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.     

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.     

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.     

Synthesis of chromophore-labelled polystyrene/poly(ethylene oxide) diblock copolymers

Polystyrene/poly(ethylene oxide) diblock copolymers bearing anthracene or phenanthrene groups at the block junctions were synthesized by sequential anionic polymerization using cumylpotassium as initiator. Labelled copolymers were characterized by means of ultraviolet (UV) spectroscopy, gel-permeation chromatography (GPC) and ¹H NMR spectroscopy. The solutions of the prepared copolymers in water and 1,2-dichloroethane/methanol mixtures were studied by fluorescence spectroscopy. The intermolecular energy transfer process confirms the presence of organized structures (micelles) in solutions.     

Mechanical relaxation in polystyrene/poly(ethylene oxide) block copolymers

Mechanical relaxation behaviors were investigated for twelve samples of polystyrene/poly(ethylene oxide) multiblock copolymers and two ABA-type copolymers using a torsion pendulum type viscometer. Logarithmic decrements of damped oscillations and shear moduli of polymer-impregnated glass braids were plotted in the temperature range between –130 and 130°C. The temperature for the maximum logarithmic decrement decreased with the molecular weight of the poly(ethylene oxide) blocks showing increased relaxation of the polystyrene blocks. Relative modulus values are also discussed in relation to the amounts of poly(ethylene oxide) blocks.     

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.     

Synthesis and characterization of poly(ethylene oxide) and poly(perfluorohexylethyl methacrylate) containing triblock copolymers

A new series of poly(perfluorohexylethyl methacrylate)‐block‐poly(ethylene oxide)‐block‐poly(perfluorohexylethyl methacrylate), PFMA‐b‐PEO‐b‐PFMA triblock copolymers has been synthesized by atom transfer radical polymerization using bifunctional PEO macroinitiators. The molecular structure of the block copolymers was confirmed by ¹H NMR spectroscopy and SEC. X‐ray scattering studies have been carried out to investigate their bulk properties. SAXS has shown cubic arrangement of spheres (bcc), hexagonally packed cylinders (hpc) and lamellar microdomain formation in the melt of triblock copolymers investigated, depending on composition. Crystallization was, however, found to destroy the ordered melt morphology and imposes a lamellar crystalline structure. WAXS, DSC and polarized light microscopy measurements confirmed the crystallization of PEO segments in block copolymers. Long PFMA blocks were found to have significant effect on PEO crystallization.

Synthesis of triblock copolymers of EO and FMA by ATRP.     

Novel amphiphilic branched copolymers based on polystyrene and poly(ethylene oxide)

Newly designed polystyrene (PS)/poly(ethylene oxide) (PEO) branched copolymers were obtained by a sequence of reactions involving both cationic and anionic polymerizations. Stars made of six PS-b-PEO arms and PS/PEO copolymers constituted of an inner PS part and an arborescent outer layer of PEO are two novel amphiphilic branched architectures that are described in this paper. The investigation of the solution properties of these copolymers by size exclusion chromatography (SEC) using tetrahydrofuran and an aqueous medium revealed that PS/PEO arborescent copolymers are more inclined to self-organize into unimicellar systems than their star homologues.     

Synthesis and characterization of polycaprolactone / poly(ethylene oxide) / polylactide tri-component copolymers

Polycaprolactonel/poly(ethylene oxide)/polylactide tri-component copolymers (PCEL) with different compositions were synthesized by copolymerization of ε-caprolactone and L-lactide in the presence of poly(ethylene glycol) using stannous octoate as a catalyst. The copolymers were purified and characterized by various analytical techniques such as GPC, FT-IR, H NMR, ¹³C NMR, DSC, and X-ray diffractometry. It was evidenced that these copolymers were pure tri-component compounds which exhibited partially random chain structures, and possessed good mechanical properties and variable biodegradability.     

Morphology,crystallization and thermal behaviour of poly(ethylene oxide)/poly(vinyl acetate) blends

Blends of poly(ethylene oxide) (PEO) and poly(vinyl acetate) (PVAc) show a unique value of the glass transition temperature, intermediate between that of plain polymers. The addition of PVAc to PEO causes a depression in both the spherulite growth rate (G) and the overall kinetic rate constant (K_n). Such depression is larger at higher undercooling and, at a given crystallization temperature, it increases with the content of PVAc. The experimental G and K_n data were analyzed by means of latest kinetic theories in order to determine the influence of composition on the process of surface secondary nucleation. The melt behaviour of PEO/PVAc blends cannot be explained only in terms of diluent effects due to the compatibility of the components in the melt. Especially, at lower undercooling it is likely that annealing and morphological effects must also be taken into account. The morphology of thin films of blends, isothermally crystallized from melt, suggests that an amorphous mixed phase (PEO + PVAc) is formed in interlamellar regions. It was found that plain PEO crystals grow according to a regime I process of surface secondary nucleation while in the case of blends the crystals of PEO grow via regime II mechanism.     

A DSC study of the miscibility of poly(ethylene oxide)-block-poly(DL-lactide) copolymers with poly(DL-lactide).

The purpose of this study was to examine the miscibility of poly(ethylene oxide)-block-poly(DL-lactide) copolymers with poly (DL-lactide). The copolymers L7E73L7 and L17E78L17 (L = carbonyloxymethylmethylene unit, OCOCH(CH3); E = oxyethylene unit, OCH2CH2) were synthesised by non-catalysed anionic polymerisation and characterised by gel permeation chromatography and 13C NMR. Blends of each of the copolymers with poly(DL-lactide) with compositions over the range from 10 to 90 wt% copolymer were cast as thin films and examined by differential scanning calorimetry (DSC) to determine glass transition temperatures (Tg) and melting temperatures (Tm). The phase diagram showed a region of miscibility above the melting point of the copolymer in the system (approx. 35-40 degrees C). Within this region the system was glassy at low mass fractions of oxyethylene in the copolymer (wE < or = 0.1) and rubbery at higher mass fractions. Below Tm a mechanically compatible glassy blend existed at low wE whilst quenching of systems of higher wE led to phase separation, the biphasic region consisting of crystalline Em-sequences of copolymer separated from non-crystalline poly(DL-lactide). The phase diagram resulting from this study provides the means for the design of drug delivery systems based on blends of poly(DL-lactide) and poly(ethylene oxide)-containing components. The crystal melt boundary can be lowered by the use of block copolymers with short poly(ethylene oxide) blocks permitting the preparation of blends which are miscible at room temperature and rubbery or glassy according to composition.     

Synthesis and characterization of poly(dimethylsiloxane)-poly(ethylene oxide)-heparin CBABC type block copolymers

Heparin and poly(ethylene oxide) were coupled to a central anchoring block of poly(dimethylsiloxane) in order to investigate its blood compatible properties. Diamino telechelic poly(dimethylsiloxane) (PDMS-(NH2)2, Mw = 20,000) was first modified to isocyanate functionalities using toluene 2,4-diisocyanate. This modified PDMS was then coupled to diamino-telechelic poly(ethylene oxide) (PEO-(NH2)2, Mw = 2000, 4000, 6000) to create BAB type copolymers having terminal free amino groups. These amino groups were covalently coupled to heparin containing terminal aldehyde groups using sodium cyanoborohydride to yield a bioactive, CBABC type block copolymer. The physical characterization of these copolymers was performed with IR, NMR, sulphur elemental analysis, Wilhelmy plate contact angle, and differential scanning calorimetry (DSC). CBABC block copolymer surfaces demonstrated heparin bioactivity in in vitro evaluation, and improved nonthrombogenic properties during ex vivo A-A shunt experiments.     

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.     

Synthesis of amphiphilic star-shaped and graft copolymers based on polystyrene and poly(ethylene oxide)

Amphiphilic block macromonomers possessing a central unsaturation were synthesized by condensation of polystyrene half-ester of maleic acid {α-[2-(3-carboxyacryloyloxy)ethyl]-ω-sec-butylpoly[1-phenylethylene]} with poly(ethylene glycol) monoether or polystyrene-block-poly(ethylene oxide). In the radical monomer cis-trans-isomerization homopolymerization of the diblock macromonomers, four-to eight-armed amphiphilic star-shaped copolymers were obtained. Radical copolymerization of the diblock macromonomers with styrene led to graft copolymers with low degree of grafting. The triblock macromonomers proved to be unable to polymerize.     

Synthesis of ethylene oxide/methyl methacrylate diblock copolymers by group transfer polymerization of methyl methacrylate with poly(ethylene oxide) macroinitiators

Ethylene oxide/methyl methacrylate diblock copolymers were prepared by group transfer polymerization (GTP) of methyl methacrylate (MMA) with poly(ethylene oxide) macroinitiators ( 1 ). The macroinitiator containing silyl ketene acetal end-groups was synthesized by hydrosilation of poly(ethylene glycol) monomethacrylate which was previously prepared by esterification of poly(ethylene glycol) monomethyl ether (PEO OH). All synthesized macroinitiators initiated the GTP of MMA in tetrahydrofuran with tetrabutylammonium cyanide or tris(dimethylamino)sulfonium difluoride as catalysts. The polymerization proceeds rapidly at room temperature to quantitative yield. The block copolymers are contaminated with small fractions of PEO homopolymer as a result of uncomplete macroinitiator synthesis. Unreacted PEO—OH, which is present in very small amounts, reduces the concentration of active centers. Therefore, the degree of polymerization is not in good agreement with the theoretical value calculated for a living polymerization taking the monomer to macroinitiator ratio. However, after extraction of the PEO homopolymer, the block copolymers show a narrow molecular weight distribution. Induction periods in time-conversion curves obtained with 1 as initiator vanished nearly, when the reaction was started with the addition product of 1 and MMA. This is an indication, that in the system under investigation a slow initiation step between 1 and the first MMA unit precedes the propagation reaction. Tacticity measurements yield about 56% syndiotactic, 40% heterotactic and 4% isotactic triads for PMMA in the block copolymers.     

Phase diagram and structural study of polystyrene — poly(ethylene oxide) block copolymers, 1. Systems polystyrene/poly(ethylene oxide)/diethyl phthalate

By means of low angle X-ray scattering and differential scanning calorimetry, we have drawn the phase diagrams of poly(ethylene oxide)-polystyrene block copolymers in the presence of diethyl phthalate as a preferential solvent of polystyrene. Such systems exhibit two liquid-cristalline structures in terms of temperature and solvent concentration. At temperatures below about 50°C, a lamellar structure (LC) with crystallized poly(ethylene oxide) chains exists. Between 50 and about 170°C, we find a structure with melted poly(ethylene oxide) chains. Like for amorphous block copolymers, the type of the latter structure is governed by the copolymer composition. We have shown that in the LC structure, the poly(ethylene oxide) chains crystallize folding in two superposed layers, and we have studied the number of folds and the crystallinity of the poly(ethylene oxide) blocks as a function of solvent concentration, composition and rel. mol. mass of the copolymer, and of the crystallization temperature.     

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 and characterization of polycaprolactone/poly(ethylene oxide)/polylactide tri-componet copolymers

Polycaprolactone/poly(ethylene oxide)/polylactide tri-component copolymers (PCEL) with different compositions were synthesized by copolymerization of epsilon-caprolactone and L-lactide in the presence of poly(ethylene glycol) using stannous octoate as a catalyst. The copolymers were purified and characterized by various analytical techniques such as GPC, FT-IR, 1H NMR, 13C NMR, DSC, and X-ray diffractometry. It was evidenced that these copolymers were pure tri-component compounds which exhibited partially random chain structures, and possessed good mechanical properties and variable biodegradability.     

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.     

In vitro and ex vivo platelet interactions with hydrophilic-hydrophobic poly(ethylene oxide)-polystyrene multiblock copolymers

Hydrophilic-hydrophobic multiblock copolymers synthesized from telechelic oligomers of poly(ethylene oxide) (PEO) and polystyrene (PS) have been used to study the influence of hydrophilic and hydrophobic balance on interfacial interactions of these surfaces with blood components. In vitro coagulation assays show no inherent ability of these amphiphilic surfaces to affect contact activation or coagulation factors. In vitro platelet adhesion and release reactions from rabbit platelet-rich plasma were shown to be greatest on Biomer and PS homopolymer surfaces and least on cross-linked PEO surfaces, with the PEO-PS block copolymers demonstrating intermediate responses. These same substrates were tested in a new low-flow, low-shear arterio-artery shunt system in rabbits. Whole blood occlusion times were not a direct function of hydrophilic content as both PEO and PS homopolymers and Biomer showed short occlusion times, while PEO-PS block copolymers prolonged occlusion times considerably, depending on composition. Overall, results suggest that PEO-PS block copolymers promote unique whole blood responses in contrast to homopolymer and Biomer controls which are more complex than direct correlations to bulk hydrophilic and hydrophobic contents.