C&EN: TODAY'S HEADLINES - CHEMISTRY IN A CO2 BUBBLE

MITCH JACOBY

Liquid carbon dioxide can be used as a medium for ultrasound-induced chemical reactions, according to researchers in the Netherlands. Scientists at Eindhoven University of Technology have demonstrated that high-molecular-weight polymers can be prepared in the high-pressure fluid.

The study broadens the range of reactions that can be conducted sonochemically and may lead to new industrial processes that are free of the hazards associated with organic solvents. In addition, by triggering reactions via sound waves, the procedure can eliminate the separation step typically required with reactions that are set off by chemical initiators or catalysts.

In ordinary solvents, the process of bubble formation and implosion (cavitation) does not occur at elevated pressure. For that reason, until now sonochemical studies have been carried out under atmospheric conditions. But the high vapor pressure of dense fluids such as CO₂ counteracts the hydrostatic pressure, allowing cavitation to occur in that medium.

Taking advantage of CO₂’s unique properties, chemists in Eindhoven’s Process Development Group—including graduate students Martijn W. A. Kuijpers and Diana van Eck, assistant professor Maartje F. Kemmere, and professor Jos T. F. Keurentjes—used ultrasound to polymerize methyl methacrylate in high-pressure solutions of CO₂. Under certain reaction conditions, the team reports, they prepared polymers with an average molecular weight of 100,000 dalton and a polydispersity of roughly five [Science, 298, 1969 (2002)].

Keurentjes and coworkers note that the solubility of poly(methyl methacrylate) (PMMA) in the CO₂-monomer solution—and thus the molecular weight of the product—depends strongly on the pressure of the system and the CO₂-to-monomer ratio. For example, at a ratio of 0.71 to 0.29, PMMA with a molecular weight of 46,000 Da is roughly 5% soluble. The solubility of PMMA drops sharply, they say, as the ratio is increased.

In addition to growing polymer chains, the cavitation process can cause chain scission. According to the Eindhoven group, depending on the ultrasound intensity and the viscosity of the solution, the product molecular weight can be limited to 30,000 Da. Thus, cavitation-induced reactions in CO₂ feature a number of parameters that can be exploited as process variables to control the properties of the product.