Non-topochemical Solid State Polymerization of Benzene Monomer into Diamond Nanothread Single Crystals*

Diamond nanothreads are a new type of 1D sp³ carbon nanomaterial, synthesized by solid state reaction of benzene (Nat. Mater., 2015). In view of their stiffness and unlike most polymers, they readily form hexagonally-packed single crystals of threads, as demonstrated by XRD (JACS, 2017). We investigated the polymerization reaction under pressure. The stacks of benzene molecules in the reactant crystal must contract dramatically along the thread axis (by 40–50%) while the symmetry increases from monoclinic to hexagonal as nanothreads form. Thus it doesn't proceed with commensuration of periodicities from reactant crystal to polymer crystal as is found in topochemical reactions. In some sense, the applied mechanochemical stress “templates” the reaction to form single crystal nanothreads from polycrystalline benzene monomer. It is surprising that well-ordered 1D crystals can be obtained from an uncatalyzed, room T reaction directly forming C-C bonds with large changes in symmetry and distances. Breaking the constraint of topochemical reaction to form single crystals of polymers and carbon nanomaterials may allow for the synthesis of a large, new family of materials.