Z-type and R-type macro-RAFT agents in RAFT dispersion polymerization – another mechanism perspective on PISA

In a common RAFT dispersion polymerization induced self-assembly (PISA) formulation, R-type macro-RAFT agents with a solvophilic chain attached at the leaving group side are normally used to mediate the polymerization, meanwhile Z-type macro-RAFT agents with a solvophilic chain attached at the Z-group side are seldom employed. In this paper, we have tried to use a Z-type macro-RAFT agent (mPEG113-BTPA) to mediate RAFT dispersion polymerization of styrene, and found that the process exhibited a poorer PISA tendency and weaker molecular weight control. In comparison, we have conducted the same process mediated with an R-type macro-RAFT agent (mPEG113-DDMAT), and found that the formulation exhibited a typical PISA process, and nanoparticles were produced in situ. Further experiments suggest that the location of RAFT groups plays a key role for the PISA process in RAFT dispersion polymerization. The RAFT groups will be embedded in the produced particles when an R-type macro-RAFT agent is employed, but locate on the particle surface in the case of the Z-type macro-RAFT agent. We have tried to employ the RAFT groups on the surface of the mPEG113-BTPA stabilized particles to modify the particle surface, and successfully removed the solvophilic block via breaking the RAFT group, and generated the poly(dimethylacrylamide) block by surface-initiated RAFT polymerization. Based on the above mechanism perspective, we have designed a preliminary experiment, and found that well-defined monodisperse nanospheres could be prepared by heterogeneous RAFT dispersion polymerization mediated with a Z-type macro-RAFT agent-based block copolymer which was synthesized via pre-polymerization of styrene in 1,4-dioxane mediated with mPEG113-BTPA.