Polymerization of rac-lactide using schiff base aluminum catalysts: Structure, activity, and stereoselectivity

A series of aluminum ethyls and isopropoxides based upon N,N,O,O-tetradentate Schiff base ligand framework have been prepared. X-ray diffraction analysis and H-1 NMR confirmed that these Schiff base aluminum ethyls and isopropoxides were all monomeric species with a five-coordinated central aluminum in their solid structures. Compared to the aluminum ethyls which all retain their monomeric structure in the solution, the dinucleating phenomenons of aluminum isopropoxides with less steric hindered substituents in the solution have also been observed. The activities and stereoselectivities of these complexes toward the ring-opening polymerization of rac-lactide have been investigated. Polymerization experiments indicated that (SB-2d)(AlOPr)-Pr-i [(SB-2d) = 2,2-dimethyl-1,3-propylenebis(3,5-di-tert-butylsalicylideneiminato)] exhibited the highest stereoselectivity and (SB-3b)(AlOPr)-Pr-i [(SB-3b) = 2,2-dimethyl-1,3-propylenebis(3,5-dichlorinesalicylideneiminato)] possessed the highest activity among these aluminum isopropoxides. The substituents and the mode of the bridging part between the two nitrogen atoms both exerted significant influences upon the progress of the polymerizations, influencing either the tacticity of isolated polymers or the rate of polymerization.

Biodegradable poly(L-lactide)/poly(epsilon-caprolactone)-modified montmorillonite nanocomposites: Preparation and characterization

New nanocomposites were prepared by melt blending poly(L-lactide) (PLLA), poly(epsilon-caprolactone) (PCL), and organically modified montmorillonite (OMMT). The obtained nanocomposites showed enhanced tensile strength, modulus and elongation at break than that of PLLA/PCL blends. The dynamic mechanical analysis showed the increasing mechanical properties with temperature dependence of nanocomposites. Wide-angle X-ray diffraction analysis and transmission electron microscopy indicated that the material formed the nanostructure. Adding OMMT improved the thermal stability and crystalline abilities of nanocomposites. The morphology was investigated by environmental scanning electron microscopy, which showed that increasing content of OMMT reduces the domain size of phase-separated particles. The specific interaction between each polymer and OMMT was characterized by the Flory-Huggins interaction parameter, B, which was determined by the equilibrium melting point depression of nanocomposites. The final values of B showed that PLLA was more compatible with OMMT than PCL.

Formation of nonextinct ring-banded textures and multistacked lamella of tetra-aniline-block-poly(L-lactide) rod-coil diblock oligomer films induced by solvent vapor treatment

The self-assembly processes of the rod-coil diblock oligomer thin film of tetra-aniline (TANI)-block-poly(L-lactide) (PLLA) with different film thicknesses induced in the coil-selective solvent of acetone vapor at room temperature were studied. The morphologies of the oligomer films were determined by the film thickness. For the thicker film (232 nm), the nonextinct concentric ring-banded textures could form. While for the thinner and appropriate film (about 6 nm), multistacked diamond-shaped appearances with the periodic thickness being about 8.5 nm(6-nm-thick extended PLLA chain and 2.5-nm-thick p-pi conjugating TANI bimolecular layer) formed. The possible formation models of those two regular morphologies were presented in detail.

Self-assembly of crystalline-coil diblock copolymer in solvents with varying selectivity: From spinodal-like aggregates to spheres, cylinders, and lamellae

We have investigated systematically the morphology of thin films spin-coated from solutions of a semicrystalline diblock copolymer, poly(L-lactic acid)-block-polystyrene (PLLA-b-PS), in solvents with varying selectivity. In neutral solvents (chloroform and tetrahydrofuran (THF)), a spinodal-like pattern was obtained and the pattern boundary was sharpened by diluting the solution. Meanwhile, loose spherical associates, together with larger aggregates composed of these associates by unimer bridges, formed partly due to crystallization of the PLLA blocks in relatively concentrated solutions. In slightly PS-selective solvent (e.g., benzene), both loose and compact spherical micelles were obtained, depending on the polymer concentration, coexisting with unimers. When enhancing the selectivity with mixed solvents, for example, mixing the neutral solvent and the slightly selective solvent with a highly PS-selective solvent, CS2, loose assemblies (nanorods in CS2/THF mixtures and polydisperse aggregates in CS2/benzene mixtures) and well-developed lamellar micelles were obtained.

Stereoselective polymerization of rac-lactide with a bulky aluminum/Schiff base complex

An aluminum/Schiff base complex {[2,2-dimethyl-1,3-propylenebis(3,5-di-tert-butylsalicylideneiminato)](isopropanolato)aluminum(III) (2)} based on a bulky ligand and aluminum isopropoxide was prepared and employed for the stereoselective ring-opening polymerization (ROP) of rac-lactide (rac-LA). The initiator was characterized with nuclear magnetic resonance (NMR), crystal structure measurements, and elemental analysis. It contained a five-coordinate aluminum atom that was trigonal bipyramidal in the solid state according to the crystal structure measurements. The two conformational stereoisomers of 2 exchanged quickly on the NMR scale. Compound 2 polymerized rac-LA into a crystalline polymer that was characterized with H-1 NMR, wide-angle X-ray diffraction, electrospray ionization mass spectrometry, and gel permeation chromatography. The kinetics of the polymerization were first-order in both the monomer and initiator, and there was a linear relationship between the rac-LA conversion and the number-average molecular weight of poly(rac-LA) with a narrow molecular distribution (1.04-1.08). These features showed that the polymerization was well controlled. The high melting temperature (196-201 degreesC) and isotacticity of poly(rac-LA) indicated that complex 2 was a highly stereoselective initiator for the ROP of rac-LA.

Stereoselective polymerization of rac-lactide using a monoethylaluminum Schiff base complex

A monoethylaluminum Schiff base complex (2) with formula LA1Et (L = N,N'-(2,2-dimethylpropylene)bis(3,5-di-tei-t-butylsalicylideneimine) was synthesized and employed for the stercoselective ring-opening polymerization of rac-lactide (rac-LA). The complex 2 was characterized by nuclear magnetic resonance, crystal structure, and elemental analysis. It contains a five-coordinate aluminum atom with distorted trigonal bipyramidal geornetry in the solid state. In the presence of 2-propanol, 2 showed high stereoselectivity for the polymerization of rac-LA. The polymerization yielded crystalline poly(rac-LA) with a high melting temperature (193-201 degreesC). NMR, differential scanning calorimetry, and wide-angle X-ray diffraction indicated that the poly(rac-LA) was highly isotactic, and a stereocomplex was formed between poly-L- and poly-D-lactide block sequences. By the analysis of electrospray-ionization mass spectrometry and H-1 NMR, the polymer was demonstrated to be endcapped in both terminals with an isopropyl ester and a hydroxy group, respectively. The polymerization was of first order in rac-LA concentration. The relationship between the rac-LA conversion and molecular weights of the polymer was linear so that the polymerization could be well controlled.

Enzymatic degradation of poly(L-lactide) and poly (epsilon-caprolactone) electrospun fibers

Poly(L-lactide) (PLLA) and poly(epsilon-caprolactone) (PCL) ultrafine fibers were prepared by electrospinning. The influence of cationic and anionic surfactants on their enzymatic degradation behavior was investigated by measuring weight loss, molecular weight, crystallinity, and melting temperature of the fibers as a function of degradation time. Under the catalysis of proteinase K, the PLLA fibers containing the anionic surfactant sodium docecyl sulfate (SDS) exhibited a faster degradation rate than those containing cationic surfactant triethylbenzylammonium chloride (TEBAC), indicating that surface electric charge on the fibers is a critical factor for an enzymatic degradation. Similarly, TEBAC-containing PCL fibers exhibited a 47% weight loss within 8.5 h whereas SDS-containing PCL fibers showed little degradation in the presence of lipase PS. By analyzing the charge status of proteinase K and lipase PS under the experimental conditions, the importance of the surface charges of the fibers and their interactions with the charges on the enzymes were revealed. Consequently, a "two-step" degradation mechanism was proposed: (1) the enzyme approaches the fiber surface; (2) the enzyme initiates hydrolysis of the polymer.

Effect of crystallization on the lamellar orientation in thin films of symmetric poly(styrene)-b-poly(L-lactide) diblock copolymer

The effect of crystallization on the lamellar orientation of poly( styrene)-b-poly(L-lactide) (PS-PLLA) semicrystalline diblock copolymer in thin films has been investigated by atomic force microscopy (AFM), transmission electronic microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). In the melt state, microphase separation leads to a symmetric wetting structure with PLLA blocks located at both polymer/substrate and polymer/air interfaces. The lamellar period is equal to the long period L in bulk determined by small-angle X-ray scattering (SAXS). Symmetric wetting structure formed in the melt state provides a model structure to study the crystallization of PLLA monolayer tethered on glassy (T-c < T-g,T-PS) or rubber (T-c > T-g,T-PS) PS substrate. In both cases, it is found that the crystallization of PLLA results in a "sandwich" structure with amorphous PS layer located at both folding surfaces. For T-c <= T-g,T- PS, the crystallization induces a transition of the lamellar orientation from parallel to perpendicular to substrate in between and front of the crystals. In addition, the depletion of materials around the crystals leads to the formation of holes of 1/2 L, leaving the adsorbed monolayer exposure at the bottom of the holes.

BCNU-loaded PEG-PLLA ultrafine fibers and their in vitro antitumor activity against glioma C6 cells

The purpose of the present study was to develop implantable BCNU-toaded poly(ethylene glycol)poly(L-lactic acid) (PEG-PLLA) diblock copolymer fibers for the controlled release of 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU). BCNU was well incorporated and dispersed uniformly in biodegradable PEG-PLLA fibers by using electrospinning method. Environmental Scanning Electron Microscope (ESEM) images indicated that the BCNU-loaded PEG-PLLA fibers looked uniform and their surfaces were reasonably smooth. Their average diameters were below 1500 nm. The release rate of BCNU from the fiber mats increased with the increase of BCNU loading amount. In vitro cytotoxicity assay showed that the PEG-PLLA fibers themselves did not affect the growth of rat Glioma C6 cells. Antitumor activity of the BCNU-loaded fibers against the cells was kept over the whole experiment process, while that of pristine BCNU disappeared within 48 h. These results strongly suggest that the BCNU/PEG-PLLA fibers have an effect of controlled release of BCNU and are suitable for postoperative chemotherapy of cancers.

Biodegradable amphiphilic triblock copolymer bearing pendant glucose residues: Preparation and specific interaction with Concanavalin A molecules

A novel biodegradable amphiphilic block copolymer PLGG-PEG-PLGG bearing pendant glucose residues is successfully prepared by the coupling reaction of 3-(2-aminoethylthio) propyl-R-D-glucopyranoside with the pendant carboxyl groups of PLGG-PEG-PLGG in the presence of N,N'-carbonyldiimidazole. The polymer PLGG-PEG-PLGG, i.e., poly {(lactic acid)-co-[(glycolic acid)-alt-(L-glutamic acid)]}-block-poly(ethylene glycol)-block-poly{( lactic acid)-co-[( glycolic acid)-alt-(L-glutamic acid)]}, is prepared by ring-opening copolymerization of L-lactide (LLA) with (3s)-benzoxylcarbonylethylmorpholine-2,5-dione (BEMD) in the presence of dihydroxyl PEG with molecular weight of 2000 as macroinitiator and Sn(Oct)(2) as catalyst, and then by catalytic hydrogenation. The glucose-grafted copolymer shows a lower degree of cytotoxicity to ECV-304 cells and improved specific recognition and binding with Concanavalin A (Con A). Therefore, this kind of glucose-grafted copolymer may find biomedical applications.

Preparation of core-sheath composite nanofibers by emulsion electrospinning

Uniform core-sheath nanofibers are prepared by electrospinning a water-in-oil emulsion in which the aqueous phase consists of a poly(ethylene oxide) (PEO) solution in water and the oily phase is a chloroform solution of an amphiphilic poly(ethylene glycol)-poly(L-lactic acid) (PEGPLA) diblock copolymer. The obtained fibers are composed of a PEO core and a PEG-PLA sheath with a sharp boundary in between. By adjusting the emulsion composition and the emulsification parameters, the overall fiber size and the relative diameters of the core and the sheath can be changed. A mechanism is proposed to explain the process of transformation from the emulsion to the core-sheath fibers, i.e., the stretching and evaporation induced de-emulsification. In principle, this process can be applied to other systems to prepare core-sheath fibers in place of concentric electrospinning and it is especially suitable for fabricating composite nanofibers that contain water-soluble drugs.

Co-electrospun poly(lactide-co-glycolide), gelatin, and elastin blends for tissue engineering scaffolds

In this study, we describe composite scaffolds composed of synthetic and natural materials with physicochemical properties suitable for tissue engineering applications. Fibrous scaffolds were co-electrospun from a blend of a synthetic biodegradable polymer (poly(lactic-co-glycolic acid), PLGA, 10% solution) and two natural proteins, gelatin (denatured collagen, 8% solution) and (x-elastin (20% solution) at ratios of 3:1:2 and 2:2:2 (v/v/v). The resulting PLGA-gelatin-elastin (PGE) fibers were homogeneous in appearance with an average diameter of 380 80 mn, which was considerably smaller than fibers made under identical conditions from the starting materials (PLGA, 780 +/- 200 nm; gelatin, 447 +/- 1.23 nm; elastin, 1060 170 nm). Upon hydration, PGE fibers swelled to an average fiber diameter of 963 +/- 132 nm, but did not disintegrate. Importantly, PGE scaffolds were stable in an aqueous environment without crosslinking, and were more elastic than those made of pure elastin fibers. To investigate the cytocompatibility of PGE, we cultured H9c2 rat cardiac myoblasts and rat bone marrow stromal cells (BMSCs) on fibrous PGE scaffolds. We found that myoblasts grew equally as well or slightly better on the scaffolds than on tissue-culture plastic. Microscopic evaluation confirmed that myoblasts reached confluence on the scaffold surfaces while simultaneously growing into the scaffolds.

Composite film of carbon nanotubes and chitosan for preparation of amperometric hydrogen peroxide biosensor

A new amperometric biosensor for hydrogen peroxide was developed based on cross-linking horseradish peroxidase (HRP) by glutaraldehyde with multiwall carbon nanotubes/chitosan (MWNTs/chitosan) composite film coated on a glassy carbon electrode. MWNTs were firstly dissolved in a chitosan solution. Then the morphology of MWNTs/chitosan composite film was characterized by field-emission scanning electron microscopy. The results showed that MWNTs were well soluble in chitosan and robust films could be formed on the surface. HRP was cross-linked by glutaraldehyde with MWNTs/chitosan film to prepare a hydrogen peroxide biosensor. The enzyme electrode exhibited excellent electrocatalytic activity and rapid response for H2O2 in the absence of a mediator. The linear range of detection towards H2O2 (applied potential: -0.2 V) was from 1.67 x 10(-5) to 7.40 x 10(-4) M with correction coefficient of 0.998. The biosensor had good repeatability and stability for the determination of H2O2. There were no interferences from ascorbic acid, glucose, citrate acid and lactic acid.

Evaluation of bioeffects of a long-term administering ChangLe on rat by proton NMR method and biochemical examination

High resolution H-1 nuclear magnetic resonance ( NMR) spectroscopy has been employed to assess long-term toxicological effects of ChangLe (a kind of rare earth complex applied in agriculture). Male Wistar rats were administrated orally with ChangLe at doses of 0, 0.1, 0.2, 2.0, 10 and 20 mg/kg body weight daily, respectively, for 6 months. Urine was collected at-day 30, 60, go and serum samples were taken after 6 months. Many low-molecular weight metabolites were identified by H-1 NMR spectra of rat urine. A decrease in citrate and an increase in ketone bodies, creatinine, DMA, DMG, TMAO, and taurine in the urine of the rats. receiving high doses were found by H-1 NMR spectra. These may mean that high-dosage of ChangLe impairs the specific region of liver and kidney, such as renal tubule and mitochondria. The decrease in citrate and the increase in succinate and alpha-ketoglutarate were attributed to a combination of the inhibition of certain citric acid enzymes, renal tubular acidosis and the abnormal fatty acid catabolism. The information of the renal capillary necrosis could be derived from the increase in DMIA, DMG and TMAO. The increase in taurine was due to hepatic mitochondria dysfunction. The conclusions were supported by the results of biochemical measure. merits and enzymatic assay.