Poly(N-isopropylacrylamide) hydrogels crosslinked by small-molecular crosslinkers through click chemistry

A well-defined azido-poly(N-isopropylacrylamide) (PNIPA) was prepared by reversible addition-fragmentation chain transfer polymerization of NIPA and glycidyl methacrylate followed by the azidization of pendant epoxyl groups. Then the polymer was crosslinked via Click chemistry by bis-or tetra-alkynyl terminated compounds, which were synthesized through esterification of 1,3-propanediol and pentaerythritol with propiolic acid and 6-heptynoic acid, to fabricate PNIPA hydrogels. And the swelling properties of obtained hydrogels were studied as functions of crosslinker dosage and species. The results showed that the decrease of crosslinker dosage or increase of chain length thereof could enhance swelling capacities of hydrogels due to bigger free volumes within networks; the functionality of crosslinkers almost had no influence on swelling properties of hydrogels. In addition, no obvious distinction was found for lower critical solution temperature and swelling response to a temperature jump during reswelling and deswelling course, no matter what and how much crosslinker was used.