| Star (miktoarm) polymers have being gained great attention over the past decades due to their unique three-dimensional shape and properties. Compared with linear polymers, star polymers manifest many advantages, such as lower solution, melt viscosities and so on, due to their compact structures. Traditional strategy for the synthesis of star (miktoarm) block copolymer relies on the well-known anionic polymerization method, however, which required relatively harsh experimental conditions. The recent advances in controlled/"living"free radical polymerization (LFRP), such as, atom transfer radical polymerization (ATRP), stead Free radical polymerization (SFRP), reversible addition-fragmentation chain transfer (RAFT) polymerization, and single-electron transfer mediated living radical polymerization (SET-LRP), facilitated the synthesis of star (miktoarm) block copolymers with predetermined chemical compositions. However, it is also difficult to prepare star (miktoarm) block copolymers by only one LFRP method in some cases. Most recently, the combinations of two or several CRP methods, or combinations of them with other polymerization methods, such as ring opening polymerization (ROP), living anionic polymerization and"Click chemistry", have been successfully employed to synthesize the star (miktoarm) block copolymers.Amphiphilic copolymers have being attracted many interests because of a wide range of potential applications in biology, colloidal science, drug and gene delivery. The aggregated morphology of the self-assembly formed by the amphiphilic copolymers is affected by many factors, such as the chemical structure and molecular weight of the copolymers, the block sequence, the relative lengths of the hydrophobic and hydrophilic blocks, and the nature of solvents. Recent research has suggested that precise nature of the block copolymer chain architectures (linear or nonlinear) also plays an important role in self-assembly behaviors of the block copolymers.Our work in this thesis can be summarized as the following:(1) The novel trifunctional reversible addition-fragmentation chain transfer (RAFT) agent, tris(1-phenylethyl) 1,3,5-triazine-2,4,6-triyl trithiocarbonate (TTA), was synthesized and used to prepare the three-armed polystyrene (PS3) via RAFT polymerization of styrene (St) in bulk with thermal initiation. The polymerization kinetic plot was first order and the molecular weights of polymers increased linearly with the monomer conversions, while keeping narrow molecular weight distributions (Mw/Mn≤1.23). The number of arms of PS3 was analyzed by gel permeation chromatography (GPC), ultraviolet visible (UV-vis) and fluorescence spectra. Furthermore, the three-armed amphiphilic thermosensitive block copolymer, poly(styrene-b-N-isopropylacrylamide)3 (PS-b-PNIPAAM)3, with controlled molecular weight and well-defined structure was also successfully prepared via RAFT chain extension method using the obtained three-armed PS as the macro-RAFT agent and N-isopropylacrylamide as the second monomer. The copolymers obtained were characterized by GPC and 1H nuclear magnetic resonance (NMR) spectra. The self-assembly behaviors of the three-armed amphiphilic block copolymers (PS-b-PNIPAAM)3 in mixed solution (DMF/CH3OH) were also investigated by high performance particle sizes (HPPS) and transmission electron microscopy (TEM). Interestingly, the lower critical solution temperature (LCST) of aqueous solutions of the three-armed amphiphilic block copolymers (PS-b-PNIPAAM)3 decreased with the increase of relative length of PS in the block copolymers.(2) The novel trifunctional initiator, 1-(4-methyleneoxy-2,2,6,6-tetramethylpiperidinoxyl)-3,5-bi(bromomethyl)-2,4,6-trimethylbenzene (TEMPO-2Br), was sucessfully synthesized and used to prepare the miktoarm star amphiphilic poly(styrene)-(poly(N-isopropylacrylamide))2 (PS(PNIPAAM)2) via combination of atom transfer radical polymerization (ATRP) and nitroxide-mediated radical polymerization (NMRP) techniques. Furthermore, the star amphiphilic block copolymer, poly(styrene)-(poly(N-isopropylacrylamide-b-4-vinylpyridine))2 (PS(PNIPAAM-b-P4VP)2), was also prepared using PS(PNIPAAM)2 as the macroinitiator and 4-vinylpyridine as the second monomer by ATRP method. The obtained polymers were well-defined with narrow molecular weight distributions (Mw/Mn≤1.29). Meanwhile, the self-assembly behaviors of the miktoarm amphiphilic block copolymers, PS(PNIPAAM)2 and PS(PNIPAAM-b-P4VP)2, were also investigated. Interestingly, the aggregate morphology changed from sphere-shape micelles (4.7 < pH < 3.0) to a mixture of spheres and rods (1.0 < pH < 3.0), and rod-shape nanorods formed when pH value was below 1.0. The LCST of PS(PNIPAAM)2 (pH = 7) was about 31 oC and the LCST of PS(PNIPAAM-b-P4VP)2 was about 35 oC (pH = 3).(3) The pH-responsive amphiphilic A2B2 miktoarm star block copolymer, poly(acrylic acid)2-poly(vinyl acetate)2 ((PAA)2(PVAc)2), with controlled molecular weight and well-defined structure was successfully synthesized via combination of single-electron transfer mediated living radical polymerization (SET-LRP) and reversible addition-fragmentation chain transfer (RAFT) polymerization methods. Firstly, the precursor two-armed poly(t-butyl acrylate) (PtBA)2 functionalized with two xanthate groups was prepared by SET-LRP of t-butyl acrylate in acetone at 25 oC using the novel tetrafunctional bromoxanthate (Xanthate2-Br2) as an Iniferter (initiator-transfer agent-terminator) agent. The polymerization behavior showed typical LRP natures by the first-order polymerization kinetics and the linear dependence of molecular weight of the polymer on the monomer conversion. Secondly, the A2B2 miktoarm star block copolymer (PtBA)2(PVAc)2 was prepared by RAFT polymerization of VAc using (PtBA-N3)2(xanthate)2 obtained as the macro-RAFT agent. Finally, the pH-sensitive A2B2 amphiphilic miktoarm star block copolymer poly(acrylic acid)2-poly(vinyl acetate)2 ((PAA)2(PVAc)2) was obtained by selectively cleavage of t-butyl esters of (PtBA)2(PVAc)2. All the miktoarm star block copolymers were characterized by GPC, 1H NMR and FT-IR spectra. The self-assembly behaviors of the amphiphilic A2B2 miktoarm block copolymers (PAA)2(PVAc)2 were also investigated by transmission electron microscopy (TEM).(4) A novel amphiphilic A3B miktoarm star copolymer, poly(N-isopropylacrylamide)3-poly(N-vinylcarbazole) ((PNIPAAM)3(PVK)), was successfully synthesized by a combination of single-electron transfer living radical polymerization (SET-LRP) and reversible addition-fragmentation chain transfer (RAFT) polymerization. Firstly, the well-defined three-armed poly(N-isopropylacrylamide) (PNIPAAM)3 was prepared via SET-LRP of N-isopropylacrylamide in acetone at 25 oC using a tetrafunctional bromoxanthate iniferter (Xanthate-Br3) as the initiator and Cu(0)/PMDETA as a catalyst system. Secondly, the target amphiphilic A3B miktoarm star copolymer ((PNIPAAM)3(PVK)) was prepared via RAFT polymerization of N-vinylcarbazole (NVC) employing (PNIPAAM)3 as the macro-RAFT agent. The architecture of the amphiphilic A3B miktoarm star copolymers were characterized by GPC, 1H NMR spectra. Furthermore, the fluorescence intensity of micelle increased with the temperature and had a good temperature reversibility, which was investigated by dynamic light scattering (DLS), fluorescent and UV-vis spectra.(5) The well-defined miktoarm star terpolymer, poly(N-isopropylacrylamide)2-poly(N-vinylpyrrolidone-b-acrylic acid)2 and (poly(N-isopropylacrylamide-b- acrylic acid)2-poly(N-vinylpyrrolidone)2, were successfully prepared via a combination of single-electron transfer mediated living radical polymerization (SET-LRP) and RAFT polymerization techniques. All the miktoarm star block copolymers were characterized by GPC, 1H NMR. Interesting, this novel double hydrophilic miktoarm star terpolymer containing pH-responsive PAA and thermo-responsive PNIPAAM segments can self-assemble into four types of micellar aggregates by adjusting solution pH and temperature, the characteristic assembled structures were observed by photo and dynamic light scattering (DLS)。(6) Homo/miktoarm star polymers were successfully synthesized via combination of the"arm-first"and"coupling-onto"strategies. Firstly, the multifunctional coupling agent (core), 2, 4, 6-tris(3-ethynylphenyl)-1,3,5-triazine-2,4,6-triamine (TPTTA), was synthesized. Secondly, the linear polystyrene-Cl (PS-Cl) and poly(2-(dimethylamino)ethyl methacrylate)-Br (PDMAEMA-Br) were prepared by atom transfer radical polymerization (ATRP) method. Then, the linear PS-Cl and PDMAEMA-Br chains were modified by a nucleophilic substitution reaction with sodium azide. Finally, homo/miktoarm star polymers PS3 and PS(PDMAEMA)2 were designed by click reaction between the core (TPTTA) and the arm precursor (PS-N3 or PDMAEMA-N3). The structures of the PS3, PS(PDMAEMA)2 and the precursors were all characterized by 1H NMR, FT-IR, UV and GPC analysis. Moreover, the self-assembly behaviors of the miktoarm amphiphilic copolymer PS(PDMAEMA)2 was also investigated by transmission electron microscopy (TEM).(7) A clickable alkyne monomer, progargyl methacrylate (PgMA), was successfully polymerized in a well-controlled manner via ambient temperature single electron transfer initiation and propagation through the radical addition fragmentation chain transfer (SET-RAFT) method. The living nature of the polymerization was confirmed by the first-order kinetic plots, the linear relationships between molecular weights and the monomer conversions while keeping relatively narrow molecular weight distributions (Mw/Mn≤1.55), and the successful chain-extension with methyl methacrylate (MMA). The better controllability of SET-RAFT than other CRP methods is attributed to the less competitive termination in view of the presence of the chain transfer agent (CTA) as well as the Cu(II) that is generated in situ. Moreover, a one-pot/one-step technique combining SET-RAFT and"click chemistry"methods has been successfully employed to prepare the side-chain functionalized polymers.
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