Characterisation of N-vinyl-2-pyrrolidone-based hydrogels prepared by a Diels–Alder click reaction in water

N-vinyl-2-pyrrolidone-based hydrogels were prepared by the Diels–Alder reaction in water for the first time. Copolymers of N-vinyl-2-pyrrolidone(VP) and furfuryl methacrylate(FM) were synthesised by free radical polymerisation in toluene at 70 °C by using 2,2′-azobisisobutyronltrile as an initiator. Polymeric dienophile (PEG–AMI) was prepared from N-alaninyl maleimide (AMI) and poly(ethylene glycol) (PEG) by using N,N′-dicyclohexylcarbodiimide (DCC) as a dehydrating agent. The prepared dienes and dienophile were then dissolved in water and mixed, leading to gelation via Diels–Alder reaction after some time. The gelation times of different copolymers and PEG–AMI in different solvents and at different temperatures were measured by the vial inversion method, and the swelling behaviour of dried hydrogels was studied using a general gravimetric method. The gelation time was shorter in water than in organic solvents, and the gelation time decreased with the increase of temperature and FM content in copolymers. Conversely, the swelling ratios increased with the decrease of temperature and FM content in the copolymers. Disassembly experiments suggested that N,N-dimethylformamide (DMF) could accelerate the retro-DA reaction.

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