Ultrasonics in polymer science: applications and challenges

Ultrasonic waves in a liquid media generate both chemical and mechanistic effects that are actively used to perform chemical reactions, polymer synthesis, nanoparticle synthesis, colloids, food processing and so on. The application of sonochemistry in polymer science has been an interesting topic of research in the recent years. Ultrasonication acts as an external stimulus to initiate free radical polymerization (FRP) by the homolysis of the solvent, thereby generating radicals. The recent utilization of high frequency ultrasound (>100 KHz) for polymer synthesis has evoked new interest in the use of sonochemistry in the field of polymer chemistry, especially in chain growth polymerization reactions including reversible-deactivation radical polymerization (RDRP) techniques and novel applications. This review presents the principles of sonochemisty and the fundamental aspects governing the cavitation process and the radical generation process. A historical overview of the development of ultrasound-assisted polymerization with a focus on chain-growth polymerizations operating under pseudo-“living” conditions including nitroxide-mediated polymerization (NMP), atom transfer radical polymerization (ATRP) and reversible addition–fragmentation chain transfer (RAFT) polymerization is provided. The utilization of ultrasound in polymer applications such as hydrogels, biomedical nanostructures, drug delivery, nanocomposite synthesis is also discussed. Unlike conventional FRP, ultrasound-initiated polymerization does not involve any external toxic chemical initiators, adds temporal control to the polymerization process, offers excellent control over the molecular weight and the microstructure of the final polymers, etc. The ultrasound assisted polymerization is a novel, clean and green technology, which can be investigated further by coupling with thermo-, mechano- or photochemical stimuli or flow chemistry. It has the potential to be scaled up into an industrial process.