Transesterification in mixtures of poly(ethylene terephthalate) and poly(ethylene naphthalene-2,6-dicarboxylate): An NMR study of kinetics and end group effects

Mixtures of poly(ethylene terephthalate) and poly(ethylene naphthalene-2,6- dicarboxylate) have been transesterified at temperatures between 553 and 57 3 K. The partially transesterified polymers were analyzed using H-1 NMR, an d the rate constants for transesterification were obtained using a kinetic expression based on a second-order reversible reaction mechanism. The hydro xyl end groups have been shown to have a significant effect on the kinetics of the reaction by comparing rate constants obtained for the same polyeste r mixture but with hydroxyl end groups quantitatively esterified by trifluo roacetic acid. Carboxyl end groups do not have such a significant role in t ransesterification, and from the variation of rate constant at one temperat ure for a series of PET polymers with differing carboxyl to hydroxyl end gr oups, it appears that transesterification by direct ester-ester interchange is very small. The NMR data have also been used to calculate the randomnes s factor and number-average sequence length of the terephthalate sequences in the copolymer as transesterification progresses. The initially immiscibl e polyester mixture becomes miscible after a critical amount of transesteri fication takes place; no matter what reaction temperature is used, this hom ogenization takes place when the randomness factor has a value of 0.5. This does not correspond to completely random copolymer formation but suggests that the number-average sequence length of the terephthalate units in the c opolyester is ca. 4-8.