Reversible Addition ] Fragmentation Chain Transfer (RAFT) Polymerization in Miniemulsion Based on In Situ Surfactant Generation S. R. Simon Ting, A,B Eun Hee Min, A,C and Per B. Zetterlund A,D A Centre for Advanced Macromolecular Design, School of Chemical Engineering, University of New South Wales, Sydney NSW 2052, Australia. B Current address: Graduate School of Biomedical Engineering, University of New South Wales, Sydney, NSW 2052, Australia. C Current address: Photonics and Optical Telecommunications Group, School of Electrical Engineering and Telecommunications, University of New South Wales, Sydney, NSW 2052, Australia. D Corresponding author. Email: p.zetterlund@unsw.edu.au Reversible addition–fragmentation chain transfer (RAFT) polymerization of styrene has been implemented in aqueous miniemulsion based on the in situ surfactant generation approach using oleic acid and potassium hydroxide in the absence of high energy mixing. The best results were obtained using the RAFT agent 3-benzylsulfanyl thiocarbonyl sufanylpro- pionic acid (BSPAC), most likely as a result of the presence of a carboxylic acid functionality in the RAFT agent that renders it surface active and thus imparts increased colloidal stability. Stable final miniemulsions were obtained with no coagulum with particle diameters less than 200 nm. The results demonstrate that the RAFT miniemulsion polymerization of styrene employing the low energy in situ surfactant method is challenging, but that a system that proceeds predominantly by a miniemulsion mechanism can be achieved under carefully selected conditions. Manuscript received: 25 March 2011. Manuscript accepted: 10 May 2011. Introduction Miniemulsion polymerization offers several important advan- tages for the synthesis of polymeric nanoparticles in the particle diameter range of 50–1000 nm. [1] The particle formation mechanism in miniemulsion polymerization relies on direct transformation of monomer droplets into polymer particles, in sharp contrast with emulsion polymerization where particles are formed in the continuous aqueous phase and the monomer droplets merely act as monomer reservoirs. As a result of this mechanistic feature, which alleviates the requirement of diffu- sion of reactants from monomer droplets to polymer particles across the aqueous phase, miniemulsion polymerization is advantageous for the synthesis of hybrid nanoparticles containing encapsulated hydrophobic solids and hollow nanoparticles containing hydrophobic liquids, [2] as well as for implementation of controlled/living radical polymerization (CLRP) [3] in aque- ous dispersed systems. [4–12] However, one significant disad- vantage of miniemulsion polymerization is the fact that traditional miniemulsion preparation entails the use of a high shear device (e.g., ultrasonication) in a very energy intensive process, which has been an impediment to commercialization and industrial use. [13] Consequently, it is a matter of priority to develop low-energy methods for miniemulsion polymerization. Several different low energy techniques exist for the genera- tion of miniemulsions (also known as nanoemulsions in the non- polymerization literature). Much research has been published on low energy miniemulsion generation based on catastrophic phase transitions that occur because of a change in the sponta- neous curvature of the surfactant [14–15] as a result of a change in physicochemical properties (temperature (phase inversion tem- perature (PIT) method), [16–18] pressure, [19–20] pH, [21] ionic strength [22–23] ) or the composition (emulsion inversion point (EIP) method) [24–27] of the system. However, very little research has been carried out on the use of such techniques for the synthesis of polymeric nanoparticles. Spernath et al. [28,29] reported conventional (non-living) radical polymerization in miniemulsion based on sequential use of high energy homo- genization and the PIT method for the synthesis of poly(lauryl acrylate) nanoparticles employing non-ionic surfactants (Brij). Sadtler et al. [30] used the EIP method with the non-ionic surfactant Brij 98 to generate a styrene (St) miniemulsion which was subsequently polymerized using potassium persulfate. The synthesis of polystyrene (PSt) nanoparticles by miniemulsion polymerization based on EIP with the anionic surfactant Dow- fax 8390 was also recently reported, featuring exceptional preservation of the initial droplet identity (‘one-to-one copy’), and thus excellent particle size control. [31] Miniemulsions can also be generated by use of in situ formation of surfactant without relying on high energy shear devices. [32,33] This method relies on the formation of a surfac- tant at the oil–water interface as a result of a neutralization reaction between an organic acid (e.g., oleic acid) in the organic CSIRO PUBLISHING www.publish.csiro.au/journals/ajc Aust. J. Chem. 2011, 64, 1033–1040 Ó CSIRO 2011 10.1071/CH11123 0004-9425/11/081033 Full Paper RESEARCH FRONT