Thermal Guanidine Metathesis for Covalent Adaptable Networks

Postpolymerization Modification by Nucleophilic Addition to Styrenic Carbodiimides

Carbodiimides are electrophilic functional groups that react with select nucleophiles under mild conditions. However, their potential as platforms for postpolymerization modification has been relatively underexplored. We describe the synthesis and radical polymerization of a styrenic carbodiimide which undergoes rapid nucleophilic addition with primary and secondary alkyl amines under ambient conditions, even in the presence of other protic nucleophiles. The monomer is amenable to both free and controlled radical (co)polymerization, and we further demonstrate the utility of this approach by preparing covalent adaptable networks through guanylation of the styrenic carbodiimide with difunctional amines. These materials exhibit a variation in relaxation times according to both the guanidine structure and concentration, providing a facile means for tuning dynamic behavior.

One-Pot Synthesis of Depolymerizable δ-Lactone Based Vitrimers

A depolymerizable vitrimer that allows both reprocessability and monomer recovery by a simple and scalable one-pot two-step synthesis of vitrimers from cyclic lactones is reported. Biobased δ-valerolactone with alkyl substituents (δ-lactone) has low ceiling temperature; thus, their ring-opening-polymerized aliphatic polyesters are capable of depolymerizing back to monomers. In this work, the amorphous poly(δ-lactone) is solidified into an elastomer (i.e., δ-lactone vitrimer) by a vinyl ether cross-linker with dynamic acetal linkages, giving the merits of reprocessing and healing. Thermolysis of the bulk δ-lactone vitrimer at 200 °C can recover 85-90 wt% of the material, allowing reuse without losing value and achieving a successful closed-loop life cycle. It further demonstrates that the new vitrimer has excellent properties, with the potential to serve as a biobased and sustainable replacement of conventional soft elastomers for various applications such as lenses, mold materials, soft robots, and microfluidic devices.

Accessing Heteroannular Benzoxazole and Benzimidazole Scaffolds via Carbodiimides Using Azide-Isocyanide Cross-Coupling as Catalyzed by Mesoionic Singlet Palladium Carbene Complexes Derived from a Phenothiazine Moiety

The coupling of aryl and aliphatic azides with isocyanides yielding carbodiimides (8-17) were efficiently catalyzed by well-defined structurally characterized trans-(MIC)PdI₂(L) [MIC = 1-CH₂Ph-3-Me-4-(CH₂N(C₆H₄)₂S)-1,2,3-triazol-5-ylidene, L = NC₅H₅ (4), MesNC (5)], trans-(MIC)₂PdI₂ (6), and cis-(MIC)Pd(PPh₃)I₂ (7) type palladium complexes, which incidentally mark the first instances of the use of mesoionic singlet palladium carbene complexes for the said application. As observed from the product yields, the catalytic activity varied in the order 4 > 5 ∼ 6 > 7 for these complexes. A detailed mechanistic studies indicated that the catalysis proceeded via a palladium(0) (4a-7 a) species. Using a representative palladium precatalyst (4), the azide-isocyanide coupling was successfully extended to synthesizing two different bioactive heteroannular benzoxazole (18-22) and benzimidazole (23-27) derivatives, thereby broadening the scope of the catalytic application.

Vat Photopolymerization Additive Manufacturing of Tough, Fully Recyclable Thermosets

To advance the capabilities of additive manufacturing, novel resin formulations are needed that produce high-fidelity parts with desired mechanical properties that are also amenable to recycling. In this work, a thiol-ene-based system incorporating semicrystallinity and dynamic thioester bonds within polymer networks is presented. It is shown that these materials have ultimate toughness values >16 MJ cm^-3, comparable to high-performance literature precedents. Significantly, the treatment of these networks with excess thiols facilitates thiol-thioester exchange that degrades polymerized networks into functional oligomers. These oligomers are shown to be amenable to repolymerization into constructs with varying thermomechanical properties, including elastomeric networks that recover their shape fully from >100% strain. Using a commercial stereolithographic printer, these resin formulations are printed into functional objects including both stiff (E ∼ 10-100 MPa) and soft (E ∼ 1-10 MPa) lattice structures. Finally, it is shown that the incorporation of both dynamic chemistry and crystallinity further enables advancement in the properties and characteristics of printed parts, including attributes such as self-healing and shape-memory.

Anion-assisted amidinium exchange and metathesis

Dynamic covalent chemistry has become an invaluable tool for the design and preparation of adaptable yet robust molecular systems. Herein we explore the scope of a largely overlooked dynamic covalent reaction – amidinium exchange – and report on conditions that allow formal amidinium metathesis reactions.

In this article, we explore the scope of a largely overlooked dynamic covalent reaction – amidinium exchange – and report on conditions that allow formal amidinium metathesis reactions.

Thermodynamic, kinetic, and mechanistic studies of the thermal guanidine metathesis reaction

We describe studies of the thermal guanidine metathesis (TGM) reaction, a reversible transformation that results in exchange of N-substituents of the guanidine functional group. By comparing the effects of discrete structural variations, we find that steric congestion is an important factor in determining both the equilibrium guanidine composition and the reaction kinetics. The alkyl versus aryl nature of N-substitution also plays an essential role in the reaction rate, up to the point that minimal TGM reactivity is observed when the guanidine contains wholly alkyl substituents. Furthermore, we demonstrate that TGM occurs under thermodynamic control and present evidence that it proceeds by a dissociative mechanism, supported by direct observation of a carbodiimide intermediate.

Reversible hetero-Diels–Alder amine hardener as drop-in replacement for healable epoxy coatings

Replacing commercial hardeners with bio-sourced fatty acids linked by hetero Diels–Alder (HDA) motifs enabled epoxy-amine coatings with intrinsic self-healing properties. The HDA-based coatings demonstrate scratch healing at 95 °C within 15 min.

Dynamic Enamine-one Bond Based Vitrimer via Amino-yne Click Reaction

Here, we report the fabrication of a dynamic enamine-one bond based vitrimer through amino-yne click chemistry. In contrast to amine-acetoacetate condensation, the amino-yne click reaction yields a dynamic enamine-one motif that is composed of cis/trans (3:1) isomers and has a relatively lower activation energy (35 ± 3 kJ/mol vs 59 ± 6 kJ/mol), owing to the absence of a methyl substituent. The resulting vitrimer network has superior mechanical properties and faster dynamic exchange than that of a reference vitrimer derived from amine-acetoacetate condensation, and they are attributed to the fewer network defects and the less sterically hindered exchange reaction, respectively. Lastly, the efficient amino-yne click reaction is demonstrated to be compatible with the secondary-amine substrate, which has a low reactivity toward the amine-acetoacetate condensation. The efficient and side product-free amino-yne reaction offers a powerful chemical tool for vitrimer fabrication and is potentially desirable for sealing and adhesion applications.

N-Heterocyclic carbene-carbodiimide (NHC-CDI) betaine adducts: synthesis, characterization, properties, and applications

N-Heterocyclic carbenes (NHCs) are an important class of reactive organic molecules used as ligands, organocatalysts, and σ-donors in a variety of electroneutral ylide or betaine adducts with main-group compounds. An emerging class of betaine adducts made from the reaction of NHCs with carbodiimides (CDIs) form zwitterionic amidinate-like structures with tunable properties based on the highly modular NHC and CDI scaffolds. The adduct stability is controlled by the substituents on the CDI nitrogens, while the NHC substituents greatly affect the configuration of the adduct in the solid state. This Perspective is intended as a primer to these adducts, touching on their history, synthesis, characterization, and general properties. Despite the infancy of the field, NHC-CDI adducts have been applied as amidinate-type ligands for transition metals and nanoparticles, as junctions in zwitterionic polymers, and to stabilize distonic radical cations. These applications and potential future directions are discussed.

Chemical recycling of poly(thiourethane) thermosets enabled by dynamic thiourethane bonds

Herein, we introduce a detailed investigation of the dynamic nature of the thiourethane bond and subsequently apply this dynamic character to formation, recycling and even additive manufacturing using cross-linked networks.