1538 Macromolecules 2005,38,1538-1541 A"Living"Radical ab Initio Emulsion Fluorinated xanthate Agent ant than 10. and if the c loss than 10 PDI ng ho for example 1.3(Cu.RAPT=3.8)and 2 (Cu.RAPT 1)can A ever.what should be an easy transition fror Austro men solutio d to an agents fo D.Box that there was a loss of collid eceed Octobe d January 6.2005 )Ab initi 10n Intro ue allows the desig of polymer e phas phas e here is tures are w al diag 整路 and n cr em sities 1.5 wh anhitectureandg in ength It shouldals versely,the us of less reactive raft of polyr Its from compart en has pro 02 ducing the amount of bim 2 and 16 100 nm the e PDI er into the 1 stem) haooeoncentationa e first,and ly the er using RAFT that init tor must be as hic for styrene using nd.the png agent should be and should theoreticall result in a PDI of 1.3 00 agent re polyme T erage molecula 20% 340m n and a PDI of1 im o: con b ini em r criteria.we ould als have to in our raf n poly make high ults will als al ow mechanistic co region ns tos t olled by the PDI)is largely Results and Discussion.In a typical RAFT-medi 1).75 gofst ne wa oa ton of water (17 o whom correspondence should be sen e-mail m.monteiro mixture was deoxygenated with nitrogen for 20 min and 10.1021/ma0478567 A “Living” Radical ab Initio Emulsion Polymerization of Styrene Using a Fluorinated Xanthate Agent Michael J. Monteiro,*,† Monique M. Adamy,‡ Bastiaan J. Leeuwen,‡ Alex M. van Herk,‡ and Mathias Destarac§ School of Molecular and Microbial Sciences, Australian Institute of Bioengineering and Nanotechnology, University of Queensland, Brisbane QLD 4072, Australia; Department of Polymer Chemistry, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands; and Rhodia Recherches, Centre de Recherches d’Aubervilliers, 52, rue de la Haie Coq, 93308 Aubervilliers Cedex, France Received October 17, 2004 Revised Manuscript Received January 6, 2005 Introduction. Emulsion polymerization is a unique technique for the synthesis of nanoscale polymer par￾ticles. The technique allows the design of polymer nanocomposites with a wide range of morphologies (e.g., core-shell,1 salami,2 hemisphere3), particle sizes, and particle size distributions.4 Such structures are widely used in a plethora of industrial applications,5 varying from protective coatings to biomedical diagnostic tests. This highlights the synthetic versatility and commercial importance of such a technique. “Living” radical polym￾erization using reversible addition-fragmentation chain transfer (RAFT)6-9 in principle should vastly increase the versatility of emulsion polymerization in the pro￾duction of new nanostructures with controlled polymer architecture and chain length.10-12 It should also pro￾vide a means of producing these nanostructures at a much faster rate of polymerization and with much better control of the molecular weight distribution than in solution or bulk.13,14 This results from compartmen￾talization of radicals within the polymer particles, reducing the amount of bimolecular termination, and lowering the amount of dead polymer. Compartmental￾ization in the case of styrene occurs when the particle size is small (<100 nm in diameter), in which only one or zero radicals can exist in any particle (a “zero-one” system).4 There are a couple of criteria that need to be followed when synthesizing a polymer using RAFT. The first, and most important, is that the ratio of RAFT agent to initiator must be kept as high as possible to minimize radical coupling that forms dead polymer chains. Sec￾ond, the ratio of monomer to RAFT agent should be chosen such that the desired number-average molecular weight (Mn) is reached. To increase the rate of polym￾erization in a solution experiment, the initiator concen￾tration could be simply increased. However, according to our criteria, we would also have to increase our RAFT agent proportionately, and this restricts the synthesis of polymer chains in the low Mn region. Therefore, emulsion polymerization should allow us to carry out rapid polymerizations to high Mn’s with less formation of dead polymer. The polydispersity (PDI) is largely controlled by the Ctr,RAFT value14-16 () ktr,RAFT/kp, where ktr,RAFT is the rate constant for transfer of propagating radicals to RAFT and kp is the rate constant for propagation to monomer). If the Ctr,RAFT value is greater than 10, a low PDI (<1.1) is found at high conversions, and if the Ctr,RAFT is less than 10, PDIs ranging between, for example, 1.3 (Ctr,RAFT ) 3.8) and 2 (Ctr,RAFT e 1) can be produced.17 However, what should be an easy transition from solution to an ab initio emulsion polymerization has proved to be quite difficult,18-21 especially for highly reactive RAFT agents (see refs 18, 19, and 22 for a detailed mechanistic description for this process). The results18 showed that there was a loss of colloidal stability, retardation in rate, and loss in the control of the molecular weight distribution (MWD). Ab initio emulsion polymerization involves the emulsification of monomer with surfactant in a continuous aqueous phase, in which three phases are present: monomer droplets, swollen monomer micelles, and the water phase containing residual monomer. There is now literature on the techniques to obviate the use of ab initio emulsion polymerization using RAFT to obtain polymer particles with controlled MWDs. These are miniemulsion,21 seeded emulsion,19 and self-aggrega￾tion,23 all of which have other limiting factors, such as little control of the particle size distribution, special solvent removal to localize the RAFT agent in the seed particles, and poor control of the MWD of butyl acrylate (i.e., polydispersities close to 1.5 when PDI’s of less that 1.1 should be expected), respectively. Conversely, the use of less reactive RAFT agents (i.e., xanthates or MADIX agents24-27sethyl 2-(O-ethylxan￾thyl)propionate which has a Ctr,RAFT of 0.68 for styrene17) has proved to be quite successful in producing polymers with controlled MWDs, fast rates of polymerization, and controllable particle size distributions.10,14,28 Although the MWD could be predicted, the PDI’s for styrene13 and n-butyl acrylate14 were 2 and 1.6, respectively. It is possible to reduce the PDI using xanthates by feeding the monomer into the reaction at a slow rate to keep the local monomer concentration as low as possible.28 Although this worked well for n-butyl acrylate, theoreti￾cally the time required to reduce the PDI for the styrene system using this procedure is not practical. The Ctr,RAFT value (3.8)17 for styrene using a fluorinated xanthate (ethyl 2-(O-trifluoroethylxanthyl)propionate)29 is much higher and should theoretically result in a PDI of 1.3 at 100% conversion. However, the solution polymeriza￾tions using this RAFT agent resulted in a conversion of only 20% after 340 min and a PDI of 1.8. The aim of this work is to use this fluorinated xanthate in a styrene ab initio emulsion polymerization to make high conver￾sion polymer (close to 100%) with PDI’s close to 1.3. The results will also allow mechanistic conclusions about the complex partitioning and transportation of the RAFT agent in a multiphase system to be made. Results and Discussion. In a typical RAFT-medi￾ated ab initio emulsion polymerization (expt 4 in Table 1), 75 g of styrene was added to a solution of water (175 g), sodium dodecyl sulfate (SDS, 1.11 g, which is above its critical micelle concentration), sodium bicarbonate (NaHCO3, 25 mg), and RAFT agent 1-(O-trifluoroeth￾ylxanthyl)ethyl propionate (F-MADIX, 0.693 g). The mixture was deoxygenated with nitrogen for 20 min and † University of Queensland. ‡ Eindhoven University of Technology. § Centre de Recherches d’Aubervilliers. * To whom correspondence should be sent: e-mail m.monteiro@ uq.edu.au. 1538 Macromolecules 2005, 38, 1538-1541 10.1021/ma0478557 CCC: $30.25 © 2005 American Chemical Society Published on Web 02/09/2005