Five crystal polymorphs of the herbicide metazachlor (MTZC) were characterized by means of hot stage microscopy, differential
scanning calorimetry, IR- and Raman spectroscopy as well as X-ray powder diffractometry. Modification (mod.) I, II and III
can be crystallized from solvents and the melt, respectively, whereas the unstable mod. IV and V crystallize exclusively from
the super-cooled melt. Based on the results of thermal analysis and solvent mediated transformation studies, the thermodynamic
relationships among the polymorphic phases of metazachlor were evaluated and displayed in a semi-schematic energy/temperature-diagram.
At room temperature, mod. III (Tfus =76C, ΔfusHIII =26.6 kJ mol-1) is the thermodynamically stable form, followed by mod. II (Tfus =80C, ΔfusHII =23.0 kJ mol-1) and mod. I (Tfus =83C, ΔfusHII=19.7 kJ mol-1). These forms are enantiotropically related showing thermodynamic transition points at ~55C (Ttrs, III/II), ~60C (Ttrs, III/I) and ~63C (Ttrs, II/I). Thus mod. I is the thermodynamically stable form above 63C, mod. III below 55C and mod. II in a small window between
these temperatures. Mod. IV (Tfus =72-74C, ΔfusHII =18.7 kJ mol-1) and mod. V (Tfus =65C) are monotropically related to each other as well as to all other forms. The metastable mod. I and II show a high kinetic
stability. They crystallize from solvents, and thus these forms can be present in commercial samples. Since metazachlor is
used as an aqueous suspension, the use of the metastable forms is not advisable because of a potential transformation to mod.
III. This may result in problematic formulations, due to caking and aggregation.