Application of Ball Milling for Highly Selective Mechanochemical Polymorph Transformations

Milling is an important secondary processing technique in the manufacture of pharmaceuticals, primarily used as a particle size reduction process. p-Aminobenzoic acid (PABA), m-aminobenzoic acid (MABA), o-aminobenzoic acid (OABA), and carbamazepine (CBZ) are pharmaceutically relevant compounds that can exist in different polymorphic forms with distinct packing motifs and thus different physicochemical properties. A comprehensive study of the effect of milling on the polymorphism of PABA, MABA, OABA, and CBZ was carried out. Milling of PABA in the presence of catalytic amounts of valeric acid or methanol yielded the β polymorph, which is otherwise difficult to obtain in bulk quantities. Milling also proved to be a more convenient method for producing MABA form IV compared with previously reported procedures. Principal component analysis of the pair distribution function-transformed X-ray powder diffraction spectra of ball-milled CBZ samples showed that the milling-induced polymorphic transformation strongly depends on the ball-to-powder ratio. Elusive CBZ form IV could be obtained in pure form by optimizing the milling conditions.