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

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Abstract
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.

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