Polymorphic Development Strategy for Rapid Pesticide Release: A Case Study of Spirotetramat

Spirotetramat has been utilized extensively in the world for its long-lasting effects. However, since form I has been reported with the disadvantages of low solubility and poor pesticide release rate in the literature, systematic crystal form screening is urgently required. In this study, a new polymorph (form II) was discovered by melt crystallization, and its single crystal grew in melt microdroplets. In comparison to form I, form II is a metastable crystal form with a lower melting point and enthalpy as well as higher lattice energy. Its solubility is higher than that of form I. The increased solubility facilitates the release of pesticides at an accelerated rate. Furthermore, a series of stability experiments have shown that form II can maintain stability at room temperature and ambient humidity for more than 3 months. The form II crystals retain their stability even after grinding or heating. Meanwhile, the phase transformations among three solid forms were presented. Finally, a solution crystallization method was proposed to prepare form II crystals with uniform particle size distribution.