Dispersion RAFT polymerization: comparison between the monofunctional and bifunctional macromolecular RAFT agents

Bifunctional macromolecular RAFT (macro-RAFT) agent mediated polymerization affords one-step synthesis of the BAB triblock copolymer, which has special morphology. However, the bifunctional macro-RAFT agent mediated polymerization under heterogeneous conditions is rarely reported. In this contribution, the dispersion RAFT polymerizations of styrene in a methanol-water mixture mediated with poly(ethylene glycol) based monofunctional and bifunctional macro-RAFT agents, which afford the AB diblock copolymer of poly(ethylene glycol)-block-polystyrene (mPEG-b-PS) and the BAB triblock copolymer of polystyrene-block-poly(ethylene glycol)-block-polystyrene (PS-b-PEG-b-PS), respectively, are compared. It is found that these two dispersion RAFT polymerizations have similar polymerization rates, and almost full monomer conversion can be achieved. The molecular weight of both the mPEG-b-PS diblock copolymer and the PS-b-PEG-b-PS triblock copolymer linearly increases with the monomer conversion, whereas the control over the polydispersity index (PDI) of the PS-b-PEG-b-PS triblock copolymer is not as good as that of the mPEG-b-PS diblock copolymer. The monofunctional macro-RAFT agent mediated dispersion polymerization affords the in situ synthesis of mPEG-b-PS colloidal nanoparticles, which can be uniformly distributed in the polymerization medium and whose size increases with the extension of the solvophobic PS block, whereas the bifunctional macro-RAFT agent mediated dispersion polymerization leads to a mixture of colloidal nanoparticles and gel-like networks of the PS-b-PEG-b-PS triblock copolymer.