SINGLE-PHASE AND HETEROPHASE SOLID-STATE CHEMICAL-KINETICS OF THERMALLY-INDUCED METHYL TRANSFER IN TETRAGLYCINE METHYL-ESTER

To better understand the general interrelationships between chemical t ransformations and physical transformations in solid-state reactions, we have studied the kinetics of methyl transfer in polycrystalline sam ples of tetraglycine methyl ester (TGME) over the temperature range of 83 degrees C-115 degrees C. Changes in the concentrations of the reac tant and various intermediates (sarcosyltriglycine methyl ester, METGM E, and tetraglycine, TG) and products (sarcosyltriglycine, METG, and N ,N-dimethyl glycyl triglycine, Me(2)TG) were measured over the entire time course of the reaction using HPLC. Corresponding measurements of physical transformations occurring during the course of the reaction w ere made using X-ray powder diffractometry and differential scanning c alorimetry. Kinetic curves for the loss of TGME in the range of 83 deg rees C-115 degrees C have a sigmoidal shape and collapse into one curv e when plotted in terms of reduced time, t/t(0.5), as do plots of inte rmediate and product concentration plotted in the same manner. The fir st 25% of the reaction proceeds homogeneously through what is believed to be the formation of a crystalline solid solution of the intermedia tes and products in the reactant. The acceleratory character of the ki netic curves in the single-phase portion of the reaction has been desc ribed by a kinetic scheme that contains a concentration-dependent rate constant. The appearance of a new crystalline phase beyond 35% of the reaction changes the reaction mechanism from a bulk reaction to an in terface-controlled process that causes further acceleration of the met hyl transfer. The apparent activation energies for both single-phase a nd heterophase stages of the reaction are about 100-130 kJ/mole. (C) 1 997 John Wiley & Sons, Inc.