KINETIC AND CATALYTIC ASPECTS IN MELT TRANSESTERIFICATION OF DIMETHYLTEREPHTHALATE WITH ETHYLENE-GLYCOL

The kinetics of melt transesterification of dimethyl terephthalate wit h ethylene glycol in the presence of zinc acetate as catalyst has been studied in semibatch conditions. We observed that this reaction occur s with the formation of many oligomers characterized from the terminal groups of the chains that can be hydroxyl-hydroxyl, methyl-hydroxyl o r methyl-methyl. Experimental runs have been performed at different te mperatures, initial reagents ratios, and catalyst concentrations, foll owing the amount of methanol released during the time as well as the c oncentration of any kind of oligomer. All the oligomers have been iden tified and determined by HPLC analysis. A classic kinetic model based on a complex reaction scheme containing four or five reaction sequence s has been developed. The scheme with four sequences foresees 24 oligo meric species involved in 58 different reactions, while the scheme wit h five sequences has 48 oligomeric species involved in 228 reactions. Despite the large number of oligomers and occurring reactions, only tw o kinetic parameters and two equilibrium constants are necessary to si mulate the kinetic behaviour of all the oligomers. A kinetic constant is related to the reaction of a methyl group with a hydroxyl of ethyle ne glycol, while the other corresponds to the reaction of a methyl gro up with a hydroxyl in a chain. Both kinetic constants show an activati on energy of about 15 kcal/mol. We observed a nonlinear correlation be tween activity and catalyst concentration and interpreted this fact by assuming two different catalytic activity levels for a dissociated an d an undissociated zinc ionic couple, respectively. (C) 1994 John Wile y & Sons, Inc.