Research

Current Research Topics and Topics for PhD and Masters

Research Topics Prof. Dr. Michael R. Buchmeiser

(1) Protected N-heterocyclic carbenes (NHCs) have been applied in conjunction with benzylic alcohol to polymerize lactones. NHCs are highly nucleophilic and basic molecules bearing an electron-sextet on a divalent carbon atom between (predominantly) two nitrogen atoms. Polymerizations were conducted solvent-free using imidazolium-, imidazolinium-, tetrahydropyrimidinium- and diazepinium-based structures with various different side and protecting groups. Most satisfying results were found for imidazolium- and tetrahydropyrimidinium structures. Electron-donating side groups such as 2-propyl and cyclohexyl turned out to be most active in case CO2 was applied as protecting group, whereas aromatic side groups (e.g. 2,4,6-trimethylphenyl ≡ Mes) showed minor reactivity. In contrast, imidazolinium- and tetrahydropyrimidinium-based NHCs with aromatic side groups displayed the best results if combined with MgCl2, ZnCl2 and SnCl2 as protecting group. This has been attributed to the additional activation of the monomer by the Lewis acids within the system and is referred to as “dual catalysis”. Under the same conditions the usage of non-toxic MgCl2 as protecting group of 1,3-bis(2,4,6-trimethyhlphenyl) imidazolin-2-ylidene showed the highest reactivity. Molecular weights of 2.6 to 17 kg mol-1 were achieved within 24 h. It is further noteworthy that good yields were only reached if the reactions were carried out at elevated temperatures (70°C).

(2) Poly(oxazolidin-2-one)s are known as high performance (thermoplastic) material and offer many benefits like high temperature stability, good availability, cheap monomers and high glass transition temperatures. Nevertheless, their synthesis is still challenging, mainly due to trimerization of the isocyanates to isocyanurates, which leads to undesirable crosslinking. The synthesis of poly(oxazolidin-2-one)s from diepoxides and diisocyanates is, among all other possibilities, the most reasonable in terms of costs, flexibility, availability but also in terms of chemistry since the reaction is a polyaddition, which does not result in the formation of any unwanted condensation by-products. However, the selective and exclusive formation of oxazolidin-2-one moieties must be addressed by the choice of the catalyst applied. NHCs in dual catalysis with Lewis acids (like LiCl or MgCl2) were found to form exclusively oxazolidin-2-one moieties if the monomers were added subsequently to the catalyst mixture. It was possible to polymerize aliphatic and even more reactive aromatic diisocyanates with different diepoxides. Aside from the selective formation of oxazolidin-2-ones, also high regioselectivity within the formed ring structures was found and confirmed by 13C NMR spectroscopy. A finding, additionally benefiting the polymerization, was that the combination of an NHC and a Lewis acid allowed for the formation of oxazolidin-2-ones from isocyanurates and epoxides, thus a certain amount of potentially formed trimers can be re-transformed into the desired structures. Molecular weights ranged from 6 to 50 kg mol‑1­.

Selected Publications:

  1. S. Naumann, M. R. Buchmeiser, Catal. Sci. Technol., 4, 2014, 2466-2479.
  2. S. Naumann, F. G. Schmidt, M. R. Buchmeiser, Polym. Chem., 4, 2013, 4172-4181.
  3. H. J. Altmann, M. Clauss, S. König, E. Frick-Delaittre, C. Koopmans, A. Wolf, S. Naumann, M. R. Buchmeiser, Macromolecules, 52, 2019, 487-494.

A series of latent, thermally- or UV-triggerable pre-catalysts based on CO2­­-protected N-heterocyclic carbenes (NHCs) and NHC-Sn+II, -Zn+II, -Mg+II, -Al+III as well as NHC-Ru and NHC-Mo alkylidene complexes for step- and chain-growth polymerization have been synthesized.

The CO2-, Sn+II-, Zn+II-, Mg+II-, Al+III-protected NHCs exhibit very good thermal latency in a wide range of applications including the ring opening polymerization of lactams and lactones, polyurethane synthesis with tailored of isocyanurate content, thereby improving the thermal and mechanical properties, polymerization of methyl methacrylate (MMA) and curing of highly crosslinked anhydride-hardened epoxy resins. The active catalyst species can only be formed at elevated temperatures, an advantage that allows premixing of batch and diluted systems, important for a broad spectrum of applications, e.g. for reaction injection molding (RIM) and resin transfer molding (RTM). CO2-protected NHCs do not only show excellent activity, but also allow for metal free synthesis and therefore circumvent toxic heavy metals such as Hg+II or organotin compounds, which have sometimes toxicities comparable to cyanide. 

Complementary, the ring-opening metatheses polymerization (ROMP) triggered by cationic NHC-Ru+II complexes in the presence of various norborn-2-ene substituted monomers can selectively be activated by UV irradiation. These initiators are of particular interest in technical applications of ROMP, they allow for premixing of a monomer/pre-catalyst mixture and its storage over long periods of time at elevated temperatures. Most importantly, these initiators allow for shaping and profiling of such mixtures prior to polymerization (“curing”).

The characteristic coalescence temperature, Tc, at which the complexes exhibit the highest activity, makes them interesting targets for applications that require latent catalyst systems. Poly(dicyclopentadiene) at present is synthesized by a two-component catalyst system, that is mixed immediately prior to use. The employment of a one-component system, which can be premixed and stored within the substrate, i.e. Poly(dicyclopentadiene) would significantly facilitate the process. Molybdenum imido alkylidene NHC complexes, e.g., based on triazol-4-ylidenes and mesoionic carbenes are also most suitable latent pre-catalysts for the polymerization of Poly(dicyclopentadiene). Storage of the catalysts with the highly reactive substrate is possible and activation of the catalysts can be implemented by simple heating of the mixture.

Selected Publications:

  1. S. Naumann, S. Epple, C. Bonten, M. R. Buchmeiser, ACS Macro Lett., 2, 2013, 609-612.
  2. S. Naumann, F. G. Schmidt, W. Frey, M. R. Buchmeiser, Polym. Chem., 4, 2013, 4172-4181.
  3. B. Bantu, G. M. Pawar, K. Wurst, U. Decker, A. M. Schmidt, M. R. Buchmeiser, Eur. J. Inorg. Chem.2009, 1970-1976.
  4. B. Bantu, G. M. Pawar, U. Decker, K. Wurst, A. M. Schmidt, M. R. Buchmeiser, Chem. Eur. J.2009, 3103-3109.
  5. S. Naumann, F. G. Schmidt, R. Schowner, W. Frey, M. R. Buchmeiser, Polym. Chem.2013, 2731-2740.
  6. S. Naumann, M. Speiser, R. Schowner, E. Giebel, M. R. Buchmeiser, Macromolecules, 47, 2015, 4548-4556.
  7. M. R. Buchmeiser, J. A. Kammerer, S. Naumann, J. Unold, R. Ghomeshi, S. K. Selvarayan, P. Weichand, R. Gadow, Macromol. Mater. Eng., 9, 2015, 937-943.
  8. D. Wang, K. Wurst, W. Knolle, U. Decker, L. Prager, M. R. Buchmeiser, Angew. Chem., 120, 2008, 3311-3314; Angew. Chem. Int. Ed., 47, 2008, 3267-3270.
  9. D. Wang, K. Wurst, M. R. Buchmeiser, Chem. Eur. J., 16, 2010, 12928-12934.
  10. J. Beerhues, S. Sen, R. Schowner, G. M. Nagy, M. R. Buchmeiser, invitation to a special issue celebrating Prof. R. H. Grubbs 75th birthday, J. Polym. Sci. A: Polym. Chem. 55, 2017, 3028-3033.

SPAN, a sulfur-containing poly(acrylonitrile)-derived composite material containing up to 55 wt.-% of chemically bound S, is used as cathode material for M-S batteries (M = Li, Mg). Different morphologies of SPAN, including fibrous, monolithic and pellicular structures, are employed. The structure of SPAN has been fully elucidated und correlated with the electrochemical performance of Li-S batteries built therefrom. In particular, their discharge / charge chemistry and the role of the electrolyte during cycling have been investigated. These investigations entail the electrochemical characterization as well as physico-chemical measurements including WAXS, ESCA, electron microscopy, FT-IR and Raman measurements and MALDI-TOF, to name just a few. Based on that knowledge, Li-S batteries stable for >1200 cycles displaying energy densities up to 2 mA.h/cm2 @ 0.5C have already been realized. In the field of Mg-S batteries, both the electrolyte system and the anode morphology play a key role for a working Mg-S cell. It is crucial to understand the electrochemical processes in the cell to optimize the electrolyte components and the anode side. Based on the results of  different measurements including XPS, SEM, EDX, XRD and cyclic voltammetry new electrolytes and optimized anodes are developed to improve the cycle stability, C-rate capability and discharge capacities of SPAN-based Mg-S cells. In summary, a comprehensive picture of the chemistry and electrochemistry of SPAN-based metal-sulfur batteries is to be created that ultimately allows for designing high-capacity devices with good cycling stability (>1500 cycles) and high energy density (>3.5 mA.h/cm2 @ 1C ).

Selected Publications:

  1. J. Fanous, M. Wegner, J. Grimminger, Ä. Andresen, M. R. Buchmeiser, Chem. Mater., 23, 2011, 5024-5028.
  2. J. Fanous, M. Wegner, J. Grimminger, M. Rolff, M. B. M. Spera, M. Tenzer and M. R. Buchmeiser, J. Mater. Chem., 22, 2012, 23240-23245.
  3. J. Fanous, M. Schweizer, D. Schawaller, M. R. Buchmeiser, Macromol. Mater. Eng., 297, 2012, 123-127.
  4. J. Fanous, M. Wegner, M. B. M. Spera, M. R. Buchmeiser, J. Electrochem. Soc., 160(8), 2013, A1169-A1170.
  5. M. Frey, R. Zenn, S. Warneke, K. Müller, A. Hintennach, R. E. Dinnebier and M. R. Buchmeiser, ACS Energy Lett., 2, 2017, 595−604.
  6. S. Warneke, A. Hintennach, Michael R. Buchmeiser, J. Electrochem. Soc., 165, 2018, A2093-A2095.
  7. S. Warneke, M. Eusterholz, R. Zenn, A. Hintennach, R. E. Dinnebier, M. R. Buchmeiser, J. Electrochem. Soc., 165, 2018, A6017-A6020.
  8. S. Warneke, R. K. Zenn, T. Lebherz, K. Müller, A. Hintennach, U. Starke, R. E. Dinnebier, M. R. Buchmeiser, Adv. Sust. Systems, 2, 2018, 1700144.
  9. T. Lebherz, M. Frey, A. Hintennach, Michael R. Buchmeiser, RSC Adv., 9, 2019,  7181-7188.
  10. P. Wang, M. R. Buchmeiser, Adv. Funct. Mater., 2019, 1905248.
  11. Fluor-basierte Elektrolyte für Lithium-Schwefel Batterien, M. Frey, M. R. Buchmeiser, A. Hintennach (Daimler AG), DE102016004643.0.
  12. Elektrolyt und elektrochemischer Energiespeicher, S. Warneke, M. R. Buchmeiser, A. Hintennach (Daimler AG), DE102016011782.6.
  13. Kathodenmaterial und Verfahren zu dessen Herstellung (2), M. Frey, M. R. Buchmeiser, A. Hintennach (Daimler AG), Offenlegungsschrift DE 10 2015 00220 A1 (25. 08. 2016).
  14. Kathodenmaterial und Verfahren zu dessen Herstellung (1), M. Frey, M. R. Buchmeiser, A. Hintennach (Daimler AG), DE 102014012468.1.

The synthesis of the first molybdenum imido alkylidene N-heterocyclic carbene (NHC) bistriflate catalysts (Figure A) in 2014 and the following mechanistic investigations resulted in the fast development of a large catalyst library. Variations in the metal or in the ligand sphere, which included the imido and the alkoxide ligand as well as the NHC moiety, paved the way for numerous applications. Remarkably, due to the excellent σ-donor properties of the NHCs, the first highly active cationic group 6 metal alkylidene NHC were prepared that allowed running various olefin metathesis reactions with turnover numbers (TONs) >500,000. These olefin metathesis reactions entail ring opening metathesis polymerization (ROMP), cross-metathesis (CM), ring-opening cross metathesis (ROCM), ethenolysis, ring closing metathesis (RCM) and ring closing ene-yne metathesis (RCEYM), thereby making the catalysts interesting targets even beyond polymer chemistry. Their immobilization on silica, their application in biphasic metathesis and their high functional group tolerance further highlight the versatile properties of this new class of olefin metathesis catalysts and led to cooperations, further widening the scope of the catalyst system.

The unprecedented catalytic activity of 16-electron molybdenum and tungsten complexes in olefin metathesis reactions raised the question about mechanistic details, since the usual Schrock-type olefin metathesis catalysts display a 14-electron architecture. In situ 19F NMR measurements revealed the reason for the unexpected catalytic properties. The synergistic effect of the s-donating NHC and the excellent leaving group character of the triflates results in the formation of the catalytically active cationic species.

Molybdenum imido alkylidene NHC bistriflate complexes display a characteristic coalescence temperature, Tc, at which the catalysts have a square pyramidal geometry and the trans-effect of the NHC ligand on the triflate ligand becomes fully operative, enabling the fast formation of the cationic active species. Variations of the carbene ligand, covering carbenes with large Tolman electronic parameters (TEP) and therefore less pronounced σ-donor properties, such as 1,2,4-triazol-4-ylidenes, and carbenes with small Tolman electronic parameters, e.g., mesoionic carbenes, allowed for the fine tuning of Tc. This opened the way to tailor-made catalysts with special operating windows.

Molybdenum imido, tungsten imido and tungsten oxo alkylidene NHC complexes can be converted into cationic complexes by the replacement of one anionic X ligand (e.g., triflate, chloride, alkoxide) by a weakly coordinating anion like BF4 or B(ArF)4 (tetrakis-(3,5-trifluoromethylphenyl)borate, Figure A). The stable tetracoordinated cationic species exhibit high activity in olefin metathesis reactions, which sets them apart from the few exiasting examples of cationic alkylidene complexes. The influence of the weakly coordinating anion on stability and activity of the cationic systems is currently investigated in our laboratories.

Molybdenum and tungsten imido alkylidene NHC complexes can be varied in a multitude of ways, since they can contain up to five different ligands, each one being able to introduce or influence unique properties. We made use of this concept in several cases, for example by introducing ligands bearing a cationic charge (Figure C). This enables the use of these complexes in biphasic systems. The catalysts are dissolved in the polar phase due to the cationic charge of the ligand, while the substrates are dissolved in the non-polar phase. After the reaction is complete, metal free metathesis products can easily be obtained by simply separating the two phases.

Another way to make use of the versatile ligand sphere of these complexes is the introduction of a chelating alkylidene, namely an o-methoxybenzylidene. This leads to the formation of hexacoordinate, air-stable complexes, which can be employed in the latent bulk polymerization of dicyclopentadiene (DCPD), an abundant monomer commonly used for reaction injection molding (RIM). The initiator and monomer can be mixed at room temperature without any polymerization occurring, upon heating the monomer will completely cure as it forms a crosslinked polymer. The temperature at which the polymerization occurs can be influenced by changing of the imido ligand (Figure G).

A particularly stable class of catalysts are cationic molybdenum imido alkylidene complexes with triflate as anionic ligand. Some of these complexes have been shown to be stable in air and they can even be employed in alcohols as solvent. Furthermore, they can metathesize protic substrates such as aliphatic alcohols and phenols (Figure H).

Stereoselectivity in olefin metathesis reactions can be addressed by forcing the intermediate metallacyclobutane into a preferred configuration. This can most possibly be realized by the employment of sterically demanding alkoxides, chiral alkoxides, chiral carbenes and bidentate (chiral) ligands. Several complexes with such ligands have already been synthesized in our laboratories. Furthermore, imido alkylidene NHC complexes of group 6 metals exist in two configurations, the syn- configuration with the substituent R at the metal carbon double bond pointing in the direction of the imido ligand and the corresponding anti- configuration. Syn- and anti- isomers can be interconverted by simple irradiation with UV- light (Figure C). Studies on the rate of syn/anti-interconversion in group 6 imido alkylidene NHC complexes and the influence on the selectivity in olefin metathesis are ongoing.

To some extent, stereoselectivity has already been achieved. We were able to show that by finetuning the ligand sphere of the molybdenum and tungsten imido alkylidene NHC complexes, stereoselective polymerization of substituted norbornenes could be achieved. Several of the catalysts described by our group are able to produce trans-isotactic poly(norbornene)s, as has been shown for a number of different chiral norbornene-derivatives (Figure E). Fortunately, this bias for the formation of trans double bonds could also be transferred to ring-opening cross metathesis (ROCM) of a number of different norbornene derivatives and cross partners (Figure F).

Molybdenum imido alkylidene NHC bistriflate catalysts have been immobilized via coordination of a silica-bound NHCs to standard bistriflate complexes (Figure B). Employment of those immobilized analogues in RCM and CM lead to the isolation of metal-free olefin metathesis products. Furthermore, cationic tungsten oxo alkylidene complexes and cationic molybdenum imido alkylidene complexes have been immobilized on silica by replacement of the X ligand with a Si-O bond in cooperation with the Copéret group at the ETH Zurich. Especially the immobilized tungsten-based catalyst 2@SiO2 exhibits unprecedented activity in self metathesis reactions with turnover numbers >1,200,000. In depth solid-state NMR investigations allowed for analyzing the geometry of the intermediary metallacyclobutanes.

Selected Publications:

  1. M: R. Buchmeiser, S. Sen, J. Unold, W. Frey, Angew. Chem. Int. Ed., 532014, 9384-9388.
  2. D. A. Imbrich, W. Frey, S. Naumann, M. R. Buchmeiser, Chem. Commun., 522016, 6099-6102.
  3. S. Sen, R. Schowner, D. A. Imbrich, W. Frey, M. Hunger, M. R. Buchmeiser, Chem. Eur. J.,21, 2015, 13778-13787.
  4. K. Herz, J. Unold, J. Hänle, R. Schowner, S. Sen, W. Frey, M. R. Buchmeiser, Macromolecules, 48, 2015, 4768-4778.
  5. M: Pucino, V. Mougel, R. Schowner, A. Fedorov, M. R. Buchmeiser, C. Copéret, Angew. Chem. Int. Ed., 55, 2016, 4300-4302.
  6. M. R. Buchmeiser, S. Sen, C. Lienert, L. Widmann, R. Schwoner, K. Herz, P. Hauser, W. Frey, D. Wang, ChemCatChem, 8, 2016, 2710-2713.
  7. J. Beerhues, S. Sen, R. Schowner, G. M. Nagy, M. R. Buchmeiser, invitation to a special issue celebrating Prof. R. H. Grubbs 75thbirthday, J. Polym. Sci. A: Polym. Chem., 55, 2017, 3028-3033.
  8. I. Elser, W. Frey, K. Wurst, M. R. Buchmeiser, Organometallics, 35, 2016, 4106-4111.
  9. R. Schowner, W. Frey, M. R. Buchmeiser, J. Am. Chem. Soc., 1372015, 6188-6191.
  10. I. Elser, R. Schowner, W. Frey, M. R. Buchmeiser, Chem. Eur. J., 23, 2017, 6398-6405.
  11. C. Lienert, W. Frey, M. R. Buchmeiser, Macromolecules, 50, 2017, 5701-5710
  12. W. -C. Liao, T.-C. Ong, D. Gajan, G. Casano, M. Yulikov, M. Pucino, R. Schowner, M. Schwarzwälder, M. R. Buchmeiser, G. Jeschke, O. Ouari, P. Tordo, A. Lesage, L. Emsley, C. Copéret, Chem. Sci., 8, 2017, 416-422.
  13. M. Pucino, M. Inoue, C. P. Gordon, R. Schowner, L. Stöhr, S. Sen, C. Hegedüs, E. Robé, F. Tóth, M. R. Buchmeiser, C. Copéret, Angew. Chem. Int. Ed., 57, 2018, 14566-14569.
  14. M: R. Buchmeiser, Chem. Eur. J., 24, 2018, 14295-14301.
  15. I. Elser, B. R. Kordes, W. Frey, K. Herz, R. Schowner, L. Stöhr, H. J. Altmann, M. R. Buchmeiser, Chem. Eur. J., 24, 2018, 12652-12659.
  16. P. Probst, I. Elser, R. Schowner, M. Benedikter, M. R. Buchmeiser, Macromol. Rapid Commun., 2019, in press.
  17. I. Elser, M. J. Benedikter, R. Schowner, M. R. Buchmeiser, Organometallics, 38, 2019, 2461-2471.
  18. M. J. Benedikter, R. Schowner, I. Elser, P. Werner, K. Herz, L. Stöhr, D. A. Imbrich, M. R. Buchmeiser, Macromolecules, 52, 2019, 4059-4066.
  19. Herz, M. Podewitz, L. Stöhr, D. Wang, W. Frey, K. R. Liedl, S. Sen, M. R. Buchmeiser, J. Am. Chem. Soc., 141, 2019, 8264-8276.
  20. R. Schowner, W. Frey, M. R. Buchmeiser, Eur. J. Inorg. Chem., 2019, 1911-1922.
  21. M. R. Buchmeiser, Macromol. Rapid Commun., 40, 2019, 1800492.

 

Our group recently synthesized novel molybdenum and tungsten alkylidyne complexes containing N‑heterocyclic carbenes. Mechanistic studies based on NMR spectroscopy point to two different active species for alkyne metathesis, depending on the nature of the incorporated carbene. In the case of strong σ-donors (e.g. 1,3-dimethylimidazol-2-ylidene) a cationic active species was hypothesized, whereas for less donating carbenes (e.g. 1,3-dimethyl-4,5-dicyanoimidazol-2-ylidene or thiazolylidenes) the NHC free complex, i.e. a classical Schrock alkylidyne was postulated to be the active species. The catalysts were tested in benchmark alkyne metathesis reactions and showed moderate to good activity.

Furthermore, immobilization of the corresponding molybdenum alkylidyne NHC complexes on silica proved to enhance reactivity, most probably due to prevention of bimolecular decomposition pathways. In order to increase reactivity, the first cationic molybdenum NHC alkylidyne complexes with monodentate as well as chelating carbenes that provide additional stability to the highly electrophilic metal center were synthesized and isolated. Investigations concerning stabilization, reactivity and further immobilization experiments as well as the application in alkyne polymerizations are under way.

Selected Publications:

  1. M. Koy, I. Elser, J. Meisner, W. Frey, K. Wurst, J. Kästner, M. R. Buchmeiser, Chem. Eur. J.2017, 15484-15490.
  2. P. Hauser, M. Hunger, M. R. Buchmeiser, ChemCatChem, 10, 2018, 1829-1834.

The supported ionic liquid phase (SILP) technique has been used for the immobilization of enzymes inside porous monolithic systems. For these purposes, enzymes like CALB as catalyst were dissolved in an ionic liquid (IL). The enzyme-containing IL was then immobilized inside the cellulose entities of porous cellulose-polyurethane monoliths. The enzyme-loaded hybrid support was then used and tested for enzyme SILP- (e-SILP) catalysed continuous gas phase transesterification of vinyl propionate and 2-propanol. The upscaling of these hybrid bioreactors is currently under investigation because enantiomerically pure components are of highly interest for the chemical and pharmaceutical industry. Enantioselective transesterification of racemic (R,S)-1-phenylethanol with vinyl butyrate and vinyl acetate, the esterification of (+/-)-2-isopropyl-5-methylcyclohexanol with propionic anhydride and the amidation of (R,S)-1-phenylethylamine with ethyl methoxyacetate were carried out using the e-SILP technique. This is the first of its kind continuously operated bioreactor that shows unique features in the synthesis of chiral esters and amides.

Selected Publications:

  1. B. Sandig, L. Michalek, S. Vlahovic, M. Antonovici, B. Hauer, M. R. Buchmeiser, Chem. Eur. J., 21, 2015, 15835-15842.
  2. B. Sandig, M. R. Buchmeiser, ChemSusChem 2016, 9, 2917-2921.
  3. L. Changhee, B. Sandig, M. R. Buchmeiser, M Haumann. Catal. Sci. Technol. 2018, 8, 2460-2466.

Sensory fiber-reinforced polymers are classified as a novel kind of composite materials showing sophisticated properties. These materials are designed in the course of an interdisciplinary project within the CRC 1244 dealing with the investigation and development of tools and methods for the planning, construction and operating of tomorrow’s built environment. In this context, the overall aim of the project is defined by the integration of transparent and non-transparent sensory fiber reinforced plastics into the facade of a ten-storied demonstrator tower enabling the detection of external stress such as surface cracks or strain.

In this regard, the fiber fabric is embedded covalently by an appropriate polymer network, which enhances tremendously the mechanical parameters of the polymer such as elasticity and strength. The sensory properties, which are the key features of the system, are generated by imprinting a conductive structure on the surface of the composite material. Thus, a continuous voltage applied on the meander surface structure can be detected consistently as electrical signal (e.g. electrical resistance). In this context, the generated electrical signal is highly dependent on length variations within the meander structure that can be caused by external stress or even damage on the surface.

Carbon fibers are made of anisotropic carbon with at least 92 wt.-% and up to 100 wt.-% carbon. Carbon fibers have high tensile strengths of up to 7 GPa with very good creep resistance, low densities (ρ = 1.75-2.20 g/cm3) and high moduli of up to 950 GPa. They lack resistance to harsh oxidizing agents as hot air and flames, but they are resistant to all other chemical species. The unparalleled mechanical properties make carbon fibers attractive for use in composites, which are commonly referred to as carbon fiber reinforced plastics (CFRP), where carbon fibers are used in the form of woven textiles, continuous fibers or chopped fibers. The CFRPs can be produced through filament winding, tape winding, pultrusion, compression molding, vacuum bagging, liquid molding, and injection molding. For the automotive industry, CFRPs allow for a significant reduction in weight. More recently, carbon fibers moved into the center of interest as a substitute for steel in reinforced concrete. Because of their much higher resistance towards oxidation, carbon fibers in reinforced concrete enable thinner concrete structures, therefore reducing the needed amount of the increasingly scarce sand.

The most important precursor (>90 % market share) for carbon fiber production is poly(acrylonitrile) (PAN), the only other commercially available precursor being pitch. PAN-based carbon fibers inhibit high tensile strength (HT) and intermediate modulus (IMS), suitable for high strength CFRP applications, while pitch-based carbon fibers inhibit a lower tensile strength and a high modulus (HM) or “ultra” high modulus (UHM) for CFRP parts with high stiffness requirements.

However, in order to reduce energy consumption, costs and the environmental impact of carbon fiber production, renewable and energy-efficient precursors like lignin, cellulose or poly(ethylene) have moved into the center of interest. Current projects focus on all these precursors as well as on more cost-effective and environmentally friendly processing techniques. The ultimate goal is to dispose over an armor of better and more efficient processes for carbon fiber production, “green precursors” as well as carbon fibers with improved properties. Research is carried out at the Institute of Polymer Chemistry and the the High-Performance Fiber Center (HPFC) at the DITF, where pilot lines for carbon fiber production enable the processing of common and novel precursors and the production of carbon fibers on a kilogram scale.

Selected Publications:

  1. Frank, M. R. Buchmeiser "Fiber, films, resins and plastics" in Encyclopedia in Polymeric Nanomaterials (S. Kobayashi, K. Müllen, Eds.), Springer, Vol.1, 2015, 306-310, ISBN: 978-642-29647-5.
  2. E. Frank, D. Ingildeev, L. M. Steudle, J. M. Spörl, M. R. Buchmeiser, Angew. Chem., 126, 2014, 5364-5403; Angew. Chem. Int. Ed., 53, 2014, 5262-5298.
  3. E. Frank, F. Hermanutz, M. R. Buchmeiser, Macromol. Mater. Eng., 297, 2012, 493-501.
  4. E. Frank, E. Giebel, M. R. Buchmeiser, Techn. Text., 2, 2015, E53-55; Chem. Fibers, Int., 4, 2015, 216-218.
  5. Frank, D. Ingildeev, M. R. Buchmeiser, “High-Performance Poly(acrylonitrile) (PAN)-Based Carbon Fibers” in Structure and Properties of High-Performance Fibers, (G. Bhat, Ed.), 1stEd. , Woodhead Publishing Ltd., 187, 2016, 7-30, ISBN 978-0-08-100550-7.
  6. J. M. Spörl, A. Ota, R. Beyer, T. Lehr, A. Müller, F. Hermanutz, M. R. Buchmeiser, J. Polym. Sci. A: Polym. Chem., 52, 2014, 1322-1333.
  7. J. M. Spörl, A. Ota, S. Sun, K. Massonne, F. Hermanutz, M. R. Buchmeiser, Mater. Today Commun., 7, 2016, 1-10.
  8. S. Son, K. Massonne, F. Hermanutz, J. Spoerl, M. R. Buchmeiser, R. Beyer (BASF E), PCT Int. Appl. WO 2015173243 A1.
  9. L. Steudle, E. Frank, A. Ota, U. Hageroth, S. Henzler, W. Schuler, R. Neupert, M. R. Buchmeiser, Macromol. Mater. Eng., 302, 2017, 1600441.
  10. M. Clauss, E. Frank, M. R. Buchmeiser (DITF Denkendorf), WO2017089585A1.
  11. E. Frank, E. Muks, M. R. Buchmeiser (DITF Denkendorf), DE102015106348A1.
  12. J. W. Krumpfer, E. Giebel, A. Müller, L. Ackermann, C. Nardi-Tironi, J. Unold,  M. Klapper, M. R. Buchmeiser, K. Müllen, Chem. Mater., 29, 2017, 780-788.
  13. M. Speiser, S. Henzler, U. Hageroth, A. Renfftlen, A. Müller D. Schawaller, B. Sandig, M. R. Buchmeiser, Carbon, 63, 2013, 554-561.
  14. M. R. Buchmeiser, J. Unold, K. Schneider, E. B. Anderson, F. Hermanutz, E. Frank, A. Müller, S. Zinn, J. Mater. Chem. A, 1, 2013, 13154-13163.
  15. S. König, M. M. Clauss, E. Giebel, M. R. Buchmeiser, Polym. Chem., 10, 2019, 469-4476.
  16. E. Frank, E. Giebel, M. R. Buchmeiser, S. König, (DITF Denkendorf), DE102017127629A1.
  17. M. R. Buchmeiser, E. Muks, E. Frank, U. Hageroth, S. Henzler, R. Schowner, J. Spörl, A. Ota, R. Beyer, Carbon, 144, 2019, 659-665.

Fibers are classified into two main groups, commodity and high performance. High performance fibers are designed for specialized technical applications with unique physical properties, including high specific stiffness, high temperature resistance, flame retardancy, and/or chemical resistance. These fibers are continuously designed to be stronger, lighter, and safer, and their unique, specialized attributes are essential for automotive, aerospace, construction, and protection applications compared to high-volume, cheaper commodity fibers. 

Poly(aromatic)s, including for example meta- and para-aramid fibers such as Nomex, Kevlar and PBO, are particularly well-known for their highly-oriented rigid structures which contribute to their excellent mechanical properties and high-temperature resistance. They are widely-used in reinforcement, protective, safety, and ballistic applications. However, many high-strength fibers are difficult to process. Although some types of high performance fibers may have excellent chemical resistance, other types are not that UV stable and display poor chemical resistance but may have superior properties for a specific application such as their strength or limiting oxygen index. To solve these problems, we are designing novel modified monomers and comonomers that provide these polymeric fibers with improved processing and/or resistance without sacrificing on mechanical properties. Our goals are to design fibers that outperform the industry standards for strength, density, and thermal and chemical resistance and to facilitate the expansion of their technical applications.

Oxide ceramic fibers are key components of ceramic matrix composites (CMCs), which form a new class of light-weight high temperature resistant materials with exceptional properties. As CMCs combine the advantages of a monolithic ceramic material (corrosion resistance, high strength and high temperature stability) with a non-brittle fracture behavior, a high damage tolerance and an extreme thermo-shock resistance, there is an increasing interest in their industrial applications, particularly by replacing highly legated steel. Important technical fields with growing requirements are: power generation with stationary gas turbines as well as combustion chambers and engines of aircrafts, rockets and space vehicles.

Oxide ceramic fibers determine the properties of CMCs and therefore have to meet special requirements such as high strength, long-term high temperature stability as well as excellent resistance against oxidation, corrosion and creep. Generally, the creep rate of the polycrystalline ceramic fibers increases with decreasing grain size and the creep resistance of the commercially available ultrafine grained fibers is comparatively low. Particularly under mechanical stress and at high temperatures exceeding 1100 °C, they tend to creep and brittleness increases due to grain growth, which can ultimately lead to failure of the entire device. The optimization of the ceramic fiber properties with regard to high creep resistance while maintaining strength and associated long-term high temperature resistance still represents a major topic in ongoing research in the field of ceramic fibers.

Research at the DITF Denkendorf focuses on the development of continuous oxide ceramic fibers of various compositions and started as early as in 1989. The complete production process has been studied intensively comprising the design of spinning dopes, the development of the dry spinning process as well as the thermal treatment including pyrolysis, calcination and sintering processes. Corundum and mullite fibers have achieved a high level of development in the past years. Recent investigations of the fiber properties by others have shown the high potential of these two fiber types. Currently, the transfer of the technology into industrial scale is under progress.
Current research projects focus on the improvement of creep resistance, the reduction of grain growth in long time applications and the improvement of the textile processability of oxide ceramic fibers. For this purpose, the chemical compositions of the oxide ceramic fibers are further varied in order to optimize the structures and the mechanical properties.

Yttrium aluminum garnet (YAG) fibers are high performance fibers with high temperature stability, high modulus and strength, high oxidation resistance and excellent creep resistance. As YAG is characterized by a very high melting point of 1940 °C it is very attractive for high temperature applications. Furthermore, it is chemically inert in reducing and oxidizing atmosphere and it is the oxide with the highest creep resistance. Therefore, YAG fibers have the potential to outperform the commercial oxide ceramic fibers in terms of creep resistance.
The microstructural optimization of corundum fibers by the incorporation of zirconia results in zirconia toughened alumina (ZTA) fibers with a substantially inhibited grain growth at high temperatures. ZTA exhibits enhanced fracture toughness and almost doubled flexural strength in comparison to alumina. ZTA fibers with these properties not only enable more complex structures due to improved textile processability but also exhibit a higher corrosion resistance compared to other fiber types.

Selected Publications:

  1. D. Schawaller, B. Clauß, M. R. Buchmeiser, Macromol. Mater. Eng., 297, 2012, 502-522.
  2. S. Pfeifer, M. Bischoff, R. Niewa, B. Clauß, M. R. Buchmeiser, J. Eur. Ceram. Soc., 34, 2014, 1321-1328.
  3. S. Pfeifer, P. Demirci, R. Duran, H. Stolpmann, A. Renfftlen, S. Nemrava, R. Niewa, B. Clauß, M. R. Buchmeiser, J. Eur. Ceram. Soc., 36, 2016, 725-731.

Non-oxide Si-C-N ceramic fibers possess interesting properties and are predestined for applications in fiber reinforced ceramics (CMCs). These are materials with outstanding properties like high resistance against heat-shock and damage tolerance, which is completely different from conventional monolithic ceramics. Hence, new technical fields are accessible for these fiber ceramics like aerospace, power engineering and automotive applications. The production of such fibers can be conducted by a feasible melt spinning procedure, if thermoplastic precursor materials with proper rheological properties can be synthesized. By using different chlorosilanes as starting compounds meltspinnable polycarbosilazanes and polysilazanes have been produced. As a part of the research work new strategies for the synthesis for high molecular weight precursors with good processability have been developed. Based on the rheological studies and NMR spectroscopy possible structures of the polycarbosilazanes are derived. These precursors are spun to fibers in a melt spinning process (above). To avoid melting during pyrolysis the green fibers are crosslinked by electron irradiation. Pyrolysis under inert conditions at 1100 °C leads to Si-C-N ceramic fibers. These fibers are stable up to very high temperatures. Beside the formation of a thin oxidation layer no further fiber degradation is observed after temperature exposition at 1500 °C in air (below).

Research Topics Dr. Stefan Naumann

The development of polymerization catalysts is not only a merit in itself, but ideally provides materials with novel properties. In this regard, we try to implement N-heterocyclic olefins (NHOs) as organocatalysts for the synthesis of polymer architectures which allow for creating a tunable, porous platform material. We have recently demonstrated the first NHO-mediated organopolymerizations. On this basis, it is possible to use a solvent- and metal-free polymerization process for the preparation of very well-defined amphiphilic polyethers. These in turn can be converted into micro- or mesoporous carbon materials. We aim at preparing these carbon materials with predictable pore sizes, all depending on the properties of the underlying polyether structures. If successful, this will allow for creating tailor-made materials for electrodes or catalytic supports. Characterization, application and development of battery/supercapacitator devices as well as catalytic experiments using these materials are part of our investigations.
Funding: Deutsche Forschungsgemeinschaft, Projekt NA 1206/2

Selected Publications:

  1. P. Walther, S. Naumann, Macromolecules 2017, 50, 8406-8416.
  2. S. Naumann, Dongren Wang, Macromolecules 2016, 49, 8869-8878.
  3. S. Naumann, P. B. V. Scholten, J. A. Wilson, A. P. Dove, J. Am. Chem. Soc. 2015, 137, 14439–14445.
  4. S. Naumann, F. G. Schmidt, W. Frey, M. R. Buchmeiser, Polym. Chem. 2013, 4, 4172-4181.

The development of polymerization catalysts is not only a merit in itself, but ideally provides materials with novel properties. In this regard, we try to implement N-heterocyclic olefins (NHOs) as organocatalysts for the synthesis of polymer architectures which allow for creating a tunable, porous platform material. We have recently demonstrated the first NHO-mediated organopolymerizations. On this basis, it is possible to use a solvent- and metal-free polymerization process for the preparation of very well-defined amphiphilic polyethers. These in turn can be converted into micro- or mesoporous carbon materials. We aim at preparing these carbon materials with predictable pore sizes, all depending on the properties of the underlying polyether structures. If successful, this will allow for creating tailor-made materials for electrodes or catalytic supports. Characterization, application and development of battery/supercapacitator devices as well as catalytic experiments using these materials are part of our investigations.

Selected Publications Organocatalysis:

  1. S. Naumann, K. Mundsinger, L. Cavallo, L. Falivene, Polym. Chem. 2017, 8, 5803-5812.
  2. S. Naumann, A. W. Thomas, A. P. Dove, ACS Macro Lett. 2016, 5, 134 – 138.
  3. S. Naumann, A. W. Thomas, A. P. Dove, Angew. Chem. Int. Ed. 2015, 54, 9550 – 9554.

Current Thesis Topics

  1. Highly porous cathode materials for Li-Sulfur batteries
  2. Preparation of Janus-type nanoparticles
  3. Synthesis of cationic molybdenum / tungsten alkylidene complexes for olefin metathesis reactions under continuous biphasic conditions
  4. Olefin metathesis reactions in confined geometries
  5. Syn/anti interconversion of Mo- and W-alkylidene NHC complexes
  6. Cis/trans selectivity of Mo and W alkylidene NHC complexes in olefin metathesis reactions
  7. Stereoselective ring-opening metathesis polymerization (ROMP) with Mo- and W-alkylidene NHC complexes
  8. Stereoselective olefin metathesis with chiral Mo- and W-alkylidene NHC complexes
  9. Polymeric electrolytes for Li-Sulfur batteries
  10. NHC-based metal complexes as catalysts for polyaddition reactions
  11. Stereospecific polymerization of tricyclic olefins
  12. Preparation of high-creep resistant YAG ceramic fibers
  1. Preparation of monolithic cathodes for Li-Sulfur batteries
  2. Preparation of Janus-type nanoparticles
  3. Polymeric electrolytes for Li-Sulfur batteries
  4. Synthesis of cationic molybdenum and tungsten alkylidene complexes for olefin metathesis reactions under continuous biphasic conditions
  5. Olefin metathesis reactions in confined geometries
  6. Syn/anti interconversion of Mo- and W-alkylidene NHC complexes
  7. Cis/trans selectivity of Mo and W alkylidene NHC complexes in olefin metathesis reactions
  8. Stereoselective olefin metathesis with chiral Mo- and W-alkylidene NHC complexes
  9. Processing of m-aramids from ionic liquids and characterization of the solution and the fiber structure
  10. Preparation of Poly(phtalamide) fibers
  11. Preparation of high-creep resistant YAG ceramic fibers
  1. Artificial DNA: Precision polymers
  2. Preparation of monolithic cathodes for Li-S batteries
  3. Preparation of Janus-type nanoparticles
  4. Olefin metathesis reactions in confined geometries
  5. Synthesis of cationic molybdenum and tungsten alkylidene complexes for olefin metathesis reactions under continuous biphasic conditions
  6. Stereo- and regioselective polymerization of chiral norbornenes, norbornadienes, 1-6-heptadiynes, and 1,7-octadiines using chiral molybdenum and tungsten alkylidene complexes with asymmetric NHC ligands
  7. Stereoselective olefin metathesis with chiral Mo- and W-alkylidene NHC complexes
  8. (Chiral) N-heterocyclic carbenes as catalysis for polymerization
  9. Preparation of polyheterocyclic high-performance fibers using ionic liquids
  10. Development new technologies for the production of high-strength carbon fibers
  1. Synthese von hochfesten high-performance Cellulosefasern durch Nassspinnprozesse: Einsatz als Verstärkungsfasern zur Anwendung in Verbundmaterialien.
  2. Lignin-basierte Carbonfasern: Vernetzungsmethoden und reaktive Sizings für die chemische Prä-Vernetzung von Lignin-Präkursor-Fasern zur Herstellung von Carbonfasern, um den Schritt der Faserstabilisierung zu beschleunigen und fiber fusing zu verhindern. Mitarbeit bei Versuchsreihen im Rahmen des EU-Projekts „LIBRE“ (Lignin-based carbon fibers).

Contact

 

Chair of Macromolecular Materials and Fiber Chemistry

Pfaffenwaldring 55, D-70569, Stuttgart

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