Abstract
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.
Highlights
The concept of click chemistry has, without doubt, revolutionized macromolecular synthesis and materials science.[1,2,3,4] The reasons for the importance of click reactions are their wide scope, their high efficiency and selectivity, and the ease of puri cation of the resulting products
In this study we investigated the in uence of pH and temperature on the cross-linking and degradation behavior of DA hydrogels
The results of our studies show that degradation of DA hydrogels occurs by rDA reaction and subsequent OHÀ-catalyzed ringopening hydrolysis of the formed maleimides to unreactive maleamic acid derivatives (Scheme 1, degradation pathway A)
Summary
The concept of click chemistry has, without doubt, revolutionized macromolecular synthesis and materials science.[1,2,3,4] The reasons for the importance of click reactions are their wide scope, their high efficiency and selectivity, and the ease of puri cation of the resulting products. While maleimide is a strong dienophile, maleamic acid (ring-open form of maleimide) is much less reactive in DA reactions[22,25] and no hydrogel formation is observed Based on these ndings, two potential pathways for the degradation of DA hydrogels can be formulated (Scheme 1). To study the in uence of the pH of the cross-linking medium on gelation kinetics and mechanical properties, oscillatory shear experiments were performed on a TA Instruments AR 2000 rheometer (TA Instruments, Eschborn, Germany) with parallel plate geometry (40 mm in diameter, 500 mm gap size). To determine the in uence of temperature on hydrogel degradation, gel cylinders were prepared as described above, immersed in 10 mL of 50 mM phosphate buffer (pH 7.4) and incubated at 20 C, 30 C, 40 C or 50 C. The differences were considered statistically signi cant at p < 0.05
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