Polymers with pharmacological activity, 5. Hydrolytical behaviour of polymethacrylic hydrogels bearing “paracetamol” side groups

The heterogeneous hydrolytical behaviour of hydrogels based on copolymers of 2-hydroxyethyl methacrylate (HEMA) with 4-(methacryloyloxy)acetanilide (MOA)b is analyzed by measuring the concentration of active metabolites released into the alkaline-buffered solution used as the hydrolytical medium. Previously, we studied the swelling of hydrogel matrices of copolymers prepared with a MOA content between 1,0 and 5,0 wt.-%, in order to know the diffusional character of the reagents and the released species. The results obtained demonstrate that the swelling phenomenon is much faster than the hydrolytical chemical reaction. Thus, the hydrolysis is studied on the basis of a kinetic scheme which considers the formation of different kinds of residues, i.e., the sodium salts of 4-hydroxyacetanilide (PC) and 4-aminophenol (PA), according to the UV spectra obtained at different times of treatment. The characteristic rate coefficients, k1, k2, kc and k′c are evaluated from the experimental data on the basis of the suggested kinetic scheme.