New route to amphiphilic core-shell polymer nanospheres: Graft copolymerization of methyl methacrylate from water-soluble polymer chains containing amino groups

A novel method has been developed to prepare amphiphilic core-shell polymer nanospheres via graft copolymerizations of methyl methacrylate (MMA) from water-soluble polymer chains containing amino groups. Thus, amine-substituted biopolymers and synthetic polymers are treated with a small amount of tert-butyl hydroperoxide (TBHP, 0.08 mM) in water at 80°C to generate free radicals on the amine nitrogens, which subsequently initiate the graft copolymerization of MMA. tert-Butoxy radicals are also generated that either initiate the homopolymerization of MMA or abstract hydrogen from the polymer backbones. The amphiphilic macroradicals generated in situ self-assemble to form polymeric micelle-like microdomains. which promote the emulsion polymerization of the monomer. Thus, well-defined, amphiphilic core-shell nanospheres, which range from 60 to 160 nm in diameter, are produced in the absence of surfactant. The conversion and grafting efficiency of the monomer strongly depend on the TBHP concentration and the structure of the amino-containing water-soluble polymer. Polymers containing primary amine groups are considerably more effective than those containing secondary or tertiary groups, while ammonium cations do not induce the polymerization. The particle size and stability strongly depend on the structure and molecular weight of the hydrophilic polymer, as well as the pH of the mixture. Transmission electron microscopic (TEM) images of the particles clearly show well-defined core-shell morphologies where PMMA cores are coated with hydrophilic polymer shells. The amphiphilic core-shell nanospheres can be produced in high concentrations (up to 22% solids content). This new method is scientifically and technologically significant because it provides a commercially viable route to a wide variety of novel amphiphilic core-shell nanospheres.