Forced to govern the unknown: on the role of the added liquid or polymer in mechanochemical solid forms screens

Mechanochemistry - the use of mechanical force to induce and sustain chemical transformations has recently been highlighted by IUPAC as "one of the ten most important chemistry innovations that will change the world". Although such discipline is relatively new with a significant growth of interest observed particularly over the last three decades, several independent studies have demonstrated mechanochemistry to be effective and often superior to other approaches for the discovery of new solid forms. The propensity of a specific molecule to give different polymorphs and/or form multicomponent crystals can be assessed mechanochemically by changes in the exact conditions of the reaction, including neat grinding (NG), variable temperature grinding (VATEG), liquid-assisted grinding (LAG), variable amount LAG (VALAG), ion liquid-assisted grinding (ILAG) and polymer-assisted grinding (POLAG). Mechanochemical reactions performed in the presence of catalytic amounts of a liquid or polymer are particularly interesting since the added liquid or polymer gives additional suitable conditions for the obtainment of the desired solid form. At the same time, however, the understanding of the mechanisms involved during mechanochemical reactions in the presence of a liquid or polymer remains vague, thus limiting our capabilities to control mechanochemical reaction. In this context, several research groups have recently produced important contributions, mainly due to serendipitous events. This presentation has particular focus on the effect of the catalysts available in mechanochemistry, and relates the outcomes both to the operational conditions such as the amount and to their chemical characteristics. Some relevant examples already available in literature will be mentioned, and the most recent result in our laboratory will be also presented