Despite the great interest that cocrystals are currently gaining for their application to the design of new supramolecular structures with desired functional properties, studies concerning new experimental strategies capable of controlling polymorphism phenomena of a given system are scarcely reported. We propose herein the use of polymer-assisted grinding (POLAG) as a new method for the selective control of the product polymorphic form in a mechanochemical cocrystallization reaction. Specifically, to the model system selected in this study formed by caffeine and glutaric acid, we demonstrate that the polymorphic outcome can be controlled by modifying the number of monomer units of the catalyst from the shortest dimer to a polymer with chains of approximately 1000 units. The characteristics of each polymorphic form were investigated by low-dose high-resolution TEM, and the mechanistic aspects of the …