Thermo- and redox-responsive dumbbell-shaped copolymers: from structure design to the LCST–UCST transition

A dumbbell-shaped copolymer (PDMAEMA)4-PCL-SS-PCL-(PDMAEMA)4 with a lower critical solution temperature (LCST) and redox responses was first synthesized by the combination of ring opening polymerization (ROP), click chemistry and atom transfer radical polymerization (ATRP). Then a dumbbell-shaped copolymer (PDMAPS)4-PCL-SS-PCL-(PDMAPS)4 with an upper critical solution temperature (UCST) and redox responses was obtained through the quaternization reaction between DMAEMA and excess 1,3-propane sultone. Both (PDMAEMA)4-PCL-SS-PCL-(PDMAEMA)4 and (PDMAPS)4-PCL-SS-PCL-(PDMAPS)4 were amphiphilic, and they could self-assemble into spherical micelles under certain conditions. Benefitting from the LCST-type thermoresponse of PDMAEMA and UCST-type thermoresponse of PDMAPS, both dumbbell-shaped copolymers (PDMAEMA)4-PCL-SS-PCL-(PDMAEMA)4 and (PDMAPS)4-PCL-SS-PCL-(PDMAPS)4 presented temperature-responsive properties. The morphology and size of their assemblies would be changed when the temperature is altered. Meanwhile, due to the redox-response of disulfide bonds, both (PDMAEMA)4-PCL-SS-PCL-(PDMAEMA)4 and (PDMAPS)4-PCL-SS-PCL-(PDMAPS)4 showed redox-responsive properties. When DL-dithiothreitol (DTT) was added into the systems, disulfide bonds were broken, causing the change of (PDMAEMA)4-PCL-SS-PCL-(PDMAEMA)4 to PCL-(PDMAEMA)4, while (PDMAPS)4-PCL-SS-PCL-(PDMAPS)4 changed into PCL-(PDMAPS)4. The distributions of Rh for both (PDMAEMA)4-PCL-SS-PCL-(PDMAEMA)4 and (PDMAPS)4-PCL-SS-PCL-(PDMAPS)4 assemblies broadened with asymmetric peaks, giving rise to curves with double peaks. After 24 h, DLS curves became symmetric with a single peak, and the Rh was about half that of the original assemblies. In the meantime, dispersed spherical micelles could be observed by TEM.