Pressure-induced polymerization of carbon monoxide: Disproportionation and synthesis of an energetic lactonic polymer

We have studied pressure-induced chemical reactions in carbon monoxide using both a diamond anvil cell and a modified large volume press. Our spectroscopic data reveal that carbon monoxide disproportionates into molecular CO2 and a solid lactone-type polymer; photochemically above 3.2 GPa, thermochemically above 5 GPa at 300 K, or at 3 GPa and similar to 2000 K as achieved by laser heating. The solid product can be recovered at ambient conditions with a high degree of conversion, measured to be up to 95% of the original CO. Its fundamental chemical structure includes beta-lactone and conjugated C=C, which can be considered a severely modified polymeric carbon suboxide with open ladders and smaller five-membered rings. The polymer is metastable at ambient conditions, spontaneously liberating CO2 gases exothermically. We find that the recovered polymer has a high energy density, 1-8 kJ/g, and is very combustible. We estimate the density of recovered CO polymer to be at least 1.65 g/cm(3).