ATRP Catalyst Removal and Ligand Recycling Using CO2-Switchable Materials

We have designed three approaches to remove copper catalyst and recycle the ligands after atom transfer radical polymerizations, all based on using materials whose properties can be switched using only CO2 and any nonacidic gas (e.g., air, nitrogen, and argon) as triggers. The first approach involves use of a CO2-switchable solvent (Cy2NMe, N, N-dicyclohexylmethylamine) as the medium for the ATRP reaction. After addition of water to the polymerized reaction mixture and sparging with CO2, the polymer precipitates while the copper salt remains in the solvent. The second approach involves using a conventional ATRP solvent such as toluene to conduct the polymerization. Following polymerization, addition of water and CO2 sparging result in the ligand (Me6TREN) and copper salt transferring into the aqueous phase, while the polymer remains in the organic phase. Finally we demonstrate the effectiveness of the approach in ARGET ATRP using less ligand and copper salt. Residual copper in the polymer was <30 ppb using the switchable solvent and <15 ppb in toluene, while residual nitrogen was <90 ppm using the switchable solvent and <35 ppb in toluene. The feasibility of recovering and reusing the ligand for subsequent polymerizations is also established.