Polycarbonyl, (also known as polymeric-CO, p-CO or poly-CO) is a solid, metastable, and explosive polymer of carbon monoxide.[1] The polymer is produced by exposing carbon monoxide to high pressures. The structure of the solid appears amorphous, but may include a zig zag of equally-spaced CO groups.[2]
Poly-CO can be produced at pressures of 5.2 GPa; it is amorphous and yellow to dark red in color.[3] Polymerisation is catalysed by blue light at slightly lower pressures in the δ-phase of solid CO.[4] Another white, crystalline phase can be made at higher temperatures at 6 or 7 GPa.[1]
R. J. Mills discovered this solid, which was first produced in a tungsten carbide anvil in 1947. Originally this was thought to be polymeric carbon suboxide, but the formation does not yield any gas byproduct such as carbon dioxide.[5] The yield of the solid can be up to 95%.[6]
The polymer is stable above about 80 K. Below this temperature the ε form of solid molecular CO is formed instead. When the pressure is released the polymer remains stable at atmospheric pressure. The solid dissolves in water, alcohol and acetone.[5] When exposed to the atmosphere it is hygroscopic, becomes gluey, and changes colour, becoming darker.[6] The reaction with water produces carboxylic groups.[7][8]
The solid stores a high energy. It can decompose explosively forming glassy carbon and carbon dioxide.[6] The energy density stored can be up to 8 kJ/g. During the decomposition the temperature can be 2500 K.[6] The density is 1.65 g/cm3, however most of the solid produced is porous, so the true density is likely to be higher.[6]
Infrared spectroscopy shows bands at 650, 1210, 1440, 1650 and 1760 cm−1. The 1760 band is likely to be due to the -C-(C=O)-C- structure.[4] The 1600 is due to vibration of a C=C double bond.[6]
The solid is electrically insulating with an electronic gap energy of 1.9 eV.[4]
Nuclear magnetic resonance for the material made from 13CO shows sharp resonance at 223 ppm due to ester or lactone attached carbon, and 151 ppm due to C=C double bonds. There is also broad resonance at 109 and 189 ppm. Over time of a few days, the 223 ppm peak reduces and all the other features increase in strength.[6]
Ideas of the structure include a zig zag chain of CO pointing in opposite directions, or five atom rings connected by CO and C−C bonds. The rings are lactones of tetronic acid: −C:−(C=O)−(C−O−)−(C=O)−O−. Interconnections between the rings are zig zags of CO.[4]
Other ideas of the structure of the solid, include graphitic carbon with carbon dioxide under pressure, and a polymer with this C3O2 monomer: −(C=O)−O−(C−)=C<. Yet other ideas are that the solid is the same as the polymer of carbon suboxide with oxalic anhydride.[9]