). Four chain transfer agents are examined with modifications to both the stabilizing group (Z group) and leaving group (R group). We investigated their impact on molecular weight evolution and the morphology of the resulting polymer products in scCO 2 . The rate retardation effect intrinsic to many RAFT reactions which provides good kinetic control was shown to be much larger than that observed in analogous solution polymerizations performed in conventional organic media. This is believed to be due to the combination of the RAFT mechanism and the dispersion mechanism in scCO 2 . Introduction Reversible addition fragmentation chain transfer (RAFT) polymerization has proven to be one of the most effective forms of controlled polymerization developed over the past 10 years. 1-3 The process allows construction of polymers with targeted molecular weights possessing low polydispersities. Preserved end groups can be reactivated, permitting the incorporation of additional monomers to produce a diverse array of block copolymers 4 in a range of architectures 5 including star, 6 branched, 7 and graft 8 topologies. The process is complementary to traditional free radical polymerization, and along with the usual components of radical initiator, monomer, and solvent the RAFT process relies on the addition of a chain transfer agent (RAFT agent). One of the biggest advantages of the RAFT process is that it is a robust system that is applicable to a variety of monomers (methacrylates, acrylates and styrenics), incorpo- rating a range of functionalities including acidic moieties. 9 In recent years the impact on the environment of many chemical processes incorporating organic solvents has come under scrutiny. 10 Novel “green” processes have been developed as environmentally friendly analogues to conventional systems. 11 One such medium that has gained significant attention is supercritical carbon dioxide (scCO 2 ). 12,13 Supercritical fluids are highly compressed gases which combine the properties of gases and liquids: the solvating power of a liquid but the mass transport properties of a gas. The physical properties are tunable; their density can be controlled via manipulation of both temperature and/or pressure. In addition, on depressurization the system reverts to its gaseous state and can be removed easily leaving a solvent free product. The majority of research has focused on scCO 2 because of its low critical parameters (T c ) 31.1 °C, P c ) 73.8 bar), 12 lack of toxicity, and nonflammability. scCO 2 has also been demonstrated to be an effective polymer- ization medium. 14 Although homogeneous polymerization is possible, it is generally restricted to a selection of monomers incorporating fluorinated and siloxane side chains leading to scCO 2 -soluble polymers. 15 Recently, scCO 2 -soluble polymers composed purely of hydrocarbon chains have been produced, i.e., vinyl acetate. 16 While many monomers are soluble in scCO 2 , the resulting polymers are not. Therefore, the bulk of polymer- izations conducted in scCO 2 have been via heterogeneous routes. 17-19 RAFT-mediated polymerizations have been shown to work in supercritical fluids, but high conversions and low polydis- persities were not observed. 20,21 Recently, we demonstrated very effective control of methyl methacrylate (MMA) in a disper- sion in the presence of the RAFT agent R-cyanobenzyl dithionaphthalate (1) and the stabilizer poly(dimethylsilo- xane monomethacrylate) (PDMS-MA). 22 The RAFT agent was able to successfully partition between the continuous scCO 2 and the dispersed polymer particles. The result was successful control over the molecular weight of the product whereby the theoretical molecular weight (M nth ) agreed closely with that obtained by gel permeation chromatography (GPC) analysis. At higher conversions the polymer product was a fine powder, free of solvent. Under scanning electron microscopy (SEM) analysis of the particles showed discrete spherical morpho- logy with particles on the scale of 1-3 μm. Importantly, we demonstrated facile removal for the RAFT terminal group from the final polymer products without loss of spherical morphology. Dispersion polymerization is a versatile tool to prepare monodisperse microspheres that can be applied to a range of applications including electronics, information technology, toners, coatings, support materials for biochemical analysis, and microelectronics. Defined size control and narrow size distribu- tions combined with the ability to introduce a range of monomer functionalities in block copolymers are the key parameters for most of these applications. The combination of scCO 2 and RAFT has allowed the formation of living microparticles with defined molecular weights and polydispersities. 22 In this report we extend the study to a range of RAFT agents in order to start developing a library of effective chain transfer agents which are able to control polymer growth in scCO 2 . Herein we show the alteration of both Z and R groups and gauge their effect on the molecular weight evolution and resultant polymer morphol- ogy. * Corresponding authors. E-mail: Steve.howdle@nottingham.ac.uk. E- mail: kristofer.thurecht@nottingham.ac.uk. 1215 Macromolecules 2008, 41, 1215-1222 10.1021/ma702017r CCC: $40.75 © 2008 American Chemical Society Published on Web 01/25/2008