Synthesis and characterization of self-catalyzed poly(ortho ester).

Poly(ortho esters) are currently under investigation as a carrier system for an antiproliferative agent in glaucoma filtering surgery. The present investigation illustrates the development of a series of self-catalyzed poly(ortho ester). These polymers contain short dimer segments of alpha-hydroxy acids in their backbone and are prepared by the addition of different polyols to the diketene acetal 3,9-diethylidene-2,4,8,10-tetra-oxaspiro-[5.5]-undecane. The structures were confirmed by NMR- and FT-IR-spectroscopy. The polymers were characterized by determination of the molecular weight, the glass transition temperature and the rheological behavior. The amount of residual solvents was also analyzed. The characteristics of the polymer can be varied by the type of polyol incorporated in its backbone. Since poly(ortho ester) is susceptible to acid-catalyzed degradation, the polymer hydrolysis can be controlled by the amount of incorporated portion of alpha-hydroxy acid. Due to the high hydrophobicity of the polymer structure, the ester bonds are more susceptible to hydrolysis than the ortho ester bonds in the polymer backbone. The hydrolysis proceeds via initial protonation of the exocyclic alkoxy group to yield pentaerythritol dipropionate and the free diol. In a next step, the pentaerythritol dipropionate hydrolysis to pentaerythritol and propionic acid. The molecular weight decrease, weight loss and the pH profile of the polymer in aqueous medium were monitored during the degradation.