Protected N-heterocyclic carbenes as latent organocatalysts for low-temperature curing of anhydride-hardened epoxy resins

Significance Statement

N-heterocyclic carbenes are the most versatile and successful group of organocatalysts developed in recent years. Their numerous benefits include high nucleophilicity and Bronsted-basicity, structural variability, and intriguing chemistry of the carbene reactive center. These benefits have enabled demanding chemical transformations such as addition to double bonds and Umpolung reactions. Polymer chemistry has significantly developed thanks to these advances in organic chemistry. N-Heterocyclic carbene-triggered organopolymerization, for examples can be implemented to synthesize polymers through ring opening polymerization, by condensation reactions and also by conjugate addition.

In most instances, the N-heterocyclic carbenes offer additional benefits, which include polymerization rates and easy control of the end-groups. Unfortunately, the change from laboratory to commercial use, large-scale implementation has been affected by inherent problems that emanate from the use of N-heterocyclic carbenes. These problems include sensitivity to protic compounds, synthetic requirements, and limited storability, all which can be prohibitive for use in the polymer industry.

Researchers led by Professor Michael Buchmeiser at University of Stuttgart in Germany, presented a rapid epoxy curing system with an intention of minimizing the problems stated above while focusing on the benefits which can be derived from using N-heterocyclic carbenes. They presented epoxy resins based on typically used epoxy compounds, carbon-dioxide protected N-heterocyclic carbenes and a number of anhydrides that combine the superior attributes of fast curing at low reaction temperatures, full homogeneity in the form of low viscosity liquids and pot life within the range of hours to days. Their research work is published in European Polymer Journal.

The authors achieved the above attributes through a meticulous tuning of the components. 1,3-Dimethylimidazolium-2-carboxylate as a thermally latent organocatalyst offered facile one-step synthesis and robustness towards atmospheric conditions and high reactivity needed once the polymerization was switched on.

The research team conducted a systematic screening of a number of epoxy compounds and 14 different anhydrides in a bid to identify monomers that were suited for rapid monomer consumption and therefore low curing temperatures. From the study, over 15 different systems were obtained where a homogeneous liquid could be formed in the absence of additional additives.

The authors were able to optimize the curing attributes of the latent one component epoxy compositions implementing thermally triggerable carbon-dioxide protected N-heterocyclic carbenes reference to low viscosity as well as fast curing kinetics at low-temperature conditions. This was possible through the implementation of a well accessible and robust protected N-heterocyclic carbene that was synthesized in a single step from commodity chemicals. The evaluation of commonly used epoxy compounds in combination with an exhaustive range of anhydride components allowed for identification of compositions that could be cured at temperatures of 120 °C or 100 °C.

The mixture provided important pot times of hours to days depending on temperature as well as catalyst loading. A prudent choice of comonomers ensured that fully homogenous, low viscosity and technically sprayable one-component mixtures were obtained, therefore, widening the range of applications. The benefits of N-heterocyclic carbene precursor in combination with the catalytic power of the free N-heterocyclic carbene therefore led to readily applicable systems that made best use of N-heterocyclic carbene organocatalysis, while avoiding the liabilities that normally curtail large-scale application of this form of a catalyst.

Protected N-heterocyclic carbenes as latent organocatalysts for low-temperature curing of anhydride-hardened epoxy resins- Advances in Engineering

About The Author

Prof. Dr. Michael R. Buchmeiser
Chair of Macromolecular Compounds and Fiber Chemistry
Institute of Polymer Chemistry, University of Stuttgart
Pfaffenwaldring 55, 70550 Stuttgart
and
Member of Board of Directors
German Institutes of Textile and Fiber Research (DITF) Denkendorf
Körschtalstr. 26, D-73770 Denkendorf
Tel: +49-(0)711-685-64074, Fax: +49-(0)711-685-64050
e-mail: [email protected]

Michael R. Buchmeiser received his Ph. D. in Organometallic Chemistry in 1993 from the University of Innsbruck, Austria (Prof. Dr. H. Schottenberger). He was awarded an “Erwin Schrödinger Fellowship” and spent one year at the MIT within the group of Prof. Richard R. Schrock (Chemistry Nobel Prize 2005). In 1998, he finished his “Habilitation” in Macromolecular Chemistry at the University of Innsbruck were he then held a Faculty Position as Associate Professor from 1998-2004. From 2000-2001, he was Visiting Professor at the Graz University of Technology, Austria. In 2004 he accepted a Faculty Position (C-4 Professor) at the University of Leipzig, Germany. In addition, from 2005-2009, he served as Vice Director and Member of Board at the Leibniz Institute of Surface Modification (IOM), Leipzig, Germany. He was offered Faculty Positions (Full Professor of Polymer Chemistry) from the University of Halle (Germany, 2004), from the University of Leoben (Austria, 2005), from the TU Dresden (Germany, 2007), and from the Saarland University, Germany, in combination with the position of the Scientific Director of the Leibniz-Institute of New Materials (INM, 2012), which he all declined. Since 2009, he holds a Faculty Position (Full Professor) at the University of Stuttgart (Germany) and is Member of the Board of Directors at the German Institutes of Textile and Fiber research (DITF) Denkendorf.

His research interests are located in the area of polymer synthesis, particularly polymerization catalysis, the synthesis of functional polymeric materials, e.g., for heterogeneous catalysis, polymers for batteries or for fiber-matrix composites as well as in the manufacturing of high-performance fibers, in particular of carbon and ceramic fibers. In addition, he is also interested in aspects of reaction engineering for both metal-catalyzed heterogeneous and biocatalysis.

So far, he has published more than 350 research papers and has filed more than 40 patents. For his scientific work, he received the “Professor Ernst Brandl Research Award 1998”, the “START Award-2001” the “Novartis Award 2001” as well as the “Otto-Roelen Medal of the German Catalytic Society”.

About The Author

Hagen J. Altmann M. Sc.
Institute of Polymer Chemistry, University of Stuttgart
Pfaffenwaldring 55, 70550 Stuttgart, Germany
e-mail: [email protected]

Hagen Altmann finished his Master in Chemistry in 2015, working on molybdenum alkylidene complexes bearing chiral N-heterocyclic carbenes (NHCs) in the group of Prof. M. R. Buchmeiser at the University of Stuttgart. In 2016 he started his PhD studies working on different types of NHC-catalyzed and NHC co-catalyzed polymerization reactions, including thermally latent, low viscosity, fast curing anhydride-hardened epoxy resins as well as the synthesis of high temperature-stable polymeric materials for fiber-matrix composites.

About The Author

Dr. Stefan Naumann
Institute of Polymer Chemistry, University of Stuttgart
Pfaffenwaldring 55, 70550 Stuttgart
Tel: +49-(0)711-685-64090, Fax: +49-(0)711-685-64050
e-mail: [email protected]

In 2010, Stefan Naumann received his diploma in chemistry from the University of Stuttgart, 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 included applications for acrylate-based monomers, polyamide production, polyether and polyester synthesis, as well as the preparation of polysiloxanes and epoxy resins.

Special emphasis was put on larger scale preparation and practicability, which resulted in novel methods to generate epoxy resin-based composite materials and “on demand” 1K polymerization systems. After finishing his PhD in 2014, he was granted a DFG research 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 the development of N-heterocyclic olefins (NHOs) and high-performing dual catalytic setups. After returning to Stuttgart in 2015, he started a habilitation process embedded in the Buchmeiser group.

His current research interests encompass polymerization catalysis (organocatalysis, dual catalysis, organometallic single-site complexes) and the preparation of mesoporous polymer/carbon materials for multiple purposes.

Reference

Hagen J. Altmann, Stefan Naumann, Michael R. Buchmeiser. Protected N-heterocyclic carbenes as latent organocatalysts for the low-temperature curing of anhydride-hardened epoxy resins. European Polymer Journal.

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