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11. Exploring the dual dynamic synergy of transesterification and siloxane exchange in vitrimers

    Sami Fadlallah, Filip Van Lijsebetten, Tapas Debsharma, Vincent Scholiers, Florent Allais, Filip E Du Prez*

    Polymer Chemistry 2024, 61 (48), e202210405

Abstract: Despite the benefits of dynamic polymer networks with multiple dynamic bonds, identifying compatible combinations of dynamic chemistries that work synergistically to achieve desirable properties, remains a significant challenge. This work focuses on the potential of utilizing both siloxane and ester dynamic bonds in epoxy-acid cured vitrimers to finetune chemical exchange reactions. We identified to what extent a common basic catalyst (TBD) can simultaneously activate siloxane and ester exchange in the corresponding epoxy-based vitrimers. Our results showed that TBD is not only able to facilitate network formation but also improved the dynamic behavior of the resulting networks dramatically, with an overall exchange rate faster than the sum of the individual exchange chemistries, as shown by stress-relaxation studies. It has been demonstrated that the siloxane and ester dynamic bonds work together in a synergistic way to facilitate topology rearrangement. The relative increase or decrease in mobility of neighboring chains, located around a specific dynamic bond within the polymer network, was investigated in detail by adjusting the concentration of dynamic bonds and catalyst. The herein reported strategy allows the production of dual dynamic polymer networks that exhibit much shorter relaxation times and thus improved (re)processability in comparison to vitrimers with one type of dynamic bond.

10. Thermomechanical characterisation of reprocessable, siloxane-based, glass-fibre-reinforced vitrimers

    Virginia Amfilochiou, Tapas Debsharma, Ives De Baere, Lode Daelemans, Filip Du Prez, Wim Van Paepegem*

    Composites Part B: Engineering 2024, 276, 111354

Abstract: Polymer composites have been extensively used for the last 30 years in the automotive, green-energy, maritime and aerospace industries. While the demands of composites' applications are increasing, complications are arising regarding their sustainability. In this work, a siloxane-based vitrimer with fast stress-relaxation, and its Glass-Fibre-Reinforced Vitrimer (GFRV) were manufactured and mechanically characterised as a sustainable alternative to thermoset composites produced by infusion. Dynamic mechanical analysis and differential scanning calorimetry were performed to identify the ageing effect on the glass-transition temperature of the materials. Tensile and in-plane shear tests were executed to investigate the materials’ performance at elevated temperature. The results were compared to the ones of a thermoset benchmark and showed that after ageing and up to 50 °C, both the neat vitrimer and its corresponding composite exhibited a thermomechanical performance comparable to their thermoset counterparts. GFRV specimens were then successfully reprocessed by hot-pressing up to two consecutive cycles. The GFRV specimen was thermoformed into an omega-stiffener profile by the first hot-pressing cycle, while its flat profile was restored during the second cycle. Finally, the reprocessing results were evaluated by optical microscopy, demonstrating that the newly developed advanced GFRV is indeed a viable, sustainable alternative.

9. BiTEMPS methacrylate dynamic covalent cross-linker providing rapid reprocessability and extrudability of covalent adaptable networks: high-yield synthesis with strong selectivity for disulfide linkages

   Tapas Debsharma, Nathan S. Purwanto, Logan M. Fenimore, Sarah Mitchell, Jayme Kennedy and John M. Torkelson*

   Polymer Chemistry 2024, 61 (48), e202210405

Abstract: The dialkylamino disulfide-based dynamic cross-linker bis(2,2,6,6-tetramethyl-4-piperidyl methacrylate) disulfide, also known as BiTEMPS methacrylate (BTMA), has been of recent interest in the preparation of various reprocessable cross-linked polymers, otherwise known as covalent adaptable networks (CANs), by free-radical polymerization or free-radical reactive processing. Here, we revised the synthesis of BTMA to produce “BTMA-S2”, i.e., BTMA with a significantly higher yield, higher purity, and less color, with ≥95% disulfide linkages compared to “BTMA-Sn”, i.e., a previous version of BTMA that was synthesized with a relatively low yield, lower purity, and more color with a mixture of oligosulfide linkages. We used a low level (5 mol%) of this BTMA-S2 to synthesize CANs with n-hexyl methacrylate (BTMA-S2-HMA CANs). The BTMA-S2-HMA CANs recover their original cross-link densities after reprocessing. Additionally, compared to the BTMA-Sn-HMA CANs, the BTMA-S2-HMA CANs exhibit much faster stress relaxation at elevated temperatures, which manifests in rapid reprocessability. Specifically, BTMA-S2-HMA CANs made with 5 mol% BTMA-S2 can be reprocessed by compression molding at 130 °C for 5 min with full recovery of cross-link density. This is a factor of six faster reprocessing than analogous BTMA-Sn-CANs made with BTMA-Sn. Additionally, we demonstrate facile melt extrusion at 180 °C of the BTMA-S2-HMA CAN material with full recovery of cross-link density after extrusion.

8. Trialkylsulfonium-Based Reprocessable Polyurethane Thermosets

   Vincent Scholiers, Benjamin Hendriks, Stephan Maes, Tapas Debsharma, Johan M. Winne*, and Filip E. Du Prez*

   Macromolecules 2023, 56 (23), 9559–9569

Abstract: We report the development of a solvent-free protocol to produce colorless, highly transparent, and glassy polyurethane-based networks containing thioether bonds using commercially available building blocks. These polyurethane networks are converted into reprocessable networks by partial alkylation of the thioether bonds, giving dynamic trialkylsulfonium bonds that are able to exchange via transalkylation at elevated temperatures, thus inducing viscoelastic flow. Reprocessability of the trialkylsulfonium networks was demonstrated for three cycles without significant degradation of material properties. Interestingly, these materials were found to be highly processable at elevated temperatures (∼140 °C) and showed excellent creep suppression up to 100 °C, a combination that is rare among dynamic covalent polymer networks. The suppression of creep can be further controlled by changing the alkylating additive. In addition, as a result of their excellent transparency and high clarity, we investigated their optical properties to assess their potential use in smart coatings and optical devices.

7. Resorcinol‐Derived Vitrimers and Their Flax Fiber‐Reinforced Composites Based on Fast Siloxane Exchange

   Tapas Debsharma, Stéphanie Engelen, Ives De Baere, Wim Van Paepegem, Filip Du Prez*

   Macromolecular Rapid Communications 2023, 44 (8), 2300020

Abstract: Natural fiber-reinforced composites are gaining increased interest for their significantly reduced carbon footprint compared to conventional glass or carbon fiber-based counterparts. In this study, natural fibers are used in a resorcinol-based epoxy resin that is thermally reshapable at higher temperatures (>180 °C) by using fast exchanging siloxane bonds, catalyzed by 1,5,7-triazabicyclo[4.4.0]dec-5-ene. Stress relaxation times of only about 6 s at 220 °C can be reached. A resorcinol-based epoxy compound is selected because it can be derived from cellulose, opening ways for more sustainable and reshapable composite materials. In a last step of the research, the low viscosity vitrimer formulation (<200 mPa s) is applied to make a flax fiber-reinforced composite using an industrially relevant vacuum-assisted resin infusion process. A section of this composite is successfully reshaped, which allows for envisioning a second life for natural fiber-reinforced composites.

6. A highly dynamic covalent polymer network without creep: mission impossible?

   Filip Van Lijsebetten, Tapas Debsharma, Johan M Winne, Filip E Du Prez*

   Angewandte Chemie International Edition 2022, 61 (48), e202210405

Abstract: Dynamic covalent polymer networks provide an interesting solution to the challenging recyclability of thermosets and elastomers. One of the remaining design constraints, however, is balancing thermal reprocessability in the form of material flow with dimensional stability during use. As a result, many chemistries are being investigated in order to improve bond reactivity control and material robustness. This Minireview highlights a number of promising concepts, with a particular emphasis on disconnecting chemical reactivity in low and high temperature regimes to obtain creep resistant, yet highly dynamic polymer networks. In addition, we will highlight the impact of sharp reactivity changes when applying extrapolation-based approaches during rheological analysis. As a result, we are confident that abandoning the myth of “permanent” reactivity will aid in the development of sustainable polymeric materials that can truly combine the benefits of thermoplastic and thermoset behaviour.

5. Fast Dynamic Siloxane Exchange Mechanism for Reshapable Vitrimer Composites

   Tapas Debsharma, Virginia Amfilochiou, Aleksandra Alicja Wróblewska, Ives De Baere, Wim Van Paepegem, and Filip E. Du Prez*

   Journal of the American Chemical Society 2022, 144 (27), 12280–12289

Abstract: To develop siloxane-containing vitrimers with fast dynamic characteristics, different mechanistic pathways have been investigated using a range of catalysts. In particular, one siloxane exchange pathway has been found to show a fast dynamic behavior in a useful temperature range (180–220 °C) for its application in vitrimers. The mechanism is found to involve 1,5,7-triazabicyclo [4.4.0] dec-5-ene (TBD) as an organic catalyst in the presence of hydroxyl groups. Using this new mechanistic approach, vitrimers with ultrafast stress-relaxation characteristics (relaxation times below 10 s) have been prepared with a readily available epoxy resin and siloxane-amine hardener. Subsequently, the low viscosity siloxane-containing vitrimer resin enabled the preparation of glass fiber-reinforced vitrimer composites using an industrially relevant vacuum-assisted resin infusion technique. The resulting composite was successfully thermoformed into a new shape, which makes it possible to envision a second life for such highly engineered materials.

4. Ring-Opening Metathesis Polymerization of Unsaturated Carbohydrate Derivatives: Levoglucosenyl Alkyl Ethers

   Tapas Debsharma, Bernd Schmidt, André Laschewsky, and Helmut Schlaad*

   Macromolecules 2021, 54 (6), 2720–2728

Abstract: A series of biomass-derived levoglucosenyl alkyl ethers (alkyl = methyl, ethyl, n-propyl, isopropyl, and n-butyl) were synthesized and polymerized by ring-opening olefin metathesis polymerization using the Grubbs catalyst C793 at room temperature. Polymerizations were successfully performed in conventional solvents such as 1,4-dioxane and dichloromethane as well as in polar aprotic “green” solvents such as 2-methyltetrahydrofuran, dihydrolevoglucosenone (Cyrene), and ethyl acetate. The prepared polyacetals with degrees of polymerization of ∼100 exhibit Schulz–Flory-type molar mass distributions and are thermoplastic materials with rather low glass transition temperatures in the range of 43–0 °C depending on the length of the alkyl substituent. Kinetic studies revealed that the polymerization proceeded rapidly to a steady state with a certain minimum monomer concentration threshold. When the steady state was reached, just about half of the [Ru] catalyst had been effective to initiate the polymerization, indicating that the initiation step was a slow process. The remaining catalyst was still active and did no longer react with monomers but with in-chain double bonds, cutting the formed polymer chains into shorter fragments. In the long term, all catalyst was consumed and propagating [Ru] chain ends were deactivated by the elimination of [Ru] from the chain ends to form inactive chains with terminal aldehyde groups.

3. Cellulose-based polyacetals by direct and sensitized photocationic ring-opening polymerization of levoglucosenyl methyl ether

   Kerem Kaya, Tapas Debsharma, Helmut Schlaad, Yusuf Yagci*

   Polymer Chemistry 2020, 11, 6884-6889

This study aims to explore the photoinitiated cationic ring-opening polymerization of levoglucosenyl methyl ether (LGME), a chemical obtained from the most abundant biomass – cellulose. Direct and sensitized photopolymerizations of LGME using photoinitiators acting at the near UV or visible range in conjunction with diphenyliodonium hexafluoroantimonate (DPI) yielded unsaturated polyacetals with varying molar masses and distributions.

Abstract: The unsaturated bicyclic acetal levoglucosenyl methyl ether was readily obtained from sustainable feedstock (cellulose) and polymerized by cationic ring-opening polymerization to produce a semicrystalline thermoplastic unsaturated polyacetal with relatively high apparent molar mass (up to ca. 36 kg mol−1) and decent dispersity (ca. 1.4). The double bonds along the chain can undergo hydrogenation and thiol–ene reactions as well as crosslinking, thus making this polyacetal potentially interesting as a reactive functional material.

2. Cellulose‐Derived Functional Polyacetal by Cationic Ring‐Opening Polymerization of Levoglucosenyl Methyl Ether

   Tapas Debsharma, Prof. Yusuf Yagci, Prof. Dr. Helmut Schlaad*

   Angewandte Chemie International Edition 2019, 58 (51), 18492-18495

1. Ring-Opening Metathesis Polymerization of Biomass-Derived Levoglucosenol

   Tapas Debsharma, Dr. Felix N. Behrendt, Prof. Dr. André Laschewsky, Prof. Dr. Helmut Schlaad

   Angewandte Chemie International Edition 2019, 58 (20), 6718-6721

Abstract: The readily available cellulose-derived bicyclic compound levoglucosenol was polymerized through ring-opening metathesis polymerization (ROMP) to yield polylevoglucosenol as a novel type of biomass-derived thermoplastic polyacetal, which, unlike polysaccharides, contains cyclic as well as linear segments in its main chain. High-molar-mass polyacetals with apparent weight-average molar masses of up to 100 kg mol−1 and dispersities of approximately 2 were produced despite the non-living/controlled character of the polymerization due to irreversible deactivation or termination of the catalyst/active chain ends. The resulting highly functionalized polyacetals are glassy in bulk with a glass transition temperature of around 100 °C. In analogy to polysaccharides, polylevoglucosenol degrades slowly in an acidic environment.