Polymer Chemistry and Catalysis

Associated Group Prof. Udo Kragl (University of Rostock)

Overview of the Department's Activities


Traditionally, tin-organyls have being used as vulcanization catalysts in sealants and adhesives with excellent results in terms of shelf life, curing time and selectivity. Nevertheless, due to its inherent toxicity and the subsequent environmental concerns, the search for alternative catalysts with comparable performance is one of our main research areas. In order to achieve this task, we design and develop new catalytic systems making use of tools from inorganic and organic synthesis and organometallic chemistry.

Fluxional behaviour of diamide-amine-supported titanium complexes
X-ray molecular structure of a binuclear diamide-amine titanium complex.

Silicone polymers, especially polymethylsiloxanes (PDMS) derivatives, are of paramount importance as adhesives, sealants, insulators, etc. Among these, those that vulcanize at low temperatures under ambient conditions represent a considerable fraction of the market. Silicone sealants display outstanding performance under harsh environmental conditions, are resistant to UV light, ozone, moisture and are characterized by their extreme frost and heat resistance. Moreover, they show excellent adhesion to the surface of typical construction substrates. Occasionally, in addition to the above mentioned requirements that sealants must satisfy, very special characteristics are required for specialty applications in the fields of machinery, automotive, electronics, etc. Hence, we strive to develop new functionalized silicones with tailored properties to meet those requirements.

Typical cross-linking (vulcanization) reaction for an alpha,omega-di-dialkoxyvinylsilyl-polydimethylsiloxane silicone

Additionally, in cooperation with our industry partners, we aim to develop new polymerization catalysts and polymeric materials to be used in coatings, sealants and adhesive technologies and to investigate process engineering issues.

Moreover, by judicious characterization and optimization of the process parameters and the catalytic system, we aim to synthesize polymers not only with narrow molecular weight distributions but also tailored physicochemical and mechanical properties.

Typical propoxylation reactor for the synthesis of polyethers

Selected Literature

1. “Catalytic Systems for the Cross-linking of Organosilicon Polymers” D. Wang, J. Klein and E. Mejía, Chem. Asian. J. , 2017, 12, 1180-1197

2. “Recent Developments on the Preparation of Silicones with Antimicrobial Properties” A. Kottmann, E. Mejía, T. Hemery, J, Klein and U. Kragl, Chem. Asian. J. , 2017, 12, 1168-1179

3. “Mono- and Binuclear Titanates Bearing Podand Diamidoamine Ligands and Their Use as Catalysts in Siloxane Cross-Linking” Martha Höhne, Andrea Gutacker, Johann Klein and Esteban Mejia, Organometallics, 2017, 36, 2452-2459

 

 

Mejia's Junior Research Group

The cross-coupling of open-shell (radical) species have become a powerful and attractive way to create new C-C bonds. Yet, the hurdles associated with this process (even though the activation energy of radical−radical coupling reactions is nearly zero), have slowed down the development of radical C-H functionalization/cross coupling processes, compared with “classical” cross-coupling methodologies. Very recently, we reported a novel methodology to achieve oxidative C–H cross-coupling of underivatized allylic substrates (cyclic and linear) and terminal alkynes (aromatic and aliphatic) (Scheme below). The intuitive evolution of this methodology is the replacement of the oxidant (ideally by air) and the introduction of chirality at the formed tertiary carbons.

Allylic alkynylation via copper catalyzed cross-dehydrogenative coupling

Recently, intense efforts have been devoted to the development of metal-free catalysts to meet the requirements of sustainable and green chemistry. We have developed different carbon-based, metal-free systems (both heterogeneous and homogeneous) as oxidation catalysts, for which we selected the synthesis of imines via oxidative homocoupling of amines as a model reaction. Imines are an important class of compounds with ample use in the pharmaceutical and chemical industry, as they are key precursors for many biologically active heterocycles. This new transformation may be expanded to other substrates and shall foster the somehow neglected use of organic radical cations as environmentally benign redox catalysts.

Oxidative coupling of amines catalyzed by pyrazine radical cations

Stereoselective polymerization is a potent synthetic method to gain access to high performance materials, since the controlled stereochemistry of the polymer microstructure where successive stereocenters are of the same (isotactic) or alternating (syndiotactic) relative configurations usually affords higher crystallinity compared to their atactic analogues and hence, improved mechanical and thermal properties. Compared to tremendous amount of research on the stereoselective polymerization of olefins, the stereoselective epoxide polymerization and copolymerization remains largely unexploited. With this precedent in mind, we want to investigate this neglected class of materials, developing efficient methods for their synthesis, especially considering that this caveat is what has precluded these polymers to be applied if an industrial scale. In this sense, we propose to use magnesium catalysts, which we have demonstrated to be active and iso-specific (see below). Beyond achieving efficient methods for their preparation, we also want to investigate the physical and mechanical properties of these ill-characterized materials in view of potential, novel applications.

Iso-enriched homo- and co-polymerization of racemic and meso epoxides using achiral organomagnesium catalysts

Selected Literature

1. "Copper‐Catalyzed Allylic C‐H Alkynylation via Cross‐Dehydrogenative Coupling" A. Almasalma and E. Mejía, Chem. Eur. J., 2018, 24, 12269-12273

2. "Pyrazine Radical Cations as Catalyst for the Aerobic Oxidation of Amines" Rok Brisar, Dirk Hollmann and Esteban Mejia, Eur. J. Org. Chem., 2017,  5391-5398

3. "Aerobic Oxidative Homo- and Cross-Coupling of Amines Catalyzed by Phenazine Radical Cations" R. Brišar, F. Unglaube, D. Hollmann, H. Jiao, and E. Mejía, J. Org. Chem., 2018, 83, 13481-13490

4. “Rediscovering the Isospecific Ring-Opening Polymerization of Racemic Propylene Oxide with Dibutylmagnesium” S. Ghosh, H. Lund, H. Jiao, and E. Mejía, Macromolecules, 2017, 50, 1245-1250

5. "Isospecific Copolymerization of Cyclohexene Oxide and Carbon Dioxide Catalyzed by Dialkylmagnesium Compounds" S. Ghosh, D. Pahovnik, U. Kragl and E. Mejía, Macromolecules, 2018, 51, 846-852