Mechanistic Investigation of the Position of Reversible Addition–Fragmentation Chain Transfer (RAFT) Groups in Heterogeneous RAFT Polymerization

Heterogeneous reversible addition–fragmentation chain transfer (RAFT) polymerization is of growing interest as an in situ route to well-defined block copolymers and block copolymer nanoparticles. Recent investigations on heterogeneous RAFT polymerization have demonstrated that the position of RAFT groups has a significant effect on the nature of heterogeneous RAFT polymerization as well as the final block copolymer nanoparticles. Herein, we investigated RAFT dispersion polymerization mediated by an R-type macromolecular RAFT (macro-RAFT) agent or a Z-type macro-RAFT agent. We found that well-controlled polymerization was achieved in the R-RAFT dispersion polymerization, while uncontrolled polymerization occurred in the Z-RAFT dispersion polymerization (i.e., low monomer conversion, broad molar mass distribution, and low blocking efficiency). This problem could be addressed by using a binary mixture of the R-type macro-RAFT agent and the Z-type macro-RAFT agent. Additionally, RAFT seeded dispersion polymerization was also employed to improve the RAFT controllability of the Z-RAFT dispersion polymerization. Well-controlled polymerization was observed in each case regardless of the position of RAFT groups. Finally, the spatial distribution of RAFT groups could significantly affect the morphologies of block copolymer nanoparticles. This study not only provides important insights into the mechanism of heterogeneous RAFT polymerization but also enables one to control the position of RAFT groups within block copolymer nanoparticles without compromising the RAFT controllability.