Synthesis of well-defined epoxy-functional spherical nanoparticles by RAFT aqueous emulsion polymerization
The environmentally-friendly synthesis of epoxy-functional spherical nanoparticles has been achieved using polymerization-induced self-assembly (PISA) in aqueous solution. Firstly, a non-ionic hydrophilic stabilizer block, poly(glycerol monomethacrylate) (PGMA), was prepared by reversible addition–fragmentation chain transfer (RAFT) solution polymerization in ethanol. This water-soluble precursor was subsequently chain-extended via RAFT aqueous emulsion polymerization of glycidyl methacrylate (GlyMA) at 50 °C and neutral pH to ensure maximum retention of the epoxy functionality. PISA leads to the formation of well-defined PGMA-PGlyMA spherical diblock copolymer nanoparticles at up to 35% w/w solids and 1H NMR spectroscopy studies indicated that virtually all of the epoxy groups survive such relatively mild conditions. DMF GPC studies confirmed that relatively low dispersities (Mw/Mn < 1.30) were obtained if the mean degree of polymerization of the core-forming PGlyMA block remained below 100. Well-defined triblock copolymer nanoparticles could also be prepared via seeded RAFT emulsion polymerization of n-butyl methacrylate, with DMF GPC analysis indicating a relatively narrow molecular weight distribution (Mw/Mn < 1.20). The epoxy groups within the nanoparticle cores were ring-opened by adding sodium azide to a 10% w/w aqueous copolymer dispersion at 50 °C, as confirmed by FT-IR spectroscopy. PGMA45-PGlyMA100 diblock copolymer nanoparticles could be conveniently converted into cationic nanogels by utilizing water-soluble diamines as crosslinkers. These nanogels were characterized by DLS and aqueous electrophoresis and remained intact when dispersed in DMF; in contrast, the corresponding linear precursor nanoparticles dissociated to form molecularly-dissolved copolymer chains under the same conditions.