Metal-Catalyzed Radical Polymerization up to High Pressure

The mechanism and kinetics of metal-catalyzed radical polymerization were investigated by spectroscopic means. A particular focus was set on Fe-mediated atom-transfer radical polymerization (ATRP) as there is a growing interest for an economic alternative to the extensively used Cu-mediated ATRP. Experiments were started with an iron bromide-based catalyst, which simply operates without any external ligands. FT-nearIR and Mössbauer spectroscopy were used to determine the structures of [Fe(II)Bru(Solv)v] and [Fe(III)Brw(Solv)x] complexes in a variety of solvents. It was found that the tetrahedral species [Fe(II)Br3(Solv)]− and [Fe(III)Br4]− essentially govern the activation−deactivation equilibrium of ATRP. The structure of these complexes is correlated with the measured ATRP activation rate coefficients, kact, and with the equilibrium constants, KATRP, for monomer-free model systems. In weakly polar solvents such as esters, ketones, and substituted benzenes, kact and also KATRP are up to two orders of magnitude higher than with strongly polar solvents, such as N-methylpyrrolidin-2-one (NMP), acetonitrile, and dimethylform-amide, where the [Fe(II)Br3(Solv)]− complex is more stabilized. Since further tuning of catalyst activity is important to access a wide range of monomers for ATRP, several types of Fe−ligand systems were tested for a potential enhancement of KATRP. The NIR spectroscopic analysis indicated that tetrahedral [Fe(II)BruLv]u+v=4 complexes also play a role with external ligands, L, such as N-heterocyclic carbenes and phosphines. However, these compounds do not significantly improve KATRP compared with solvent molecules being the ligands. Nevertheless, the studies were helpful to clarify the role of phosphines in ATRP. The highly Lewis basic tris(2,4,6-trimethoxy-phenyl)phosphine (TTMPP) may coordinate to Fe(II), but primarily acts as a reducing agent for [Fe(III)Br4]−, thus transforming TTMPP to TTMPP-Br+. Triphenylphosphine (TPP) is a less effective reducing agent. An enhanced KATRP was found for amine–bis(phenolate) iron complexes. A combined Mössbauer, EPR, NMR, and online VIS/NIR spectroscopic analysis was carried out to determine the relevant Fe species. An interplay between ATRP and organometallic-mediated radical polymerization (OMRP), which is based on the reaction of propagating radicals with Fe(II), may occur depending on the monomer under investigation. Styrene polymerization operates via ATRP, whereas an interplay between ATRP and OMRP occurs for MMA polymerization. The kinetics of ATRP and OMRP were quantitatively measured by highly time-resolved EPR spectroscopy in conjunction with pulsed-laser application for radical production, i.e., the so-called SP–PLP–EPR method. ATRP deactivation of methacrylate-type radicals by an amine–bis(phenolate)iron catalyst was monitored without interference by organometallic reactions. Toward higher temperatures, the ratio of deactivation to propagation rate increases, which is beneficial for ATRP control. SP–PLP–EPR was also applied to quantify the catalytic termination (CRT) of two propagating radicals by Fe(II) via an organometallic intermediate. In case of the [Fe(II)Br3(Solv)]− catalyst, the organometallic reaction plays a role for acrylate rather than for methacrylate polymerization, where CRT is by about three orders of magnitude slower. As a consequence, ATRP of acrylates should be carried out with low levels of the Fe(II) catalyst to avoid CRT and thus improve the living character of ATRP. The investigations into metal-catalyzed radical polymerization were expanded up to pressures of 6000 bar. Applying pressure results in a redistribution of iron bromides in favor of the charged species [FeBr4]2− and [Fe(Solv)6]2+, which is particularly pronounced in polar solvents such as NMP or acetonitrile. As a consequence, the reaction volume, ΔrV(KATRP), is positive for [Fe(II)Xu(Solv)v] catalysts (up to 18 cm3 mol−1). The studies demonstrated the advantage of the well-defined amine–bis(phenolate)iron system: ΔrV(KATRP) is negative, (−17 ± 2) cm3 mol−1, which is associated with a favorable shift of the ATRP equilibrium toward the side of the activated radical. Along with the increase in propagation rate, ATRP rate is thus enhanced by more than two orders of magnitude between 1 and 6000 bar. ATRP also benefits from an improved living character under high pressure, which is due to the lowering of diffusion-controlled termination. This facilitates the synthesis of polystyrenes and polyacrylates with molar masses above 100,000 g mol−1 and dispersities below 1.29 under either Fe or Cu catalysis. These advantages were not compromised by an increase in the rate of intramolecular transfer, i.e., the backbiting reaction during acrylate polymerization under high pressure, which was deduced from modeling the ATRP experiments. This thesis has improved the understanding of the mechanism and kinetics of Fe-mediated ATRP, in particular, of the potential interplay with OMRP. Moreover, the studies provide guidance for the selection of suitable reaction conditions that yield predominantly ATRP-mediated polymerizations with improved control.2016-09-0

recent strides in new directions

Are they still electrifying? Electrochemically switchable rotaxanes are well known for their ability to efficiently undergo changes of (co-)conformation and properties under redox-control. Thus, these mechanically interlocked assemblies represent an auspicious liaison between the fields of molecular switches and molecular electronics. Since the first reported example of a redox-switchable molecular shuttle in 1994, improved tools of organic and supramolecular synthesis have enabled sophisticated new architectures, which provide precise control over properties and function. This perspective covers recent advances in the area of electrochemically active rotaxanes including novel molecular switches and machines, metal-containing rotaxanes, non-equilibrium systems and potential applications

Tetrathiafulvalene – a redox-switchable building block to control motion in mechanically interlocked molecules

With the rise of artificial molecular machines, control of motion on the nanoscale has become a major contemporary research challenge. Tetrathiafulvalenes (TTFs) are one of the most versatile and widely used molecular redox switches to generate and control molecular motion. TTF can easily be implemented as functional unit into molecular and supramolecular structures and can be reversibly oxidized to a stable radical cation or dication. For over 20 years, TTFs have been key building blocks for the construction of redox-switchable mechanically interlocked molecules (MIMs) and their electrochemical operation has been thoroughly investigated. In this review, we provide an introduction into the field of TTF-based MIMs and their applications. A brief historical overview and a selection of important examples from the past until now are given. Furthermore, we will highlight our latest research on TTF-based rotaxanes

Investigations on the Turbulent Wake of a Generic Space Launcher Geometry in the Hypersonic Flow Regime

The turbulent wake flow of generic rocket configurations is investigated experimentally and numerically at a freestream Mach number of 6.0 and a unit Reynolds number of 10 x 10^6. The flow condition is based on the trajectory of Ariane V at an altitude of 50 km, which is used as baseline to address the overarching tasks of wake flows in the hypersonic regime like fluid-structural coupling, reverse hot jets and base heating. Experiments using pressure transducers and high-speed schlieren measurement technique were conducted to gain insight into the local pressure fluctuations on the base and the oscillations of the recompression shock. This experimental configuration features a wedge-profiled strut orthogonally mounted to the main body. Additionally, the influence of cylindrical nozzle extensions attached to the base of the rocket is investigated, which is the link to the numerical investigations. Here, the axisymmetric model possesses a cylindrical sting support of the same diameter as the nozzle extensions. The sting support allows investigations of a undisturbed wake flow. A time-accurate zonal RANS/LES approach was applied to identify shocks, expansion waves, and the highly unsteady recompression region numerically. Subsequently, experimental and numerical results in the strut-averted region are opposed with regard to the wall pressure and recompression shock frequency spectra. For the compared configurations, experimental pressure spectra exhibit dominant Strouhal numbers at about S rD = 0.03 and 0.27 and the recompression shock oscillates at 0.2. In general, the numerical pressure and recompression shock fluctuations agree satisfactorily to the experimental results. The experiments with a blunt base reveal base-pressure spectra with dominant Strouhal numbers at 0.08 at the center position and 0.145, 0.21 − 0.22 and 0.31 − 0.33 at the outskirts of the base

Simultaneous Contacting and Interconnection of Passivated Emitter and Rear Solar Cells

The back end process of passivated emitter and rear cells (PERC) consists of at least one laser process and three screen-printing steps followed by the stringing and tabbing of the cells. To reduce the number of steps we have developed a process that metallizes the rear side including contact formation and simultaneously interconnects the cells. We attach an Al foil to an encapsulant layer. By laser processing we form 'laser-fired and bonding contacts' (LFBC) on the passivated rear side of the solar cells. The Al foil contacting the rear is laser welded to the Ag screen-printed front side metallization of the next cell and thus forms the cell interconnection. The laser contacts on the rear show a surface recombination velocity Scont for the contact regions of cm/s and a contact resistivity of 3.52 m?cm2. We present a first proof-of concept module combining the in-laminate Ag-Al laser welding and the LFBC reaching an efficiency of 18.4%. In accelerated aging test modules show no degradation (< 1% in efficiency) after 100 humidity-free cycles.Federal Ministry for Environment, Nature Conservation, and Nuclear Safety/FKZ/0325192State of Lower Saxon

Documentary professionalization research related to teacher training courses. A study review

Das Studienreview beschäftigt sich mit der Frage, welchen Beitrag die Dokumentarische Methode für die professionalisierungsbezogene Forschung hinsichtlich des Lehramtsstudiums leistet, d.h. welches erkenntnisgenerierende Potenzial, aber auch welche entsprechenden Limitationen mit diesem methodisch-methodologischen Zugang einhergehen. Um dieser Frage nachzugehen und das entsprechende Feld der dokumentarisch orientierten Forschung zu kartieren, bietet der Beitrag eine systematische Synopse der vorliegenden, thematisch anschlussfähigen empirisch basierten Publikationen der Jahre 2007 bis 2023 als Ausgangspunkt für weitere Argumentationen. Ausgehend vom Gegenstandsfeld der Forschung zur Professionalisierung von (angehenden) Lehrpersonen im deutschsprachigen Raum, werden die dokumentarisch fundierten Studien innerhalb dieses Feldes und innerhalb der professionstheoretischen Ansätze verortet. Resümierend ergeben sich daraus Implikationen für anstehende Forschungen. (DIPF/Orig.)The study review focuses on the question of what contribution the Documentary Method makes to research related to the professionalization of teacher education, i.e. what potential for generating knowledge, but also what corresponding limitations come with this methodological approach. In order to address this question and map the relevant field of documentary-oriented research, the article provides a systematic synopsis of the existing, thematically relevant empirically based publications from 2007 to 2023 as a starting point for further arguments. Starting from the subject area of research on the professionalization of (prospective) teachers in the German-speaking area, the documentary-based studies are situated within this field and within the professional theoretical approaches. In summary, this leads to implications for upcoming research. (DIPF/Orig.

Generalized tetanus in a 4-year old boy presenting with dysphagia and trismus: a case report

This is an Open Access article distributed under the terms of the Creative Commons Attribution Licens

Evaluation of multivalency as an organization principle for the efficient synthesis of doubly and triply threaded amide rotaxanes

Mono-, di- and trivalent pseudorotaxanes with tetralactam macrocycle hosts and axles containing diamide binding stations as the guests have been synthesised. Their threading behaviour was analyzed in detail by NMR experiments and isothermal titration calorimetry. An X-ray crystal structure of the monovalent pseudorotaxane confirms the binding motif. Double mutant cycle analysis provides the effective molarities and insight into the chelate cooperativity of multivalent binding. While the second binding event in a trivalent pseudorotaxane exhibits a slightly positive cooperativity, the third binding is nearly non-cooperative. Nevertheless, the enhanced binding affinities resulting from the multivalent interaction are the basis for a highly efficient synthesis of di- and trivalent rotaxanes through stoppering the axle termini by “click” chemistry. Evidence for the multiply threaded geometry comes from NMR spectroscopy as well as tandem mass-spectrometric fragmentation experiments of mass-selected rotaxane ions in the gas phase. Furthermore, the trivalent rotaxane can be controlled by external stimuli (chloride addition and removal) which lead to an elevator-type movement of the wheel along the axle

Impact of Myeloid p38α/MAPK on Orthodontic Tooth Movement

Objectives: Myeloid p38α/MAPK regulate and coordinate osteoclastogenesis. The present study was conducted to investigate the role of myeloid p38α/MAPK during orthodontic tooth movement. Methods: Orthodontic tooth movement was performed in wildtype and p38αΔmyel mice lacking p38α/MAPK expression in myeloid cells. First, bone parameter as well as osteoblast and osteoclast number were determined in tibiae. RNA was isolated from the untreated and orthodontically treated maxillary jaw side and expression of genes involved in inflammation and bone remodelling were analysed. Finally, periodontal bone loss, alveolar bone density and extent of orthodontic tooth movement were assessed. Results: Bone density was increased in p38αΔmyel mice compared to wildtype mice in tibiae (p = 0.043) and alveolar bone (p = 0.003). This was accompanied by a reduced osteoclast number in tibiae (p = 0.005) and TRAP5b in serum (p = 0.015). Accordingly, expression of osteoclast-specific genes was reduced in p38αΔmyel mice. Extent of tooth movement was reduced in p38αΔmyel mice (p = 0.024). This may be due to the higher bone density of the p38αΔmyel mice. Conclusions: Myeloid p38α/MAPK thus appears to play a regulatory role during orthodontic tooth movement by regulating osteoclastogenesis