Our research group has developed a worldwide-recognized reputation for the synthesis of functional polymers through efficient modification strategies. The aim of these efforts was and is to develop highly efficient, but at the same time easy to carry out, syntheses. The chemistry developed in our group has been used intensively by many other research groups – often in close cooperation with our group. This indirectly demonstrates the efficiency of our chemistry for the synthesis of functional polymers and materials.

The ultimate goal for every synthetic polymer chemist is the extensive control on the final properties of a desired product. Thus, control on the molecular level is inevitable.

Our group is specialized on the synthesis of precisely tailored polymers. Therefore, taking advantage of modern polymerization techniques (e.g., ATRP, NMP, RAFT polymerization, ROMP, ROP, ADMET), subsequently, we are combining this approach with post-polymerization modification techniques, often on the basis of click chemistry. As a consequence, we achieve full control of polymer architecture and the polymers functionality.

There are several possibilities to alter polymeric functionalities. Changing the monomer is one option, modifying the monomer prior to polymerization is another one. However, utilizing polymer analogues reactions, i.e., post-polymerization modifications, has the advantage of using solely one monomer for polymerization and subsequently modifying the defined polymer chain with multiple functionalities. Thereby, the polymerization degree maintains unchanged.

In order to achieve full control of the polymer architecture and functionality, we concentrate on the development of new (reactive) monomers that enable an efficient and robust post-polymerization functionality. An inherent reactive character is necessary in order to guarantee a quantitative modification under mild conditions, i.e., room temperature.

In the past, we have thus been able to promote the use of activated esters within polymer science, namely the use of pentafluorophenyl (PFP) esters. PFP esters have been prepared from acrylates, methacrylates, 4-vinylbenzoates, norbornene-carboxylic acids, etc.

Recently, we were able to present a new approach for the synthesis of sulfur copolymers (so-called „Inverse Vulcanization“) by heating elemental sulfur with unsaturated compounds. We have further expanded this synthetic approach and will continue to do so in the future. The following approaches are to be taken into account:

• Use of renewable unsaturated compounds. In first preliminary studies, we could already show that plant oils are suitable for this purpose. Such sulfur-containing vegetable oil rubbers have just been filed as a patent. However, a precise understanding of the molecular structure is still lacking in order to optimize the synthesis of these new cathode materials.
• Aromatic natural compounds, e.g. Eugenol and derivatives.
• Aromatic alkynes that partially form conductive polythiophenes in the network,

as preliminary studies have shown, which would increase the conductivity and lead to a material improvement.

• Nanostructuring of polysilicone cathode materials.
• Post-functionalization of sulfur copolymers

Smart Polymers and Actuators

Polymers whose properties can be influenced by an external stimulus are of central interest in our research group. The synthetic methods of reactive polymers and block copolymers and their subsequent functionalization make it possible to combine different chemical groups. This has allowed us to produce multi-responsive polymers, which respond to at least three stimuli temperature, light and oxidation.

In addition to the controlled solubility of such responsive polymers, we are currently working on the modification of optical properties of polymers by targeted external influences. For this, the use of photochromic groups in polymers, particularly novel electrochromic polymers are subject of current research in the research group.

The variety of smart polymers has also been employed in the fabrication of hydrogel actuators. Recent advancements in materials science have sparked a heightened interest in smart and responsive materials, attributed to their unique ability to autonomously execute functions or respond in a controlled manner to external stimuli. Such materials are pivotal in creating advanced, efficient devices and systems, including sensors, actuators, and drug delivery systems. Within the diverse spectrum of stimuli-responsive materials, those exhibiting sensitivity to light command particular attention for their swift and precise control over material properties. However, the integration of varied stimuli-reactive attributes within a sole polymer structure presents a significant challenge which is essential for replicating the intricate behaviors observed in natural systems. As such, we have and continue to drive forward the development of smart materials with tunable properties and using state-of-the-art 3D printing techniques to fabricate complex polymer networks with actuating functions.

Research on materials that enable energy storage has been blooming recently. As such, our group has established the use of novel and uniquely functionalized polymers for application rechargeable batteries as well as fuel cells in recent years. The penetration of everyday life with electronic devices (e.g., mobile telephone, smart watch, etc.) requires an ever-increasing use, and thus the technical realization of batteries with high energy storage densities. However, the demand for efficient energy storage for electric vehicles and/or so-called grids is also steadily increasing. Alternatively, the development of polymer membranes for fuel cells has been conducted. We address this research area with several aspects:

• Novel polymeric cathode materials for Li-S batteries

It is currently a trend to develop new electrode materials. Here, sulfur-based cathode materials have triggered a „gold rush“ in the research landscape. The high specific energy of Li-S batteries with a theoretical capacity of 2600 Wh kg-1 correspond to a multiplication of the currently achievable energy density. The optimization of the cycle stability of sulfur-based batteries is still subject of intensive research. One of the main reasons are the formation of soluble lithium-oligosulfides

(Li2Sn, 3≤n≤8), which diffuse very well through the electrolyte and the membrane (so- called shuttle effect), as well as the formation of insoluble and insulating Li2S2 and/or Li2S that often deposit as a passivating layer on the surface of the lithium electrode so that they no longer participate in the redox cycle and thus lead to a decrease in the Coulomb efficiency of the Li-S cells. As such, we investigate inverse-vulcanized materials as successful cathode materials for Li-S batteries.

• Novel polymer electrolytes for Li, Na, K, and Mg-ion batteries

Special attention is given to the further development of electrolytes in batteries. The ionic conductivity of the Li+ ions (or other ions) through the „solid-electrolyte interface“ SEI and the actual electrolyte contributes significantly to the limited battery performance. In principle, different types electrolytes can be distinguished in the lithium-ion and post-lithium battery technology. The focus of our group lays on polymer electrolytes (PE) because they feature a number of advantages: (a) PE are non-volatile, (b) have a limited destructive decomposition at electrodes, (c) contribute to a reduction of dendritic growth; (d) increased leak tightness, (e) mechanical stabilization of the cells, (f) increased shock resistance, (g) increased overall safety, (h) better protection against overheating and overcharging. The main challenges here are the costs and lifetime as well as the safety in the application. The most intensively studied class of PE is based on polyethylene oxide (PEO). Especially in solid PE, a high lithium transmittance number t+ is crucial. The high crystallinity of PEO/salt complexes at room temperature results in a low conductivity of just over 10-5 S/cm, while practical applications require values of at least 10-3 S/cm. In order to achieve increased ionic conductivities, fillers have also been used since these increase both the mechanical stability of the PE and also the lithium-ion conductivity. It has been found that fillers exhibiting acidic groups on their surface have a high lithium transfer number t+ and an excellent stabilization of the Li metal electrode over long periods of time. It is furthermore known that lower anion mobility results in an increased cation mobility and thus also results in an increase in the lithium transfer number t+. It was therefore proposed to complex anions to increase the dissociation of the lithium salt, the solubility and also the number of free cations in solution. Noteworthy, we have developed unique synthetic routes toward side-chain PEO block copolymers that tackle the above aspects and have proven to be very successful in ion conduction of Li+, Na+, K+, and Mg2+, which allows us to draw first structure-property relationships for the future development of optimized polymer electrolytes.

• Synthesis of Polymeric Organic Radical Batteries

Batteries for mobile devices often require high mechanical flexibility, fast charging times and lightness. For this reason, organic radical batteries (ORBs) are suitable for such areas. However, to date, reliable, reproducible and scalable syntheses are missing. The central building block of most ORBs is the stable 2,2,6,6-tetramethylpiperidine-1-oxy (TEMPO) radical, which can be oxidized and reduced by one-electron processes. In addition to the nitroxide radicals, N-based triarylaminium cation radicals and diphenylpicrylhydrazyl derivatives as well as O-based phenoxyl radicals have also been investigated as stable radicals. Post-functionalization of polymers has been introduced to prepare such polymeric ORBs utilizing the novel redox active moieties of squaric acid quinoxalines (SQXs).

• Proton Exchange membranes (PEMs) for Fuel Cells

In the past our group has investigated branched poly(arylene ether ketone sulfone) as the basic skeleton for the development of branched sulfonated polymer PEMs, achieving superior performance to commercial PFSA membranes ultimately. Again, here the expertise in efficient post-polymerization functionalization chemistries had been advantageous. Increasing the degree of branching and ion exchange capacity exerted the same impact on promoting water absorption, proton conductivity, single-cell performance, and formation of denser distribution of ionic clusters while reducing the mechanical properties of the membranes. Distinctly different effects of degree of branching and ion exchange capacity occurred on dimensional variation and oxidative stability, as elevated degree of branching led to isotropic swelling change and enhanced oxidative stability, whereas the influence of ion exchange capacity was totally the opposite. Optimization of membranes led to a proton conductivity and swelling change comparable to Nafion 117, as well as satisfactory oxidative stability, achieving a trade- off between proton conductivity, dimensional stability, and durability.

In the age of digitalization, control over chemical syntheses has become very important. Therefore, we have aimed to implement a combined – periphery-independent – scientific approach for online monitoring, improvement and reproduction of polymerization processes in advanced reaction vessels. We could transfer literature approaches in small molecule chemistry to polymerizations that are less forgiving in terms of side reactions and solution properties. It allowed us to develop methods for the (co)polymerization of reactive monomers and the subsequent post-functionalization of the obtained reactive polymers in flow. Thereby, we have established a synthetic platform that allows us to combine to design and 3D print our own reaction ware for conducting successful in- series chemistries for the preparation of defined functional polymers.