Oxygen Initiated and Accelerated PET-RAFT in Ionic Liquid Solutions

Deoxygenation is required for most radical-based polymerizations. Photoinduced electron/energy-transfer-RAFT (PET-RAFT) polymerization provides alternative pathways for oxygen-tolerant polymer synthesis via light and a photoredox catalyst. Room-temperature ionic liquids (ILs) are liquid salts that comprise large, delocalized ions. Here, we employ 1-ethyl-3-methylimidazolium ethyl sulfate ([EMIM][EtSO4]) as a reaction medium and cosolvent with water for oxygen-initiated PET-RAFT polymerization using organic photocatalysts pheophorbide A (Pheo A), chlorophyll, or eosin Y, under blue, green, or red light. We determined that these photocatalysts reduced molecular oxygen to superoxide radical anions, which then reacted with the imidazolium cation of the IL to form hydrogen peroxide. Hydrogen peroxide can then be converted to hydroxyl radicals that enable an additional reversible addition–fragmentation chain-transfer (RAFT) polymerization mechanism. Contrary to conventional radical polymerizations, our system can initiate conventional polymerization without deoxygenation. We also demonstrated that polymerization could be achieved at lower irradiance through thin barriers using Pheo A under penetrative red light. The oxygen-initiated approach was further demonstrated using chlorophyll extracted from spinach leaves, mimicking natural photosynthesis in producing biomacromolecules.