Self-assembly behavior of pH- and thermo-responsive hydrophilic ABCBA-type pentablock copolymers synthesized by consecutive RAFT polymerization

Abstract

A multi-responsive hydrophilic ABCBA-type pentablock copolymer, consisting of poly(ethylene glycol), poly(N-isopropylacrylamide), and poly(2-(diethylamino)ethyl methacrylate), PDEA-b-PNIPAM-b-PEG-b-PNIPAM-b-PDEA, was synthesized by consecutive reversible addition–fragmentation chain transfer (RAFT) polymerization. The polymerization showed all of the expected features of living radical polymerization and allowed the synthesis of copolymers with controlled lengths of the PNIPAM and PDEA blocks. Due to the multi-component and multi-functional nature of the pentablock copolymer, the solution properties can be manipulated by the changing of parameters such as temperature, pH etc. For example, the copolymer could be dissolved in acidic aqueous solution at room temperature, since the pH was below the pKa of the PDEA block, and the temperature was below the lower critical solution temperature (LCST) of the PNIPAM block. While in alkaline solution at room temperature, the copolymer self-assembled into core–shell–corona micelles, with the hydrophobic PDEA block as the core, the thermoresponsive PNIPAM block as the shell and the hydrophilic PEG block as the corona. At elevated temperatures in acidic media, the copolymer self-assembled into PNIPAM-core micelles with mixed hydrophilic PEG and pH-responsive PDEA coronas. Moreover, the self-assembly behavior of the pentablock copolymers in aqueous solution was investigated by varying the length of PDEA blocks.