New insights into the cross-linking and degradation mechanism of Diels-Alder hydrogels.

Eight-armed poly(ethylene glycol) was functionalized with furyl and maleimide groups. The two macromonomers were cross-linked by Diels-Alder (DA) reactions and the degradation behavior of the formed hydrogels was investigated. UV spectroscopy showed that maleimide groups were subject to ring-opening hydrolysis above pH 5.5, with the reaction rate depending on the pH and temperature. As a result of this, the gelation kinetics and stiffness of DA hydrogels were dependent on the temperature and the pH of the cross-linking medium, as demonstrated by rheological experiments. The gel time varied between 87.8 min (pH 3.0, 37 °C) and 374.7 min (pH 7.4, 20 °C). Values between 420 Pa (pH 9.0, 37 °C) and 3327 Pa (pH 3.0, 37 °C) were measured for the absolute value of the complex shear modulus. Hydrogel swelling and degradation were influenced by the same parameters. With increasing pH and temperature the degradation time was reduced from 98 days (pH 7.4, 20 °C) to 2 days (pH 7.4, 50 °C); no degradation was observed at pH 3.0 and 5.5. Molecular modeling studies of the DA and retro-Diels-Alder (rDA) moieties revealed that hydrogel degradation occurred by rDA reaction followed by OH--catalyzed ring-opening hydrolysis of maleimide groups to unreactive maleamic acid derivatives.