Universität Stuttgart
Permanent URI for this communityhttps://elib.uni-stuttgart.de/handle/11682/1
Browse
Search Results
Item Open Access Macrocyclization of dienes under confinement with cationic tungsten imido/oxo alkylidene N‐heterocyclic carbene complexes(2023) Ziegler, Felix; Bruckner, Johanna R.; Nowakowski, Michal; Bauer, Matthias; Probst, Patrick; Atwi, Boshra; Buchmeiser, Michael R.Macrocyclization reactions are still challenging due to competing oligomerization, which requires the use of small substrate concentrations. Here, the cationic tungsten imido and tungsten oxo alkylidene N-heterocyclic carbene complexes [[W(N-2,6-Cl2-C6H3)(CHCMe2Ph(OC6F5)(pivalonitrile)(IMes)+ B(ArF)4-] (W1) and [W(O (CHCMe2Ph(OCMe(CF3)2)(IMes)(CH3CN)+ B(ArF)4-] (W2) (IMes=1,3-dimesitylimidazol-2-ylidene; B(ArF)4-=tetrakis(3,5-bis(trifluoromethyl)phenyl borate) have been immobilized inside the pores of ordered mesoporous silica (OMS) with pore diameters of 3.3 and 6.8 nm, respectively, using a pore-selective immobilization protocol. X-ray absorption spectroscopy of W1@OMS showed that even though the catalyst structure is contracted due to confinement by the mesopores, both the oxidation state and structure of the catalyst stayed intact upon immobilization. Catalytic testing with four differently sized α,ω-dienes revealed a dramatically increased macrocyclization (MC) and Z-selectivity of the supported catalysts compared to the homogenous progenitors, allowing high substrate concentrations of 25 mM. With the supported complexes, a maximum increase in MC-selectivity from 27 to 81 % and in Z-selectivity from 17 to 34 % was achieved. In general, smaller mesopores exhibited a stronger confinement effect. A comparison of the two supported tungsten-based catalysts showed that W1@OMS possesses a higher MC-selectivity, while W2@OMS exhibits a higher Z-selectivity which can be rationalized by the structures of the catalysts.Item Open Access Dualkatalytische Synthese von maßgeschneiderten Reverse Pluronics zur Herstellung von geordneten, mesoporösen Kohlenstoff- und Silikamaterialien(2023) Vogler, Charlotte; Naumann, Stefan (PD Dr.)Item Open Access Autonomous adaption of intelligent humidity‐programmed hydrogel patches for tunable stiffness and drug release(2023) Pflumm, Stephan; Wiedemann, Yvonne; Fauser, Dominik; Safaraliyev, Javidan; Lunter, Dominique; Steeb, Holger; Ludwigs, SabineIntelligent humidity‐programmed hydrogel patches with high stretchability and tunable water‐uptake and ‐release are prepared by copolymerization and crosslinking of N‐isopropylacrylamide and oligo(ethylene glycol) comonomers. These intelligent elastomeric patches strongly respond to different humidities and temperatures in terms of mechanical properties which makes them applicable for soft robotics and smart skin applications where autonomous adaption to environmental conditions is a key requirement. It is shown that beyond using the hydrogel in the conventional state in aqueous media, new patches can be controlled by relative humidity. This humidity programming of the patches allows to tune drug release kinetics, opening potential application fields such as skin wound therapy and personalized medication. In situ dynamic‐mechanical measurements show a huge dependence on temperature and humidity. The glass transition temperature Tg shifts from around 60 °C at dry conditions to below 0 °C for 75% r.h. and higher. The storage modulus is tunable over more than four orders of magnitude from 0.6 up to 400 MPa. Time‐temperature superposition in master curves allows to extract relaxation times over 14 orders of magnitude. With strains at break of over 200% the patches are compliant with human skin and therefore patient‐friendly in terms of adapting to movements.Item Open Access The effect of mPEGA/EHA ratio and copolymer composition on the solution behavior of amphiphilic, comb‐shape copolymers synthesized via Cu(0)‐mediated SET‐LRP for potential drug delivery applications(2023) Pourdakheli Hamedani, Yasaman; Çakırefe, Semanur; Fietz, Agnes; Hurst, José; Schnichels, Sven; Adams, FriederikeComb-shape, block copolymers from hydrophilic poly(ethylenglycol) monomethyl ether acrylate (mPEGA, A) and hydrophobic 2-ethylhexyl acrylate (EHA, B) are synthesized by copper(0)-mediated single-electron transfer living radical polymerization (SET-LRP) via sequential addition of the two monomers, resulting in different compositions (AB, ABA, BAB, BA), molar masses, and mPEGA/EHA ratios. All polymers show narrow molar mass distributions and molecular weights of 7.7-25.50 kg mol-1, demonstrating precise control over the polymerization and molecular weights through the utilization of SET-LRP. Kinetic experiments are conducted to investigate the polymerization behavior of mPEGA and EHA in N,N-dimethylformamide as a rather uncommon solvent for SET-LRP further underlining a living-type polymerization. Amphiphilic properties are investigated by critical micelle concentration (CMC) measurements and formation of micelles in water. A reverse relation between mPEGA/EHA ratio and CMC values reveals that an increased hydrophobicity leads to decreased CMC values. The self-assembly behavior of polymers in water confirms the formation of uniform and stable micelles in water with a size between 12 and 184 nm depending on the composition of the polymers. With increased hydrophilicity, micelle sizes increase as well. In vitro tests of the obtained polymers show excellent biocompatibility even at high concentrations further affirming their suitability for drug delivery applications.Item Open Access Electrochemical crosslinking strategies using redox-active units for optoelectronic thin film applications(2023) Malacrida, Claudia; Ludwigs, Sabine (Prof. Dr.)In this dissertation the electrochemical reactivity of different classes of redox molecules of optoelectronics interest, including thiophene and arylamine derivatives, is studied and exploited to crosslink precursor systems endowed with multiple dimerization sites to obtain functional electroactive films. The latter are characterized by a well-defined and controlled π-conjugation and redox behavior, which can be directly related to the one of the dimer of the starting redox unit. In the first section of the thesis, the electrochemical crosslinking from solution of different multimeric systems, including arylamine based push-pull molecules was exploited. In the second part of the thesis, insight of oxidative crosslinking as a post-solution deposition step to obtain electroactive films is provided. The dimerizing ability of triarylamine (TPA) and carbazole (Cbz) in the form of redox-active pendant units on a saturated polymer backbone as crosslinkers was studied upon oxidation.Item Open Access Sulfur‐composites derived from poly(acrylonitrile) and poly(vinylacetylene) : a comparative study on the role of pyridinic and thioamidic nitrogen(2023) Kappler, Julian; Klostermann, Sina V.; Lange, Pia L.; Dyballa, Michael; Veith, Lothar; Schleid, Thomas; Weil, Tanja; Kästner, Johannes; Buchmeiser, Michael R.Sulfurized poly(acrylonitrile) (SPAN) is a prominent example of a highly cycle stable and rate capable sulfur/polymer composite, which is solely based on covalently bound sulfur. However, so far no in‐depth study on the influence of nitrogen in the carbonaceous backbone, to which sulfur in the form of thioketones and poly(sulfides) is attached, exists. Herein, we investigated the role of nitrogen by comparing sulfur/polymer composites derived from nitrogen‐containing poly(acrylonitrile) (PAN) and nitrogen‐free poly(vinylacetylene) (PVac). Results strongly indicate the importance of a nitrogen‐rich, aromatic carbon backbone to ensure full addressability of the polymer‐bound sulfur and its reversible binding to the aromatic backbone, even at high current rates. This study also presents key structures, which are crucial for highly cycle and rate stable S‐composites.Item Open Access Characterization and optimization of sulfurized poly(acrylonitrile) cathodes and silicon anodes(2023) Niesen, Stefan; Buchmeiser, Michael R. (Prof. Dr.)Item Open Access Olefin metathesis under spatial confinement and continuous flow : investigation of isomeric side reactions with a Grubbs-Hoveyda type catalyst(2023) Böth, André; Roider, Thomas; Ziegler, Felix; Xie, Xiulan; Buchmeiser, Michael R.; Tallarek, UlrichA 2nd‐generation Grubbs-Hoveyda type catalyst was immobilized inside mesoporous silica and used in the ring‐closing metathesis (RCM) of an α,ω‐diene to a large macro(mono)cycle. The goal was to investigate the relationship between substrate concentration, reaction time, and overall experiment time on the rate of isomerization under spatial (mesopore space) confinement with continuous‐flow microreactors. RCM reactions are commonly monitored by 1H NMR analysis, however, elucidation of reaction mixtures yielding large rings with a difference of only a single carbon atom remains difficult, because NMR signals are sometimes indistinguishable. In this work, an analytical platform with on‐line separation and detection of UV‐active substrate as well as (side) products by high‐performance liquid chromatography and a UV/Vis‐diode array detector (DAD) plus mass spectrometry served as enabling technology to quantify yield and selectivity under the respective reaction conditions. Using this setup, competitive reaction equilibria and isomerization reactions, in particular, could be resolved. Identification and quantification of relevant compounds of the reaction scheme under spatial confinement became possible despite chemical similarity. Kinetic data revealed that isomerization increases with higher substrate concentrations (up to 250 mM) and longer reaction times (from 1.2 to 18.6 min), but shows a distinct decline for prolonged overall experiment times (up to ∼250 min).Item Open Access Lignin/poly(vinylpyrrolidone) multifilament fibers dry‐spun from water as carbon fiber precursors(2023) Kreis, Philipp; Frank, Erik; Clauß, Bernd; Bauch, Volker; Stolpmann, Heiko; Kuske, Lisa; Schneck, Tanja; König, Simon; Buchmeiser, Michael R.The preparation of lignin-based carbon fibers by dry spinning from aqueous solution followed by stabilization and continuous carbonization to endless yarns is reported. The influence of carbonization temperature and draw ratio on the morphology and mechanical properties of the final carbon fibers is investigated by single-fiber testing, wide-angle X-ray scattering, scanning electron microscopy, and Raman spectroscopy. A draw ratio of 5% (1.05) with a carbonization temperature of 1400 °C leads to the best mechanical properties. The resulting multifilament carbon fibers have an average diameter between 10-12 µm, an average tensile strength of 1.30 ± 0.32 GPa, a Young's modulus of 101 ± 18 GPa, and an elongation at break of 1.31 ± 0.23%. The maximum Weibull strength (𝜎0) is 1.04 GPa with a Weibull modulus (m) of 5.1. The use of a water-soluble system is economically advantageous; also, unlike melt-spun lignin fibers, the dry-spun precursor fibers can be thermally converted without any additional crosslinking step.Item Open Access Modular approach to creating functionalized surface arrays of molecular qubits(2023) Tesi, Lorenzo; Stemmler, Friedrich; Winkler, Mario; Liu, Sherri S. Y.; Das, Saunak; Sun, Xiuming; Zharnikov, Michael; Ludwigs, Sabine; Slageren, Joris vanThe quest for developing quantum technologies is driven by the promise of exponentially faster computations, ultrahigh performance sensing, and achieving thorough understanding of many‐particle quantum systems. Molecular spins are excellent qubit candidates because they feature long coherence times, are widely tunable through chemical synthesis, and can be interfaced with other quantum platforms such as superconducting qubits. A present challenge for molecular spin qubits is their integration in quantum devices, which requires arranging them in thin films or monolayers on surfaces. However, clear proof of the survival of quantum properties of molecular qubits on surfaces has not been reported so far. Furthermore, little is known about the change in spin dynamics of molecular qubits going from the bulk to monolayers. Here, a versatile bottom‐up method is reported to arrange molecular qubits as functional groups of self‐assembled monolayers (SAMs) on surfaces, combining molecular self‐organization and click chemistry. Coherence times of up to 13 µs demonstrate that qubit properties are maintained or even enhanced in the monolayer.