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We report an advanced analysis protocol that allows to quantitatively study the course of step-growth reactions by size exclusion chromatography on the example of the depolymerization of a Diels-Alder polymer based on a furane/maleimide couple at elevated temperatures. Frequently occurring issues of molar mass calibrations and overlap of monomer with solvent signals are addressed for determining reliable molar masses. Thereby, even kinetic parameters (e.g., rate coefficients) can be derived that otherwise would require performing additional spectroscopic experiments. Our results confirm first-order behavior of the rDA reaction with an activation energy of 33 kJ mol -1 .

We show that, all other conditions being equal, bond cleavage in the middle of molecules is entropically much more favored than bond cleavage at the end. Multiple experimental and theoretical approaches have been used to study the selectivity for bond cleavage or dissociation in the middle versus the end of both covalent and supramolecular adducts and the extensive implications for other fields of chemistry including, e.g., chain transfer, polymer degradation, and control agent addition are discussed. The observed effects, which are a consequence of the underlying entropic factors, were predicted on the basis of simple theoretical models and demonstrated via high-temperature (HT) NMR spectroscopy of self-assembled supramolecular diblock systems as well as temperature-dependent size-exclusion chromatography (TD SEC) of covalently bonded Diels-Alder step-growth polymers., (© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

A rapid and catalyst-free cycloaddition system for visible-light-induced click chemistry is reported. A readily accessible photoreactive 2H-azirine moiety was designed to absorb light at wavelengths above 400 nm. Irradiation with low-energy light sources thus enables efficient small-molecule synthesis with a diverse range of multiple-bond-containing compounds. Moreover, in order to demonstrate the efficiency of the current approach, quantitative ligation of the photoactivatable chromophore with functional polymeric substrates was performed and full conversion with irradiation times of only 1 min at ambient conditions was achieved. The current report thus presents a highly efficient method for applications involving selective cycloaddition to electron-deficient multiple-bond-containing materials., (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

We report the transfer of entropic chain length effects into the realm of supramolecular chemistry and thereby demonstrate a macromolecular method to tune the reaction equilibria of hydrogen bonding motifs via the application of substituents of differing lengths and masses while not altering the actual recognition units to achieve a difference in the degree of association. The supramolecular adducts are characterized via temperature-dependent nuclear magnetic resonance (NMR) spectroscopy.

We report the investigation of fundamental entropic chain effects that enable the tuning of modular ligation chemistry - for example dynamic Diels-Alder (DA) reactions in materials applications - not only classically via the chemistry of the applied reaction sites, but also via the physical and steric properties of the molecules that are being joined. Having a substantial impact on the reaction equilibrium of the reversible ligation chemistry, these effects are important when transferring reactions from small molecule studies to larger or other entropically very dissimilar systems. The effects on the DA equilibrium and thus the temperature dependent degree of debonding (% debond ) of different cyclopentadienyl (di-)functional poly(meth-)acrylate backbones (poly(methyl methacrylate), poly(iso-butyl methacrylate), poly( tert -butyl methacrylate), poly(iso-butyl acrylate), poly( n -butyl acrylate), poly( tert -butyl acrylate), poly(methyl acrylate) and poly(isobornyl acrylate)), linked via a difunctional cyanodithioester (CDTE) were examined via high temperature (HT) NMR spectroscopy as well as temperature dependent (TD) SEC measurements. A significant impact of not only chain mass and length with a difference in the degree of debonding of up to 30% for different lengths of macromonomers of the same polymer type but - remarkably - as well the chain stiffness with a difference in bonding degrees of nearly 20% for isomeric poly(butyl acrylates) is found. The results were predicted, reproduced and interpreted via quantum chemical calculations, leading to a better understanding of the underlying entropic principles.

A facile, fast and ambient-temperature avenue towards highly fluorescent polymers is introduced via polymerizing non-fluorescent photoreactive monomers based on light-induced NITEC chemistry, providing a platform technology for fluorescent polymers. The resulting polypyrazolines were analyzed in depth and the photo-triggered step-growth process was monitored in a detailed kinetic study.

Dynamic bonding materials are of high interest in a variety of fields in material science. The reversible nature of certain reaction classes is frequently employed for introducing key material properties such as the capability to self-heal. In addition to the synthetic effort required for designing such materials, their analysis is a highly complex--yet important--endeavor. Herein, we critically review the current state of the art analytical methods and their application in the context of reversible bonding on demand soft matter material characterization for an in-depth performance assessment. The main analytical focus lies on the characterization at the molecular level., (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

A novel adaptable network based on the reversible hetero Diels-Alder reaction of a cyanodithioester and cyclopentadiene is presented. Reversible between 50-120 °C, the adjustable and self-healing features of the network are evidenced via temperature dependent rheology experiments and repetitive tensile tests whereas the network's chemical structure is explored by temperature dependent (1) H MAS-NMR spectroscopy., (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Polymers capable of dynamic bonding/debonding reactions are of great interest in modern day research. Potential applications can be found in the fields of self-healing materials or printable networks. Since temperature is often used as a stimulus for triggering reversible bonding reactions, an analysis operating at elevated temperatures is very useful for the in situ investigation of the reaction mechanism, as unwanted side effects can be minimized when performing the analyses at the same temperature at which the reactions occur. A temperature-dependent size exclusion chromatographic system (TD SEC) has been optimized for investigating the kinetics of retro Diels-Alder-based depolymerization of Diels-Alder polymers. The changing molecular weight distribution of the analyzed polymers during depolymerization gives valuable quantitative information on the kinetics of the reactions. Adequate data interpretation methods were developed for the correct evaluation of the chromatograms. The results are confirmed by high-temperature dynamic light scattering, thermogravimetric analysis, and time-resolved nuclear magnetic resonance spectroscopy at high temperatures. In addition, the SEC system and column material stability under application conditions were assessed using thermoanalysis methods, infrared spectroscopy, nitrogen physisorption, and scanning electron microscopy. The findings demonstrate that the system is stable and, thus, we can reliably characterize such dynamically bonding/debonding systems with TD SEC.

Making light work of RAFT conjugation: a non-activated RAFT agent at the end of RAFT polymers can readily be coupled with ortho-quinodimethanes (photoenols) in a photo-triggered Diels-Alder reaction under mild conditions without catalyst. The method is universal and opens the door for the conjugation of a large number of RAFT-prepared polymers with photoenol-functionalized (macro)molecules. (RAFT=reversible addition-fragmentation chain transfer.)., (Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

The cytoskeletal protein filament F-actin has been treated in a number of recent studies as a model physical system for semiflexible filaments. In this work, we studied the viscoelastic properties of entangled solutions of the filamentous bacteriophage fd as an alternative to F-actin with similar physical parameters. We present both microrheometric and macrorheometric measurements of the viscoelastic storage and loss moduli, G'(f ) and G"(f ), respectively, in a frequency range 0.01

We present a systematic comparison of microrheological and macrorheological measurements of the viscoelastic storage and loss moduli, G'(f) and G"(f), respectively, of solutions of the semiflexible biopolymer F-actin. Using magnetic tweezers microrheometry and rotating disk macrorheometry, we show that microscopic values for G'(f) and G"(f) are significantly smaller than macroscopic results over the frequency range f = 0.004-4 Hz, whereas the qualitative shape of the spectra is similar. These findings confirm recent theoretical predictions [A. C. Maggs, Phys. Rev. E 57, 2091 (1998)]. The discrepancy affects not only absolute values of G'(f) and G"(f): although microscopic and macroscopic plateau regime are found in the same frequency range, the two methods yield different values for the entanglement time which determines the high-frequency end of the plateau. By investigating F-actin solutions of different mean filament lengths, we show that microscopic and macroscopic G'(f) and G"(f) converge, if the probe particle used in microrheometry becomes large compared to the length of actin filaments.

An improved magnetic bead microrheometer based on phase contrast microscopy allowing high resolution measurements of local deformations within macromolecular network is applied to study local viscoelastic properties of cross-linked actin networks. By embedding nonmagnetic colloidal beads as probes into the networks, the spatial variation of the strain field within cross-linked actin networks can be mapped. Moreover, the Poisson ratio and shear modulus can be measured locally.

Our investigations have shown that depletion of brown fat is prevented by its complete denervation. The cutting off either the spinal nerves or the perivascular nerves only leads to a partial depletion. From this we conclude that the brown fat is innervated via these two nerve paths. The depletion of brown fat is accompanied by the development of different cell types that are described as different stages of this process. The investigations eliminate the possibility that brown fat may change into white fat.

The enzymic formation of guanosine diphosphate 2-deoxy-D-glucose and uridine diphosphate 2-deoxy-D-glucose from synthetically prepared 2-deoxy-D-glucose 1-phosphate is described. Incubation of 2-deoxy-D-glucose 1-phosphate with an enzyme preparation from bovine mammary glands and either GTP or UTP gives rise to the corresponding nucleoside-diphosphate derivative of 2-deoxy-D-glucose. Uridine diphosphate 2-deoxy-D-glucose could also be obtained by incubation of 2-deoxy-D-glucose 1-phosphate with UTP and UDP glucose pyrophosphorylase from beef liver.