Synthesis And Self-assembly Of Dendritic Star Copolymer Containing Multi Arms

Recently, much interest has been aroused in the research of dendritic star copolymers because they have the properties of both linear polymers and dendritic polymers. These polymers are generally prepared by living polymerization, such as living cationic/ anionic polymerization, cationic ring-opening polymerization (CROP), atom transfer radical polymerization (ATRP) and reversible addition–fragmentation transfer (RAFT) polymerization. In comparison with other living radical polymerization techniques, RAFT polymerization has the advantages of utilizing versatile functional vinyl monomers and easy operations. So it becomes a powerful tool to synthesize novel copolymers with complicated architectures. In addition, the self-assembly behaviors of dendritic star copolymers is still at the very early beginning compared with that of linear polymer. There are still many key questions to resolve, such as synthesizing more dendritic star copolymers with novel structure by living polymerization, studying the relationship of the polymer molecular structure and self-assembly behaviors, carefully investigating the micro-phase separation of the dendritic star copolymer, obtaining new assemble shape and establishing new characterization methods.All the above facts lead to the origin and impetuses of this thesis. The main research work in this thesis is the synthesis of several different dendritic star copolymers as well as their self-assembly behaviors in selective solvents. The main results obtained in this thesis are as follows:1. The dendritic macroRAFT agent has been successfully synthesized by a two-step approach. The first step involved the reaction of terminal hydroxyl groups in the dendritic core with Maleic anhydride (MAh) to obtain the dendritic core with 16 terminal vinyl groups. The second step involved the reaction of double bond in the obtained dendritic core with dithiobenzoic acid (DTBA) to form the dendritic macroRAFT agent with nearly 16 dithiobenzoate groups. By the'core-first'approach, the macroRAFT agent was used to induce the RAFT polymerization of 2-(Dimethylamino)ethyl methacrylate(DMAEMA) to obtain the dendritic multiarm copolymer of H20-star-PDMAEMA. H20-star-PDMAEMA undergoes a thermosensitive phase transitions at the LCST of PDMAEMA, and the phase transition behaviors of the polymer with a moderate concentration in water are examined by DLS and fluorescence spectroscopy.2. RAFT copolymerization of Acrylic acid initiated by fractionated dendritic polyester (Bolton H20) based macroRAFT agent was conducted to obtain a dendritic multiarm star copolymer of H20-star-PAA. The pH-responsive phase transition and self-assembly behavior of H20-star-PAA at different solution pH were also studied by using dynamic light scattering (DLS) and transmission electron microscopy (TEM). The results show that at the lower polymer concentration, the dendritic macromolecules exist as unimolcular core-shell micelles with hydrophobic BoltornH20 as the cores and swollen poly (acrylic acid) (PAA) as the shells, and the PAA shells will collapse on the surface of the micelles with decreasing the solution pH. While in the high concentration of the polymer, H20-star-PAA self-assemble into large multimolecular micelles due to the secondary aggregation of unimolecular micelles stabilized by the intermolecular hydrogen bonding at a low solution pH.3. By the'core-first'approach, we synthesized a heteroarm star polymer with a dendritic core and multi alternating mixed polymer arms by combination of sequential CROP and RAFT polymerization. A special dendritic core with multi-alternating carboxylic acid and dithiobenzoate groups was synthesized from a dendritic polyester with 16 terminal hydroxyl groups. And then it was used to initiate the sequential CROP of tetrahydrofuran (THF) in the presence of ethylene oxide (EO) as the polymerization promoter and BF3 OEt2 as catalyst, and RAFT polymerization of methyl methacrylate (MMA). Finally, a heteroarm star copolymer with a dendritic core and multi-alternating arms of P(EO-THF) and PMMA were successfully prepared. In addition, the work also demonstrated a facile method for the ROP of THF in the presence of the polymerization promoter of ethylene oxide (EO). Carboxylic acid groups were directly used as the initiators to perform the one-step ROP copolymerization of EO and THF, where EO polymerized at an early stage and then the resulted PEO served as the macro-initiator to initiate the block copolymerization of THF by utilizing the great difference of the polymer activity between EO and THF.4. A new dendritic amphiphilic heteroarm copolymer containing multi alternating arms of P(EO-THF) and PDMAEMA on a dendritic core was synthesized by combination of sequential CROP and RAFT polymerization initiated by a dendritic macroinitiator capped with nearly 32 alternating terminal carboxyl acid and dithiobenzoate groups. The obtained mixed-arms dendritic star copolymer can self-assemble into spherical micelles with a core-shell-corona structure in mixed solvent of DMF/water. The self-assembly behavior was investigated by TEM, DLS and NMR spectrum and a possible self-assembly process is put forward. The hydrophobic dendritic H20 core and the PTHF segments associate into the hydrophobic insoluble core, and the extended PDMAEMA arms form the hydrophilic corona, while the hydrophilic PEO segments are restricted in the periphery of the hydrophobic insoluble core and form the hydrophilic shell. The obtained polymer micelles undergo a phase transitions at the LCST of PDMAEMA. During he LCST transition, the PDMAEMA chains shrike on the surface of the micelles core and are partially wrapped inside the PEO segments. Thus, the core-shell-corona structure of the micelles transformed into a core-shell structure with the PDMAEMA and PEO segments as the shell. The diameter of the micelles decreases greatly during the LCST transition, but no aggregation is found. Such a reversible phase transition behaviors of the micelles are examined by DLS and fluorescence spectroscopy.5. The dendritic copolymers with multiarm polymer brush have been successfully synthesized by RAFT of poly(ethylene glyco1)methyl ether methacrylate (PEGMA) initiated by dendritic macroinitiator. The dendritic multiarm polymer brush can self-assemble into spherical micelles in water. The self-assembly behavior was investigated by 1H NMR, DLS and TEM measurements, and a"multimicelle aggregate (MMA)"mechanism was suggested for the self-assembly process. The driving forces of aggregation are the hydrogen-bonding interactions between molecules and the hydrophobic interaction. In water, the dendritic brush polymers spontaneously self-assemble into small micelles with a core-shell structure driven by hydrophobic interaction, and then the small micelles further aggregate into larger MMAs by intermicellar interactions. The obtained polymer micelles undergo phase transitions at the LCST of PEGMA and the temperature is about 92 oC. When the solution was heated to the temperature above the LCST, hydrogen-bonding interactions between the water molecules and the PEGMA segments on the surface of the micelles were destroyed and the multimolecular micelles joined together to form the even larger aggregates driven by the increasing intermicellar hydrophobic interaction.