Biodegradable and thermoreversible hydrogels of poly(ethylene glycol)-poly(epsilon-caprolactone-co-glycolide)-poly(ethylene glycol) aqueous solutions

The aqueous solutions of triblock copolymers of poly(ethylene glycol)-poly(epsilon-caprolactone-co-glycolide)-poly(ethylene glycol) [PEG-P(CL-GA)-PEG] undergoing sol-gel transition as the temperature increases from 20 to 60 degrees C were successfully prepared. The thermogelling block copolymers were synthesized by subtle control of the hydrophilic/hydrophobic balance and the chain microstructures. The amphiphilic block copolymer formed micelles in aqueous solution, and the micelle aggregated as the temperature increased. The sol-gel transition of the copolymer aqueous solutions was studied focusing on the structure-property relationship. GA was incorporated into the polymer chain to prevent crystallization of PCL component and increase the polymer degradation. It is expected to be a promising long-term delivery system for pH-sensitive drugs, proteins, and genes.     

Electrospinning of chitosan solutions in acetic acid with poly(ethylene oxide)

Electrospinning of chitosan solutions with poly(ethylene oxide) (PEO) in an aqueous solution of 2 wt% acetic acid was studied. The properties of the chitosan/PEO solutions, including conductivity, surface tension and viscosity, were measured. Morphology of the electrospun chitosan/PEO was observed by using scanning electron micrographs. Results showed that the ultrafine fibers could be generated after addition of PEO in 2:1 or 1:1 mass ratios of chitosan to PEO from 4-6 wt% chitosan/PEO solutions at 15 kV voltage, 20 cm capillary-collector distance and flow rate 0.1 ml/h. During electrospinning of the chitosan/PEO solutions, ultrafine fibers with diameters from 80 nm to 180 nm were obtained, while microfibers with visually thicker diameters could be formed as well. Results of X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy and differential scanning calorimeter exhibited the larger electrospun microfibers were almost entirely made from PEO, while the electrospun ultrafine fibers mainly contained chitosan.     

Electrospinning of chitosan solutions in acetic acid with poly(ethylene oxide)

Electrospinning of chitosan solutions with poly(ethylene oxide) (PEO) in an aqueous solution of 2 wt% acetic acid was studied. The properties of the chitosan/PEO solutions, including conductivity, surface tension and viscosity, were measured. Morphology of the electrospun chitosan/PEO was observed by using scanning electron micrographs. Results showed that the ultrafine fibers could be generated after addition of PEO in 2:1 or 1:1 mass ratios of chitosan to PEO from 4-6 wt% chitosan/PEO solutions at 15 kV voltage, 20 cm capillary-collector distance and flow rate 0.1 ml/h. During electrospinning of the chitosan/PEO solutions, ultrafine fibers with diameters from 80 nm to 180 nm were obtained, while microfibers with visually thicker diameters could be formed as well. Results of X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy and differential scanning calorimeter exhibited the larger electrospun microfibers were almost entirely made from PEO, while the electrospun ultrafine fibers mainly contained chitosan.     

Aggregation of poly(methacrylic acid)-poly(ethylene oxide) complex in aqueous medium

It is known that poly(methacrylic acid) and poly(ethylene oxide) interact with each other through hydrogen bonds and form polymer complexes in an aqueous medium. The initially formed polymer complexes were considered to exist in a semi-stable state, and they aggregated through desolvation and hydrophobic interaction. A successive aggregation of polymer complexes was observed following the rapid initial complexation. The aggregation was affected by some chemical factors e.g., polymer concentration, temperature, pH and so on. These effects were measured as the changes of the molecular shape by means of laser-light scattering, turbidity measurements, and scanning electron microscopy. It was observed that lower pH and higher temperature made the aggregation faster within such experimental conditions as pH 2–7, 20–50°C. The aggregates were nearly spherical with almost the same diameter (200 nm), and spontaneously grew larger with time.     

Reverse thermo-responsive poly(ethylene oxide) and poly(propylene oxide) multiblock copolymers

Aiming at developing new reverse thermo-responsive polymers, poly(ethylene oxide)-poly(propylene oxide) multiblock copolymers were synthesized by covalently binding the two components using carbonyl chloride and diacyl chlorides as the coupling molecules. The appropriate selection of the various components allowed the generation of systems displaying much enhanced rheological properties. For example, 15 wt% aqueous solutions of an alternating poly(ether-carbonate) comprising PEO6000 and PPO3000 segments, achieved a viscosity of 140,000 Pas, while the commercially available Pluronic F127 displayed 5,000 Pas only. Furthermore, the structure of the chain extender played a key role in determining the sol-gel transition. While poly(ether-ester)s containing therephtaloyl (para) and isophtaloyl (metha) coupling units failed to gel at any concentration, a 15 wt% aqueous solution of the polymer chain-extended with phtaloyl chloride (ortho) gelled at 43 degrees C. The water solutions were also studied by dynamic light scattering and a clear influence of the PEO/PPO ratio on the aggregate size was observed. By incorporating short aliphatic oligoesters into the backbone, prior to the chain extension stage, reverse thermal gelation-displaying biodegradable poly(ether-ester-carbonate)s, were generated.     

Complex formation between poly(vinyl ether) of ethylene glycol and poly(acrylic acid) in aqueous and organic solutions

Interpolymer reactions between poly(acrylic acid) and poly(vinyl ether) of ethylene glycol were studied by viscometric and spectroturbidimetric methods in aqueous and organic solutions of different nature. It is shown that the formation of interpolymer complexes strongly depends on the strength of the polymer-solvent interaction. A decrease of the thermodynamical quality of the solvents should be favorable for the complexation process.     

Association and crystallization of poly(ethylene oxide)

The discontinuous change of the lamellar thickness with crystallization temperature was studied for low molecular weight fractions of OH-terminated poly(ethylene oxide) (PEO). IR analyses demonstrated that almost all of the molecular chain ends were associated in the molten state, whereas a large part of their ends were free in dilute solution. Discontinuous changes were observed for low molecular weight PEO fractions crystallized from the melt, whereas continuous changes were found both for PEO's crystallized from dilute solution and those with phenylated end groups crystallized from the melt. Accordingly, it was pointed out that the association of the end groups could play an important role in the crystallization mechanism and the conformation of the resultant PEO crystals.     

Synthesis of star-shaped poly(ethylene oxide)

Three different methods to synthesize star-shaped poly(ethylene oxide) are discussed in the present article. In all three cases, the branches are grown from a plurifunctional initiator. It is established that even though the early stages of the polymerization occur in heterogeneous phase, the consequences on the polymers formed are of minor importance. The most significant method is a core-first process, involving multifunctional poly-DVB cores as the initiating species, made anionically in dilute solution. Although strong association phenomena are occurring during the growth of the branches, star-shaped poly(ethylene oxide)s with a high number of functionalized branches are obtained. The polymers arising from all three methods were characterized accurately.     

Unique morphologies found in freeze-dried poly(ethylene oxide) prepared from dilute solutions

Freeze-dried particles of poly(ethylene oxide) (PEO) were prepared from sublimation of a 1 × 10⁻² wt.-% solution of PEO in benzene in an ice-salt bath. After isothermal crystallization at 318.2 ± 0.1 K for 2 h, an unusual planar zigzag form of PEO was found. A variety of unique spherulite-like morphologies were also observed, and their formation is discussed on the basis of the interaction between solvent molecules and segment of the polymer chains.     

The heats of dilution of poly(ethylene oxide)/water solutions

The heats of dilution of poly(ethylene oxide)/water solutions were measured by use of a micro twin calorimeter at 25°C. The degrees of polymerization of the polymers were 2, 5, 15, 25, 100, 500, 2500, and 15,000. For the analysis of the experimental data, the simple VAN LAAR formula has been used, together with a modified one; so the interaction parameters between the polymer and the solvent have been derived. It was found that the interaction parameters are considerably dependent not only on the concentration, but also on the molecular weight of the polymer, and that they correspond qualitatively with the theory of dilute polymer solutions.     

Influence of poly(ethylene oxide)‐poly(propylene oxide)‐poly(ethylene oxide) triblock copolymers on stereoselective catalysis in water

The first application of (EO)_n‐(PO)_m‐(EO)_n triblock copolymers as surfactants in asymmetric hydrogenation is described. We observed a dependence of the activity and enantioselectivity on the asymmetric hydrogenation of methyl (Z)‐α‐acetamidocinnamate on the length of the (PO)_m domain of the copolymer. The activity and enantioselectivity were observed to be independent of the hydrophilic‐lipophilic‐balance (HLB) or the critical micelle concentration (cmc) of the copolymer. The size of the (EO)_n unit does not play an important role, however the addition of a small amount of SDS enhances the enantioselectivities significantly. The triblock copolymers were also successfully used as phase transfer reagent in two‐phase Suzuki carbon‐carbon coupling reactions. The use of polymeric reagents in micellar and phase‐transfer systems has the advantage that these reagents can be easily separated from the reaction mixture by means of a membrane.     

Compatibility of poly(ethylene oxide) and poly(methyl methacrylate) chains in solutions of their blends,diblock and triblock copolymers

Solvent activity data were obtained from vapour pressure measurements at 50°C, 70°C, 100°C and reduced to χ-functions in systems consisting of benzene or toluene on the one hand and blends or block copolymers of poly(ethylene oxide) and poly(methyl methacrylate) on the other. The χ_BC-values are negative for all polymer subsystems, but miscibility on a segmental scale is somewhat different for block copolymers and blends. χ_BC depends on the concentration of solvent and is mainly influenced by the Δχ-effect in solution and correlated with the syndiotacticity of the PMMA chains. There is only a small influence of temperature and blend composition or block ratio.     

Additive effect of poly(ethylene oxide), 1 acceleration effect of poly(ethylene oxide) in several nucleophilic reactions

The additive effect of poly(ethylene oxide) (PEO) in several nucleophilic reactions, such as the oxidation of trans-stilbene with potassium permanganate, the alkylation of potassium acetate and diethyl benzylsodiomalonate as well as the Williamson reaction of sodium phenoxide with alkyl bromides was investigated and compared with that of dibenzo-18-crown-6 (crown ether). A marked effect by PEO was observed under certain conditions, which was explained by a cooperative coordination of the oxygen atoms of PEO with metal cations promoting ion dissociation and resulting in the observed remarkable rate acceleration.     

Biodegradable poly(ethylene oxide)/poly(epsilon-caprolactone) multiblock copolymers.

A series of poly(ethylene oxide) (PEO)/poly(epsilon-caprolactone) (PCL) containing biodegradable poly(ether ester urethane)s, covering a wide range of compositions, were synthesized and characterized. The synthesis consisted of a two-step process. During the first step, the ring-opening reaction of epsilon-caprolactone was carried out, initiated by the hydroxyl terminal groups of the PEO chain. The second step involved the chain extension of these PCL-PEO-PCL trimers with hexamethylene diisocyanate. By varying either the ethylene oxide/epsilon-caprolactone ratio or the length of both segments, we obtained a series of polymers having different morphologies and displaying a broad range of properties.     

The phase behaviour of a poly(ether sulfone) with poly(ethylene oxide)

Blends of poly(ethylene oxide) with a poly(ether sulfone) were prepared by casting from a common solvent and were found to be miscible and show a single, composition dependent, glass transition temperature. Mixtures in both cyclohexanone and N,N-dimethylformamide phase, separated on heating and thus conditions need to be carefully chosen to obtain homogeneous blends. At higher PEO contents, PEO crystallised from the blends at lower temperatures. The melting point depression, as determined by trubidity measurements, was used to calculate an interaction parameter which was negative, as expected for miscible polymers. The blends also phase separated on heating, and the cloud point curve could be measured by turbidity measurements and confirmed by both visible and electron microscopy. The cloud point curve was very skew with a minimum at around 10 wt.-% PES content. This was not a strong function of the molecular weight and the skew nature was thus presumably due to differences in the state parameters of the pure components. The blends showed a very high mobility with sharp and reproducible could points which might make them ideal for future miscibility studies.     

Fractionation of functional polystyrenes, poly(ethylene oxide)s and poly(styrene)-b-poly(ethylene oxide) by liquid chromatography at the exclusion-adsorption transition point

The paper reports the fractionation of functional polystyrenes (PSs) and poly(ethylene oxide)s (PEOs) as well as their block copolymers, by liquid chromatography at the exclusion adsorption transition point (EATP-LC), also called "critical conditions" mode. In this specific elution mode (EATP-LC), the fractionation is only governed by the nature and the number of functions attached to the polymer backbone, independent of the molar mass distribution of the whole sample. Functional polystyrenes (alpha- and/or alpha,omega-alcohol-, acetal-, aldehyde- and acidic-PS) could be readily separated from non-functional polystyrenes under various chromatographic conditions. The technique also allowed the fractionation of poly(ethylene oxide)s and PS-PEO block copolymers. In the latter cases, moderately polar columns (grafted silica) and water-based polar eluents were required to obtain a satisfactory fractionation.     

Additive effect of poly(ethylene oxide), 2. An acceleration effect of poly(ethylene oxide) in a williamson reaction

The additive effect of poly(ethylene oxide) (PEO) in the Williamson reaction between sodium phenolate and butyl bromide was investigated. Under certain conditions a remarkable rate acceleration by PEO was observed in this reaction, which is explained by the fact that a cooperative coordination of oxygen atoms of PEO with metal cations promotes ion dissociation. The relationship between the formation of a PEO-sodium phenolate complex and the reaction rate, and also the dependency of the reaction rate on the molecular weight of PEO were examined.