| Biodegradable and pH sensitive hydrogels containing azoaromatic crosslinks were synthesized by a polymer-polymer reaction. To minimize cyclization, a three-step synthesis was employed. First, a polymeric precursor (P-ONp) containing side-chains terminated in reactive p-nitrophenyl ester groups was synthesized by free radical precipitation copolymerization of N,N-dimethylacrylamide, acrylic acid, N-tert-butylacrylamide, and N-methacryloylglycylglycine p-nitrophenyl ester. In the second step, the p-nitrophenyl ester groups of P-ONp were converted to NH{dollar}sb2{dollar} groups (P-NH{dollar}sb2{dollar}) by a polymeranalogous reaction with an excess of a diamine, namely N,N{dollar}spprime{dollar}-({dollar}varepsilon{dollar}-aminocaproyl)-4.4{dollar}spprime{dollar}-diaminoazobenzene. The hydrogel was then prepared in the third step by a polymer-polymer reaction of P-ONp with P-NH{dollar}sb2{dollar}. Hydrogels were characterized by equilibrium degree of swelling, modulus of elasticity in compression at pH 7.4, and detailed network structure (i.e., content of crosslinks, unreacted pendent groups, and cycles). The characteristics of hydrogels depended on their crosslinking density. Cyclization was restricted, and it was shown that it is possible to quantitate the distribution of azobonds in the hydrogel networks synthesized by a polymer-polymer reaction. These hydrogels have a potential for oral colon-specific protein and peptide delivery.; In vitro degradation of hydrogels synthesized by a polymer-polymer reaction was compared with similar ones prepared by crosslinking of polymeric precursors. Results indicated that despite the difference in the distribution of side reactions, as a result of using two different synthetic methods, the degradation rates were not significantly different.; The influence of molecular geometry of hydrophilic probes on in vitro permeability across rabbit distal colonic epithelia was evaluated using the Ussing chamber technique. It appears that for the model probes under investigation, the theoretical estimations of the hydrodynamic radii, which assumes the molecules to be spherical in shape, in general provide a good basis for geometry dependence of permeability. |
