Abstract We report the synthesis, and the thermal, viscoelastic and dynamic properties of a series of dioxaborolane-based poly(n-butyl methacrylate) (PBMA) vitrimers, as a function of molar mass and cross-linking density. By combining calorimetry, dielectric spectroscopy and rheology, we explored the different characteristic temperatures that influence the dynamics of vitrimers. Three main processes could be identified by dielectric spectroscopy, the local β-process, the segmental α-process and the merged αβ-process at higher temperatures. Pressure was employed to examine their sensitivity through an apparent activation volume. We found that the local β-process was only weakly affected by pressure, and that the rubber-to-glass temperature continuously increased with vitrimer cross-linking density. Our results revealed that the dynamic bonds in vitrimers contribute to an effective unit of relaxation (e.g. an effective segment) that is larger than that in the corresponding homopolymer. In addition, the ratio of activation energies at constant volume and at constant pressure revealed that the segmental dynamics in the vitrimers is controlled by intra-molecular barriers. The viscoelastic properties are influenced by the topology freezing temperature, TV, separating segmental-controlled dynamics at lower temperatures from the bond exchange mechanism at higher temperatures initiating the flow. The derivative approach applied to the viscoelastic shift factors was found to be a practical method to locate TV. In the vitrimers, the bond exchange mechanism had a high apparent activation energy (E ∼ 90 kJ/mol) similar to that of the segmental dynamics. We present the first high-pressure investigation of the relaxation dynamics in vitrimers, elucidating the origin of the segmental process and re-defining the fundamental effective unit of relaxation.

Supramolecular metallogels based on platinum(II) complexes have attracted growing interest due to their tunable photophysical properties. However, the potential of Pt(II)-salphen complexes as low-molecular-weight gelators remains largely unexplored. Here, we report a series of Pt(II)-salphen complexes bearing alkyl chains of varying lengths and their organogelation properties. The complexes with the longer chain gelled aromatic solvents, forming hierarchical fibrillar networks, whose mechanical strength scales with alkyl chain length. Multi-scale analysis by cryo-TEM, SAXS, rheology, XRD, and solid-state NMR revealed the contributions of Pt-Pt metallophilic interactions alongside π-π stacking in the formation of the fibrils. This work opens a new avenue for tailored luminescent soft materials.

Supramolecular copolymers constitute a promising class of modular and dynamic materials. While most efforts thus far have been focused on tailoring their synthesis, structure, and function, their ability to impose...

Many functional polymeric systems have been inspired by the principle of compartmentalization, a ubiquitous form of hierarchical assembly found in Nature. For instance, polymers have been elaborated with microcapsules that release a chemical payload upon activation by an external stimulus. A fascinating challenge it to design a polymeric system to perform the opposite function, i.e., to load a cargo into a dispersed minor phase. Herein, this concept is exemplified in the context of a semicrystalline polymer matrix that contains two nucleating agents, one based on a supramolecular assembly and another based on a covalent triblock copolymer. These two additives compete to nucleate the semicrystalline matrix according to their competitive nucleation efficiency. A functional copolymer is rationally designed to induce these two additives to spatially assemble together. By adding a small amount of the functional copolymer, the supramolecular assembly remarkably segregates to the block copolymer phase, which profoundly impacts the interface between the block copolymer phase and the matrix. The compartmentalized additives no longer exhibit nucleating properties for the matrix, and the resulting material shows essentially the same thermomechanical properties as the virgin matrix.

Controlling the motion of molecular machines to influence higher-order structures is well-established in biological systems but remains a significant challenge for synthetic analogs. Herein, we aim to harness the mechanical switching of switchable molecular tweezers to modulate their self-assembly and produce stimuli-responsive organogels. We report a series of terpy(Pt-salphen)2 molecular tweezers functionalized with alkyl chains that act as low-molecular-weight gelators (LMWGs) in their open conformation. The resulting organogels were thoroughly characterized by SEM, cryo-TEM, SAXS, and rheology. The macroscopic transition from gel to solution was achieved by the cation-induced closing of the tweezers, which triggers their substantial structural reorganization. Reversible sol-gel transitions were achieved through the sequential addition of chemical stimuli or by a decomposable acid in a time-controlled operation. Such transient disassembly process regulated by a chemical fuel enables multiple gelation cycles with minimal waste while maintaining stable rheological properties. These results underscore the potential of switchable molecular tweezers in creating advanced stimuli-responsive materials.

The increasing consumption of polyethylene (PE) is an important concern for society because the rate of PE recycling remains low compared to the scale of PE production. There is thus great interest in strategies for revalorizing PE waste. In this context, our group recently reported an approach for transforming PE into nanostructured covalent adaptable networks (CANs) with azidotriazine grafting agents using reactive extrusion. These CANs feature polyazane cross-links that undergo thermomechanically activated exchange reactions, permitting the material to flow when subjected to high temperature and strain. Herein, we exploited this chemistry to develop modified PE materials with intriguing thermomechanical properties and exceptional processability. A family of compositions was prepared by reactively extruding PE with varying amounts of the azidotriazine grafting agent, and the resulting materials were studied using a combination of electron microscopy, small-angle X-ray scattering, calorimetry, rheometry, and mechanical testing. As the loading of the azidotriazine grafting agent was increased from 0.05 to 1.0 mol %, the materials transitioned from being thermoplastic-like to thermoset-like, revealing a balance of mechanical properties, melt strength, and high-temperature creep resistance. Surprisingly, all samples-even the CANs prepared with high loadings of the azidotriazine-could be easily shaped by injection molding. We believe that this approach is a promising, industrially relevant way to enhance the thermomechanical properties of polyolefins.

Supramolecular copolymers constitute a promising class of modular and dynamic materials. While most efforts thus far have been focused on tailoring their synthesis, structure, and function, their ability to impose order on the surrounding matrix remains poorly understood and relatively unexploited. In this context, we were intrigued by the potential of supramolecular copolymers to guide the outcome of the liquid-solid phase transition of a matrix. We hypothesized that the microstructure of a supramolecular copolymer could influence the local ordering of matrix molecules in the liquid state as it is cooled and solidified, thereby dictating its solid-state structure. Herein, we exemplify this concept using a matrix of isotactic polypropylene (PP) and supramolecular copolymers based on benzene tricarboxamides (BTAs). Two BTA structures were chosen for this study, one that exclusively nucleates the α-form of PP and another that preferentially nucleates the β-form. We demonstrate that the stoichiometric imbalance of the BTA comonomers and their microstructure within the copolymer play crucial roles in biasing the crystallization of the matrix.

Carbonate- and ether-based oligodiols have been converted into macro-RAFT agents via ATRAF reactions. They have been utilized in RAFT polymerization of styrene and maleic anhydride (MA) to obtain triblock copolymers (TBCs) with poly(styrene-co-maleic anhydride) side blocks. These copolymers have been used as chain extenders and tensile strength modifiers for polyamides (PAs) during reactive extrusion, which mimics mechanical recycling. TBCs of different chain lengths, number of MA moieties per chain, and stiff-to-elastic block ratio were utilized. The additives offset PA degradation during polymer processing. After 25 min of mixing (similar to 5 recycling acts), the melt viscosity of PAs have increased at least by over five times (from 55 to around 300 Pa s for PA12 and from 5 to 25 Pa s for PA6). The tensile strength of the blends was unchanged for PA12 and increased for PA6 (up to 17 %). The materials were stiffened by the presence of rigid side blocks (up to 40 % rise in Young's modulus). The type and weight fraction of oligodiol and the amount of TBC used affected the results. The presence of TBCs did not influence the morphology of the materials.

Boronic ester-based vitrimers have garnered significant attention in the polymer science community owing to their chemical stability, ease of synthesis, and recyclability. However, like most vitrimers, these materials tend to exhibit high viscosity at high temperature, making them difficult to process. Moreover, the dynamicity of boronic ester exchange at room temperature can result in poor creep resistance under service conditions for elastomeric vitrimers. Herein, we sought to address the balance of processability and mechanical performance by exploiting dioxazaborocane groups, which are a scarcely explored class of boronic esters featuring a dative nitrogen-boron bond. Both dioxazaborocane- and dioxaborolane-based vitrimers were prepared from low glass transition temperature (T g) polymethacrylate precursors bearing pendant complementary functional groups. Compared to dioxaborolane vitrimers, dioxazaborocane vitrimers exhibit faster relaxation dynamics at high temperatures, leading to more processable materials. The dioxazaborocane vitrimers also display improved tensile properties and competitive creep resistance, especially when using highly entangled precursors. This combination of enhanced processability and mechanical performance renders the dioxazaborocane group as an attractive motif for implementing into vitrimers.

Vitrimers are permanent polymer networks with dynamic covalent bonds that allow them to be reshaped and recycled. Although they are often promoted as an ideal class of materials situated between thermosets and thermoplastics, vitrimers are usually far more difficult to process than thermoplastics because their melt viscosity gradually decreases with increasing temperature. They are thus subject to a challenging trade-off between processability and mechanical performance, limiting their practical utility. Herein, we report the concept of a supramolecularly masked cross-linker that enables a vitrimer precursor to be processed as a thermoplastic before being thermally cured to form a vitrimer. The cross-linker features hydrogen bonding groups that promote aggregation and phase separation, allowing for it to be melt-blended with a functional thermoplastic at low temperature with minimal cross-linking. After shaping, the loaded thermoplastic is heated to a suitably high curing temperature to disassemble the aggregated cross-linker, thereby unmasking the cross-linker for reacting with the functionalized matrix. The resulting cured elastomeric vitrimers exhibit excellent thermomechanical properties along with mechanical and chemical recyclability. The latter consists of recovering the vitrimer precursor, which can be further reprocessed as a thermoplastic and converted back to a vitrimer after melt processing.

The revalorization of incompatible polymer blends is a key obstacle in realizing a circular economy in the plastics industry. Polyolefin waste is particularly challenging because it is difficult to sort into its constituent components. Untreated blends of polyethylene and polypropylene typically exhibit poor mechanical properties that are suitable only for low-value applications. Herein, we disclose a simple azidotriazine-based grafting agent that enables polyolefin blends to be directly upcycled into high-performance materials by using reactive extrusion at industrially relevant processing temperatures. Based on a series of model experiments, the azidotriazine thermally decomposes to form a triplet nitrene species, which subsequently undergoes a complex mixture of grafting, oligomerization, and cross-linking reactions; strikingly, the oligomerization and cross-linking reactions proceed through the formation of nitrogen-nitrogen bonds. When applied to polyolefin blends during reactive extrusion, this combination of reactions leads to the generation of amorphous, phase-separated nanostructures that tend to exist at polymer-polymer interfaces. These nanostructures act as multivalent cross-linkers that reinforce the resulting material, leading to dramatically improved ductility compared with the untreated blends, along with high dimensional stability at high temperatures and excellent mechanical recyclability. We propose that this unique behavior is derived from the thermomechanically activated reversibility of the nitrogen-nitrogen bonds that make up the cross-linking structures. Finally, the scope of this chemistry is demonstrated by applying it to ternary polyolefin blends as well as postconsumer polyolefin feedstocks.

Group II-VI semiconductor nanoplatelets (NPLs) with atomically defined thicknesses and extended atomically flat (001) facets are used for ligand binding and chiro-optical effects. In this study, we demonstrate that tartrate ligands, anchored by two carboxylate groups, chelate the (001) facets of NPLs at an average ratio of one tartrate molecule to two cadmium (Cd) surface atoms. This assembly of chiral molecules on inorganic nanocrystals generates a circular dichroism g-factor as high as 1.3 x 10-2 at the first excitonic transition wavelength of NPLs. Tartrate ligands induce an orthorhombic distortion of the initially "cubic" crystal structure, classifying the NPLs within the 222-point group. Unlike spherical nanocrystals, where it is difficult to discern whether chiral ligands affect only the surface atoms or the entire crystal structure, our findings unequivocally show that the crystal structure of NPLs is modified due to their thinness and atomically precise thickness. The in-plane lattice parameters experience compressive and tensile stresses, significantly splitting the heavy-hole and light-hole bands. Additionally, tartrate ligands adopt different conformations on the NPL surface over time, resulting in dynamic changes in the circular dichroism signal, including an inversion of its sign.

Water molecules can play a striking role in dictating the structure of supramolecular polymer networks in apolar media, but the consequences on their viscoelasticity are not completely understood. Herein, we compare two synthetic supramolecular polymer networks based on hydrogen bonding motifs that coassemble with water molecules in different ways. The first is a biphenyl tetracarboxamide (BPTA) that forms three different helical structures, two of which feature intercalated water molecules. The second is 2,4-bis(2-ethylhexylureido)toluene (EHUT), for which water molecules act as chain stoppers. Networks of each motif in n-dodecane were studied by light scattering, linear viscoelasticity, and passive microrheology while controlling the environmental conditions. At low temperatures in the presence of traces of water, both motifs form networks of dynamic, "living" supramolecular polymers. At high temperatures, in striking contrast to EHUT networks, BPTA networks behave like conventional covalent polymer chains. The counterintuitive behavior of BPTA networks is proposed to originate from enhanced dynamicity enabled by intercalated water molecules at low temperatures.

Vitrimers are a promising class of polymer networks that feature thermally activated dynamic covalent bonds. Although promising for many applications in material science, vitrimers tend to be difficult to process because of high viscosity. Processability is commonly enhanced at the expense of mechanical performance at service temperature, and thus this trade-off remains an important challenge in the development of these materials. Herein, we report a new approach based on thermoreversible organic nanofillers (TRONs), which we define as low molar mass molecules that act as reinforcing fillers at the service temperature and plasticizers at the processing temperature. This concept is exemplified in the context of an elastomeric dioxaborolane-based polybutadiene vitrimer as a model matrix, and two different dibenzylidene sorbitol derivatives are evaluated as TRONs. We find that vitrimers loaded with TRONs exhibit improved creep resistance and tensile properties at service temperatures while also exhibiting lower viscosity at processing temperatures.

We explore the behavior of a supramolecular polymer subjected to repeated shear-startup runs. This interesting test was proposed 50 years ago as a measure of disentanglement kinetics in linear polymers. A key experimental finding was that the recovery time (originally attributed exclusively to the relaxation of partially disentangled state) decreased with increasing imposed shear rate. Recent modeling and simulations put the work in perspective by showing that the recovery time mainly reflects chain reorientation. However, for the present supramolecular polymer, the recovery time was found to be rate independent. This drastic difference is attributed to the different mechanism of stress relaxation, which involves a combination of bond breaking/recombination and reptation processes. Hence, the repeated shear-startup protocol may serve as a sensitive indicator of (supramolecular) polymer signatures and a tool to evaluate stress relaxation models.

Vitrimers are polymer networks with dynamic covalent bonds that allow the network to reconfigure its connectivity while maintaining a constant number of chemical bonds at all temperatures. The melt viscosity of vitrimers thus gradually decreases with temperature. This behavior makes vitrimers more difficult to process than typical thermoplastics using conventional processing techniques, such as extrusion. Although many strategies have been reported to address this issue, it remains challenging to overcome a key tradeoff between improving the processability or the mechanical performance. Herein, this work presents a new strategy for overcoming this tradeoff in the context of elastomeric vitrimers. The approach entails the cross‐linking of a functionalized low‐glass transition ( T g ) polymer matrix with an incompatible high‐ T g polymer featuring pendant groups with complementary reactivity. When compared to a conventional homogeneous vitrimer, the microphase‐separated materials prepared by reactive extrusion present improved tensile properties and creep resistance at room temperature while also exhibiting enhanced processability at high temperature. These enhanced properties are a consequence of the combination of the phase separation between the soft and hard phases, the restriction of dynamic cross‐linking reactions within the interfacial zones, and the judicious selection of the T g of the hard phase to be in between the use and processing temperatures.

Vitrimers have captured broad interest in academia and industry because they offer a compelling combination of mechanical performance, thermal stability, and recyclability. They are permanent polymer networks that contain dynamic covalent bonds that undergo thermally activated exchange reactions without decreasing the connectivity of the network. The introduction of such functionality to commercial thermoplastics via reactive processing is envisioned to be an economical way to access high-performance, recyclable materials. Polyolefins are of particular interest because they are the most widely used class of polymers today. Although reactive processing is widely practiced in the industry, it remains a nascent approach for accessing polyolefin vitrimers. Herein, we report a dimaleimide bis(dioxaborolane) that allows for commercial polyethylene (PE) to be transformed into a vitrimer in a single step without any small-molecule byproducts. The miscibility of the grafting agent with the PE melt is a critical consideration for optimizing the grafting protocol, as efficient mixing is essential for achieving quantitative grafting and a high gel content. We find that the degree of incompatibility of the grafting agent in the PE melt strongly affects the nano- and macrostructure of the resulting PE vitrimers, which in turn dictates the thermal and mechanical properties. Finally, the material properties can be finely tuned using a reactive processing aid.

Covalent adaptable networks (CANs) are polymeric networks containing covalent crosslinks that are dynamic under specific conditions. In addition to possessing the malleability of thermoplastics and the dimensional stability of thermosets, CANs exhibit a unique combination of physical properties, including adaptability, self-healing, shape-memory, stimuli-responsiveness, and enhanced recyclability. The physical properties and the service conditions (such as temperature, pH, and humidity) of CANs are defined by the nature of their constituent dynamic covalent bonds (DCBs). In response to the increasing demand for more sophisticated and adaptable materials, the scientific community has identified dual dynamic networks (DDNs) as a promising new class of polymeric materials. By combining two (or more) distinct crosslinkers in one system, a material with tailored thermal, rheological, and mechanical properties can be designed. One remarkable ability of DDNs is their capacity to combine dimensional stability, bond dynamicity, and multi-responsiveness. This review aims to give an overview of the advances in the emerging field of DDNs with a special emphasis on their design, structure-property relationships, and applications. This review illustrates how DDNs offer many prospects that single (dynamic) networks cannot provide and highlights the challenges associated with their synthesis and characterization.

The study of synthetic organic polymers rapidly expanded since Staudinger’s recognition of the covalent structure of macromolecules. Today, these materials are far from just an academic concept—they are produced industrially and have become ubiquitous in everyday life because of their low cost and desirable physical properties. Nevertheless, a difficult choice between durability and reprocessability continues to hamper efforts to design synthetic organic polymers to be more recyclable. A new class of materials, vitrimers, has emerged as an intriguing approach to circumvent this tradeoff. Vitrimers are permanent networks of polymer chains connected via dynamic covalent bonds, which allow the network to change its topology while maintaining a constant number of chemical bonds at all temperatures. Characterized by both high mechanical performance and facile processing, vitrimers are well positioned to transition from academic labs to industrial production. The aim of this Trend article is to review the concept of vitrimers, describe their most unique properties, and present our outlook on outstanding challenges that must be met to realize vitrimers as a next generation solution for recyclable high performance materials.