Lijian Liu

Wuhan University, Wu-han-shih, Hubei, China

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Publications (49)136.76 Total impact

  • Feifei Li, Longqiang Xiao, Lijian Liu
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    ABSTRACT: Metalloenzyme-inspired polymeric palladium catalyst, which was prepared by self-assembly of polymeric imidazole ligand with PdCl2, promoted carbene polymerization with high catalytic activity and reusability. Besides the atactic polycarbethoxycarbene (PCEC), small amount of stereoregular PCEC was also obtained with poly(imidazole-Pd)-mediated polymerization of ethyl diazoacetate (EDA). This is the first example that stereoregular polycarbene in isolated form is obtained with a palladium-initiating system, suggesting that the steric polymeric imidazole ligand could affect the carbene insertion process. Detailed end group analyses on the obtained atactic and stereoregular PCEC revealed the chain initiation and chain termination mechanisms. XPS analyses on the oxidation state changes of palladium center exhibited that PdⅡ was reduced to PdⅠ, which suggests an intramolecular electron transfer process occurred during the interaction of palladium and EDA. Kinetic study shows a zero order kinetic dependent on the concentration of EDA, which indicates a strong binding of EDA on the catalyst preceding carbene formation. With the addition of radical inhibitor, the reaction rate was found to be greatly decreased. In the presence of spin trap, a striking electron paramagnetic resonance (EPR) spectrum could be detected, which suggests the involvement of radical intermediates. DFT calculations on the proposed Pd-carbene radical species supported the EPR analyses, which assigned a significant amount of spin density on the carbene-carbon atom. Based on these results, we proposed an unprecedented metal-carbene radical polymerization mechanism, which occurrs through a stepwise carbene radical migration and insertion process to afford polycarbene.
    07/2015; DOI:10.1039/C5PY00934K
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    ABSTRACT: Abundant research efforts have been devoted to meet the demands for high-strength hydrogels in biomedical applications. Double-network (DN) hydrogel and homogeneous hydrogel are two typical samples. In this study, a novel ultra-strong and resilient double macromolecular network (DMMN) hydrogel system has been developed via two-step sequential cross-linking process using hyaluronan (HA) and high molecular weight poly(ethylene glycol) (PEG) for the first and second network, respectively. The lower concentration of HA precursor solution and higher concentration of PEG precursor solution as well as higher molecular weight of PEG precursor is beneficial to produce high-strength DMMN gels. Dynamic light scattering measurements demonstrate that the DMMN gels possess the more evenly distributed polymer networks; the distinctive double and relative evenly distributed networks of DMMN gel makes it combine the current DN and homogeneous network strategies for preparing robust hydrogels. The optimized DMMN gel is capable of sustaining up to 50 MPa of compressive stress. Besides, DMMN gels exhibit excellent cytocompatibility. This study expands DN principle in designing and fabricating high-strength hydrogels with biocompatible macromolecules that show a promising prospect for biomedical applications.
    07/2015; DOI:10.1039/C5TB00867K
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    ABSTRACT: Poly(imidazole-Cu)-mediated C1/C1N2 copolymerization of ethyl diazoacetate involved copper-based carbene radical and α-carbonyl diazomethane radical intermediates, which could be captured by radical traps and detected by room-temperature EPR spectroscopy. The obtained C1/C1N2 copolymer could be used as a new kind of upconversion agent for non-invasive bioimaging study.
    Chemical Communications 06/2015; DOI:10.1039/C5CC03716F · 6.72 Impact Factor
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    ABSTRACT: A double-network (DN) hydrogel was prepared through the sequential photopolymerization of oligo(trimethylene carbonate) (TPT)-block-poly(ethylene glycol)-block-oligo(trimethylene carbonate) diarylate and methacrylated chitosan (CS–MA). The swelling behavior and mechanical properties of the hydrogels were tunable via the control of the concentration of CS–MA. Under physiological conditions, the fracture stress of the DN hydrogel reached 3.4 MPa; this was more than twice that of the corresponding TPT-block-poly(ethylene glycol)-block-TPT single network (1.6 MPa). At high ionic strength, the fracture stress of the DN hydrogel reached 6.4 MPa. The DN hydrogel exhibited good cytocompatibility, as revealed by a live–dead assay. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 42459.
    Journal of Applied Polymer Science 06/2015; DOI:10.1002/app.42459 · 1.64 Impact Factor
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    ABSTRACT: Conventionally, each fluorophore is only illuminated with the wavelength of incident light shorter (downconversion) or longer (upconversion) than that of emitted light. However, the oligopyrazolines presented in this paper are the down- and upconversion fluorescent molecules with a maximum emission at 445 nm, which are facilely synthesized from diazoacetates and maleic anhydride (MA) without any catalyst by a newly developed one-pot process. The intermediates of oligopyrazolines are formed through 1,3-dipolar cycloaddition and a cascade ring-opening insertion, which transform into oligomers in situ with molecular weight around 2 × 103 g mol−1. The structures of the subunits and resulting oligopyrazolines are clarified clearly by MALDI-TOF-MS and the quantum yield with excitation at 350 nm is in the range of 12–19%. A cell experiment with HeLa cells demonstrates its promising potential for bioimaging applications.
    04/2015; 6(22). DOI:10.1039/C5PY00336A
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    ABSTRACT: Abstract Hyaluronic acid (HA) is made up of repeating disaccharide units ((-1,4-D-glucuronic acid and (-1,3-N-acetyl-D-glucosamine) and is a major constituent of the extracellular matrix (ECM). HA and its derivatives which possess excellent biocompatibility and physiochemical properties have been studied in drug delivery and tissue engineering applications. Tyramine (Tyr)-based HA hydrogel with good compatiblity to cell and tissue has been reported recently. However, inferior mechanical property may limit the biomedical application of the HA hydrogel. In this study, HA/graphene oxide (GO) nanocomposite (NC) hydrogel was prepared through a horseradish peroxidase (HRP) catalyzed in situ crosslinking process. As compared with pure HA hydrogels, incorporation of GO to the HA matrix could significantly enhance the mechanical properties (storage moduli 1800 Pa) of the hydrogel and prolong the release of rhodamine B (RB) as the model drug from the hydrogel (33 hours) as well. In addition, due to the multiple interactions between graphene oxide and RB, the NC hydrogels showed excellent pH responsive release behavior. The release of RB from the NC hydrogel was prolonged at low pH (pH 4.0) in the presence of GO, which could be attributed to the enhanced interactions between GO and HA as well as with RB. In situ three-dimensional encapsulation of mouse embryonic fibroblasts (BALB 3T3 cells) in the NC hydrogels and cytotoxicity results indicated the cytocompatibility of both the enzymatic crossslinking process and HA/GO NC hydrogels (cell viability 90.6±4.25%). The enzymatically catalyzed fabrication of NC hydrogels proved to be an easy and mild approach, and has great potential in the construction of both tissue engineering scaffolds and stimuli-responsive drug release matrices.
    Journal of Biomaterials Science Polymer Edition 01/2015; 26(6):1-31. DOI:10.1080/09205063.2015.1007413 · 1.36 Impact Factor
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    ABSTRACT: Kinetic studies of carbene polymerization of ethyl diazoacetate (EDA) by palladium (Pd) or rhodium (Rh) catalysts were investigated by real-time fourier transform infrared (FTIR) spectroscopy. Both (L-prolinate)RhI(1,5-cyclooctadiene) and (L-prolinate)RhI(2,5-norbornadiene) mediated EDA polymerization were proved as first order reactions, suggesting that the formation of “Rh-carbenoid” is the rate determining step. The activation energy (11.24 kJ•mol−1) of the polymerization of “carbenes” generated from EDA with (L-prolinate)RhI(1,5-cyclooctadiene) as the catalyst was calculated from kinetic data via Arrhenius equation. On the other hand, the polymerizations of EDA catalyzed by three kinds of Pd-catalysts were revealed as zero order reactions, suggesting that the rate determining step involves the formation of EDA-Pd transition state complex through a coordinated step. Refilling more EDA to the (bis(acetonitrile)dichloropalladium)-mediated carbene polymerization system did not change the reaction order. The rate constant increases gradually with the enlargement of the dosage of the catalyst and decreases with the cycle-index, which proves the formation of “EDA-Pd transition state complex” and the propagating species with Pd-C bonds at the end of the polymer chain.
    RSC Advances 08/2014; 4(79). DOI:10.1039/C4RA05455E · 3.84 Impact Factor
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    ABSTRACT: Pseudopeptide oligomers as newly developing fluorescent macromolecules are of significant importance and fairly attractive in bioimaging of living cells. We report that a type of efficient fluorescent pseudopeptide oligo(3,4,5-triethoxycarbonyl-2-pyrazoline) is prepared by the novel cyclo-polycondensation of 3,4,5-triethoxycarbonyl-2-pyrazoline that is a weakly-fluorescent chromophore. The pseudopeptide oligomer exhibits a strong red-shift and remarkable fluorescence enhancement compared with the monomer 2-pyrazoline and images living cells in good quality, even though it only consists of fewer than 70 glycine residues whereas for the famous green fluorescent protein (GFP) the amount of the various natural amino acid residues is 238. Oligo(3,4,5-triethoxycarbonyl-2-pyrazoline) is a GFP-mimetic but structurally-simplified fluorescent pseudopeptide with simple chemistry, generated from the only natural resource of glycine.
    07/2014; 5(16). DOI:10.1039/C4PY00315B
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    ABSTRACT: The present contribution describes the innovation in the copolymerization of cyclic monomers, ε-caprolactam (ε-CL) and 2,2-dimethyltrimethylene carbonate (DTC), with ethyl diazoacetate (EDA). The characterizations of the obtained copolymers, poly(EA-ran-EDA-ran-ε-CL), poly(EA-ran-EDA-ran-DTC) (EA refers to ethyl acetate group CH3CH2OCOCH= from EDA by nitrogen-releasing), were performed by hydrogen nuclear magnetic resonance spectroscopy (1H NMR), carbon nuclear magnetic resonance spectroscopy (13C NMR) and size exclusion chromatography (SEC). In optimized conditions, the copolymer of ε-CL with EDA possessing Mn (number-average molar mass) of 1300 g/mol and PDI of 2.12 as well as that of DTC with EDA presenting Mn of 8000 g/mol and PDI of 1.47 were obtained. The incorporation of the azo group in the obtained copolymers was determined by the results of elemental analysis (in the range of 3.30-10.22%) and fourier transformed infrared (FT-IR) spectroscopy. Furthermore, the thermal properties of the obtained copolymers were examined by differential scanning calorimeter (DSC). X-ray diffraction (XRD) results showed that the synthesized copolymers were amorphous.
    Polymer International 07/2014; 63(7). DOI:10.1002/pi.4740 · 2.25 Impact Factor
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    ABSTRACT: Lipophilic 6-O-poly (ϵ-caprolactone)-L-ascorbic acid (AA-6-PCL) is synthesized through ROP of ϵ-caprolactone (CL). The number of repeating CL units in the polymer chain varies from 6 to 19. AA-6-PCL loaded supramolecular polymer micelles (SMPMs) are constructed with β-cyclodextrin (β-CD) and PCL as blocks. Transmission electron microscopy images show a nanospheric morphology of the micelles with a size range of 43.3 ± 5.0 nm. The drug loading contents are 22.53-39.23% for AA-6-PCL. AA-6-PCL exhibits high radical scavenging capacity (93.96-96.73%) and efficient scavenging potency, and a cytotoxicity study proves the excellent cytocompatibility of AA-6-PCL loaded β-CD/PCL SMPMs, which altogether herald their potential application in the study of the induced pluripotent stem cells.
    Macromolecular Bioscience 02/2014; 14(2). DOI:10.1002/mabi.201300197 · 3.65 Impact Factor
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    ABSTRACT: Polypyrazoles with molecular weights in the range of 6500–9300 g mol−1 and yields in the range of 71.4–82.5% were synthesized successfully by catalyst-free 1,3-dipolar cycloaddition of bisdiazo compounds to bisalkynes under thermal conditions and lost merely 5% of their weights at temperatures higher than 298 °C. The porous polypyrazole–copper complex with pore diameters in the range of 1–2 μm was prepared by the coordination of polypyrazole and copper sulfate (CuSO4), which is a reusable catalyst for the Huisgen 1,3-diploar cycloaddition of alkynes and organic azides to give the corresponding triazoles with yields over 90%.
    12/2013; 5(2). DOI:10.1039/C3PY01105D
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    ABSTRACT: Cyclopropane derivatives were prepared by the reaction of diazocompounds and alkene without catalyst under microwave irradiation, which demonstrated that it is an efficient method in organic chemistry. Compared with traditional metal catalyzed cyclopropane, this method is convenient and less harmful to environment. A second-order process was involved in the cyclopropanation as shown by the kinetic study.
    Chinese Journal of Organic Chemistry 12/2013; 33(12):2602. DOI:10.6023/cjoc201307030 · 0.86 Impact Factor
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    ABSTRACT: The ring-opening polymerization (ROP) of a cyclic ester using alkyl acetate carbene (ROCOCH:) is generated from diazoacetate as organocatalyst under microwave irradiation, which enables the one-pot preparation of copolymers of polyester and polyolefin. The chemical structure of the polymerized product is characterized by NMR, Fourier transformed infrared (FTIR), and UV-vis spectroscopy. The incorporation of the azo group into the obtained copolymer is determined by elemental analysis, which indicates that 1.38-6.21% nitrogen is contained in the obtained copolymers. The influences of catalyst and microwave irradiation parameters on the polymerization are investigated. Both the microwave power and irradiation time have great influences on the copolymerization. Moreover, the molar mass of the obtained polymers is calculated with polystyrene standards, which gradually increases from 600 to 36 100 g mol(-1) as the reaction temperature increases from 60 to 120 degrees C. Polymer with (M) over bar (n) of 36 100 g mol(-1) and PDI of 1.86 is produced under optimized conditions. The combination of ROP and carbene polymerization offers a new and convenient pathway to synthesize copolymers of polyesters and polyolefins.
    Macromolecular Chemistry and Physics 11/2013; 214(21). DOI:10.1002/macp.201300412 · 2.45 Impact Factor
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    ABSTRACT: Supramolecular polymer micelles were prepared on basis of the inclusion complexation between cyclodextrin carbamates and cellulose derivatives in aqueous media. Cyclodextrin carbamates were synthesized by microwave-assisted method from cyclodextrin and urea. The urea modified cyclodextrin shows the higher yield than the physical mixture of urea/cyclodextrin in the micellization with cellulose derivatives. The supramolecular structure of the core-shell micelles was demonstrated by (1)H NMR spectra, TEM images, and fluorescence spectra. The drug release behavior of the supramolecular polymer micelles was evaluated using prednisone acetate as a model drug. The drug loaded micelles showed steady and long time drug release behavior. With these properties, the supramolecular polymer micelles are attractive as drug carriers for pharmaceutical applications.
    10/2013; 98(1):982-7. DOI:10.1016/j.carbpol.2013.06.075
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    ABSTRACT: Nanocomposite hydrogels with tunable mechanical properties and drug release behavior were prepared by halloysite nanotube (HNT), oligo(trimethylene carbonate)-poly(ethylene glycol)-oligo(trimethylene carbonate) diacrylate (TPT), and alginate sodium (AG). The mechanical properties of the hydrogel were greatly enhanced by the addition of small amount of HNT (2 wt%). The drug release behavior of the hydrogels was examined using bovine serum albumin (BSA) as model drug, and TPT hydrogel showed a fickcian diffusion controlled release mechanism, while the BSA transport mechanism in TPT-HNT, TPT-AG, and TPT-AG-HNT hydrogels were anomalous transport. The cytocompatibility of the hydrogel were verified by the MTT assay, and the hydrogel showed comparable cytocompatibility with the widely recognized poly(ethylene glycol) hydrogel.
    Composites Science and Technology 08/2013; 85:126-130. DOI:10.1016/j.compscitech.2013.06.011 · 3.63 Impact Factor
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    ABSTRACT: To develop hydrogels with high mechanical strength for cell encapsulation and three-dimensional culture is a critical challenge for cartilage tissue engineering. In this study, novel double network (DN) hydrogels were fabricated and phosphate buffer solution (PBS) was used as the solvent to facilitate cell encapsulation. The tough DN hydrogels were prepared through a two-step photopolymerization using neutral oligo(2,2-dimethyltrimethylene carbonate)–poly(ethylene glycol)–oligo(2,2-dimethyltrimethylene carbonate)–diacrylate (DPD–DA) chains for the first network and methacrylated hyaluronic acid (HA–GMA) chains for the second network. The fracture stress values of the optimized and cell-laden DN hydrogels were 8.38 ± 0.67 MPa and 6.28 ± 1.26 MPa, respectively, which showed comparable mechanical strength with natural articular cartilage tissue. The live/dead cell viability assay of cell-laden DN hydrogels demonstrated the tough DN hydrogels have comparable cytocompatibility with PEG hydrogels. After long-term culture, cartilage-specific extra cellular matrix accumulated in these DN hydrogels. The results indicated the potential of the novel DN hydrogels as cartilage tissue engineering scaffolds.
    08/2013; 1(34). DOI:10.1039/C3TB20600A
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    ABSTRACT: Bisdiazo compounds were synthesized and underwent denitrogen alkene polymerization (DNAP) via the formation of an sp2 carbon bond in the presence of copper(II) catalysts, affording unsaturated polymers with molecular weights in the range of 2900 to 35400 Da. The copper-catalyzed denitrogen alkene polymerization of bisdiazo compounds allows the efficient synthesis of unsaturated polyesters.
    New Journal of Chemistry 06/2013; 37(7):1874-1877. DOI:10.1039/C3NJ00339F · 3.16 Impact Factor
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    ABSTRACT: The structure of graphene oxide (GO) enables it to act as the olefin in cyclopropanation reaction with ethyl diazoacetate (EDA) under microwave irradiation. This versatile synthetic method offers a powerful method for the rapid chemical modification of GO. The attachment of carbethoxycarbene (CEC) to GO was confirmed by the results of FTIR, XPS, Raman, and UV–vis analysis.
    Chemical Physics Letters 01/2013; 556:376–379. DOI:10.1016/j.cplett.2012.11.084 · 1.99 Impact Factor
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    ABSTRACT: A series of biodegradable hydrogels based on oligo(2,2-dimethyltrimethylene carbonate)-block-poly(ethylene glycol)-block-oligo(2,2-dimethyltrimethylene carbonate) diacrylate (DPD-DA) precursor with varied length of hydrophilic poly(ethylene glycol) (PEG) segment and hydrophobic oligo(2,2-dimethyltrimethylene carbonate) (ODTC) segment were prepared by photopolymerization. Hydrophobic interaction was found to affect the properties of the hydrogel. The elastic modulus and toughness of the hydrogel could be tuned by altering the lengths of the hydrophobic ODTC segment as well as the hydrophilic PEG segment. In a monolayer culture, the number of swine cartilage chondrocytes (SCCs) attached to the hydrogel surface increased along with an increase in the length of ODTC segment in the precursor. SCCs cultured on the surface of hydrogel and photo-encapsulated in the hydrogel demonstrated comparable cytocompatibility with the widely recognized PEG hydrogel.
    RSC Advances 11/2012; 3(1):157-165. DOI:10.1039/C2RA22622G · 3.84 Impact Factor
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    ABSTRACT: Supramolecular polymer micelles (SMPMs) were constructed from natural and natural-derived polymers: β-cyclodextrin (β-CD)/maleic anhydride modified β-cyclodextrin (MAh-β-CD) and methylcellulose (MC) in aqueous solution by one-pot self-assembly procedure, in which, β-CD and MAh-β-CD inclusion complexes were used as the hydrophilic shell and the free MC as the core. The shapes of the SMPMs were regular spheres with diameters of 25 ± 5 nm. The critical micelle concentrations, calculated from steady-state fluorescence emission spectra, were around 15.13 and 20.89 mg/L for MC/β-CD and MC/MAh-β-CD SMPMs, respectively. The in vitro drug release behaviors of the micelles were studied using prednisone acetate as a model drug, and the results showed that the MC/MAh-β-CD micelle had a drug-enrichment core and excellent drug released behaviors with a sustaining release time of 700 h.
    Carbohydrate Polymers 09/2012; 90(1):569–574. DOI:10.1016/j.carbpol.2012.05.079 · 4.07 Impact Factor