Ea. Schmitt et al., IMPORTANCE OF DISTINCT WATER ENVIRONMENTS IN THE HYDROLYSIS OF POLY(DL-LACTIDE-CO-GLYCOLIDE), Macromolecules, 27(3), 1994, pp. 743-748
A detailed examination of the role of water in the hydrolysis of poly(
DL-lactide/glycolide) esters is reported. The hydrolysis rate of polye
ster pellets was independent of moderate changes in ionic strength, pH
, and buffer concentration. Total water uptake by the polymer depended
on ionic strength but not pH. The hydrolysis rates were independent o
f the total water content of the polymer. Solid-state H-2-NMR qualitat
ively demonstrated the presence of two types of water ((H2O)-H-2) envi
ronments within the polymer, bulk water with free rotation and bound w
ater with hindered rotation. Differential scanning calorimetry also sh
owed the presence of nonfreezing water, confirming different water env
ironments within the polymers. Quantitative solid-state 2H-NMR showed
that the polymer contained a constant amount of water ((H2O)-H-2) with
hindered rotation. The molar quantity of ester groups in the polymer
is 25-fold higher than the molar quantity of water with hindered rotat
ion, suggesting that water's immobility and reactivity result from its
selective hydrogen bonding to the oxygen of ester carbonyl groups. Th
e increased hydrolysis rate, observed for polyesters with higher glyco
lide content, correlates with an observed increase in bound, reactive
water.