Ultrasound-induced polymerization of methyl methacrylate in liquid carbon dioxide : a clean and safe route to produce polymers with controlled molecular weight

Ultrasound-induced cavitation is known to enhance chemical reactions as well as mass transfer at ambient pressures. Ultrasound is rarely studied at higher pressures, since a high static pressure hampers the growth of cavities. Recently, we have shown that pressurized carbon dioxide can be used as a medium for ultrasound-induced reactions, because the static pressure is counteracted by the higher vapor pressure, which enables cavitation. With the use of a dynamic bubble model, the possibility of cavitation and the resulting hot-spot formation upon bubble collapse have been predicted. These simulations show that the implosions of cavities in high-pressure fluids generate temperatures at which radicals can be formed. To validate this, radical formation and polymerization experiments have been performed in CO2-expanded methyl methacrylate. The radical formation rate is approximately 1.5*1014 s-1 in this system. Moreover, cavitation-induced polymerizations result in high-molecular weight polymers. This work emphasizes the application potential of sonochemistry for polymerization processes, as cavitation in CO2-expanded monomers has shown to be a clean and safe route to produce polymers with a controlled molecular weight.