Catalytic copolymerization, carbon dioxide with epoxides

Supercritical carbon dioxide represents an inexpensive, environmentally benign alternative to conventional solvents for chemical synthesis. In this chapter, we delineate the range of reactions for which supercritical CO2 represents a potentially viable replacement solvent based on solubility considerations and describe the reactors and associated equipment used to explore catalytic and other synthetic reactions in this medium. Three examples of homogeneous catalytic reactions in supercritical CC are presented the copolymerization of CO2 with epoxides, ruthenium>mediated phase transfer oxidation of olefins in a supercritical COa/aqueous system, and the catalyic asymmetric hydrogenation of enamides. The first two classes of reactions proceed in supercritical CO2, but no improvement in reactivity over conventional solvents was observed. Hythogenation reactions, however, exhibit enantioselectivities superior to conventional solvents for several substrates. 

Naphthaleneytterbium demonstrates a high activity not only in stoichiometric reactions but also in many catalytic processes. It catalyzes at room temperature the polymerization of styrene, methylmetacrylate, ethylacrylate, isoprene, ethylene oxide, the copolymerization of ethylene oxide with styrene, piperilene and carbon dioxide [65], the hydrogenation of alkenes, alkynes, ketones, aldehydes [78], the formation of alkylenecarbonates from epoxides and CO2 [65]. 

A novel catalytic system based on aluminum porphyrins has been developed. It is particularly effective for the living ring-opening polymerization of epoxides and lactones and for the copolymerization of epoxides with carbon dioxide and with cyclic anhydrides. HNMR has demonstrated that the growing species of this initiating system is that shown in (3). 

---