ABC or ACB triblock copolymers? Changing the RAFT group position in diblock copolymer macro-RAFT agents leads to different PISA behaviors in RAFT dispersion polymerization

Abstract

Polymerization-induced self-assembly (PISA) via reversible addition–fragmentation chain transfer (RAFT) dispersion polymerization has been established as an efficient method for the synthesis of well-defined block copolymer nanoparticles. However, RAFT groups are always buried inside nanoparticles that limits the applications and further structural control over block copolymer nanoparticles. In this study, diblock copolymer macro-RAFT agents with the RAFT group located at the center were synthesized by RAFT solution polymerization using a Z-type macro-RAFT agent. These diblock copolymer macro-RAFT agents were used to mediate RAFT dispersion polymerization, which enabled the efficient synthesis of well-defined ACB triblock copolymer nanoparticles with RAFT groups on the surface. It was found that the chain extension of a short solvophilic block to the Z-type macro-RAFT agent could significantly improve the controllability of RAFT dispersion polymerization. Additionally, diblock copolymer macro-RAFT agents with the RAFT group located at the chain end were also synthesized by RAFT solution polymerization using an R-type macro-RAFT agent. Using the R-type diblock copolymer macro-RAFT agents in RAFT dispersion polymerization, ABC triblock copolymer nanoparticles were successfully synthesized. With the established RAFT dispersion polymerization, the effect of the block sequence of triblock copolymers on RAFT-PISA was investigated in detail. It was demonstrated that higher-order morphologies (e.g. vesicles) were favored in the case of ACB triblock copolymers, while kinetically trapped spheres were favored in the case of ABC triblock copolymers. Finally, we demonstrated that RAFT groups of ACB triblock copolymer nanoparticles could be completely removed by treating with excess initiators without disturbing morphologies. This study not only develops a well-controlled RAFT-PISA method for the synthesis of block copolymer nanoparticles with RAFT groups on the surface, but also provides mechanistic insights into the block sequence effect on RAFT-PISA.