Synthesis of Nanosized Polymer Particles in Nonaqueous Emulsion
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Synthesis of Nanosized Polymer Particles in Nonaqueous Emulsion Robert Dorresteijn, Robert Haschick, Kevin Müller, Markus Klapper and Klaus Müllen Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany. E-mail: [email protected] ABSTRACT In nonaqueous emulsion, moisture-sensitive polymerizations are performed in order to generate nanoparticles, which are not accessible by common aqueous emulsion polymerization. A nonaqueous emulsion, consisting of two immiscible aprotic organic solvents, is stabilized by amphiphilic block copolymers, such as PI-b-PEO or PI-b-PMMA copolymer, and lead to formation of nanosized dispersed droplets. They act as dispersed “nanoreactors” for the one-step synthesis of poly(urethane) nanoparticles in a polyadditon reaction as well as poly(L-lactide) nanoparticles through ring-opening polymerization, catalyzed by a moisture-sensitive catalyst. The well-dispersed particles possess average diameters below 100 nm and have narrow size distributions owing to the long-term stability of the dispersed droplets in the continuous phase. INTRODUCTION Over the past years organic nanoparticles gained research interest and have found manifold applications owing to their well-defined morphology, their tunable size and the possibility of surface-functionalization. They are commonly used as coatings1, as paints and pigments2, in drug-delivery3, diagnostics4 and in catalysis5-6. However, the number of monomers compatible to common aqueous emulsion polymerizations is limited to moisture-stable compounds. Thus, common aqueous emulsion polymerizations prohibit the use of isocyanates in step-growth polymerizations or the application of moisture-sensitive catalysts, such as N-heterocyclic carbenes (NHC) for the ring-opening polymerization (ROP) of L-lactide in a single step and at mild temperatures. Where the presence of water in a step-growth polymerization leads to formation of side-product urea, the described lactide polymerization fails completely. Hence, a nonaqueous system is strongly needed to carry out such sensitive polymerizations. Examples for oil-in-oil emulsions, containing two immiscible organic solvents, have been described in the literature based on alcohols etc..7 However, acidic protons, as they are present in such emulsions, might cause side reactions. Hence, emulsions consisting of two immiscible and aprotic organic solvents must be designed. Riess et al. introduced solvent mixtures of DMF / n-hexane or cyclohexane / acetonitrile, respectively, in 1970.8 These solvent mixtures were stabilized by PS-b-PI and PS-b-PMMA copolymers, respectively. However, droplet diameters below 1 µm were not achievable. In order to generate nanosized dispersed droplets, the stabilizing block copolymer had to be modified regarding the polarity difference of both building blocks. PI-b-PMMA as well as PI-b-PEO copolymers were proven to be able of stabilizing heterogeneous solvent mixtures with narrow size-distributed “nanoreactors”.9-11
EXPERIMENT In our standard procedure for parti
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