Determination of specific rate constants of specific oligomers during polyester hydrolysis.

Aliphatic polyesters have great utility as temporary prosthetics and surgical aids, and in drug delivery systems. Knowledge of the mechanism and pathways of hydrolysis can form a basis for the design and selection of a controlled-release system. Because of the importance of hydrolysis to the properties of such a system, a technique was developed to accurately determine the specific rate constants and relevant kinetic parameters for the specific oligomers from the reaction mixture of polyesters undergoing hydrolysis. A detailed kinetic analysis of the acid hydrolysis of well-characterized oligomers of polyesters is presented. Several oligomers of poly(butylene tartrate)s were synthesized and their kinetic behavior was studied under acidic conditions. The oligomers and their degradation products were monitored by HPLC and identified by fast-atom bombardment mass spectrometry. From the rates of hydrolysis measured from pH 1 to 3.0 and at temperature of 75 degrees C, it becomes evident that the degradation of the oligomers obeys pseudo-first-order kinetics. The rate of hydrolysis among the homologous series of oligomers increases as the molecular weight increases. The hydrolysis was catalyzed by hydrogen ion; the catalytic rate constant increased predictably with the number of ester linkages present in the molecule. The reaction is first order with respect to the catalyst concentration and the number of ester linkages. Good agreement was obtained with a model in which the individual rate of bond cleavage depends only on the statistical factor. The technique described in this paper is unique for the determination of polyester hydrolysis pathways and isolation of any structural effects on the rates of hydrolysis.