RAFT synthesis and micellization of a photo-, temperature- and pH-responsive diblock copolymer based on spiropyran

A photo-, temperature- and pH-responsive diblock copolymer comprised of poly(2-spiropyranpropyl methacrylate)-b-poly(N-isopropylacrylamide) (PSPPMA-b-PNIPAM) containing a solvophobic photo- and pH-responsive block of PSPPMA and a thermoresponsive PNIPAM block was synthesized via RAFT polymerization. This successful synthesis of the block copolymer was ascribed to introducing a 8-atom spacer between the CC and rigid spiropyran (SP) moiety to reduce the steric repulsion. The self-assembly of PSPPMA-b-PNIPAM containing a rigid SP moiety resembled a rod-coil diblock copolymer. The size of the PSPPMA-b-PNIPAM nano-assemblies was relatively large and the morphology of the PSPPMA-b-PNIPAM nano-assemblies changed from micelles to vesicles upon increasing the degree of polymerization in the PSPPMA block. The PSPPMA-b-PNIPAM nano-assemblies are photo-, temperature- and pH-responsive, and the micelles-to-vesicles transition took place under 365 nm light irradiation or acidification of the polymer solution. This micelles-to-vesicles transition was ascribed to the light or pH induced spiropyran-to-merocyanine transformation in the PSPPMA block.