CHAIN-END SCISSION IN ACID-CATALYZED HYDROLYSIS OF POLY(D,L-LACTIDE) IN SOLUTION

A progressively increasing rate of hydrolysis was observed for backbone ester bonds (60 degrees C) of poly(D,L-lactide) (PLA) in a p-dioxane-d(8)/D2O mixture containing deuterochloric acid (DCl). Replacement of DCl with excess lactic acid did not result in appreciable degradation of the polymer, suggesting that the phenomenon was not caused by autocatalysis. To model this behavior, a general theoretical argument was developed. The bases of the argument are that hydrolysis of an erodible polymer by a random scission mechanism would result in a maximum chain-end concentration (E(max)) equal to 25% of the total repeating units, whereas chain-end scission (unzipping) would result in a very low chain-end concentration throughout the course of degradation. Experimentally, E(max) = 7.9%, which suggested a substantial contribution from chain-end scission in the hydrolysis of PLA. The degradation data indicated that the hydrolysis of chain-ends was approximately 10 times faster than the hydrolysis of internal PLA bonds. The accelerated chain-end scission rate was attributed to a short-range substitution effect. This contrasts with the hydrolysis of poly(epsilon-caprolactone) (PCL) which occurred by a random scission mechanism.