Synthesis And Assembly Behavior Of Amphiphilic Multi-arm Star Polymers

Polymerization-induced self-assembly（PISA）has become a common method for preparing nanoparticles with diverse morphologies because it overcomes the disadvantages of traditional self-assembly process,such as complex steps,low solid content and long-time consumption.The effects of sequence and topological structure of amphiphilic block copolymer on the self-assembly process and the obtained morphologies have drawn great attention in PISA research.In this paper,four three-armed（（mPEG）₃-CDPA）macro-chain transfer agents（CTAs）with different lengths of hydrophilic chains including（mPEG550）₃-CDPA,（mPEG750）₃-CDPA,（mPEG1000）₃-CDPA and（mPEG2000）₃-CDPA were synthesized for the first time.Reversible addition-fragmentation chain transfer（RAFT）dispersion polymerization of styrene（St）mediated by the four（mPEG）₃-CDPA and the obtained morphology were investigated systematically.The present study was focused on the topological effect of（mPEG750）₃-CDPA（versus linear mPEG2000-CDPA）and the chain length effect of（mPEG）₃-CDPA.The main results are as follows:1.At 15%solid content,the kinetics study of the RAFT dispersion polymerization of St mediated by（mPEG750）₃-CDPA was carried out in the mixture of CH₃OH/H₂O=4/1（w/w）at 70℃.The results showed that the polymerization process can be divided into a slow and homogeneous stage as well as a fast and heterogeneous stage.When the polystyrene（PS）segment reached the critical degree of polymerization,nanoassemblies began to form and polymerization rate was enhanced,reaching a conversion of 85%within 6 h.2.At 25%solid content,RAFT dispersion polymerization mediated by both（mPEG750）₃-CDPA and mPEG2000-CDPA underwent sphere-wormlike-vesicle transition.The differences were as follows:（mPEG750）₃-CDPA had a wider wormlike morphology range（DP=80～120）,while wormlike morphology only existed near DP=60 in the case of mPEG2000-CDPA.At similar DP of St,the number average particle size of（mPEG750）₃-PS vesicles was larger and the particle size distribution was wider than in the case of linear mPEG2000-b-PS.Therefore,A₃B topology has a dramatic influence on the morphology of nanoassemblies.3.The morphologies of nanoassemblies obtained in the RAFT dispersion polymerization of St mediated by（mPEG）₃-CDPA with different chain lengths at different solid contents was investigated systematically.Diverse morphologies of nanoassemblies were obtained for（mPEG550）₃-PS,including some morphologies that have not been observed in the literature（columnar vesicles,oversized vesicles and multi-layer vesicles）.The assembled morphology of（mPEG2000）₃-PS was the simplest.At the same solid content and the ratio of hydrophilic/hydrophobic segments,more complex assembled morphologies were formed when the hydrophilic and hydrophobic segments were shorter.The stabilizing ability of solvophilic segment to solvophobic segment,the mobility of solvophobic segment and the packing parameters jointly determine the morphology of the nanoassemblies.This study has developed a facile method for obtaining nanoassemblies of A₃B miktoarm star polymer with diverse morphologies by RAFT dispersion polymerization.It can provide a deeper understanding of the topological effect in RAFT PISA,which can benefit the construction and application of nanoassemblies with complex morphologies.