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- Dual Catalysis with an N-Heterocyclic Carbene and a Lewis Acid: Thermally Latent Pre-Catalysts for the Polymerization of Azepan-2-one
H. J. Altmann, M. Steinmann, G. Mourgas, I. Elser, M. J. Benedikter, S. Naumann, M. R. Buchmeiser, submitted (2020)
- Polymere & Nachhaltigkeit - Minitrendbericht 2020
M. Winnacker, S. Naumann (in press)
- Dual Catalytic Ring-Opening Polymerization of Ethylene Carbonate for the Preparation of Degradable PEG
N. von Seggern, T. Schindler, S. Naumann, Biomacromolecules, 2020, 21, 2661
- Ultra-High Molecular Weight Poly(Propylene Oxide): Preparation and Perspectives
P. Walther, C. Vogler, S. Naumann Synlett, 2020, 31, 641
- Polar Olefins for Catalysis: Synthesis, Properties & Applications of N-Heterocyclic Olefins
S. Naumann, Chem. Commun., 2019, 55, 11568
- Controlled Synthesis of "Reverse Pluronic"-Type Block Copolyethers with High Molar Masses for the Preparation of Hydrogels with Improved Mechanical Properties
F. Markus, J. Bruckner, S. Naumann, Macromol. Chem. Phys. 2020, 221, 1900437
- Lewis Pair Polymerization of Epoxides via Zwitterionic Species as a Route to High-Molar Mass Polyethers
P. Walther, A. Krauss, S. Naumann, Angew. Chem. Int. Ed. 2019, 58, 10737; Angew. Chem. 2019, 131, 10848
- Proton Affinities of N-Heterocyclic Olefins and their Implications for Organocatalyst Design
R. Schuldt, J. Kästner, S. Naumann, J. Org. Chem. 2019, 84, 2209
- Synthesis of Linear Poly(oxazolidin-2-one)s by Cooperative Catalysis Based on N-Heterocyclic Carbenes and Simple Lewis Acids
H. J. Altmann, M. Clauss, S. König, E. Frick-Delaittre, C. Koopmans, A. Wolf, S. Naumann, M. R. Buchmeiser, Macromolecules 2019, 52, 487
- Organic Catalysis for Polymerisation Edited by A. Dove, H. Sardon and S. Naumann, RSC Polymer Chemistry Series (2018). ISBN: 978-1-78801-184-6
- Polarized Olefins as Enabling (Co)Catalysts for the Polymerization of gamma-Butyrolactone
P. Walther, W. Frey, S. Naumann, Polym. Chem., 9, 3674 (2018)
- The Lewis Pair Polymerization of Lactones Using Metal Halides and N-Heterocyclic Olefins: Theoretical Insights
J. Meisner, J. Karwounopoulos, P. Walther, J. Kästner, S. Naumann, Molecules, 23, 432 (2018)
- Controlled Preparation of Amphiphilic Triblock-Copolyether in a Metal- and Solvent-Free Approach for Tailored Structure-Directing Agents
A. Balint, M. Papendick, M. Clauss, C. Müller, F. Giesselmann, S. Naumann, Chem. Commun., 54, 2220-2223 (2018)
- N-Heterocyclic Olefin-Based (Co)Polymerization of a Challenging Monomer: ω-Pentadecalactone and its Co-polymers with γ-Butyrolactone, δ-Valerolactone and ε-Caprolactone
P. Walther, S.Naumann, Macromolecules, 50, 8406-8416 (2017)
- N-Heterocyclic Olefins as Initiators for the Polymerization of (Meth)Acrylic Monomers: A Combined Experimental and Theoretical Approach
S. Naumann, K. Mundsinger, L. Cavallo, L. Falivene, Polym. Chem. 8, 5803-5812 (2017)
- Protected N-Heterocyclic Carbenes as Latent Organocatalysts for the Low-Temperature Curing of Anhydride-hardened Epoxy Resins
H. J. Altmann, S, Nauman, M. R. Buchmeiser, invited paper, Eur. Polym. J., 95, 766-774 (2017), highlighted in Adv. Eng., Oct. 20, 2017
- Dual Catalysis Based on N-Heterocyclic Olefins for the Copolymerization of Lactones: High Performance and Tunable Selectivity
S. Naumann, D. Wang, Macromolecules, 49, 8869-8878 (2016)
- Application of Imidazolinium Salts and N-Heterocyclic Olefins for the Synthesis of Anionic and Neutral Tungsten Imido Alkylidene Complexes
D. A. Imbrich, W. Frey, S. Naumann, M. R. Buchmeiser, Chem. Commun., 52, 6099-6102 (2016)
- In Situ Copolymerization of Lactams for Melt Spinning
K. Jovic, J. Unold, S. Naumann, M. Ullrich, F. G. Schmidt, M. R. Buchmeiser, Macromol. Mater. Eng., 301, 423-428 (2016)
- Highly Polarized Alkenes as Organocatalysts for the Polymerization of Lactones and Trimethylene Carbonate
S. Naumann, A. W. Thomas, A. P. Dove, ACS Macro Lett., 5, 134 - 138 (2016)
- N-Heterocyclic Carbenes for Metal-Free Polymerization Catalysis: An Update
S. Naumann, A. P. Dove, Polym. Int., 65, 16 - 27 (2016)
- Dual Catalysis for Selective Ring-Opening Polymerization of Lactones: Evolution toward Simplicity
S. Naumann, P. B. V. Scholten, J. A. Wilson, A. P. Dove, J. Am. Chem. Soc., 137, 14439–14445 (2015)
- Convenient Preparation of High Molecular Weight Poly(dimethylsiloxane) Using Thermally Latent NHC-Catalysis: A Structure-Activity Correlation
S. Naumann, J. Klein, D. Wang, M. R. Buchmeiser, Beilstein J. Org. Chem. 11, 2261-2266 (2015)
- N-Heterocyclic Olefins as Organocatalysts for Polymerization: Preparation of Well-Defined Poly(propylene oxide)
S. Naumann, A. W. Thomas, A. P. Dove, Angew Chem. Int. Ed., 54, 9550 - 9554 (2015)
- N-Heterocyclic Carbenes as Organocatalysts for Polymerizations: Trends and Frontiers
S. Naumann, A. P. Dove, Polym. Chem., 6, 3185 - 3200 (2015)
- Latent CO2-Protected N-Heterocyclic Carbene-Based Single-Component System-Derived Epoxy/Glass Fiber-Composites
M. R. Buchmeiser, J. A. Kammerer, S. Naumann, J. Unold, R. Ghomeshi, S. K. Selvarayan, P. Weichand, R. Gadow, Macromol. Macromol. Mater. Eng., 300, 937-943 (2015)
- Heterogenization of Ferrocene Palladacycle Catalysts on ROMP-Derived Monolithic Supports and Application to a Michael Addition
M. Sudheendran, S. H. Eitel, S. Naumann, M. R. Buchmeiser, R. Peters, New J. Chem., 38, 5597-5607 (2014)
- Air Stable and Latent Single-Component Curing of Epoxy/Anhydride-Resins Catalyzed by Thermally Liberated N-Heterocyclic Carbenes
S. Naumann, M. Speiser, R. Schowner, E. Giebel and M. R. Buchmeiser, Macromolecules, 47, 4548-4556 (2014)
- Liberation of N-Heterocyclic Carbenes (NHCs) from Thermally Labile Progenitors: Protected NHCs as Versatile Tools in Organo- and Polymerization Catalysis
S. Naumann, M. R. Buchmeiser, Catal. Sci. Technol., 4, 2466-2479 (2014)
- Latent and Delayed Action Polymerization Systems
S. Naumann, M. R. Buchmeiser, Macromol. Rapid Commun.,35, 682-701 (2014)
- Anionic Ring-Opening Homo- and Copolymerization of Lactams by Latent, Protected N-Heterocyclic Carbenes for the Preparation of PA 12 and PA 6/12
S. Naumann, F. G. Schmidt, M. Speiser, M. Böhl, S. Epple, C. Bonten, M. R. Buchmeiser, Macromolecules, 46, 8426-8433 (2013)
- Polymerization of ε-Caprolactame by Pre-Catalysts based on Protected N-Heterocyclic Carbenes: The Importance of Basicity
S. Naumann, S. Epple, C. Bonten, M. R. Buchmeiser, ACS Macro Lett., 2, 609-612 (2013)
- Protected N-Heterocyclic Carbenes as Latent Pre-Catalysts for the Polymerization of ε-Caprolactone
S. Naumann, F. G. Schmidt, M. R. Buchmeiser, Polym. Chem., 4, 4172-4181 (2013)
- Ionically tagged Ru-alkylidenes for metathesis reactions under biphasic liquid-liquid conditions
B. Autenrieth, F. Willig, D. Pursley, S. Naumann, M. R. Buchmeiser, ChemCatChem., 5, 3033-3040 (2013)
- Protected N-heterocyclic Carbenes as Latent Catalysts for Polymerizations
S.Naumann, F. G. Schmid, M. R. Buchmeiser, 23. Stuttgarter Kunststoff-Kolloquium, 6.-7. 3. 2013, Stuttgart
- Polymerization of Methyl Methacrylate by Latent Pre-Catalysts Based on CO2-Protected N-Heterocyclic Carbenes
S. Naumann, F.-G. Schmidt, R. Schowner, W. Frey, M. R. Buchmeiser, Polym. Chem., 4, 2731-2740 (2013)
- Regioselective Cyclopolymerization of 1,7-Octadiynes
S. Naumann, J. Unold, W. Frey, M. R. Buchmeiser, Macromolecules, 44, 8380-8387 (2011), highlighted in SYNFACTS, 8 (2), 0150, 2012 by T. M. Swager and J. R. Cox
- Lactone polymerization catalyzed by protected Nheterocyclic carbenes and main group metal carbene complexes as latent initiators (Evonik Industries)
F. G. Schmidt, M. R. Buchmeiser, S. NaumannWO 2014154427 A1 20141002
- Method for polymerizing ε-caprolactam (DITF Denkendorf)
S. Naumann, M. R. BuchmeiserWO2014118076 A1 20140807
- Anionic ringopening polymerization catalyst precursors for ring-opening laurolactam polymerization (Evonik Industries)
F. G. Schmidt, S. Reemers, K. Burger, M. Ullrich, M. R. Buchmeiser, S. Naumann DE 102013210424 A1 20141211
- Polymerizable reaction mixture for producing epoxy resins, and the use thereof (DITF Denkendorf)
M. R. Buchmeiser, S. Naumann,DE 102013008723 A1 20141127
- Bulk or solution polymerization using latent initiators based on thermally activatable N-heterocyclic carbene compounds (Evonik Indistries)
F. G. Schmidt, M. R. Buchmeiser, S. Naumann WO 2014001007 A1 20140103
- Catalyst system for the preparation of highmolecular weight polyether and application thereof (Universität Stuttgart)
S. Naumann, P. Walther, M. R. Buchmeiser, EP19151061
- Catalyst mixtures comprising N-heterocyclic carbenes and Lewis acids for the preparation of oxazolidinones (Covestro)
H. Altmann, S. Naumann, M. R. Buchmeiser, K. Koopmans, C. Gürtler, WO 2020025805 A1 20200206
Stefan Naumann received his diploma in chemistry from the University of Stuttgart in 2010, working on metathesis-derived conjugated polymers. This was followed by PhD studies under the guidance of Prof. M. R. Buchmeiser, investigating protected N-heterocyclic carbenes (NHCs) for thermally latent polymerization processes, which resulted in “on demand” 1K polymerization systems (lactams, epoxides, lactones). After finishing his PhD in 2014, he was granted a DFG PostDoc stipend and joined the group of Prof. Andrew P. Dove at the University of Warwick (UK). There, he focused on novel ways to catalyze polymerization reactions, including organopolymerization and dual catalytic setups. Returning to Stuttgart in 2015, he started a habilitation process at the Institute of Polymer Chemistry (IPOC). Since 2018 he is PI in the CRC 1333 (“Molecular Heterogeneous Catalysis in Defined Geometries”). He was awarded with the Advancement Stipend 2019 of the Macromolecular Division of the German Chemical Society (GDCh), and in 2020 received the Thieme Journal Award.
Stefan’s research interests broadly encompass polymerization catalysis (organocatalysis, dual catalysis, organometallic single-site complexes) and functional materials. This includes the synthesis and development of organocatalysts (1-1), the application of cooperative polymerization setups (1-2), novel perspectives for polyether synthesis (1-3), the utilization of “non-polymerizable” g-butyrolactone as a sustainable resource (1-4) and the preparation of mesoporous carbon materials with defined porosity and surface chemistry (1-5).
1-1: Olefins as Polymerization Catalysts
N-heterocyclic olefins (NHOs) constitute a class of very electron-rich, strongly polarized olefins. The heterocyclic backbone is prone to accommodating a positive (partial) charge, which results in charge separation; the corresponding electron excess is located on the exocyclic carbon atom. This renders the NHO a potent base and capable nucleophile, hence its general applicability in catalysis. During the past ~ 5 years, we have identified the crucial tuning parameters, enabling us to turn NHOs into powerful polymerization (co-)catalysts. Thus, the importance of aromatization was studied, rendering the backbone a crucial site for managing the NHO’s reactivity. Less obviously, but of comparable importance, is the impact of the N-substituents; beyond the expected stereoelectronic effects, these also control the planarity of the NHO and thus the degree of conjugation of its p electron system. By exploiting these properties, we succeeded in separately addressing nucleophilicity and basicity, thereby enabling different polymerization pathways. Overall, as we found from experiments and calculations, NHOs are superbases and can also add to moderately activated double bonds. Accordingly, we employ NHOs for the polymerization of epoxides, lactones, acrylic monomers and carbonates.
See for example: R. Schuldt, J. Kästner, S. Naumann* J. Org. Chem. 2019, 84, 2209-2218; S. Naumann* Chem. Commun. 2019, 55, 11658-11670.
1-2: Lewis Acid/Base Cooperativity in Dual Catalytic Polymerizations
A recent development in polymerization catalysis does not aspire to maximize the reactivity of, for example, the propagating chain end, but rather in opposite aims at employing propagating species with attenuated reactivity. This is counterbalanced by in turn activating the monomer. The benefits are especially notable if one achieves to selectively activate the monomer, while the polymer chain itself remains largely unaffected. To this end, my research activity has focused on developing simple, yet highly tunable catalyst pairings of Lewis bases (organobases) and Lewis acids for the ring-opening polymerization (ROP) of O-heterocyclic monomers. Multiple benefits have resulted from this approach, including among others:
- A very rapid, yet highly selective (PDI < 1.0x) method for polymerizing lactones, where crucially the copolymerization parameters can be influenced, even switched by choice of the Lewis acid.
- The ability to chemically simplify both catalyst components, resulting in more competitive applications (i.e. ethylene carbonate polymerization, recycling applications).
- Realization of ultra-high molecular weight (UHMW) poly(propylene oxide) (Mn > 1 Mio. g/mol) and functional derivatives, by quantitatively suppressing transfer-to-monomer (see 1-3).
See for example: S. Naumann, P. B. V. Scholten, J. A. Wilson, A. P. Dove* J. Am. Chem. Soc. 2015, 137, 14439-14445; S. Naumann*, D. Wang Macromolecules 2016, 49, 8869-8878; P. Walther, A. Krauss, S. Naumann*, Angew. Chem. Int. Ed. 2019, 58, 10737-10741.
1-3: Functional Polyethers by Mechanistically Divergent ROP of Epoxides
Two very different polymerization pathways, namely organocatalytic, anionic polymerization and zwitterionic polymerization can be operated by application of NHO catalysis. The different mechanisms have fundamental repercussions for the resulting polyethers. While in the former case highly defined polyether results (typically PDI ≤ 1.03), in the latter case ultra-high molar masses are generated (up to 1.6*106 g/mol), notably with the support of a Mg(II)-based monomer-activating agent. This mechanistic dualism opened up novel material applications for polyethers. As an example, we have used the metal-free route to synthesize amphiphilic block copolyethers (“Pluronics”-type) for creating improved hydrogels with a better mechanical robustness. Very differently, the resulting UHMW-polymers from the zwitterionic approach (i.e., poly(propylene oxide, PPO) are themselves very appealing (forming entangled elastomers, suitable for low-temperature application). The overall aim of my research regarding polyether formation is my conviction that, by perspective, polyethers can be developed into valuable performance polymers; to this end, a combination of advanced catalysis and process technology is necessary, encompassing not only control over molar mass and molar mass distribution, but also tacticity and degradability of the polyethers.
See for example: S. Naumann*, A. W. Thomas, A. P. Dove* Angew. Chem. Int. Ed. 2015, 54, 9950-9954; P. Walther, A. Krauss, S. Naumann*, Angew. Chem. Int. Ed. 2019, 58, 10737-10741; F. Markus, J. Bruckner, S. Naumann*, Macromol. Chem. Phys. 2020, 221, 1900437.
1-4: γ-Butyrolactone as a Sustainable, Recyclable Monomer
γ-Butyrolactone (GBL), as a rare exception among many monomers, combines several aspects which can render its application truly sustainable and economically attractive. This lactone can be bio-sourced and crucially, its thermodynamically favoured, (almost) strainless five-membered structure facilitates ready depolymerization (by simple heating). This typically occurs without any side products as demonstrated by Chen and co-workers, thus constituting an ideal candidate for a circular economy. However, to realize this aim, capable catalysis is required: until very recently, GBL was considered “non-polymerizable” and still remains challenging to do so. Our research therefore followed a two-pronged approach: developing more practicable ways to generate the homopolymer (poly(GBL)) and secondly copolymerizing GBL with other lactones to create tailored polyesters. In this case, the dual catalytic methodology proved especially successful, allowing for the selective realization of different poly(GBL) architectures and for controlling GBL incorporation in copolymers by choice of the co-catalyzing Lewis acid.
See for example: P. Walther, S. Naumann*, Macromolecules 2017, 50, 8406-8416; P. Walther, W. Frey, S. Naumann*, Polym. Chem. 2018, 9, 3674-3683.
1-5: Ordered Mesoporous Carbon Materials with Controlled Pore Diameter and Surface Chemistry
This research is conducted within the framework of CRC 1333 “Molecular Heterogeneous Catalysis in Defined Geometries”. The overarching target of this initiative is to exploit defined mesoporous support materials to host molecular catalysts (exclusively inside the pores) and impact their reactivity by exerting confinement effects and diffusion/transport phenomena. In this context, I am PI of a project investigating the porous supports, focusing on carbon materials. To this end, we employ an organic self-assembly process, using amphiphilic block copolyethers as structure directing agents. During a so-called EISA-process (Evaporation Induced Self-Assembly) different morphologies can be constructed (in our case 2D hexagonal), fixed (phenolic resin cross-linking) and carbonized (700°C, under nitrogen). Crucially, we employ our ability for precise polyether preparation (see 1-3) to in turn realize highly ordered carbon structures. By fine-tuning the structure-directing polymers with regard to molar mass and hydrophilic-to-lipophilic balance, we aim to control the diameter of the generated mesopores. We further aim to control the surface chemistry of the mesoporous material.
See for example: https://www.crc1333.de/; A. Balint, M. Papendick, M. Clauss, C. Müller, F. Giesselmann, S. Naumann*, Chem. Commun. 2018, 54, 2220-2223.