Studies On Structural Design, Formation Machnism And Applications Of Hyperbranched Polymers

Highly branched polymer, because of its smaller hydrodynamic radius and more terminal functional groups comparing with linear polymer, is widely used in coatings, adhesives, drug delivery and so on. In recent years, long-subchain hyperbranched polymer, as a new member of highly branched polymer family, has drawn great attention and widespread research. Long-subchain hyperbranched polymer, as a new polymer material, combines the advantages of both linear polymers and conventional hyperbranched polymers. Long-subchain hyperbranched polymers have a highly branched structure and a long subchain, which determines that they have more terminal functional groups and smaller hydrodynamic radius comparing with linear polymers with the same molecular weight. Furthermore, the long-subchain structure indicates a greater probability of occurrence of intermolecular entanglement between the polymer chains, so that long-subchain hyperbranched polymers have the superior mechanical properties than conventional hyperbranched polymers. Now, more researchers and engineers recognize the importance of long-subchain hyperbranched polymers, and a stream of new synthetic methods have also been developed and utilized. However, there are still many problems to be solved, such as how to simplify the synthesis process, how to synthesize long-subchain hyperbranched polymers with complex structures, compositions or functionality and how to amplify their application. This thesis focuses on the formation machnism of long-subchain hyperbranched polymers, the synthesis of novel hyperbranched polymers and the application of these hyperbranched polymers, aimed at developing a simplified synthetic method to synthesize multi-component hyperbranched polymers having a complicated structure and high functionality, exploring their applications and the relationship between molecular structure and their properties, which mainly includes the following five areas:1. Self-cyclization during the formation of long-subchain hyperbranched polymer and its influence.Much work focuses on studying the machanism of self-condensation of ABx macromonomers, discussing how reaction conditions, such as reaction time, temperature, solvent polarity, the type and content of the ligand, influence the structure and molecular weight of the final product, but few of them concerns self-cyclization of macromonomer, which is a side reaction throughout the entire polymerization process and very difficult to avoid. In the actual course of the reaction, the effect of self-cyclization on propagating reaction is quite complicated. The author is trying to give (?) explanation based on the molecular structure. First of all, the long-subchain hyperbranched polymers can be considered as ABx polymers, where A and B are reactive. It still has the possibility of occurrence of intramolecular reaction, therefore, the self-cyclization may not only occur at the stage of macromonomers, but also at the stage of long-subchain hyperbranched polymers with different degrees of polymerization. Secondly, since a self-cycled macromonomer or self-cycled hyperbranched polymer can be regarded as monofunctional or multifunctional core of Bn, which could further react with the ABx hyperbranched propagating chains. In this work, we synthesized polystyrene with one alkynyl group at the center of the polymer chain and two bromide groups at each chain end via ATRP mechanism, which could be transfer to seesaw-type AB2 macromonomers by converting bromine groups into azide groups. Seesaw-type AB2 macromonomers having different molecular weight were prepared. The measurement of 1H-NMR, GPC and FT-IR were introduced to investigate the structure of long-subchain hyperbranched polymers prepared from AB2 macromonomers with different molecular weight, and to explore the mechanism about how self-cyclization side reaction influences the structure and the degree of polymerization of the synthsized long-subchain hyperbranched polymers.2. Synthetic mimics of antimicrobial peptides with potent antimicrobial activity and low hemolytic toxicityBacterial antibiotic-resistance and their rapid spread not only has posed a threat to global public health and caused widespread concern, but also provides a great impetus to the development of novel potent antimicrobial agents. Antimicrobial peptides (AMPs), as an important part of the innate immunity of multi-cellular organisms, are widely considered as a promising source of novel antimicrobials. Note that cationicity and amphipathicity are two structural motifs common to most AMPs and may account for their membrane lytic activity - the former may facilitate AMP-bacterium association while the latter may facilitate the subsequent membrane-destabilization processes. However, higher hydrophobicity leads to exponentially stronger hemolytic toxicity, resulting in weakly preferential toxicity to bacterial cells. In this chapter, we report on the development of long-subchain hyperbranched poly(aminoethyl acrylate), that was prepared via a facile synthesis procedure and effectively killed bacteria in suspension while exhibiting low hemolytic toxicity. Such potent long-subchain hyperbranched polycations may have implications in the fight against bacterial antibiotic-resistance.3. Biodegradable long-subchain hyperbranched block copolymers:synthesis, surface modification and drug loadingComparing with linear block copolymers, hyperbranched block copolymers can self-assemble to form uni-molecule or multi-molecular micelles at lower concentrations, which exhibit a higher stability as drug carriers. In addition, hyperbranched block copolymers also contain a lot of terminal functional groups, which could be further modified with targeting molecules or the drug molecule through responsive weak chemical bonds and so on to mediate the drug release process. In this chapter, we designed and synthesized seesaw-type triblock copolymer macromonomers with one alkynyl group in the middle of the polymer chain and two bromide groups in each chain end, Through self-condensation of macromonomers and deprotection of Boc groups in trifluoroacetic acid, we successfully synthesized biodegradable long-subchain hyperbranched block copolymers. By the combination of hydrophobic interaction of polylactide and charge interaction of positive poly (aminoethyl acrylate), charge-negative drugs, such as DNA or RNA, could be loaded efficiently. Furthermore, these nano-drugs could exhibit synergistic release behavior under acidic conditions. In order to avoid the immune response, the hyperbranched block copolymers were further surface-modified with PEG, to shield positive charge of long-subchain hyperbranched block copolymers and improve vehicle stability in aqueous solution.4. Synthesis of the novel fluorescent hyperbranched polymersExploration and development of novel polymerization reaction and new monomers play a vital role on the development of new polymer science. As we all know, the structure and properties of the polymer used for the synthesis depends largely on their monomers’ molecular structure. Moreover, without effective way of polymerization, conversion of monomers to polymers can not be achieved. The monomers of poly(triazole)s always contain alkynyl groups and organic azide groups. However, organic azides, particularly azide groups in small organic molecules, are not only of poor stability, prone to explosion and of high-risk operation, but also difficult to store and transport. In this work, we committed to the development of new monomers used to prepare hyperbranched poly(triazole)s, which not only improved the stability of the monomers and the efficiency of the reaction, but also reduced the operating risk. Interestingly, hyperbranched poly(triazole)s synthesized by this method, although themselves do not have fluorophores, still exhibit fluorescent properties. We studied hyperbranched poly(triazole)s solutions of different solvents, and the results show that the hyperbranched poly(triazole)s in methanol, DMF, water, and water/THF mixed solvent, exhibited their maximum emission wavelength at 400 nm, whereas in THF or methylene chloride their maximum emission wavelength was at 310 nm.5. Dual pH- and thermo-responsive hydrogels for drug deliveryStimuli-responsive polymers are capable to change their volume, permeability or other properties under external stimulus. The stimulus may be temperature, pH, electric field, or chemical substances. Based on these characteristics, the stimuli-responsive hydrogels are widely available in drug delivery, biosensors, catalysts, tissue engineering and other fields. Endowed hydrogels with dual pH- and thermo-responsiveness, there are more ways to mediate hydrogels’behavior, which extends the application of intelligent hydrogels. In this chapter, we use copolymers of DMAEMA (or NIPAM) and acrylic acid propynyl ester (ProA) to react with macromolecular cross-linking agent of PNIPAM (or PDMAEMA) with two azide groups at one chain end via click reaction. We prepared two types of comb-type grafted hydrogels with reverse crosslinkage-graft architecture and compared their responsive behavior and controlled release behavior, which reveals not only pH- and temperature-responsive behaviors, but also drug release behavior are strongly dependent on crosslinkage-graft architecture.