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Covalent organic frameworks (COFs) have emerged as a new class of molecularly precise, porous functional materials characterized by a broad structural and chemical versatility, leading to a diverse range of applications. Despite the increasing popularity of COFs, fundamental aspects of their formation are poorly understood and profound experimental insights into their formation processes are still lacking. Here we use a combination of in situ X-ray powder diffraction and Raman spectroscopy to elucidate the reaction mechanism of mechanochemically synthesized imine COFs, leading to the observation of key reaction intermediates. Real-time monitoring provides experimental evidence of templating effects by the liquid additive for the subsequent pore formation and layer assembly. Moreover, the solid-state catalyst scandium triflate Sc(OTf)3 is revealed to be instrumental in directing the reaction kinetics and mechanism, resulting in products with crystallinity and porosity en par with solvothermally synthesized COFs. This work highlights the potential of mechanochemistry as a green synthetic route towards COF synthesis, and emphasizes the subtle interplay between choice of liquid additives, catalysts, and activation procedure.