Synthesis Of Brush Thermo-responsive Polymer And Its Application In The RAFT Dispersion Polymerization

Thermo-responsive polymers have been widely investigated for their potential applications in drug delivery, catalysis, tissue engineering scaffolds, biological separation and biochemical sensors. In recent decades, the nonlinear PEG analogues are found to exhibit LCST in water and exhibit UCST in alcohols. These thermo-responsive nonlinear PEG analogues have received tremendous attention due to their rich morphological textures, well-established biocompatibility and potential applications in biomedical materials. Recently, benefiting from the controlled radical polymerization (CRP) techniques, such as reversible addition-fragmentation chain transfer (RAFT) polymerization, various multiply thermo-responsive block copolymers have been synthesized. The adoption of the reversible addition fragmentation chain transfer (RAFT) technique into the dispersion polymerization is potential to synthesize well-defined polymer particles with controlled and narrowly distributed molecular weight. However, the growth of block copolymer nano-objects during the polymerization-induced self-assembly under dispersion polymerization is not clarified yet. Herein, we synthesize doubly thermo-responsive brush-linear diblock copolymers by RAFT polymerization and investigate their doubly thermo-responsive micellization. Then we aim to clarify the growth of block copolymer nanoparticles in the polymerization-induced self-assembly, and the dispersion RAFT polymerization of styrene in the ethanol/water mixture mediated with brush or linear macro-RAFT agent is checked as typical example. The content of this dissertation involves the following three aspects:(1) Synthesis of doubly thermo-responsive brush-linear diblock copolymers of PmPEGV-b-PNIPAM and formation of core-shell-corona micellesIn this section, doubly thermo-responsive brush-linear diblock copolymer of poly[poly(ethylene glycol) methyl ether vinylphenyl]-block-poly(N-isopropylacrylamide)(PmPEGV-b-PNIPAM) is prepared by RAFT polymerization. The obtained brush-linear diblock copolymer exhibits two lower critical solution temperatures (LCSTs) corresponding to the linear poly(N-isopropylacrylamide)(PNIPAM) block and the brush poly[poly(ethylene glycol) methyl ether vinylphenyl](PmPEGV) block in water. This brush-linear diblock copolymer undergoes a two-step temperature sensitive micellization. At temperature above the first LCST, the brush-linear diblock copolymer self-assembles into core-corona micelles with the dehydrated PNIPAM block forming the core and the solvated brush PmPEGV block forming the corona. When temperature increases above the second LCST, the polystyrene backbone in the brush PmPEGV block collapses onto the dehydrated PNPAM core to form core-shell-corona micelles, in which the dehydrated PNIPAM block forms the core, the collapsed polystyrene backbone in the brush PmPEGV block forms the shell and the solvated poly(ethylene glycol) side-chains forms the corona. The effect of the length of the PNIPAM block and the length of the poly(ethylene glycol) side-chains on the thermo-responsive micellization and the size of core-shell-corona micelles is investigated.(2) Synthesis of thermo-responsive brush-linear block copolymer of PMEO2MA-b-PVEA with tunable UCST and LCST in the alcohol/water mixtureIn this section, the multi-thermo-responsive block copolymer of poly[2-(2-methoxyethoxy)ethyl methacrylate]-block-poly[N-(4-vinylbenzyl)-N,N-diethylamine](PMEO2MA-b-PVEA) displaying phase transition at both the lower critical solution temperature (LCST) and the upper critical solution temperature (UCST) in the alcohol/water mixture is synthesized by reversible addition-fragmentation chain transfer (RAFT) polymerization. The poly[2-(2-methoxyethoxy)ethyl methacrylate](PMEO2MA) block exhibits the UCST phase transition in alcohol and the LCST phase transition in water, while the poly[N-(4-vinylbenzyl)-N,N-diethylamine](PVEA) block shows the UCST phase transition in isopropanol and the LCST phase transition in the alcohol/water mixture. Both the polymer molecular weight and the co-solvent/non-solvent exert great influence on the LCST or UCST of the block copolymer. By adjusting the solvent character including the water content and the temperature, the block copolymer undergoes multi phase transition at LCST or UCST, and various block copolymer morphologies including inverted micelles, core-corona micelles and corona-collapsed micelles are prepared.(3) The RAFT dispersion polymerization of styrene mediated by brush macro-RAFT agent of PmPEGV-TTC and linear macro-RAFT agent of PDMA-TTCPolymerization-induced self-assembly is demonstrated to be a valid strategy to prepare highly concentrated block copolymer nano-objects. In this section, the investigation on the growth of block copolymer nanoparticles through the macro-RAFT agent mediated dispersion polymerization was made by employing the brush macro-RAFT agent of poly[poly(ethylene oxide) methyl ether vinylphenyl] trithiocarbonate (PmPEGV-TTC) and the linear poly(dimethylacrylamide) tiithiocarbonate (PDMA-TTC) as typical example. Well-controlled dispersion RAFT polymerization employing either the brush or linear macro-RAFT agent was achieved and uniform block copolymer nanoparticles were obtained. The decreasing number of the block copolymer nanoparticles (Np) in the polymerization medium and the increasing aggregation number (Nagg) of the block copolymer nanoparticles during the nanoparticle growth were detected, and both the particle-disassembly/reassembly and the chain extension of the block copolymer contributing to the growth of the block copolymer nanoparticles were concluded. The present study is anticipated to be helpful to clarify the growth of block copolymer nanoparticles via the polymerization-induced self-assembly under dispersion conditions.