Structures and properties of temperature-responsive gels with homogeneous networks prepared by photogelation of four-armed star-shaped poly(N-isopropylacrylamide)

The network structures of polymer gels strongly influence their mechanical properties and stimuli responsiveness. Recently, many scientists have attempted to prepare well-defined gels with homogeneous networks from four-armed star-shaped polymers such as four-armed poly(ethylene glycol) (Tetra-PEG). In this study, we synthesized a four-armed star-shaped block copolymer by the copolymerization of N-isopropylacrylamide (NIPAAm) and allylacrylamide (AllAm) via single-electron transfer living radical polymerization (SET-LRP) using a four-branched initiator. The resulting four-armed star-shaped copolymer (Tetra-PNIPAAm-b-PAllAm) was crosslinked with hexa(ethylene glycol) dithiol upon visible light exposure. The resulting gels (Tetra-PNIPAAm gels) exhibited slower temperature responsiveness than the general PNIPAAm gel prepared by the standard free radical copolymerization of NIPAAm and N,N’-methylenebisacrylamide. Dynamic light scattering (DLS) measurements revealed that the Tetra-PNIPAAm gels had more homogeneous networks than the general PNIPAAm gels. The slow temperature responsiveness of the Tetra-PNIPAAm gels is attributable to their more homogeneous network structure than that of the general PNIPAAm gels. Thus, four-armed star-shaped PNIPAAm is a useful building block for designing well-defined temperature-responsive gels with homogeneous networks.