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ABSTRACT: A Michael addition strategy involving the reaction between a maleimide double bond and amine groups is investigated for the synthesis of cryogels at subzero temperature. Low-molecular-weight PEG-based building blocks with amine end groups and disulfide-containing building blocks with maleimide end groups are combined to synthesize redox-responsive PEG cryogels. The cryogels exhibit an interconnected macroporous morphology, a high compressive modulus and gelation yields of around 95%. While the cryogels are stable under physiological conditions, complete dissolution of the cryogels into water-soluble products is obtained in the presence of a reducing agent (glutathione) in the medium. Cell seeding experiments and toxicologic analysis demonstrate their potential as scaffolds in tissue engineering.
Macromolecular Bioscience 03/2012; 12(3):383-94. · 3.89 Impact Factor
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ABSTRACT: A synthetic platform for the preparation of various norbornenyl (Nb) containing reversible addition–fragmentation chain transfer (RAFT) agents has been developed. The design of the chain transfer agents (CTAs) is based on the desymmetrization of α,α′-dibromo-p-xylene by monosubstitution with an alkoxide anion and subsequent replacement of the residual bromine atom in the benzylic bromide to create several series of RAFT CTA including trithiocarbonate, xanthate and dithiobenzoate CTAs, allowing for the possibility to introduce other functional groups besides Nb, such as an allyl group. While a norbornene functionality was chosen as most reactive functional group toward thiols in radical-mediated thiol–ene chemistry, an allyl group was introduced for the sake of direct comparison of the double bond reactivity in the thiol–ene reaction. Control of the radical polymerization of acrylates, styrene and vinyl acetate has been achieved by using this novel family of CTAs. The results indicate that the Nb group remained intact at low monomer conversions (e.g., below 50% for styrene and vinyl acetate, below 30% for acrylates) and at optimal reaction temperatures (e.g., 70 °C for styrene and vinyl acetate, 62 and 65 °C for 1-ethoxyethyl acrylate and methyl acrylate, respectively), while the monomer-to-CTA ratio was kept high. Polymers with high end-group fidelity were modified with a series of thiol-containing compounds, leading to α-semitelechelics with different chain-end structures. While allyl-containing polymers exhibited a significantly lower reactivity, modification of the Nb-containing semitelechelics was rapid and fully accomplished under the same reaction conditions. However, for the given conditions, dodecanethiol and benzyl mercaptan showed a lower reactivity toward Nb-containing polymers, as evidenced by the obtained modification efficiency of 70% and 45%, respectively.
06/2011;
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Angewandte Chemie International Edition 01/2011; 50(1):60-2. · 13.45 Impact Factor
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Journal of Polymer Science Part A Polymer Chemistry 03/2010; 48(9):2016 - 2023. · 3.92 Impact Factor
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ABSTRACT: In this work, we report our findings on the use of radical thiol-ene chemistry for polymer-polymer conjugation. The manuscript combines the results from the Preparative Macromolecular Chemistry group from the Karlsruhe Institute of Technology (KIT) and the Polymer Chemistry Research group from Ghent University (UGent), which allowed for an investigation over a very broad range of reaction conditions. In particular, thermal and UV initiation methods for the radical thiol-ene process were compared. In the KIT group, the process was studied as a tool for the synthesis of star polymers by coupling multifunctional thiol core molecules with poly(n-butyl acrylate) macromonomers (MM), employing thermally decomposing initiators. The product purity and thus reaction efficiency was assessed via electrospray ionization mass spectrometry. Although the reactions with 10 or 5 equivalents of thiol with respect to macromonomer were successful, the coupling reaction with a one-to-one ratio of MM to thiol yielded only a fraction of the targeted product, besides a number of side products. A systematic parameter study such as a variation of the concentration and nature of the initiator and the influence of thiol-to-ene ratio was carried out. Further experiments with poly(styrene) and poly(isobornyl acrylate) containing a vinylic end group confirmed that thermal thiol-ene conjugation is far from quantitative in terms of achieving macromolecular star formation. In parallel, the UGent group has been focusing on photo-initiated thiol-ene chemistry for the synthesis of functional polymers on one hand and block copolymers consisting of poly(styrene) (PS) and poly (vinyl acetate) (PVAc) on the other hand Various functionalization reactions showed an overall efficient thiol-ene process for conjugation reactions of polymers with low molecular weight compounds (similar to 90% coupling yield). However, while SEC and FTIR analysis of the conjugated PS-PVAc products indicated qualitative evidence for a successful polymer-polymer conjugation, (1)H NMR and elemental analysis revealed a low conjugation efficiency of about 23% for a thiol-to-ene ratio equal to one. Blank reactions using typical thiol-ene conditions indicated that bimolecular termination reactions occur as competitive side reactions explaining why a molecular weight increase is observed even though the thiol-ene reaction was not successful. The extensive study of both research groups indicates that radical thiol-ene chemistry should not be proposed as a straightforward conjugation tool for polymer-polymer conjugation reactions. Head-to-head coupling is a major reaction pathway, which interrupts the propagation cycle of the thiol-ene process. (C) 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 1699-1713,2010
Journal of Polymer Science Part A Polymer Chemistry 03/2010; 48(8):1699 - 1713. · 3.92 Impact Factor
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ABSTRACT: The quickly developing field of "click" chemistry would undoubtedly benefit from the availability of an easy and efficient technology for product purification to reduce the potential health risks associated with the presence of copper in the final product. Therefore, solvent-resistant nanofiltration (SRNF) membranes have been developed to selectively separate "clicked" polymers from the copper catalyst and solvent. By using these solvent-stable cross-linked polyimide membranes in diafiltration, up to 98 % of the initially present copper could be removed through the membrane together with the DMF solvent, the polymer product being almost completely retained. This paper also presents the first SRNF application in which the catalyst permeates through the membrane and the reaction product is retained.
Chemistry 12/2009; 16(3):1061-7. · 5.93 Impact Factor
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ABSTRACT: The fabrication of micrometer-sized monodisperse highly porous polymer particles, of both spherical and rodlike shapes, using a simple microfluidic setup is demonstrated. Droplets were generated in a coflow device from a water-in-oil high internal phase emulsion (HIPE), hereby creating a water-in-oil-in-water (W/O/W) emulsion. The individual droplets of monomer HIPE were polymerized downstream in the channel through photopolymerization. The polymer particles produced via this strategy possess very large macropores in comparison with the more conventional porous polymer beads synthesized by inducing in situ phase separation throughout the polymerization process through the use of porogenic solvents. Epoxy-functionalized porous particles made using the HIPE microfluidic method showed superior performance in a consecutive azide and cycloaddition "click"-"click" modification procedure monitored by IR. Our micro-fluidic approach led to the successful miniaturization of monodisperse submillimeter spherical poly(HIPE) beads, down to diameters of 400 μm. More strikingly is the production of poly(HIPE) rods, which were obtained by using a viscous HIPE, which in coflow emulsification formed an unstable jet that broke up into rodlike sections. These rodlike droplets maintained their shapes throughout the microfluidic channel and did not relax back into spherical droplets, allowing for production of poly(HIPE) rods upon photopolymeriza-tion. The nonspherical shape in this case is not determined by confined channel geometries, which to the best of our knowledge is unprecedented as a strategy to produce nonspherical polymer particles with micro-fluidics.
Macromolecules 11/2009; 42(23):9289–9294. · 5.17 Impact Factor
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ABSTRACT: Chemical modification reactions of alkyne containing polyHEMA-based macroporous network structures (cryogels) by Cu(I) catalyzed azide-alkyne 'click' cycloaddition reactions and their monitoring and quantification with high-resolution magic angle spinning (hr-MAS) NMR spectroscopy are reported. Complete conversion is obtained when benzylazide is reacted with the grafted alkyne function, but only partial conversion is observed when using azide-modified poly(ethylene glycol) (PEG-N(3) ). Subsequent addition of benzylazide consumes all remaining alkyne groups. All chemical modifications are easily monitored at each stage using hr-MAS NMR spectroscopy. The alkyne functionality and the resulting triazole ring provide well resolved (1) H resonances to monitor and quantify the progress of such 'click' reactions in general.
Macromolecular Rapid Communications 08/2009; 30(15):1328-33. · 4.60 Impact Factor
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Macromolecular Chemistry and Physics 01/2009; 210(3‐4):287 - 298. · 2.36 Impact Factor
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ABSTRACT: A novel heterogeneous copper(I) catalyst system, which is based on readily available poly(ethyleneimine), has been used as a recyclable catalyst for Cu(I) catalyzed "click" 1,3 dipolar cycloaddition reactions of azides and alkynes in organic media. Branched poly(ethyleneimine) was first methylated and then cross-linked with 1,9-dibromononane. Subsequently, after the immobilization of Cu(I)Br, this system was applied for heterogeneous copper catalyzed click chemistry of a few model reagents and polymeric compounds.
Macromolecular Rapid Communications 01/2009; 30(1):34-8. · 4.60 Impact Factor
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ABSTRACT: In this chapter, we report on the synthesis and characterization of well-defined amphiphilic block copolymers, ‘block-like’
copolymers and star copolymers composed of poly(isobornyl acrylate) (PiBA) and poly(acrylic acid) (PAA) by ATRP. As PiBA polymers
exhibit interesting physical characteristics, we report first a detailed study of the homopolymerization of iBA. The precursor
monomers 1-ethoxyethyl acrylate as well as tert-butyl acrylate have been used to synthesize the precursor polymers for the
PiBA-co-PAA block copolymers. Furthermore, a combination of ATRP and ‘click’ chemistry was used to prepare block and graft
copolymers using a modular approach. The PiBA-PAA block and ‘block-like’ copolymers were investigated as pigment stabilizers
for aqueous pigment dispersions. In the second part of the research, well-defined PiBA star copolymers were prepared, and
reactive nanoparticles were obtained by end group modification of the PiBA star polymers with reactive moieties. Finally,
the control of the visco-elastic properties by the incorporation of these nanoparticles in an acrylate polymer matrix was
investigated.
KeywordsBlock copolymer-Block-like copolymer-Star polymer-ATRP-“Click” chemistry-Nanoparticle-Isobornyl acrylate-1-Ethoxyethyl acrylate-Acrylic acid-Pigment stabilization
12/2008: pages 111-132;
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Macromolecular Rapid Communications 05/2008; 29(12‐13):1111 - 1118. · 4.60 Impact Factor
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ABSTRACT: Dextrans modified with alkyne and azide groups through hydrolysable carbonate esters form degradable microcapsules after Cu(I) catalysed 'click' reaction between azides and alkynes yielding triazole cross-links.
Chemical Communications 02/2008; · 6.17 Impact Factor
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Journal of Polymer Science Part A Polymer Chemistry 01/2008; 46(5):1649 - 1661. · 3.92 Impact Factor
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ABSTRACT: Diblock copolymers poly(tetrahydrofuran-b-tert-butyl acrylate) (PTHF-b-PtBA) and poly(tetrahydrofuran-b-1-ethoxyethyl acrylate) (PTHF-b-PEEA) were successfully synthesized by the dual initiator 4-hydroxybutyl-2-bromoisobutyrate (HBBIB). The isobutyrate and alcohol function of HBBIB were used for the atom transfer radical polymerization of tBA (or EEA) and the living cationic ring-opening polymerization of THF, respectively. Hydrolysis or thermolysis of the aforementioned diblock copolymers results in amphiphilic pH-responsive copolymers PTHF-b-poly(acrylic acid) (PTHF-b-PAA). Matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) and nuclear magnetic resonance spectroscopy (1H NMR) were used to analyze the PTHF macroinitiator, while clear evidence for the formation of well-defined block copolymer structures was obtained by 1H NMR, gel permeation chromatography (GPC), and infrared spectroscopy (FT-IR). The amorphous PtBA block in PTHF-b-PtBA resulted in a decrease of the crystallinity and the melting point of PTHF, as shown by differential scanning calorimetry (DSC). Self-assembly of PTHF-b-PAA copolymers in water into aggregates and micelles when exposed to specific pH values was confirmed by dynamic light scattering, infrared, and NMR spectroscopies.
04/2006;
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08/2004;
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ABSTRACT: The effect of elevated light treatment (25 degrees C, PPFD 360 mumol m-2 sec-1) or chilling temperatures combined with elevated light (5 degrees C, PPFD 360 mumol m-2 sec-1) on the activity of six antioxidant enzymes, guaiacol peroxidases, and glutathione peroxidase (GPx, EC 1.11.1.9) protein accumulation were studied in tobacco Nicotiana tabacum cv. Petit Havana SR1. Both treatments caused no photooxidative damage, but chilling caused a transient wilting. The light treatment increased the activities of ascorbate peroxidase (APx, EC 1.11.1.11) and guaiacol peroxidases while catalase (EC 1.11.1.6), superoxide dismutase (SOD, EC 1.15.1.1), monodehydroascorbate reductase (MDHAR, EC 1.6.5.4), dehydroascorbate reductase (DHAR, EC 1.8.5.1), and glutathione reductase (EC 1.6.4.2) were unchanged. In contrast, chilling treatment did not increase any of the antioxidant enzyme activities, but decreased catalase and to a lesser extent DHAR activities. Glutathione peroxidase protein levels increased sporadically under light treatment and constantly under chilling. Both chilling and light stress caused induction of glutathione synthesis and accumulation of oxidised glutathione, although the predominant part of the glutathione pool remained in the reduced form. Antioxidant enzymes from the chilling treated plants were measured at both 25 degrees C and 5 degrees C. Measurements at 5 degrees C revealed a 3-fold reduction in catalase activity, compared with that measured at 25 degrees C, indicating that the overall reduction in catalase after four days of chilling was approximately 10-fold. The overall reduction in activity for the other antioxidant enzymes after four days of chilling was 2-fold for GR and APx, 1.5-fold for MDHAR, 3.5-fold for DHAR. The activity of SOD was the same at 25 and 5 degrees C. These results indicate that catalase and DHAR are most strongly affected by the chilling treatment and may be the rate-limiting factor of the antioxidant system at low temperatures.
Journal of Plant Physiology 06/2003; 160(5):509-15. · 2.79 Impact Factor
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ABSTRACT: The molecular mechanisms by which plants acclimate to oxidative stress are poorly understood. To identify the processes involved in acclimation, we performed a comprehensive analysis of gene expression in Nicotiana tabacum leaves acclimated to oxidative stress. Combining mRNA differential display and cDNA array analysis, we estimated that at least 95 genes alter their expression in tobacco leaves acclimated to oxidative stress, of which 83% are induced and 17% repressed. Sequence analysis of 53 sequence tags revealed that, in addition to antioxidant genes, genes implicated in abiotic and biotic stress defenses, cellular protection and detoxification, energy and carbohydrate metabolism, de novo protein synthesis, and signal transduction showed altered expression. Expression of most of the genes was enhanced, except for genes associated with photosynthesis and light-regulated processes that were repressed. During acclimation, two distinct groups of coregulated genes ("early-" and "late-response" gene regulons) were observed, indicating the presence of at least two different gene induction pathways. These two gene regulons also showed differential expression patterns on an oxidative stress challenge. Expression of "late-response" genes was augmented in the acclimated leaf tissues, whereas expression of "early-response" genes was not. Together, our data suggest that acclimation to oxidative stress is a highly complex process associated with broad gene expression adjustments. Moreover, our data indicate that in addition to defense gene induction, sensitization of plants for potentiated gene expression might be an important factor in oxidative stress acclimation.
Proceedings of the National Academy of Sciences 09/2002; 99(16):10870-5. · 9.68 Impact Factor
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ABSTRACT: Transgenic tobacco deficient in the H2O2-removing enzyme catalase (Cat1AS) was used as an inducible and noninvasive system to study the role of H2O2 as an activator of pathogenesis-related (PR) proteins in plants. Excess H2O2 in Cat1AS plants was generated by simply increasing light intensities. Sustained exposure of Cat1AS plants to excess H2O2 provoked tissue damage, stimulated salicylic acid and ethylene production, and induced the expression of acidic and basic
PR proteins with a timing and magnitude similar to the hypersensitive response against pathogens. Salicylic acid production
was biphasic, and the first peak of salicylic acid as well as the peak of ethylene occurred within the first hours of high
light, which is long before the development of tissue necrosis. Under these conditions, accumulation of acidic PR proteins
was also seen in upper leaves that were not exposed to high light, indicating systemic induction of expression. Short exposure
of Cat1AS plants to excess H2O2 did not cause damage, induced local expression of acidic and basic PR proteins, and enhanced pathogen tolerance. However,
the timing and magnitude of PR protein induction was in this case more similar to that in upper uninfected leaves than to
that in hypersensitive-response leaves of pathogen-infected plants. Together, these data demonstrate that sublethal levels
of H2O2 activate expression of acidic and basic PR proteins and lead to enhanced pathogen tolerance. However, rapid and strong activation
of PR protein expression, as seen during the hypersensitive response, occurs only when excess H2O2 is accompanied by leaf necrosis.
Proceedings of the National Academy of Sciences 05/1998; 95(10):5818-5823. · 9.68 Impact Factor
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ABSTRACT: Hydrogen peroxide (H2O2) has been implicated in many stress conditions. Control of H2O2 levels is complex and dissection of mechanisms generating and relieving H2O2 stress is difficult, particularly in intact plants. We have used transgenic tobacco with 10% wild-type catalase activity to study the role of catalase and effects of H2O2 stress in plants. Catalase-deficient plants showed no visible disorders at low light, but in elevated light rapidly developed white necrotic lesions on the leaves. Lesion formation required photorespiratory activity since damage was prevented under elevated CO2. Accumulation of H2O2 was not detected during leaf necrosis. Alternative H2O2-scavenging mechanisms may have compensated for reduced catalase activity, as shown by increased ascorbate peroxidase and glutathione peroxidase levels. Leaf necrosis correlated with accumulation of oxidized glutathione and a 4-fold decrease in ascorbate, indicating that catalase is critical for maintaining the redox balance during oxidative stress. Such control may not be limited to peroxisomal H2O2 production. Catalase functions as a cellular sink for H2O2, as evidenced by complementation of catalase deficiency by exogenous catalase, and comparison of catalase-deficient and control leaf discs in removing external H2O2. Stress analysis revealed increased susceptibility of catalase-deficient plants to paraquat, salt and ozone, but not to chilling.
The EMBO Journal 08/1997; 16(16):4806-4816. · 9.20 Impact Factor
