Claas Hövelmann - Academia.edu (original) (raw)

Papers by Claas Hövelmann

Research paper thumbnail of Oxy-Functionalization of Nucelophilic Re(I) Metal Carbon Bonds Catalyzed by Se

General Considerations: All air and water sensitive procedures were carried out either in a Vacuu... more General Considerations: All air and water sensitive procedures were carried out either in a Vacuum Atmosphere glove box under argon, or using standard Schlenk techniques under argon. Labeled solvents, D2O and CD3CN were purchased from Cambridge Isotopes and used without further purification. CD3CN was dried over CaH2 and distilled for NMR used for compound characterizations. 2,2’-bipyridine (Lancaster Synthesis, Inc. 99+%) was sublimed at 100-120°C and stored under argon prior to use. CO5ReBr and rhenium carbonyl ,Re2(CO)10, were purchased from Strem. THF (Mallinckrodt) was refluxed over Na/benzophenone and distilled under argon before use in synthesis or for column chromatography. Basic alumina (EMD) was dried for 2 days at 100 o C in a vacuum oven, and then kept in the glove box antechamber under vacuum overnight before use. 48% aqueous HBr, GOLD LABEL SeO2 (99.9+%), methaneseleninic acid (CH3Se(O)OH, 95%), CH3MgI (Grignard, 3.0M in diethyl ether), CH3I (95.5% stabilized with Cu) ...

Research paper thumbnail of Nachruf auf Kilian Muniz (1970 ‐ 2020)

Nachrichten aus der Chemie

Research paper thumbnail of Discussion of paper by M. Staropoli, A. Raba, C. H. Hovelmann, M.-S. Appavou, J. Allgaier, M. Krutyeva, W. Pyckhout-Hintzen, A. Wischnewski, and D. Richter, entitled ‘Melt dynamics of supramolecular comb polymers: Viscoelastic and dielectric response’

Journal of Rheology, Nov 1, 2017

Discussion of paper by H. Watanabe, Y. Matsumiya and Y. Kwon, entitled 'Dynamics of rouse chains ... more Discussion of paper by H. Watanabe, Y. Matsumiya and Y. Kwon, entitled 'Dynamics of rouse chains undergoing head-to-head association and dissociation: Difference between dielectric and viscoelastic relaxation'

Research paper thumbnail of Discussion of paper by B. Gold, C. H. Hovelmann, N. Luhmann, W. Pyckhout-Hintzen, A. Wischnewski, and D. Richter, entitled ‘The microscopic origin of the rheology in supramolecular entangled polymer networks’

Journal of Rheology, Nov 1, 2017

Research paper thumbnail of In operando monitoring of wood transformation during pretreatment with ionic liquids

Cellulose

The conversion of lignocellulosic biomass or wood into chemicals still poses a challenge due to t... more The conversion of lignocellulosic biomass or wood into chemicals still poses a challenge due to the recalcitrance of this composite-like material consisting of lignin, hemicellulose and cellulose. A very high accessibility of cellulose is reported by a pretreatment with ionic liquids that enables high conversion rates by enzymatic hydrolysis. However, the underlying mechanisms have not yet been monitored in operando nor are they fully understood. We monitored the transformation of wood in ionic liquids using small-angle neutron scattering to observe changes in the material in operando and to elucidate the intrinsic effects. The data analysis shows three different stages that is (1) impregnation, (2) the formation of voids and (3) increasing structure size within cellulose fibrils. This consecutive mechanism coincides with macroscopic disintegration of the tissue. The analysis further reveals that the reduction of order in longitudinal direction along the fiber axis is a prerequisite...

Research paper thumbnail of The microscopic origin of the rheology in supramolecular entangled polymer networks

Journal of Rheology

Supramolecular groups in polymeric systems lead to responsive materials which are ideally suited ... more Supramolecular groups in polymeric systems lead to responsive materials which are ideally suited for applications in dynamic environments. The key to their advanced properties such as shape-memory or self-healing is the reversibility of secondary interactions which can be triggered by external stimuli such as temperature, light, or pH-value. Controlling the (mechanical) behavior of such systems requires a precise understanding of intrinsic properties. We present a multimethod study of transient polyisoprene networks that were functionalized with different amounts of hydrogen bonding urazole groups. This work aims at understanding rich rheological features on the basis of their microscopic origin. First, the thermorheological simple behavior is validated experimentally. Subsequently, we characterize the underlying microscopic processes by broadband dielectric spectroscopy (a-process and a Ã-process), differential scanning calorimetry (glass transition behavior), and Fourier-transform infrared spectroscopy (thermodynamics of group association/dissociation). Based on these results, the influence of the supramolecular groups on the rheological response is analyzed. The observed features such as the onset of elastomeric properties in the flow regime, a drastic increase in the chain relaxation time with an increasing amount of functional groups, and the occurrence of a second rheological relaxation process, which is the most prominent effect, are discussed and related to their physical origin. V

Research paper thumbnail of Melt dynamics of supramolecular comb polymers: Viscoelastic and dielectric response

Journal of Rheology

Discussion of paper by H. Watanabe, Y. Matsumiya and Y. Kwon, entitled 'Dynamics of rouse chains ... more Discussion of paper by H. Watanabe, Y. Matsumiya and Y. Kwon, entitled 'Dynamics of rouse chains undergoing head-to-head association and dissociation: Difference between dielectric and viscoelastic relaxation'

Research paper thumbnail of Scale-Up Procedure for the Efficient Synthesis of Highly Pure Cyclic Poly(ethylene glycol)

Macromolecules

Poly(ethylene glycol) (PEG) is ideally suited for the synthesis of cyclic polymers. The cyclizati... more Poly(ethylene glycol) (PEG) is ideally suited for the synthesis of cyclic polymers. The cyclization reaction of PEG via its tosylate intermediate is well established. We improved the cyclization reaction and obtained cyclic raw products in high yields. The quantities of linear precursor and higher molecular weight condensation byproducts were low. The latter byproducts can be removed efficiently by classical fractionation using chloroform/heptane as solvent/nonsolvent pair. For the removal of linear precursor a process was developed which comprises the quantitative oxidation of alcoholic PEG chain ends to carboxyl groups and their subsequent removal with the help of a basic ion-exchange resin. The efficient cleaning processes allowed carrying out the ring closure reaction at relatively high concentrations and so increasing sample quantities. As a result, cyclic poly(ethylene glycol) was obtained in high purity up to a molecular weight of 20 000 g/mol in quantities of several grams. In order to monitor the...

Research paper thumbnail of Importance of Compact Random Walks for the Rheology of Transient Networks

ACS Macro Letters

Controlling the mechanical behavior of novel supramolecular materials is of the utmost importance... more Controlling the mechanical behavior of novel supramolecular materials is of the utmost importance and requires a fundamental understanding of the underlying physical processes. We present a multimethods approach to the dynamics of entangled transient polyisoprene networks. Small-angle neutron scattering (SANS) on randomly functionalized chains shows homogeneous supramolecular melts with Gaussian chain conformations. The H-bond lifetimes (dielectric α*-process) and the rheological response in terms of the loss modulus G″ differ by 2 orders of magnitude in time. Within the concept of a compact random walk (RW), where the random walker (urazole group acting as a sticker) undergoes multiple returns to its starting point and following the concept of theoretical proposed renormalized sticky bond lifetimes, we quantitatively solve this longstanding and unexplained large discrepancy: While the bond opening gives rise to the dielectric response, for rheological relaxation the association with a new partner is rele...

Research paper thumbnail of Hydrogen Bonding in a Reversible Comb Polymer Architecture: A Microscopic and Macroscopic Investigation

Macromolecules, 2016

In this work an investigation of the hydrogen bonding mechanism in a transiently branched comb-li... more In this work an investigation of the hydrogen bonding mechanism in a transiently branched comb-like polymer system in the melt is reported. The system under investigation consists of a polybutylene oxide (PBO)-based backbone, randomly functionalized with thymine (thy) groups, in combination with shorter PBO graft chains, end-functionalized with diaminotriazine (DAT) groups. The functional groups are able to associate through hydrogen bonding. The hetero-complementary association of these groups leads to the formation of a transiently branched comb-like polymer system. Since recently virtually exclusive heterocomplementary association could be observed in the supramolecular association of telechelically-modified oligomeric PEG chains, here we aim to extend the supramolecular assembly mechanism towards branched structures. The present work combines Small Angle Neutron Scattering (SANS) experiments on a selectively labeled system with macroscopic dynamics measured in linear rheology response. The association of thy-and DAT-modified components was studied as a function of temperature and composition. The scattering function reveals the formation of a block copolymer and can be exclusively attributed to heterocomplementary association of the hydrogen bonding groups. Scattering functions of nonfunctionalized blends are also reported as references and evidence the change in the microstructure induced by the hetero-complementary association. All scattering profiles were described by means of the Random Phase Approximation (RPA) formalism from which the average aggregation number i.e. comb arm functionality and the equilibrium association constant could be determined directly in the melt state as a function of temperature. On the other hand, rheological measurements were performed in the melt state to study the influence of the reversible bonds on the macroscopic dynamics of the polymer system. The rheology data are in good agreement with the SANS results and confirm the transient comb-like branched architecture. The supramolecular association exhibits characteristic bonding times of the groups in the order of 1 s at-25 °C and therefore makes the thy-DAT pair an ideal candidate for the development of responsive materials that combine permanent and transient linkages for novel applications and self-healing properties.

Research paper thumbnail of Association Behavior, Diffusion, and Viscosity of End-Functionalized Supramolecular Poly(ethylene glycol) in the Melt State

Macromolecules, 2015

A. Supramolecular polymer synthesis and characterization Materials. Anhydrous dichloromethane (Si... more A. Supramolecular polymer synthesis and characterization Materials. Anhydrous dichloromethane (Sigma Aldrich, ≥99.8%), triethylamine (Sigma Aldrich, ≥99%), methanesulfonyl chloride (Fluka, ≥98%), THF (KMF, ≥99.9%), ammonium hydroxide solution (28-30%, Sigma Aldrich), N,N-dimethylformamide (DMF) (Sigma Aldrich, over molecular sieve, ≥99.8%), O-(Benzotriazol-1-yl)-N,N,N′,N′-tetramethyluronium tetrafluoroborate (TBTU) (Sigma Aldrich, ≥ 97 %), N,N-diisopropylethylamine (Sigma Aldrich, ≥99%), thymine-1-acetic acid (Sigma Aldrich, 98%), 2-chloro-4,6-diamino-1,3,5-triazine (DAT-Cl) (Sigma Aldrich, 98%), benzyl bromide (Sigma Aldrich, 98%), ethylene glycol-d4 (Eurisotop, 98%), and potassium tert-butoxide (Sigma-Aldrich, 99%) were used as received. General synthetic techniques. All synthetic manipulations with air sensitive agents were carried out in a glove box, filled with argon [M Braun, Unilab] or at a high vacuum line using standard cryogenic distillation techniques. Air and moisture sensitive agents were handled in pressure tested flasks equipped with teflon stopcocks that allowed transferring materials between vacuum line and glove box without contamination with air. Drying and purification procedures. Polyethylene glycol 2.000 (h-PEG, Merck) was dried under high vacuum conditions for two days. The molecular weight was determined by 1 H-NMR spectroscopy in pyridine-d5. The comparison of the main signal at 3.6 ppm with the signal of the methylene groups next to the OH-end groups at 3.95 ppm yielded a molecular weight of Mn =

Research paper thumbnail of Synthesis and rheological behavior of poly(1,2-butylene oxide) based supramolecular architectures

RSC Adv., 2016

We describe the synthesis, analysis and rheological behavior of differently functionalized poly(1... more We describe the synthesis, analysis and rheological behavior of differently functionalized poly(1,2-butylene oxide) based supramolecular polymers.

Research paper thumbnail of Wissenschaftliche Arbeiten auf Englisch: Scientific English für Mediziner und Naturwissenschaftler. Von Christian Hrdina, Robert Hrdina. Langenscheid, München 2006.152 Seiten, brosch. 24,90 Euro. ISBN 3-86117-257-7

Nachrichten aus der Chemie, 2006

Research paper thumbnail of Diamination of Alkenes by Use of a Bisimidoosmium Reagent

Research paper thumbnail of Towards the microscopic understanding of self-healing mechanisms

Research paper thumbnail of Synthesis and mechanical analysis of transient-covalent double networks

Nature uses a combination of dynamic hydrogen bonds and static covalent bonds in e.g. the muscle ... more Nature uses a combination of dynamic hydrogen bonds and static covalent bonds in e.g. the muscle sarcomer to achieve toughness in otherwise elastic materials. The dynamic bonds can open when a force is applied, thereby providing a stress-relieve mechanism to prevent rupturing of the covalent links. Upon release of external force the hydrogen bond network can heal to restore the former properties. To mimic these astonishing effects we envisioned a similar combination in elastomeric compounds by creating a dual network of hydrogen bonded and covalently linked polyisoprene. We synthesized a number of dual networks by first modifying polyisoprene with hydrogen bond forming urazole groups to form supramolecular networks with a cross-linking density between 0 and 15 mol%. In a second step covalent crosslinks were added by hydrosilylation with a bissilane linker. Linear rheological analysis of the transient network showed a dissipation mechanism extrapolated to be on the order of 0.01-0.10 s at ambient temperature. In combination with neutron scattering, which provides unrivalled and first insights on the chain level, the self-healing properties of these novel semi-transient networks are studied on the molecular level in static and dynamic deformation through a selective labelling of chains. Understanding of the self-healing mechanism in these mixed covalent and transient systems will allow the development of new polymeric materials with advanced functionality.

Research paper thumbnail of Methane-to-Methanol Conversion

Encyclopedia of Inorganic Chemistry, 2006

ABSTRACT The purpose of this project is to develop a novel process by which natural gas or methan... more ABSTRACT The purpose of this project is to develop a novel process by which natural gas or methane from coal gasification products can be converted to a transportable liquid fuel. It is proposed that methanol can be produced by the direct, partial oxidation of methane utilizing air or oxygen. It is anticipated that, compared to present technologies, the new process might offer significant economic advantages with respect to capital investment and methane feedstock purity requirements. Results to date are discussed. 6 refs.

Research paper thumbnail of Metal-free diamination of alkenes employing bromide catalysis

Chemical Science, 2012

ABSTRACT A unique metal-free intramolecular diamination of alkenes based on bromide catalysis is ... more ABSTRACT A unique metal-free intramolecular diamination of alkenes based on bromide catalysis is reported that uses only potassium bromide and sodium chlorite avoiding any use of transition metal. This unprecedented halide catalysis is of general applicability, uses economic reagents, can be conveniently up-scaled and proceeds under mild and selective conditions that surpass all conventional transition metals in scope.

Research paper thumbnail of Molecular Approach to Supramolecular Polymer Assembly by Small Angle Neutron Scattering

Research paper thumbnail of Making Water the Exciting Way: A Classroom Demonstration of Catalysis

Journal of Chemical Education, 2014

ABSTRACT An understanding of the factors that govern the rate of chemical reactions has proven el... more ABSTRACT An understanding of the factors that govern the rate of chemical reactions has proven elusive for many students who begin a survey course in the chemical sciences. Inquiry-based curricula built upon an understanding of common student misconceptions related to chemical kinetics have proven to be a more effective means by which to develop student understanding than traditional lecture formats. To facilitate teacher-guided discussion regarding the subject of catalysis, we have developed a simple and safe demonstration, whereby the hydrogen and oxygen components evolved upon electrolysis of water are recombined at room temperature with the aid of a platinum/ruthenium catalyst. This demonstration is designed to draw students’ attention toward the dramatic change in reaction rate that can be affected by catalyst involvement and to provoke dialog regarding the mechanistic changes that accompany reaction catalysis.

Research paper thumbnail of Oxy-Functionalization of Nucelophilic Re(I) Metal Carbon Bonds Catalyzed by Se

General Considerations: All air and water sensitive procedures were carried out either in a Vacuu... more General Considerations: All air and water sensitive procedures were carried out either in a Vacuum Atmosphere glove box under argon, or using standard Schlenk techniques under argon. Labeled solvents, D2O and CD3CN were purchased from Cambridge Isotopes and used without further purification. CD3CN was dried over CaH2 and distilled for NMR used for compound characterizations. 2,2’-bipyridine (Lancaster Synthesis, Inc. 99+%) was sublimed at 100-120°C and stored under argon prior to use. CO5ReBr and rhenium carbonyl ,Re2(CO)10, were purchased from Strem. THF (Mallinckrodt) was refluxed over Na/benzophenone and distilled under argon before use in synthesis or for column chromatography. Basic alumina (EMD) was dried for 2 days at 100 o C in a vacuum oven, and then kept in the glove box antechamber under vacuum overnight before use. 48% aqueous HBr, GOLD LABEL SeO2 (99.9+%), methaneseleninic acid (CH3Se(O)OH, 95%), CH3MgI (Grignard, 3.0M in diethyl ether), CH3I (95.5% stabilized with Cu) ...

Research paper thumbnail of Nachruf auf Kilian Muniz (1970 ‐ 2020)

Nachrichten aus der Chemie

Research paper thumbnail of Discussion of paper by M. Staropoli, A. Raba, C. H. Hovelmann, M.-S. Appavou, J. Allgaier, M. Krutyeva, W. Pyckhout-Hintzen, A. Wischnewski, and D. Richter, entitled ‘Melt dynamics of supramolecular comb polymers: Viscoelastic and dielectric response’

Journal of Rheology, Nov 1, 2017

Discussion of paper by H. Watanabe, Y. Matsumiya and Y. Kwon, entitled 'Dynamics of rouse chains ... more Discussion of paper by H. Watanabe, Y. Matsumiya and Y. Kwon, entitled 'Dynamics of rouse chains undergoing head-to-head association and dissociation: Difference between dielectric and viscoelastic relaxation'

Research paper thumbnail of Discussion of paper by B. Gold, C. H. Hovelmann, N. Luhmann, W. Pyckhout-Hintzen, A. Wischnewski, and D. Richter, entitled ‘The microscopic origin of the rheology in supramolecular entangled polymer networks’

Journal of Rheology, Nov 1, 2017

Research paper thumbnail of In operando monitoring of wood transformation during pretreatment with ionic liquids

Cellulose

The conversion of lignocellulosic biomass or wood into chemicals still poses a challenge due to t... more The conversion of lignocellulosic biomass or wood into chemicals still poses a challenge due to the recalcitrance of this composite-like material consisting of lignin, hemicellulose and cellulose. A very high accessibility of cellulose is reported by a pretreatment with ionic liquids that enables high conversion rates by enzymatic hydrolysis. However, the underlying mechanisms have not yet been monitored in operando nor are they fully understood. We monitored the transformation of wood in ionic liquids using small-angle neutron scattering to observe changes in the material in operando and to elucidate the intrinsic effects. The data analysis shows three different stages that is (1) impregnation, (2) the formation of voids and (3) increasing structure size within cellulose fibrils. This consecutive mechanism coincides with macroscopic disintegration of the tissue. The analysis further reveals that the reduction of order in longitudinal direction along the fiber axis is a prerequisite...

Research paper thumbnail of The microscopic origin of the rheology in supramolecular entangled polymer networks

Journal of Rheology

Supramolecular groups in polymeric systems lead to responsive materials which are ideally suited ... more Supramolecular groups in polymeric systems lead to responsive materials which are ideally suited for applications in dynamic environments. The key to their advanced properties such as shape-memory or self-healing is the reversibility of secondary interactions which can be triggered by external stimuli such as temperature, light, or pH-value. Controlling the (mechanical) behavior of such systems requires a precise understanding of intrinsic properties. We present a multimethod study of transient polyisoprene networks that were functionalized with different amounts of hydrogen bonding urazole groups. This work aims at understanding rich rheological features on the basis of their microscopic origin. First, the thermorheological simple behavior is validated experimentally. Subsequently, we characterize the underlying microscopic processes by broadband dielectric spectroscopy (a-process and a Ã-process), differential scanning calorimetry (glass transition behavior), and Fourier-transform infrared spectroscopy (thermodynamics of group association/dissociation). Based on these results, the influence of the supramolecular groups on the rheological response is analyzed. The observed features such as the onset of elastomeric properties in the flow regime, a drastic increase in the chain relaxation time with an increasing amount of functional groups, and the occurrence of a second rheological relaxation process, which is the most prominent effect, are discussed and related to their physical origin. V

Research paper thumbnail of Melt dynamics of supramolecular comb polymers: Viscoelastic and dielectric response

Journal of Rheology

Discussion of paper by H. Watanabe, Y. Matsumiya and Y. Kwon, entitled 'Dynamics of rouse chains ... more Discussion of paper by H. Watanabe, Y. Matsumiya and Y. Kwon, entitled 'Dynamics of rouse chains undergoing head-to-head association and dissociation: Difference between dielectric and viscoelastic relaxation'

Research paper thumbnail of Scale-Up Procedure for the Efficient Synthesis of Highly Pure Cyclic Poly(ethylene glycol)

Macromolecules

Poly(ethylene glycol) (PEG) is ideally suited for the synthesis of cyclic polymers. The cyclizati... more Poly(ethylene glycol) (PEG) is ideally suited for the synthesis of cyclic polymers. The cyclization reaction of PEG via its tosylate intermediate is well established. We improved the cyclization reaction and obtained cyclic raw products in high yields. The quantities of linear precursor and higher molecular weight condensation byproducts were low. The latter byproducts can be removed efficiently by classical fractionation using chloroform/heptane as solvent/nonsolvent pair. For the removal of linear precursor a process was developed which comprises the quantitative oxidation of alcoholic PEG chain ends to carboxyl groups and their subsequent removal with the help of a basic ion-exchange resin. The efficient cleaning processes allowed carrying out the ring closure reaction at relatively high concentrations and so increasing sample quantities. As a result, cyclic poly(ethylene glycol) was obtained in high purity up to a molecular weight of 20 000 g/mol in quantities of several grams. In order to monitor the...

Research paper thumbnail of Importance of Compact Random Walks for the Rheology of Transient Networks

ACS Macro Letters

Controlling the mechanical behavior of novel supramolecular materials is of the utmost importance... more Controlling the mechanical behavior of novel supramolecular materials is of the utmost importance and requires a fundamental understanding of the underlying physical processes. We present a multimethods approach to the dynamics of entangled transient polyisoprene networks. Small-angle neutron scattering (SANS) on randomly functionalized chains shows homogeneous supramolecular melts with Gaussian chain conformations. The H-bond lifetimes (dielectric α*-process) and the rheological response in terms of the loss modulus G″ differ by 2 orders of magnitude in time. Within the concept of a compact random walk (RW), where the random walker (urazole group acting as a sticker) undergoes multiple returns to its starting point and following the concept of theoretical proposed renormalized sticky bond lifetimes, we quantitatively solve this longstanding and unexplained large discrepancy: While the bond opening gives rise to the dielectric response, for rheological relaxation the association with a new partner is rele...

Research paper thumbnail of Hydrogen Bonding in a Reversible Comb Polymer Architecture: A Microscopic and Macroscopic Investigation

Macromolecules, 2016

In this work an investigation of the hydrogen bonding mechanism in a transiently branched comb-li... more In this work an investigation of the hydrogen bonding mechanism in a transiently branched comb-like polymer system in the melt is reported. The system under investigation consists of a polybutylene oxide (PBO)-based backbone, randomly functionalized with thymine (thy) groups, in combination with shorter PBO graft chains, end-functionalized with diaminotriazine (DAT) groups. The functional groups are able to associate through hydrogen bonding. The hetero-complementary association of these groups leads to the formation of a transiently branched comb-like polymer system. Since recently virtually exclusive heterocomplementary association could be observed in the supramolecular association of telechelically-modified oligomeric PEG chains, here we aim to extend the supramolecular assembly mechanism towards branched structures. The present work combines Small Angle Neutron Scattering (SANS) experiments on a selectively labeled system with macroscopic dynamics measured in linear rheology response. The association of thy-and DAT-modified components was studied as a function of temperature and composition. The scattering function reveals the formation of a block copolymer and can be exclusively attributed to heterocomplementary association of the hydrogen bonding groups. Scattering functions of nonfunctionalized blends are also reported as references and evidence the change in the microstructure induced by the hetero-complementary association. All scattering profiles were described by means of the Random Phase Approximation (RPA) formalism from which the average aggregation number i.e. comb arm functionality and the equilibrium association constant could be determined directly in the melt state as a function of temperature. On the other hand, rheological measurements were performed in the melt state to study the influence of the reversible bonds on the macroscopic dynamics of the polymer system. The rheology data are in good agreement with the SANS results and confirm the transient comb-like branched architecture. The supramolecular association exhibits characteristic bonding times of the groups in the order of 1 s at-25 °C and therefore makes the thy-DAT pair an ideal candidate for the development of responsive materials that combine permanent and transient linkages for novel applications and self-healing properties.

Research paper thumbnail of Association Behavior, Diffusion, and Viscosity of End-Functionalized Supramolecular Poly(ethylene glycol) in the Melt State

Macromolecules, 2015

A. Supramolecular polymer synthesis and characterization Materials. Anhydrous dichloromethane (Si... more A. Supramolecular polymer synthesis and characterization Materials. Anhydrous dichloromethane (Sigma Aldrich, ≥99.8%), triethylamine (Sigma Aldrich, ≥99%), methanesulfonyl chloride (Fluka, ≥98%), THF (KMF, ≥99.9%), ammonium hydroxide solution (28-30%, Sigma Aldrich), N,N-dimethylformamide (DMF) (Sigma Aldrich, over molecular sieve, ≥99.8%), O-(Benzotriazol-1-yl)-N,N,N′,N′-tetramethyluronium tetrafluoroborate (TBTU) (Sigma Aldrich, ≥ 97 %), N,N-diisopropylethylamine (Sigma Aldrich, ≥99%), thymine-1-acetic acid (Sigma Aldrich, 98%), 2-chloro-4,6-diamino-1,3,5-triazine (DAT-Cl) (Sigma Aldrich, 98%), benzyl bromide (Sigma Aldrich, 98%), ethylene glycol-d4 (Eurisotop, 98%), and potassium tert-butoxide (Sigma-Aldrich, 99%) were used as received. General synthetic techniques. All synthetic manipulations with air sensitive agents were carried out in a glove box, filled with argon [M Braun, Unilab] or at a high vacuum line using standard cryogenic distillation techniques. Air and moisture sensitive agents were handled in pressure tested flasks equipped with teflon stopcocks that allowed transferring materials between vacuum line and glove box without contamination with air. Drying and purification procedures. Polyethylene glycol 2.000 (h-PEG, Merck) was dried under high vacuum conditions for two days. The molecular weight was determined by 1 H-NMR spectroscopy in pyridine-d5. The comparison of the main signal at 3.6 ppm with the signal of the methylene groups next to the OH-end groups at 3.95 ppm yielded a molecular weight of Mn =

Research paper thumbnail of Synthesis and rheological behavior of poly(1,2-butylene oxide) based supramolecular architectures

RSC Adv., 2016

We describe the synthesis, analysis and rheological behavior of differently functionalized poly(1... more We describe the synthesis, analysis and rheological behavior of differently functionalized poly(1,2-butylene oxide) based supramolecular polymers.

Research paper thumbnail of Wissenschaftliche Arbeiten auf Englisch: Scientific English für Mediziner und Naturwissenschaftler. Von Christian Hrdina, Robert Hrdina. Langenscheid, München 2006.152 Seiten, brosch. 24,90 Euro. ISBN 3-86117-257-7

Nachrichten aus der Chemie, 2006

Research paper thumbnail of Diamination of Alkenes by Use of a Bisimidoosmium Reagent

Research paper thumbnail of Towards the microscopic understanding of self-healing mechanisms

Research paper thumbnail of Synthesis and mechanical analysis of transient-covalent double networks

Nature uses a combination of dynamic hydrogen bonds and static covalent bonds in e.g. the muscle ... more Nature uses a combination of dynamic hydrogen bonds and static covalent bonds in e.g. the muscle sarcomer to achieve toughness in otherwise elastic materials. The dynamic bonds can open when a force is applied, thereby providing a stress-relieve mechanism to prevent rupturing of the covalent links. Upon release of external force the hydrogen bond network can heal to restore the former properties. To mimic these astonishing effects we envisioned a similar combination in elastomeric compounds by creating a dual network of hydrogen bonded and covalently linked polyisoprene. We synthesized a number of dual networks by first modifying polyisoprene with hydrogen bond forming urazole groups to form supramolecular networks with a cross-linking density between 0 and 15 mol%. In a second step covalent crosslinks were added by hydrosilylation with a bissilane linker. Linear rheological analysis of the transient network showed a dissipation mechanism extrapolated to be on the order of 0.01-0.10 s at ambient temperature. In combination with neutron scattering, which provides unrivalled and first insights on the chain level, the self-healing properties of these novel semi-transient networks are studied on the molecular level in static and dynamic deformation through a selective labelling of chains. Understanding of the self-healing mechanism in these mixed covalent and transient systems will allow the development of new polymeric materials with advanced functionality.

Research paper thumbnail of Methane-to-Methanol Conversion

Encyclopedia of Inorganic Chemistry, 2006

ABSTRACT The purpose of this project is to develop a novel process by which natural gas or methan... more ABSTRACT The purpose of this project is to develop a novel process by which natural gas or methane from coal gasification products can be converted to a transportable liquid fuel. It is proposed that methanol can be produced by the direct, partial oxidation of methane utilizing air or oxygen. It is anticipated that, compared to present technologies, the new process might offer significant economic advantages with respect to capital investment and methane feedstock purity requirements. Results to date are discussed. 6 refs.

Research paper thumbnail of Metal-free diamination of alkenes employing bromide catalysis

Chemical Science, 2012

ABSTRACT A unique metal-free intramolecular diamination of alkenes based on bromide catalysis is ... more ABSTRACT A unique metal-free intramolecular diamination of alkenes based on bromide catalysis is reported that uses only potassium bromide and sodium chlorite avoiding any use of transition metal. This unprecedented halide catalysis is of general applicability, uses economic reagents, can be conveniently up-scaled and proceeds under mild and selective conditions that surpass all conventional transition metals in scope.

Research paper thumbnail of Molecular Approach to Supramolecular Polymer Assembly by Small Angle Neutron Scattering

Research paper thumbnail of Making Water the Exciting Way: A Classroom Demonstration of Catalysis

Journal of Chemical Education, 2014

ABSTRACT An understanding of the factors that govern the rate of chemical reactions has proven el... more ABSTRACT An understanding of the factors that govern the rate of chemical reactions has proven elusive for many students who begin a survey course in the chemical sciences. Inquiry-based curricula built upon an understanding of common student misconceptions related to chemical kinetics have proven to be a more effective means by which to develop student understanding than traditional lecture formats. To facilitate teacher-guided discussion regarding the subject of catalysis, we have developed a simple and safe demonstration, whereby the hydrogen and oxygen components evolved upon electrolysis of water are recombined at room temperature with the aid of a platinum/ruthenium catalyst. This demonstration is designed to draw students’ attention toward the dramatic change in reaction rate that can be affected by catalyst involvement and to provoke dialog regarding the mechanistic changes that accompany reaction catalysis.