Effect of solvent and molecular structure on the crystallization of polymorphs of BPT esters

The polymorphic crystallization behaviors for methylester (Me-est) and isobutyl ester (i-But-est) of BPT in cyclohexane (c-Hxn) and acetonitrile (MeCN) solutions at 298 K were investigated, and the solvent effect was discussed in comparison with the previous results.In the crystallization of Me-est, only one form with needle-like morphology was obtained in c-Hxn and MeCN solutions at any supersaturation. Two polymorphs of A and B forms appeared in the crystallization of i-But-est in c-Hxn solutions and solution-mediated transformation occurred. The morphology of the stable B form is prismatic and that of the metastable A form is needle-like. From MeCN solutions only, the stable B form with the prismatic morphology crystallized. The polymorphic crystallization behavior of i-But-est is similar to that of Pr-est and different from that of Me-est. The solubility of each ester increases with the dipole moment of the solvents, indicating that solute–solvent interaction increases with dipole moment. In the case of i-But-est and Pro-est, the meta-stable form can crystallize from solvents with weak interaction (EtOH and c-Hxn), but only the stable form crystallizes from solvents with large interaction (MeCN). In the crystallization of Me-est, no polymorphs appear, even in EtOH and c-Hxn. It is presumed that the especially strong hydrogen bonding between two molecules may disturb the formation of the polymorphs.