Latent catalysts are a versatile strategy to temporally and locally control the rate of bond exchange reactions (and the related material flow) in dynamic polymer networks. Herein, we explored novel families of reversibly switchable catalysts, which undergo a distinctive shift in their pKa values due to an external trigger (either light or a change in temperature). In a first step, the reversible activation/deactivation of a photoswitchable nitrogen superbase undergoing light-induced isomerization of dithienylethene groups was studied in thiol-ene photopolymers.[1] Depending on the color of light used (visible versus UV light), a substantial difference in the pKa value was observed, which was exploited to kinetically control the base-catalyzed bond exchange between thioester and thiol groups.[2] The reversibility and local control of the isomerization reaction (and the related bond exchange kinetics) was demonstrated by stress relaxation, reshaping, micro imprint and tensile testing experiments. In a further approach, photochromic merocyanine-based photoacids were synthetized and applied as reversibly switchable catalysts for acid-driven dynamic polymer networks.[3] Upon visible light exposure, the catalyst was activated by undergoing a spirocyclization, while the deactivation reaction took place under dark conditions at elevated temperature. The photoacid was placed in a thiol-ene photopolymer comprising ample -OH and ester moieties and a reversible control of the transesterification kinetics was demonstrated by stress relaxation studies.

Phlorotannins are special classes of polyphenols occuring in brown algae (Phaeophyta) in which they contribute to a substantial amount to the dry mass (3 % in the investigated samples of Fucus vesiculosus and Ascophyllum nodosum). Phloroglucinol trigylcidyl ether as model compound was reported to form thermosets with high glass transition temperature and mechanical strength when crosslinked with several acid anhydrides [1]. In this contribution we show our efforts to use phlorotannins that are directly extracted from brown algae. The yield of isolated phlorotannins was optimized by changing grinding and extraction parameters. Taking phloroglucinol as reference, differences in obtained hydroxyl content and epichlorohydrin functionalization are discussed. Crosslinking is performed with succinic acid anhydride, admerginic acid and other anhydrides, with the implementation of catalysts suitable for adaptable network formation [2, 3]. Activation energies for network reorganisation above the glass transition are deduced from rheological relaxation experiments. This project (Algoform) is funded by the German Federal Ministry of Food and Agriculture.

The development of thermally induced healing materials has garnered significant attention due to their potential for waste reduction and enhanced polymer product lifespans. Vitrimers, as covalent adaptable networks, exhibit thermo-activated dy-namic associative exchange reactions. [1] Above their topology freezing transition temperature (Tv), exchange reactions accelerate, allowing the material to flow like a viscoelastic liquid while remaining in a crosslinked solid state. Vitrimers with dynam-ic hydroxyl ester links require transesterification catalysts to enable rapid bond ex-change above Tv. [2] A library of thermally latent bases for catalyzing transesterifica-tion in vitrimers was investigated. Synthesized thermo-base generators (TBGs) were analyzed for thermal decomposition and thermally triggered base release using thermogravimetric analysis (TGA), evolved gas analysis (EGA), nuclear magnetic resonance spectroscopy (NMR), and infrared spectroscopy (IR). These catalysts were selectively thermally activated and deactivated, enabling multiple thermally switchable dynamic polymer networks. The tailored activation and deactivation strat-egy was validated through stress relaxation, reshaping experiments, self-healing studies, and tensile testing. The molecular architecture of thermally latent catalysts allowed precise control of decomposition temperature and induction period across a wide range. Consequently, different activation and deactivation temperatures sup-port diverse applications in vitrimers with varying Tv. This innovation offers promis-ing avenues for broadening the applicability of vitrimers in the polymer industry, pav-ing the way for future advancements in sustainable material design.

Thermosets are known for their poor (re-)processing and recycling capabilities due to their permanently crosslinked microstructure. As a result, their end-of-life options are limited to landfill or incineration, preventing thermosetting materials from meeting the demand for sustainable and circular materials. In recent years, the introduction of dynamic covalent bonds in polymeric networks has formed a promising solution to the challenge of developing circular thermosets. So-called covalent adaptable networks (CANs) bear reversible bonds that can form or break upon an external stimulus (i.e. heat, mechanical stress, UV-radiation etc.), thus producing materials with inherent recycling and reprocessing capabilities. While research initially mainly focused on the preparation of CANs from finite petroleum-based resources, conflicting with fully circular design, the focus now turned to the use of alternative and renewable resources. In this regard, applying cellulose as a resource for the synthesis of renewable CANs is an attractive alternative. Cellulose is the most abundant natural polymer and is associated with multiple advantages including low cost, renewability, biodegradability and great mechanical performance. Chemical modification of cellulose presents a challenge, nevertheless the large number of hydroxy groups makes cellulose potentially suitable for a wide range of chemical modifications. We have explored the design and synthesis of cellulose-based CANs, focusing on the modification and crosslinking of cellulose with different dynamic chemistries (i.e. imine and Diels-Alder). The resulting materials were evaluated for their physicochemical properties, thermomechanical properties, dynamic nature and reprocessing capabilities.

Keywords: cellulose, recyclable thermosets, dynamic chemistry

The development of bio-based vitrimer materials is essential for environmental preservation. This study presents UV-curable resins formulated from rapeseed and linseed oils with various functional groups. The first series of materials involved synthesizing epoxidized rapeseed and linseed oils, followed by epoxy ring-opening to introduce acrylic groups. The second series involved synthesizing acrylated rapeseed and linseed oils, followed by epoxidation of the remaining fatty acid double bonds. Therefore one- and two- step acrylation syntheses as well as the role of hydroxyl and epoxy moieties in vitrimeric systems were compared. Both acrylic and epoxy functional groups participated in dynamic transesterification reactions (DTERs) as confirmed by real-time photorheology measurements. Moreover, rheological studies revealed that the stress relaxation rate slows down with increasing crosslink density and lower amount of hydroxyl moieties. The properties of vitrimer such as self-healing, weldability, and reprocessability have been investigated.

The three-dimensional network architecture of hydrogels significantly influences their mechanical and physical properties, making their understanding essential for designing optimized hydrogel-based biomaterials. This study presents a comparative analysis of two konjac glucomannan (KGM)/kappa carrageenan (KCAR) hybrid hydrogels with similar stiffness (5.2-5.7 kPa and 1.6-1.7 kPa) but distinct network architectures: a classic network formed by extended polysaccharide interactions and a nanogel junction network where cross-linked KCAR nanogels (KCAR-NGs) link KGM chains. The classic network exhibited superior tensile resistance, elongation, and solvent-induced swelling resistance, leading to slower dissolution and higher viscosity. In contrast, the nanogel junction network allowed greater permeability for small molecules and faster dissolution, suggesting a more open structure.
Addressing the challenge of balancing mechanical stability with controlled dissolution, this research also demonstrates how tuning hydrogen-bonded nanogel interactions can modulate hydrogel properties. The nanogel-based hydrogel exhibited self-healing and shear-thinning behaviors, with dissolution rates influenced by temperature, nanogel concentration, and potassium ions. Adding KCl further enhanced mechanical properties, increasing compression and tensile moduli. These findings highlight the potential of nanogel junction networks in biomedical applications requiring stability, permeability, and rapid dissolution under physiological conditions without the need for high temperatures or chelating agents. This work underscores the ability to tailor hydrogel properties by leveraging nanogel interactions, advancing the design of functional biomaterials.