Fast Dynamic Siloxane Exchange Mechanism for Reshapable Vitrimer Composites

Efficiency and Mechanism of Catalytic Siloxane Exchange in Vitrimer Polymers: Modeling and Density Functional Theory Investigations

Of late, siloxane-containing vitrimers have gained significant interest due to their fast dynamic characteristics over a reasonable temperature range (180-220 °C), making them well-suited for diverse applications. The exchange reaction pathway in the siloxane vitrimers is accountable for the covalent adaptive network, with the reaction's effectiveness being regulated by either organic or organometallic catalysts. However, directly studying the exchange reaction pathway in the bulk phase using experimental approaches is challenging because of the intricate and interconnected structure of these vitrimers. Here, we perform comprehensive density functional theory (DFT) and experimental investigations to discover the detailed catalytic efficacy of siloxane exchange and provide direction for the reaction process using a 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) catalyst. The calculated transition barrier energy and catalytic efficiency of hexamethyldisiloxane and dihydroxy-dimethylsilane exchange derived from the nudged elastic band with transition-state calculations strongly agree with the experimental findings. In addition, Fukui indices, along with partial charges, are employed to evaluate the nucleophilic and electrophilic behaviors of silanol and siloxane molecules. Our analysis revealed that by utilizing the Fukui indices of both the acid and the base, we can make an approximate estimation of the respective kinetics of the SN2 process in the siloxane exchange reaction mechanism. These findings establish a foundation for comprehending a crucial aspect of the exchange mechanism in siloxane vitrimer systems and could aid in the development of novel catalysts.

Is a Vitrimer with a High Glass Transition Temperature Available? A Case Study on Rigid Polyimides Cross-Linked with Dynamic Ester Bonds

Vitrimers, possessing associative covalent adaptable networks, are cross-linked polymers exhibiting malleable (glass-like) feature and recyclable and reprocessable (thermoplastics-like) properties. The dynamic behaviors of vitrimer are dependent on both chain/molecular mobility (glass transition temperature, Tg) and dynamic bond-exchanging reaction rate (topology freezing transition temperature, Tv). This work aims on probing the effect of high Tg on the stress relaxation and physical recyclability of vitrimers, employing a polyimide cross-linked with dynamic ester bonds (Tg: 310 °C) as the example. Due to its high Tg and chain rigidity, the cross-linked polyimide does not exhibit a high extent of stress relaxation behavior at 320 °C (10 °C above its Tg), even though the temperature is much higher than the hypothetical Tv. While raising the processing temperature to 345 °C, the cross-linked polyimide exhibits a stress relaxation time of about 3300 s and physical malleability. Nevertheless, side reactions may occur in the recycling and reprocessing process under the harsh condition (high temperature and high pressure) to alter the thermal properties of the recycled sample. The diffusion control plays a critical role on the topography transition of a vitrimer having a high Tg. The Tg ceiling is noticeable for developments of vitrimers.

Dynamic Chemistry Toolbox for Advanced Sustainable Materials

Developing dynamic chemistry for polymeric materials offers chemical solutions to solve key problems associated with current plastics. Mechanical performance and dynamic function are equally important in material design because the former determines the application scope and the latter enables chemical recycling and hence sustainability. However, it is a long-term challenge to balance the subtle trade-off between mechanical robustness and dynamic properties in a single material. The rise of dynamic chemistry, including supramolecular and dynamic covalent chemistry, provides many opportunities and versatile molecular tools for designing constitutionally dynamic materials that can adapt, repair, and recycle. Facing the growing social need for developing advanced sustainable materials without compromising properties, recent progress showing how the toolbox of dynamic chemistry can be explored to enable high-performance sustainable materials by molecular engineering strategies is discussed here. The state of the art and recent milestones are summarized and discussed, followed by an outlook toward future opportunities and challenges present in this field.

Comparing Surface and Bulk Curing Processes of an Epoxy Vitrimer

We used atomic force microscopy-based infrared spectroscopy (AFM-IR) and nanomechanical mapping (AFM-NM) to image the surface of a vitrimer, specifically dicarboxylic acid-cured diglycidyl ether of bisphenol A (DGEBA), to assess the curing process of a surface layer and compared this to the process in the bulk. We identified the β-hydroxy esters with various functionalities that are the key to form the cross-links for a system, including difunctional DGEBA and carboxylic acids. The IR peaks of the carbonyl group in generated ester groups are distinguished clearly from those in acids, allowing us to quantitatively assess the curing process at the surface and in the bulk. The initial curing at the surface exhibits a gradual cross-linking and is found to be lower than a rapid cross-linking in the bulk due to a relatively lower concentration of the β-hydroxy esters with high functionalities. This curing process leads to a smaller chemically and mechanically heterogeneous nanostructure at the surface relative to the bulk. After multiple reprocessings, a substantial number of esters lacking dynamic exchange capability form in the bulk, which decrease the flowability and reprocessability of the vitrimers and therefore the mechanical properties.

Vitrimer Nanocomposites for Highly Thermal Conducting Materials with Sustainability

Vitrimers, as dynamic covalent network polymers, represent a groundbreaking advancement in materials science. They excel in their applications, such as advanced thermal-conductivity composite materials, providing a sustainable alternative to traditional polymers. The incorporation of vitrimers into composite fillers enhances alignment and heat passway broadly, resulting in superior thermal conductivity compared to conventional thermosetting polymers. Their dynamic exchange reactions enable straightforward reprocessing, fostering the easy reuse of damaged composite materials and opening possibilities for recycling both matrix and filler components. We review an overview of the present advancements in utilizing vitrimers for highly thermally conductive composite materials.

Simply accessible platinum(II) complexes enabling alkene hydrosilylation at ppm catalyst loadings

An efficient olefin hydrosilylation protocol utilising Pt(II)-thioether-based pre-catalysts is reported. These simple and readily available complexes exhibit excellent catalytic performance and offer significant advantages over existing alternatives, enabling rapid and high conversions at ppm-level catalyst loadings.

Reprocessable Polyurethane Foams Using Acetoacetyl-Formed Amides

Like any other thermosetting material, polyurethane foams (PUFs) contain permanent cross-links that hinder their reprocessability and make their recyclability a tedious and environmentally unfriendly process. Herein, we introduce acetoacetyl-formed amides, formed by the reaction of isocyanates with acetoacetate groups, as dynamic units in the backbone of PUFs. By extensive variation of the foam composition, optimum parameters have been found to produce malleable foams above temperatures of 130 °C, without the requirement of any solvent during the foaming process. The PU cross-linked material can be compression-molded at least three times, giving rise to PU elastomers and thus maintaining a cross-linked network structure. Characterization of the original foams shows comparable properties to standard PUFs, for example, having a density of 32 kg/m3, while they show similar chemical and thermal properties upon reprocessing to strong PU elastomers, exhibiting Tg ranging from -42 to -48 °C. This research provides a straightforward method to produce thermally reprocessable PUFs as a promising pathway to address the recycling issues of end-of-life foams.

Thermally Robust yet Deconstructable and Chemically Recyclable High-Density Polyethylene (HDPE)-Like Materials Based on Si-O Bonds

Polyethylene (PE) is the most widely produced synthetic polymer. By installing chemically cleavable bonds into the backbone of PE, it is possible to produce chemically deconstructable PE derivatives; to date, however, such designs have primarily relied on carbonyl- and olefin-related functional groups. Bifunctional silyl ethers (BSEs; SiR2 (OR'2 )) could expand the functional scope of PE mimics as they possess strong Si-O bonds and facile chemical tunability. Here, we report BSE-containing high-density polyethylene (HDPE)-like materials synthesized through a one-pot catalytic ring-opening metathesis polymerization (ROMP) and hydrogenation sequence. The crystallinity of these materials can be adjusted by varying the BSE concentration or the steric bulk of the Si-substituents, providing handles to control thermomechanical properties. Two methods for chemical recycling of HDPE mimics are introduced, including a circular approach that leverages acid-catalyzed Si-O bond exchange with 1-propanol. Additionally, despite the fact that the starting HDPE mimics were synthesized by chain-growth polymerization (ROMP), we show that it is possible to recover the molar mass and dispersity of recycled HDPE products using step-growth Si-O bond formation or exchange, generating high molecular weight recycled HDPE products with mechanical properties similar to commercial HDPE.

Reversible Amidation Chemistry Enables Closed-Loop Chemical Recycling of Carbon Fiber Reinforced Polymer Composites to Monomers and Fibers

Highly efficient recycling of carbon fiber reinforced polymer composites into monomers and fibers is a formidable challenge. Herein, we present a closed-loop recycling approach for carbon fiber reinforced polymer composites using reversible amidation chemistry, which enables the complete recovery of intact carbon fibers and pure monomers. The polymer network, synthesized by amidation between a macromonomer linear polyethyleneimine and a bifunctional maleic anhydride cross-linker, serves as a matrix for the construction of composites with exceptional mechanical properties, thermal stability and solvent resistance. The matrices can be fully depolymerized under the acidic condition at ambient temperature, allowing the effective separation and recovery of both carbon fibers and the two monomers. The reclaimed carbon fibers retain nearly identical mechanical properties to pristine ones, while pure monomers are recycled with high separation yields (>93 %). They can be reused in for multiple cycles for the manufacture of new composites, whose mechanical properties recover over 95 % of their original properties. This line of research presents a promising approach for the design of high-performance and sustainable thermoset composites, offering significant environmental and economic benefits.

Reprocessible Triketoenamine-Based Vitrimers with Closed-Loop Recyclability

Development of thermosets that can be repeatedly recycled via both chemical route (closed-loop) and thermo-mechanical process is attractive and remains an imperative task. In this work, we reported a triketoenamine based dynamic covalent network derived from 2,4,6-triformylphloroglucinol and secondary amines. The resulting triketoenamine based network does not have intramolecular hydrogen bonds, thus reducing its π-electron delocalization, lowering the stability of the tautomer structure, and enabling its dynamic feature. By virtue of the highly reversible bond exchange, this novel dynamic covalent bond enables the easy construction of highly crosslinked and chemically reprocessable networks from commercially available monomers. The as-made polymer monoliths exhibit high mechanical properties (tensile strength of 79.4 MPa and Young's modulus of 571.4 MPa) and can undergo a monomer-network-monomer (yields up to 90 %) recycling mediated by an aqueous solution, with the new-generation polymer capable of restoring the material strength to its original state. In addition, owing to its dynamic nature, a catalyst-free and low-temperature reprogrammable covalent adaptable network (vitrimer) was achieved. The design concept reported herein can be applied to the development of other novel vitrimers with high repressibility and recyclability, and sheds light on future design of sustainable polymers with minimal environmental impact.

Circularity in polymers: addressing performance and sustainability challenges using dynamic covalent chemistries

The circularity of current and future polymeric materials is a major focus of fundamental and applied research, as undesirable end-of-life outcomes and waste accumulation are global problems that impact our society. The recycling or repurposing of thermoplastics and thermosets is an attractive solution to these issues, yet both options are encumbered by poor property retention upon reuse, along with heterogeneities in common waste streams that limit property optimization. Dynamic covalent chemistry, when applied to polymeric materials, enables the targeted design of reversible bonds that can be tailored to specific reprocessing conditions to help address conventional recycling challenges. In this review, we highlight the key features of several dynamic covalent chemistries that can promote closed-loop recyclability and we discuss recent synthetic progress towards incorporating these chemistries into new polymers and existing commodity plastics. Next, we outline how dynamic covalent bonds and polymer network structure influence thermomechanical properties related to application and recyclability, with a focus on predictive physical models that describe network rearrangement. Finally, we examine the potential economic and environmental impacts of dynamic covalent polymeric materials in closed-loop processing using elements derived from techno-economic analysis and life-cycle assessment, including minimum selling prices and greenhouse gas emissions. Throughout each section, we discuss interdisciplinary obstacles that hinder the widespread adoption of dynamic polymers and present opportunities and new directions toward the realization of circularity in polymeric materials.

Closed-loop recyclable and biodegradable thioester-based covalent adaptable networks

Closed-loop recyclable and biodegradable aliphatic covalent adaptable networks (CANs) based on dynamic β-CO thioester linkages that exhibit a service temperature beyond 100 °C are reported. These CANs possessing tensile strength and modulus values of up to 0.3 and 3 MPa, respectively, effectively undergo stress relaxation above 100 °C. The samples exhibit creep resistance ability and low hysteresis loss, and are repeatedly reprocessable at 120 °C. These CANs are depolymerizable to monomers under mild conditions and lose notable mechanical strength (92.4%) and weight (76.5%) within ∼35 days under natural biodegradation conditions.

Deciphering Siloxane Bond Exchanges: From a Molecular Study to Vitrimerization and Recycling of Silicone Elastomers

The activity of various additives promoting siloxane equilibration reactions is examined and quantified on model compounds. We found in particular that the "superbase" phosphazene derivative P₄ -^t Bu can promote very fast exchanges (a few seconds at 90 °C) even at low concentration (<0.1 wt %). We demonstrate that permanent silicone networks can be transformed into reprocessable and recyclable dynamic networks by mere introduction of such additives. Annealing at high temperature degrades the additives and deactivates the dynamic features of the silicone networks, reverting them back into permanent networks. A simple rheological experiment and the corresponding model allow to extract the critical kinetic parameters to predict and control such deactivations.

Degradable and Reprocessable Resins from a Dioxolanone Cross-Linker

Chemically cross-linked polymers offer excellent temperature and solvent resistance, but their high dimensional stability precludes reprocessing. The renewed demand for sustainable and circular polymers from public, industry, and government stakeholders has increased research into recycling thermoplastics, but thermosets have often been overlooked. To address this need for more sustainable thermosets, we have developed a novel bis(1,3-dioxolan-4-one) monomer, derived from the naturally occurring l-(+)-tartaric acid. This compound can be used as a cross-linker and copolymerized in situ with common cyclic esters such as l-lactide, ε-caprolactone, and δ-valerolactone to produce cross-linked, degradable polymers. The structure-property relationships and the final network properties were tuned by both co-monomer choice and composition, with properties ranging from resilient solids with tensile strengths of 46.7 MPa to elastomers with elongations up to 147%. In addition to exhibiting properties rivalling those of commercial thermosets, the synthesized resins could be recovered at end-of-life through triggered degradation or reprocessing. Accelerated hydrolysis experiments showed the materials fully degraded to tartaric acid and the corresponding oligomers from 1 to 14 days under mild basic conditions and in a matter of minutes in the presence of a transesterification catalyst. The vitrimeric reprocessing of networks was demonstrated at elevated temperatures, and rates could be tuned by modifying the concentration of the residual catalyst. This work develops new thermosets, and indeed their glass fiber composites, with an unprecedented ability to tune degradability and high performance by creating resins from sustainable monomers and a bio-derived cross-linker.

Dual-Dynamic Chemistries-Based Fast-Reprocessing and High-Performance Covalent Adaptable Networks

Covalent adaptable networks (CANs) possess multiple functions including reprocessing (or recyclability), self-healing, welding, shape shifting, 3D printing, etc., due to the network rearrangement from dynamic bonds, and favorable performance from their cross-linked feature, and they are supposed to be as sustainable alternatives to thermosets. However, the thermal and mechanical properties, and stability of CANs are often sacrificed for rapid network rearrangement. In this paper, fast-reprocessing CANs with high performance are facilely constructed by in situ polymerization and dynamic cross-linking of styrene (St), maleic anhydride (MA), and acetal diol (BHAD). The rigid and hydrophobic polymer backbone endow the materials with high glass transition temperatures, mechanical performance, and water resistance. Besides, carboxylic group-catalyzed dual dynamic ester and acetal-based networks exhibit faster stress relaxation and realize extrusion reprocessing. This work provides an ingenious and simple strategy of construction of CANs combining rapid network rearrangement and excellent comprehensive performance, which is beneficial for the application of CANs.

Remolding and Deconstruction of Industrial Thermosets via Carboxylic Acid-Catalyzed Bifunctional Silyl Ether Exchange

Convenient strategies for the deconstruction and reprocessing of thermosets could improve the circularity of these materials, but most approaches developed to date do not involve established, high-performance engineering materials. Here, we show that bifunctional silyl ether, i.e., R'O-SiR₂-OR'', (BSE)-based comonomers generate covalent adaptable network analogues of the industrial thermoset polydicyclopentadiene (pDCPD) through a novel BSE exchange process facilitated by the low-cost food-safe catalyst octanoic acid. Experimental studies and density functional theory calculations suggest an exchange mechanism involving silyl ester intermediates with formation rates that strongly depend on the Si-R₂ substituents. As a result, pDCPD thermosets manufactured with BSE comonomers display temperature- and time-dependent stress relaxation as a function of their substituents. Moreover, bulk remolding of pDCPD thermosets is enabled for the first time. Altogether, this work presents a new approach toward the installation of exchangeable bonds into commercial thermosets and establishes acid-catalyzed BSE exchange as a versatile addition to the toolbox of dynamic covalent chemistry.

Self-Healable Covalently Adaptable Networks Based on Disulfide Exchange

Introducing dynamic covalent bonding into thermoset polymers has received considerable attention because they can repair or recover when damaged, thereby minimizing waste and extending the service life of thermoset polymers. However, most of the yielded dynamic covalent bonds require an extra catalyst, high temperature and high-pressure conditions to trigger their self-healing properties. Herein, we report on a catalyst-free bis-dynamic covalent polymer network containing vinylogous urethane and disulfide bonds. It is revealed that the introduction of disulfide bonds significantly reduces the activation energy (reduced from 94 kJ/mol to 51 kJ/mol) of the polymer system for exchanging and promotes the self-healing efficiency (with a high efficiency of 86.92% after being heated at 100 °C for 20 h) of the material. More importantly, the mechanical properties of the healed materials are comparable to those of the initial ones due to the special bis-dynamic covalent polymer network. These results suggest that the bis-dynamic covalent polymer network made of disulfide and inter-vinyl ester bonds opens a new strategy for developing high-performance vitrimer polymers.