There is one caveat. All these polymorphic transformations are only possible when a small amount of 1-1 is either present or is added to the powder as an impurity. The amount of 1-1 present or added to the powder of the polymorph turnover cycles displayed in Figure is 1.5%mol mol -1 (see ESI Section 4 for further experimental details and comments). This shows the importance of exceptionally small amounts of contaminants in enabling polymorph transformations by ball mill grinding. This level of contaminant is present in many chemicals, including those that are commercially available, and this phenomenon may be much more widespread than generally realised. Such small concentration of impurities is virtually undetectable by PXRD and was thus identified and quantified by high-performance liquid chromatography (HPLC, see ESI Section 2) only, which shows that a multitechnique approach is crucial in these solid state investigations. The role of this 1-1 contaminant in the polymorph conversion is puzzling. One possible explanation is that such impurity alters the relative polymorph stability by substituting the heterodimer in its crystal structures, akin to forming a solid solution, but such hypothesis is largely excluded by accurate PXRD analyses which do not indicate molecular substitution within any of the three polymorphs (see ESI Section 2.2 for details). We believe instead that the contaminant acts as a template for the nucleation of the different polymorphs. Importantly, as no base catalyst was used in these experiments, no covalent bonds are expected to be broken during the turnover cycles displayed in Figure . The chemical composition is therefore conserved throughout the entire three-polymorph turnover cycle and cannot explain the change in relative polymorph stability under the different milling conditions used or the reversibility of the process.