Democratization of Sono-RAFT: Reversible-Deactivation Radical Polymerization with Low-Frequency Ultrasound

Sonochemically initiated reversible addition-fragmentation chain transfer polymerization (sono-RAFT) is achieved by employing low-frequency ultrasound (f = 40 kHz) at high monomer concentrations, including bulk, for the first time. The utilization of cost-effective low-frequency sonication baths can promote the use of sono-RAFT for polymer and materials discovery. In this report, we continually replenish argon or nitrogen in solution to induce transient cavitation events, which create initiating radical species from the resulting monomer pyrolysis. Well-controlled polymerizations of acrylates and acrylamides were achieved with good agreement between theoretical and experimental molecular weights, low dispersity, and temporal control using a variety of chain-transfer agents (CTAs). Solution viscosity increased throughout the polymerization, ultimately limiting the final monomer conversion to 25–36% in bulk monomer. This limitation could be mitigated by dilution with N,N-dimethylacetamide (DMAc), an organic solvent capable of radical formation during sonication, allowing for 76 and 90% conversion at 1.94 and 0.97 M, respectively. Chain-end analysis elucidated the radical initiating species, confirmed the CTA retention, and demonstrated efficient chain extension. Overall, this method advances sono-RAFT polymerization by increasing its accessibility as a method for polymer synthesis.