Small-angle neutron scattering has been used to determine the rate constant
s for transesterification reactions between hydrogenous and deuterated poly
(ethylene terephthalate)s. Intimate mixtures of the two polymers at a weigh
t fraction composition of 0.5 were pressed into plaques and heated for a ra
nge of times at temperatures of 543, 553, and 563 K. The small-angle-scatte
ring cross sections were collected for the plaques on quenching to ambient
temperature. These data were analyzed using a procedure based on second-ord
er kinetics developed earlier and which has been applied to a range of poly
esters. From the rate constants obtained the activation energy and preexpon
ential factor for the transesterification reaction have been obtained. Aspe
cts of the reaction that have been investigated were the molecular weight o
f the hydrogenous polymer and the ratio of hydroxyl to carboxyl end groups
in a hydrogenous polymer, the molecular weight of which was close to that o
f the deuterio polymer. Although there is an increase in the rate constant
as the average molecular weight decreases, there is no effect of molecular
weight on the activation energy that has an average value of 168 kJ mol(-1)
. The increase in rate constant with a decrease in molecular weight indicat
es the importance of the role of end groups in the transesterification reac
tion. The relative contribution of hydroxyl or carboxyl groups to the react
ion mechanism could not be discerned from these data. It is speculated that
this is due to the dominant role of the hydroxyl end group in the reaction
since the concentration of this varied but little in the mixtures.