Liquid or supercritical carbon dioxide (scCO(2)) is a versatile reaction medium for ring-opening metathesis polymerization (ROMP) and ring-closing olefin metathesis (RCM) reactions using well-defined metal catalysts. The molybdenum alkylidene complex 1 and ruthenium carbenes 2 and 3 bearing PCy(3) or N-heterocyclic carbene ligands, respectively, can be used and are found to exhibit efficiency similar to that in chlorinated organic solvents. While compound 1 is readily soluble in scCO(2), complexes 2 and 3 behave like heterogeneous catalysts in this reaction medium. Importantly, however, the unique properties of scCO(2) provide significant advantages beyond simple solvent replacement. This pertains to highly convenient workup procedures both for polymeric and low molecular weight products, to catalyst immobilization, to reaction tuning by density control (RCM versus acyclic diene metathesis polymerization), and to applications of scCO(2) as a protective medium for basic amine functions. The latter phenomenon is explained by the reversible formation of the corresponding carbamic acid as evidenced by (1)H NMR data obtained in compressed CO(2). Together with its environmentally and toxicologically benign character, these unique physicochemical features sum up to a very attractive solvent profile of carbon dioxide for sustainable synthesis and production.

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The product distribution can be controlled by varying the density of the reaction medium when olefin metatheses of acyclic dienes are carried out in supercritical CO₂ (scCO₂), as shown below. Substrates with NH groups, which are generally not tolerated in conventional solvents, are compatible with Ru-based metathesis catalysts in scCO₂. Avoidance of potentially hazardous organic solvents, simple isolation of the products, and the recovery of the catalysts in active form are additional practical advantages associated with using compressed CO₂ as reaction medium for ring-closing metathesis (RCM) and ring-opening metathesis polymerization (ROMP)

The article contains sections titled: 1. Introduction 2. Physical Properties 3. Chemical Properties 4. Production 4.1. Phosgenation of Free Amines 4.2. Other Phosgenation Procedures 4.3. Non‐Phosgene Processes 5. Environmental Protection 6. Quality Specifications and Analysis 7. Storage and Transportation 8. Uses 9. Economic Aspects 10. Toxicology and Occupational Health