Unexpected Switching of the Photogelation Chemistry When Cross-Linking Poly(2-oxazoline) Copolymers

The photohydrogelation reaction of functional poly(2-oxazoline)s can be significantly accelerated by the presence of weak hydrophobic interactions. Here we describe the synthesis and cross-linking of water-soluble poly(2-oxazoline) copolymers containing vinyl groups in the side chains by copolymerizing 2-methyl-2-oxazoline and 2-undecenyl-2-oxazoline or 2-(3-butenyl)-2-oxazoline. An improved synthetic pathway to the 2-(3-butenyl)-2-oxazoline monomer based on alpha-deprotonation of 2-methyl-2-oxazoline is also included. When exposed to radical thiol-ene conditions in the presence of dithiothreitol in water, all of the copolymers produced homogeneous hydrogels, but the nature of the copolymers greatly influenced the cross-linking kinetics. The polymers with the vinyl groups on short alkyl chains cured slowly, and the physical properties of the hydrogels were strongly dependent on the molar ratio of thiol and ene groups. Conversely, the polymers with the vinyl groups on long alkyl chains cured extremely rapidly, whereby the process was nearly independent of the thiol concentration. A model of hydrophobic interactions actually enhancing cure kinetics, but at the expense of the thiol ene reaction, is proposed.