Synthesis of thermoresponsive PNIPAm-b-PVP-b-PNIPAm hydrogels via aqueous RAFT polymerization

Abstract

Stimuli-responsive BAB type triblock copolymers of poly(N-isopropyl acrylamide) and poly(N-vinylpyrrolidone) i.e. PNIPAm-b-PVP-b-PNIPAm were readily synthesized via aqueous reversible addition–fragmentation chain transfer (RAFT) mediated polymerization using a difunctional xanthate RAFT agent, with redox initiation, under mild conditions. The thermoresponsive behavior and temperature-induced self-assembly in aqueous media was studied using turbidimetry, variable temperature dynamic light scattering (VT DLS), VT ¹H NMR spectroscopy and transmission electron microscopy. With increasing temperature, above the critical solution temperature of the PNIPAm segments, the triblock copolymers self-assemble into flower-like micelles with a PNIPAm core. Concentrated aqueous solutions (>20 wt%) displayed a thermoreversible sol–gel transition, as shown by test-tube inversion and injectability tests. Thermogellation was further confirmed via variable temperature rheology. This study establishes the potential for PNIPAm-b-PVP-b-PNIPAm systems to be incorporated in biomedical research as potential drug delivery devices via injectable hydrogel systems.