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

A progressively increasing rate of hydrolysis was observed for backbon e 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 auto catalysis. To model this behavior, a general theoretical argument was developed. The bases of the argument are that hydrolysis of an erodibl e polymer by a random scission mechanism would result in a maximum cha in-end concentration (E(max)) equal to 25% of the total repeating unit s, whereas chain-end scission (unzipping) would result in a very low c hain-end concentration throughout the course of degradation. Experimen tally, E(max) = 7.9%, which suggested a substantial contribution from chain-end scission in the hydrolysis of PLA. The degradation data indi cated that the hydrolysis of chain-ends was approximately 10 times fas ter than the hydrolysis of internal PLA bonds. The accelerated chain-e nd 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.