Studies On The Synthesis And Properties Of Highly Branched Long-subchain Polymers

Highly branched polymers have been widely used in adhensives, coatings and drug delivery resulting from its smaller hydrodynamic radius and higher functional end groups. In recent years, as the member of the highly branched polymer family, long chain hyperbranched polymers and long chain dendritic polymers were highlighted by experts. As one type of new materials, long chain hyperbranched polymer and long chain dendritic polymer combine the advantages of both linear polymer and highly branched polymer. The highly branched structure endows them with smaller hydrodynamic radius and higher functional end groups; and the long subchain between each branching points increases the probability of the intermolecular entanglement, which leads to impoved mechanical properties. Nowadays, more and more people have recognized the importance of long chain hyperbranched polymer and long chain dendritic polymer, and many new synthesis strategies have been also developed by experts. Even though, many problems concerning with the synthesis of long chain hyperbranched polymer and long chain dendritic polymer do still exist, such as too tedious synthesis routes, too simple composition and structure of the polymer and not outstanding functional performance of the polymer. In this thesis work, we have managed to explore the synthesis strategies of new type long chain hyperbranched polymer and long chain dendritic polymer and the dependences of properties on polymer structure. The work is concluded as follows:1. The Synthesis of "Defect-free" Long-chain Hyperbranched Polystyrene.For conventional hyperbranched polymer, some branching points would not be formed at the expected branching units due to the structure imperfection of monomers, the inhibited activity of the reactive groups and the steric hindrance. Thus, the expected branching units would exist as linear untis. The existence of linear untis would lead to the difference subchain length between each branching points and the defect of polymer structure, causing negtive influence on the performance of the hyperbranch polymer. In this work, we synthesized "defect-free" hyperbranch polystyrene with long subchain. Firstly, we synthesized seesaw type macromonomers by atom transfer radical polymerization. Then,"defect-free" hyperbranched polystyrene was synthesized by one step synthese via click chemistry. Unexpectedly, we found that the intramolecular cyclization of the seesaw type macromonomers would also occur. 2. Influence of chain conformation on Formation of Long-Subchain Hyperbranched Polystyrene from Seesaw-Type AB2MacromonomersThe reactivity of groups and spatial hindrance are two key factors to affect the organic reaction, while the spatial hindrance in polymer synthesis is strongly dependent on chain conformation. Thus, in this piece of work, chain conformation was specially considered in the synthesis of hyperbranched polystyrenes. Through atom transfer radical polymerization of styrene with1,3-dibromomethyl-5-propargyloxy-benzene as initiator followed by the conversion of bromine end-groups into azide end-groups, well-defined seesaw-type polystyrene (PSt) macromonomers with two molecular weights (Mn=8.0and28.0k) were obtained. Thus, a series of long-subchain hyperbranched (lsc-hp) PSt with high overall molar masses and regular subchain lengths were obtained via copper-catalyzed azide-alkyne cycloaddition click chemistry performed in THF and DMF, respectively. The polycondensation of seesaw-type macromonomers was monitored by gel permeation chromatography. Because DMF is the reaction medium with higher polarity, click reaction proceeds more easily in DMF. Therefore, the growth of lsc-hp PSt in DMF has faster rate than that in THF for the shorter seesaw-type macromonomer (Seesaw-8k). However, THF is the solvent with better solubility to PSt and leads to looser conformation of PSt chains. Thus, for the longer seesaw macromonomer (Seesaw-28k), lsc-hp PSt in THF has higher overall molar mass. As well, the self-cyclization of seesaw-type macromonomers also depends on both solvent and molar mass of macromonomer. The self-cyclization degrees of Seesaw-8k in DMF and THF are almost the same while that of Seesaw-28k macromonomer is obviously lower in THF. The experimental results suggest a physical consideration to control the growth of hyperbranched polymers.3. Solvent Replacement to Thermo-Responsive Nanoparticles from Long-Subchain Hyperbranched PSt Grafted with PNTPAM for EncapsulationLong-subchain hyperbranched polystyrene (lsc-hp PSt) with uniform subchain length was obtained through copper-catalyzed azide-alkyne cycloaddition click chemistry from seesaw macromonomer of PSt having one alkynyl group anchored at the chain centre and two azido group attached to both chain ends [alkynyl-(PSt-N3)2]. After precipitation fraction, different portions of lsc-hp PSt having narrow overall molecular weight distribution were obtained for further grafting with alkynyl-capped poly(N-isopropylacrylamide)(alkynyl-PNIPAM), which was obtained via single-electron transfer living radical polymerization of NIP AM with propargyl2-bromoisobutyrate as the initiator and grafted onto the peripheral azido groups of lsc-hp PSt via click chemistry. Thus, amphiphilic lsc-hp PSt grafted with PNIPAM chains (lsc-hp PSt-g-PNIPAM) was obtained and would have star-like conformation in tetrahydrofuran (THF). By replacing THF with water, lsc-hp PSt-g-PNIPAM was dissolved at molecular level in aqueous solution due to the hydrophilicity of PNIPAM and exhibited thermal induced shrinkage of PNIPAM arms. The water-insoluble lsc-hp PSt would collapse densely and could be served as a reservoir to absorb hydrophobic chemicals in aqueous solution. The influence of overall molecular weight of lsc-hp PSt on the absorption of pyrene was studied.4. Janus Long-Chain Hyperbranched Copolymers from Self-Assembly Mediated Alternating Click Reaction and Their Witting Behavior Adjustment with Film-Casting SolventSelf-Assembly Mediated Synthesis strategy was successfully adopted into the synthesis of Janus long-chain hyperbranched copolymer composed of polystyrene (PSt) and poly[oligo(ethylene glycol) methacrylate](POEGMA) hemispheres. Briefly, seesaw macromonomers with one alkynyl group at the chain center and two azido groups at the chain ends of PSt and POEGMA were used to prepare their own long-chain hyperbranched homopolymers (lhp-PSt and lhp-POEGMA) and the peripheral azido groups were converted into amino groups. Then, with the help of self-assembly in different selective solvents, lhp-PSt and lhp-POEGMA were linked alternatively onto one four-armed star copolymer of PSt and POEGMA with four azido end groups through click chemistry. The techniques of nuclear magnetic resonance, Fourier transfer infrared resonance, gel permeation chromatograph and dynamic light scattering (DLS) were used to confirm the successful preparation of μ-(lhb-PSt)(lhb-POEGMA) and its different precursors. The self-assembly behavior of different amphiphilic copolymers was investigated by transmission electron microscopy and DLS. Also, the component location of self-assemblies in different solvents was investigated with contact angle measurement on the films of amphiphilic copolymers. Variation of contact angle with the casting solvent was correlated with the component location. This report opens a novel route to prepare and confirm Janus long-chain hyperbranched copolymers.5. Physical Confinement to Janus Dendritic Copolymers with long Subchains and Their Generation-dependent Microphase Separation In this paper, we put forward a synthetic strategy combined with chemical and physical confinement to prepare Janus dendritic copolymers with long subchains. Taking advantage of this strategy, Janus dendritic copolymers with long poly(tert-butyl acrylate)(PtBA) and polystyrene (PSt) segments as each half-dendrons have been successfully obtained after careful purification. Herein, seesaw macronomers of PtBA and PSt with one alkynyl group at the chain centre and two bromo groups at the chain ends were used as the constructural units. Briefly, the first generation of Janus dendritic copolymer, J-(PSt)1/(PtBA)1,was synthesized under chemical confinement through click chemistry onto one core molecule with two end azido groups. Afterwards, the next two generations of Janus dendritic copolymers, J-(PSt)2/(PtBA)2and J-(PSt)2/(PtBA)2, have been obtained under alternating physical and chemical confinement. Precipitation fraction and thin layer chromatograph were used to separate the aimed products. The variations of chemical composition, end groups and molecular weight were monitored with the techniques of nuclear magnetic resonance, Fourier transfer infrared resonance and gel permeation chromatograph. Differential scanning calorimetry, small angle X-ray scattering, atom force microscopy as well as laser light scattering were used to investigate the microphase separation behavior in bulk and physical confinement in the selective solvent. The research results will push the pathway to polymer synthesis under physical principles and the study of self-assembly of copolymer with complicated architecture.6. A Bacterial Lipase-Sensitive PEGylated Long-Chain Dendritic Polycation:An Antimicrobial with Neutral Surface Charge but Targeted Membrane-Activity.Lipase-responsive long-chain dendritic polycation (lcd-polycation) with well-defined structure was successfully synthesized. Firstly, poly(t-butyloxycarbonyl-aminoethyl acrylate (PBoc-AEA) with three arms was synthesized from a tri-functional core via single electron transfer living radical polymerization (SET-LRP). Afterwards, led polycation of poly(aminoethyl acrylate)(lcd-PAEA) was synthesized by divergent synthetic strategy via step-wise SET-LRP followed by the removal of t-butyloxycarbonyl groups. The well-defined architecture of lcd-PAEA was confirmed by proton nuclear magnetic resonance spectroscopy, gel permeation chromatograph, Fourier transform infrared spectroscopy and laser light scattering (LLS). Plating killing assays indicated that lcd-PAEA showed high antibacterial activity against E. coil, P. aeruginosa, S. aureus and B. subtilis comparing with linear PAEA. Besides, lcd-PAEA demonstrates greatly diminished hemolytic toxicity. To improve the stability of lcd-PAEA in plasma, the positive charge was shielded by grafting ploy(ε-caprolactone)-b-poly(ethylene glycol)(PCL-b-PEG) onto lcd-PAEA via CuAAC click chemistry. The obtained lcd-PAEA-g-PCL/PEG was sensitive to lipase and the enzyme-mediated degradation was observed by dynamic LLS. Plating killing assays indicated that lcd-PAEA-g-PCL/PEG also had high antibacterial activities to S. aureus and E. coil. Both lcd-PAEA and lcd-PAEA-g-PCL/PEG disabled the bacterial cytoplasmic membranes of S. aureus and E. coil. This long-chain dendritic polycation with high antibacterial activity, lipase response and high plasma stability would be used in vivo.