DEGRADATION KINETICS CF METHACRYLATED DEXTRANS IN AQUEOUS-SOLUTION

The kinetics of the hydrolysis of glycidyl methacrylate derivatized de xtran (dex-MA), hydroxyethyl methacrylate derivatized dextran (dex-HEM A), and hydroxyethyl methacrylate (HEMA) were systematically investiga ted in aqueous solution in the H-0/pH range of -1.8 to 10.4 at 37 degr ees C. The degradation products were quantified with reversed-phase HP LC and used to calculate the residual amount of dextran-bound methacry late esters. In all compounds the degradation reactions follow first-o rder kinetics, the rate constants being susceptible to both specific a cid and specific base catalysis. The reaction rate constant was indepe ndent of both the dex-MA concentration and the degree of substitution. The log k(obs)-pH profiles can be divided into three parts: a proton- catalyzed, a solvent-catalyzed, and a hydroxyl-catalyzed section. At h igh acidities, dex-HEMA and HEMA are equally stable, but about seven t imes less stable than dex-MA. At alkaline pH, the order of stability i s HEMA > dex-MA > dex-HEMA. This demonstrates that at alkaline pH dex- HEMA is predominantly degraded by hydrolysis of the carbonate ester, w hereas at low pH, hydrolysis of the methacrylate ester is the main deg radation route of this compound.