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

Abstract

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 R_h 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 R_h was about half that of the original assemblies. In the meantime, dispersed spherical micelles could be observed by TEM.