Sonochemically Initiated RAFT Polymerization in Organic Solvents

Abstract

Sonochemically initiated RAFT polymerization (sonoRAFT) in organic solvents is presented for the first time. Acoustic cavitation, induced by high-frequency ultrasound (US) (490 kHz), caused pyrolysis of organic solvent molecules providing a source of initiating radicals. In this way, polymers could be prepared without conventional radical initiators (e.g., AIBN) or additives (e.g., piezoelectric nanoparticles). The polymerization of acrylates and acrylamides was well controlled with the prepared polymers displaying good agreement between theoretical and experimental molecular weights with low dispersities. Methacrylates were found to be less controlled. Several factors were found to influence the final monomer conversion (25–92%) and the upper limit of molecular weight (20–30 kDa) such as the monomer vapor pressure and solubility, solvent choice, and polymer type. No polymerization occurred in the absence of US demonstrating excellent temporal control, and the polymer products were found to possess high chain-end fidelity via chain extension and MALDI-TOF mass spectroscopy.

Mechanically Initiated Bulk‐Scale Free‐Radical Polymerization

“…The M n decreased slightly over time. We conjecture this decrease is due to the chain terminations and chain scissions caused by ultrasound . No polymerization was observed without piezoelectric nanoparticles (Figure ; see Figure S3), suggesting the ultrasound‐mediated solvent cavitation is not sufficient to initiate the polymerization at low frequency ultrasound (40 kHz) .…”

“…We conjecture this decrease is due to the chain terminations and chain scissions caused by ultrasound . No polymerization was observed without piezoelectric nanoparticles (Figure ; see Figure S3), suggesting the ultrasound‐mediated solvent cavitation is not sufficient to initiate the polymerization at low frequency ultrasound (40 kHz) . Moreover, the polymerization was not successful in the presences of BaTiO 3 (Figure ; see Figure S4), probably because of the weak reactivity between BaTiO 3 and the Fe III complexes .…”

Recent Developments and Future Challenges in Controlled Radical Polymerization: A 2020 Update

“…The concept was then transferred to organic media, although much more specialized conditions and tedious optimizations were required for selecting suitable monomers and solvents. 38 Overall, the aqueous system appears to function better as opposed to the organic counterpart in terms of control over the polymerization and final dispersities. Nevertheless, sono-RAFT still requires extensive optimization and design of specific conditions, such as the monomer concentration, the vapor pressure of both monomer and solvent and the need for varied frequencies depending on the structure of monomer/solvent.…”

Progress and Perspectives Beyond Traditional RAFT Polymerization

“…[ 57 ] Sonochemical initiation has since proven effective in a range of aqueous and organic systems where •OH radicals or various carbon‐centered radicals constitute the predominant initiating species, respectively. [ 58 ] Solvents exhibiting both low vapor pressure and low room‐temperature viscosity (such as dimethylformamide and dimethylacetamide) seemed best suited for sono‐derived radical generation and organic sono‐RAFT. More recently, sono‐RAFT has been applied for the preparation of controlled polymers with targeted self‐assembly properties, yielding thermoresponsive nanogels when applied to dispersed media.…”