Hydrolysis and erosion studies of autocatalyzed poly(ortho esters) containing lactoyl-lactyl acid dimers

Poly(ortho esters) based on 3,9-diethylidene-2,4,8,10-tetraoxaspiro[5.5]und ecane, 1,10-decanediol, and 1,10-decanediol dilactate were prepared and the ir hydrolysis in phosphate buffer at 37 degrees C and pH 7.4 was determined as a function of time. Specifically, kinetics of polymer weight loss, poly mer molecular weight changes, release of lactic acid and release of propion ic acid were determined. A significant decrease in molecular weight occurre d within about 20 days. Weight loss and evolution of hydrolysis product occ urred after an induction period that decreased with increasing content of t he lactoyl-lactyl segment in the polymer. After the induction period, a con comitant rate of weight loss and release of hydrolysis products was noted. The rate increased with increasing content of the lactoyl-lactide segment. After a certain time period, size-exclusion chromatography revealed the for mation of a peak at an apparent molecular mass of about 550 Da. The compoun ds under the peak were analyzed by electrospray ionization/mass spectrometr y (ESI/MS) and found to consist of a multiplicity of lower molecular weight polymer hydrolysis fragments. On the basis of expected hydrolysis products , it was possible to assign a structure to most molecular weight peaks in t he ESI/MS spectrum. The data were found to be consistent with a hydrolysis that takes place predominantly in the surface layers of the solid polymer.