Miniemulsion Copolymerization of Methyl Methacrylate and Butyl Acrylate by Ultrasonic Initiation Melanie A. Bradley, † Stuart W. Prescott, † Harold A. S. Schoonbrood, ‡ Katharina Landfester, § and Franz Grieser* ,† Particulate Fluids Processing Centre, School of Chemistry, The University of Melbourne, Victoria 3010, Australia; Wacker Chemicals Australia, Unit 18/20 Duerdin St, Clayton North Victoria 3168, Australia; and Max-Planck Institute for Colloids and Interfaces, Forschungscampus Golm, 14424 Potsdam, Germany Received December 21, 2004; Revised Manuscript Received May 19, 2005 ABSTRACT: The ultrasonically initiated batch miniemulsion copolymerizations of methyl methacrylate (MMA) and butyl acrylate (BA) are studied at different MMA:BA ratios (including homopolymerizations), and the physical properties and chemical composition of the polymers formed are investigated. Trends in the evolution of the particle number are rationalized with reference to the mechanical properties of the polymer particles, with the number concentration of the softer, BA-rich particles reducing with continued sonication. Molecular weight data are consistent with high radical fluxes entering the particles, with the radical entry frequency calculated from the peroxide yield in a model system to be ∼1.5 × 10 -2 s -1 . To within experimental uncertainty, the copolymer composition is found to be consistent with the terminal model for propagation reactions and previously published reactivity ratios; hence, it is concluded that ultrasound has little effect on the propagation step in a free-radical polymerization process. The results obtained also support a miniemulsion polymerization pathway for sonochemically synthesized latex particles. Introduction The effects of ultrasound on a chemical process are both mechanical and chemical in origin, although mechanistic understanding of sonochemical reactions has only been developed relatively recently. 1 Irradiation of liquids with ultrasound causes enhanced mass trans- port, emulsification, and bulk thermal heatingseffects that have been exploited in the preparation of mini- emulsion polymerizations. 2,3 The chemical effects of ultrasound derive primarily from acoustic cavitation involving the formation, growth, and implosive collapse of bubbles. At an ultrasound frequency of 20 kHz, the local temperature of the collapsed microbubble in an aqueous solution is ∼4300 K. 4 These high temperatures lead to the homolysis of water within the bubbles, creating • OH and H • radicals, 5 which have previously been shown to initiate polymer- ization with a variety of monomers. 6 Miniemulsions consist of stable nanometer-sized drop- lets, and particle formation in the polymerization of such systems is by droplet nucleation. 7 Droplet stability is typically maintained by the use of a costabilizer/sur- factant system; added ionic or nonionic surfactant stabilizes the droplets against collisional growth, while an added costabilizer (a hydrophobe such as hexade- cane) stabilizes against Ostwald ripening. 2 Recent advances in miniemulsion polymerization have demon- strated that with a suitable choice of stabilizing group 8 or with constant agitation 9 the costabilizer may not be necessary for a miniemulsion polymerization to be undertaken. The free radicals produced during cavitation are used here in the ultrasonically initiated miniemulsion po- lymerization of methyl methacrylate. Previous reports of ultrasonically initiated dispersed-phase polymeriza- tion indicate that monomer conversion at ambient temperature is possible. 9-14 It is interesting that mini- emulsion polymerizations described elsewhere have frequently made use of ultrasound for agitation, but not for initiation, with chemical initiators being added to the reaction after a period of sonication. 2 Moreover, the same irradiation frequency and power (even the same make and model sonicator) have been used. The key differences between this work and studies of “classical” miniemulsions for which ultrasound has been only for agitation are that here (i) argon sparging is used to increase the cavitation temperature (hence, increasing the radical flux), as compared with, for example, nitro- gen; (ii) oxygen is excluded during sonication; (iii) the temperature is maintained at ∼25 °C, whereas during many (but not all) conventional miniemulsion prepara- tions temperature escalations are quite common, hence the radical flux here is higher (increasing the water temperature reduces the cavitation temperature, lower- ing the chemical yield of radicals 15 ); (iv) the droplets are produced in the absence of a costabilizer, are unstable, and require continual agitation until polymerization commences within them (balancing Ostwald ripening), whereas miniemulsions may normally be stored for some time before use. In this work, the miniemulsion copolymerization and homopolymerization of methyl methacrylate and butyl acrylate are studied, with initiating radicals derived from an ultrasonic field. The physical properties of these copolymers are compared to conventionally produced emulsion copolymers, and the compositions of the copolymers are investigated and discussed in relation to a conventional batch emulsion copolymerization model. The role of the surfactant in the ultrasonically initiated miniemulsion process is discussed. † The University of Melbourne. ‡ Wacker Chemicals Australia. § Max-Planck Institute for Colloids and Interfaces. * Author for correspondence. 6346 Macromolecules 2005, 38, 6346-6351 10.1021/ma0473622 CCC: $30.25 © 2005 American Chemical Society Published on Web 06/18/2005