Profound Effect of the Milling Assembly on Polymorphism in Mechanochemical Cocrystallization

Mechanochemistry provides a highly efficient, but still poorly understood route to synthesize and screen for polymorphs of organic solids. We present a hitherto unexplored approach to control the mechanism and outcome of mechanochemical cocrystallization through changes to the milling assembly, i.e. milling jar and balls. Whereas polymorph control of mechanochemical cocrystallization is typically discussed in terms of liquid additives, real-time synchrotron X-ray diffraction studies reveal a direct impact of the choice of milling media on the rate of formation and interconversion of cocrystal polymorphs. This effect enabled the discovery of a new polymorph of a cocrystal of nicotinamide and adipic acid, whose formation and conversion to the previously known, enantiotropically-related form, was readily controlled by milling in jars made of different materials.