Living Radical Polymerization in Aqueous Dispersed Systems with Water-Soluble Catalysts
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Living radical polymerization is an important technique for synthesizing advanced macromolecules including block copolymers. Since its discovery in the early 1990s the capability of the field has expanded with new types of chemistry and techniques. One of the most widely used chemistries is atom transfer radical polymerization (ATRP) also known as “metal mediated living radical polymerization” (Mt-LRP). Mt-LRP has also expanded its use to aqueous dispersed systems including emulsion, miniemulsion and microemulsion, with the biggest advancements seen in miniemulsion where the droplets act as nanosized reactors. Extremely hydrophobic catalyst complexes are typically used in miniemulsion. While effective in controlling the polymerization, these hydrophobic catalyst complexes also get trapped in the final polymer particles and are difficult to remove. Herein I report the progress made using thermoresponsive polymer bound catalysts for Mt-LRP reactions in miniemulsion, which allow the successful LRP in miniemulsion with facile catalyst removal. Polymers could be prepared with less than 10 ppm of ruthenium in the final polymer compared to >500 ppm in the reaction mixture. The polymerizations were improved by the addition of ferrocene (FeCp2) in miniemulsion, to give almost complete conversion in significantly shorter times. The addition of ferrocene adds a second catalytic cycle that is ionic in nature and requires excess halogens present for a successful polymerization to occur. The ionic species in this catalytic cycle meant that the use of cationic surfactants with halogen counter-ions could directly affect the polymerization chemistry, which was shown by increasing rates with the addition of a bromine counter-ion versus a chlorine counter ion in the cationic surfactant used. Water-soluble FeCp2 derivative cocatalysts were also used to improve the rates and conversions of Mt-LRP of ruthenium catalyzed polymerizations while also allowing colourless polymers to be synthesized. Finally, the ligand EHA6TREN, was found to be active in bulk or solution (anisole) when complexed to iron (III) bromide with reverse ATRP, or to iron (II) bromide with AGET ATRP, yielding an iron-mediated living radical polymerization.