High pressure solid-state polymerization of molecules having a triple bond

Simple molecules that exist in a gas phase at ordinary temperature and pressure, can form crystals at high pressures beyond 1 GPa. At these pressures, molecules approach each other in solid by compression and hence intermolecular interactions change significantly. Intermolecular distances shrink roughly by 10% by compression to 10 GPa, for example, in van der Waals crystals. Such a molecular approach may cause reconstruction of chemical bonds or rearrangement of molecules. In this paper, we describe high pressure solid-state polymerization of several molecules with C��N triple bonds, such as hydrogen cyanide (HCN). Polymerization into conjugated polymers was observed for HCN, acetonitrile, and tetracyanoethylene (TCNE). Prior to polymerization, structural phase transitions with molecular rearrangement often took place. Raman measurements for HCN at 90 K showed band splitting above 2 GPa, indicating a structural phase transition with lowering symmetry. HCN molecules polymerized above 4 GPa. Acetonitrile solidified to phase I at 0.4 GPa and further transformed to phase at 0.6 GPa at room temperature. The phase �K was shown to be stable up to 15 GPa, where polymerization was induced. TCNE is known to form two crystalline phases at atmospheric pressure. An unstable monoclinic phase transformed to a new high-pressure phase near 3.5 GPa. TCNE molecules polymerized near 6 GPa.