Atom Transfer Radical Polymerization in Microemulsion
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Living/controlled radical polymerization (L/CRP) techniques in aqueous based systems were studied. The main focus of this research was adapting an ATRP (atom transfer radical polymerization) to a microemulsion polymerization in order to form nano-size particles with low concentration of surfactant. In conventional microemulsion polymerization, the cationic surfactant cetyltrimethylammonium bromide (CTAB) was successfully employed with a low weight ratio of surfactant/monomer of 1:10 to produce polymer particles with mean diameters less than 40 nm. Poor control resulted when this microemulsion polymerization was used with reverse ATRP, but using acetone as a phase transfer agent improved the results. The AGET (activators generated by electron transfer) initiation technique was also employed in microemulsion ATRP. In this “two-step” procedure, a reducing agent, ascorbic acid, was used to reduce the higher oxidation state catalyst in situ during the first stage to initiate a microemulsion polymerization. Monomer was then continuously fed to the microemulsion ATRP to form the final polymer latex. In an effort to improve this microemulsion polymerization, factors such as the catalyst concentration, temperature, and surfactant concentration were studied. Two monomers, butyl acrylate (BA) and butyl methacrylate (BMA) were investigated. When BA was used, linear first-order kinetic plots and relatively narrow molecular weight distribution (Mw/Mn~1.5) were observed. The final latex had a particle size ~20 nm. When BMA was employed, very fast reaction rates were obtained, leading to poorly controlled polymerizations with quite high polydispersity (Mw/Mn~2). The two-step AGET ATRP procedure in microemulsion provides options for synthesizing polymer nano-particles with low concentration surfactant in aqueous dispersed media.