Microporous Polymer Networks Made by Cyclotrimerization of Commercial, Aromatic Diisocyanates

Fabrication of High-Performance Polyisocyanurate Aerogels through Cocyclotrimerization of 4,4'-Methylene Diphenyl Diisocyanate and Its Mono-Urethane Derivatives

Aromatic polyisocyanurate (PIR) aerogels are recognized as advanced porous materials and extensively studied due to their lightweight nature, high porosity, and specific surface area, which attribute to their outstanding thermal insulation properties. The inherent thermal stability of the PIR moieties, combined with great insulating performance, renders PIR aerogels highly suitable for building insulation applications. Nevertheless, materials containing isocyanurate obtained through direct trimerization of aromatic isocyanates exhibit brittleness, resulting in inferior mechanical performance. In order to enhance the processability of the PIR aerogels, we propose a cocyclotrimerization approach involving mixtures of mono- and difunctional aromatic isocyanates. This approach is designed to develop a PIR network with decreased cross-linking density and brittleness. Herein, we developed an array of PIR aerogels from different alkyl chain-modified isocyanate mixtures. The resulting PIR aerogels exhibited high porosity (>89%), a large surface area (∼300 m2/g), superinsulating performance with ultralow thermal conductivity (∼16.8 mW m-1 K-1), notable thermal stability (Td5% ∼ 250 °C), improved mechanical performance, and intrinsic hydrophobicity without the need for postmodification. These high-performance organic aerogels hold significant promise for applications requiring superinsulating materials.

Current State and Perspectives of Simulation and Modeling of Aliphatic Isocyanates and Polyisocyanates

Aliphatic isocyanates and polyisocyanates are central molecules in the fabrication of polyurethanes, coatings, and adhesives and, due to their excellent mechanical and stability properties, are continuously investigated in advanced applications; however, despite the growing interest in isocyanate-based systems, atomistic simulations on them have been limited by the lack of accurate parametrizations for these molecular species. In this review, we will first provide an overview of current research on isocyanate systems to highlight their most promising applications, especially in fields far from their typical usage, and to justify the need for further modeling works. Next, we will discuss the state of their modeling, from first-principle studies to atomistic molecular dynamics simulations and coarse-grained approaches, highlighting the recent advances in atomistic modeling. Finally, the most promising lines of research in the modeling of isocyanates are discussed in light of the possibilities opened by novel approaches, such as machine learning.

Switching from Single to Simultaneous Free-Radical and Anionic Polymerization with Enamine-Based Organic Electron Donors

Although most monomers can polymerize through different propagation pathways, polymerization-initiating systems that can switch from one mode to another are rare. In this study, we demonstrate that enamine-based organic electron donors (OEDs) constitute the first systems able to initiate either free-radical or anionic polymerization under simple, mild, and safe conditions. While direct electron-transfer reduction of monomers by OEDs results in the initiation of anionic chain-growth polymerization, introduction of a competing oxidant with a higher reduction potential than the monomer switches the former anionic propagation to a clean radical-propagation process. The benefit of this dual-mode activator is highlighted in the synthesis of an interpenetrating polymer network through simultaneous initiation of radical and anionic propagation processes.

Molten Salt Templated Synthesis of Covalent Isocyanurate Frameworks with Tunable Morphology and High CO2 Uptake Capacity

The use of reactive molten salts, i.e., ZnCl₂, as a soft template and a catalyst has been actively investigated in the preparation of covalent triazine frameworks (CTFs). Although the soft templating effect of the salt melt is more prominent at low temperatures, close to the melting point of ZnCl₂, leading to the formation of abundant micropores, a significant mesopore formation is observed that is due to the partial carbonization and other side reactions at higher temperatures (>400 °C). Evidently, high-temperature synthesis of CTFs in various eutectic salt mixtures of ZnCl₂ with alkali metal chloride salts also leads to mesopore formation. We reasoned that using the isocyanate moieties instead of cyano groups in the monomer, 1,4-phenylene isocyanate, could enable efficient interactions between carbonyl moieties and alkali metal ions to realize efficient salt templating to form covalent isocyanurate frameworks (CICFs). In this direction, the trimerization of 1,4-phenylene diisocyanate was carried out under ionothermal conditions at different reaction temperatures using ZnCl₂ (CICF) and the eutectic salt mixture of KCl/NaCl/ZnCl₂ (CICF-KCl/NaCl) as the reactive solvents. We observed notable differences in the morphologies of the two polymers, whereas CICF showed irregular-shaped micrometer-sized particles, the CICF-KCl/NaCl exhibited a filmlike morphology. Moreover, favorable ion-dipole interactions between alkali metal cations and oxygen atoms of the monomer facilitated two-dimensional growth and the formation of a purely microporous framework in the case of CICF-KCl/NaCl along with a near theoretical retention of the nitrogen content at 500 °C. The CICF-KCl/NaCl showed a BET surface area of 590 m² g^-1 along with a CO₂ uptake capacity of 5.9 mmol g^-1 at 273 K and 1.1 bar because of its high microporosity and nitrogen content. On the contrary, in the absence of alkali metal ions, CICF showed high mesopore content and a moderate CO₂ uptake capacity. This study underscores the importance of the strength of the interactions between the salts and the monomer in the ionothermal synthesis to control the morphology, porosity, and gas uptake properties of the porous organic polymers.

The mechanochemical Scholl reaction as a versatile synthesis tool for the solvent-free generation of microporous polymers

Herein we report the mechanochemical Scholl polymerization of 1,3,5-triphenylbenzene in a high speed ball mill. The reaction is conducted solvent-free, solely using solid FeCl3. The resulting porous polymer was obtained in >99% yield after very short reaction times of only 5 minutes and exhibits a high specific surface area of 658 m2 g-1, which could be further enhanced up to 990 m2 g-1 by liquid assisted grinding. Within this study we illuminate the origin of porosity by investigating the impact of various milling parameters and milling materials, temperature and pressure, and different liquids for LAG as well as post polymer milling. Finally we expand the procedure to different monomers and mills, to present the mechanochemical Scholl reaction as a versatile synthesis tool for porous polymers.

Thermal stabilities and conformational behaviors of isocyanurates and cyclotrimerization energies of isocyanates: a computational study

Isocyanurates are cyclic trimers of isocyanate molecules. They are generally known as highly thermostable compounds. However, it is interesting how the thermal stabilities of the isocyanurate molecules will be altered depending on the substituents of their three nitrogen atoms. We performed computational investigations on the thermochemical behaviors of isocyanurate molecules with various alkyl and phenyl substituents. The cyclotrimerization processes of isocyanates are highly exothermic. Our best estimate of the enthalpy change for the cyclotrimerization of methyl isocyanate into trimethyl isocyanurate was -66.4 kcal mol-1. Additional negative cyclotrimerization enthalpy changes were observed for n-alkyl-substituted isocyanates. This trend was enhanced with an extension of n-alkyl chains. Conversely, low negative cyclotrimerization enthalpy changes were shown for secondary and tertiary alkyl-substituted isocyanates. The n-alkyl-substituted isocyanurates were shown to be stabilized due to attractive dispersion interactions between the substituents. Meanwhile, the branched alkyl-substituted isocyanurates were destabilized due to the deformation of their isocyanurate rings. For various alkyl-substituted isocyanates, the sum of the deformation energy of the isocyanurate ring and the intramolecular inter-substituent nonbonding interaction energies was found to be linearly correlated with their cyclotrimerization energies. The cyclotrimerization energy for phenyl isocyanate was shown to have significantly deviated from the linear relationship observed for the alkyl-substituted isocyanurates. This is probably attributable to a remarkable change in the orbital resonance interactions during the cyclotrimerization of phenyl isocyanate to triphenyl isocyanurate.

Highly efficient cyclotrimerization of isocyanates using N-heterocyclic olefins under bulk conditions

With a catalyst loading as low as 0.005%, high to excellent yields of isocyanurates could be achieved from N-heterocyclic olefin mediated organocatalytic cyclotrimerization of a wide range of isocyanates under bulk conditions. Experimental details coupled with structural characterization of the key intermediates led to comprehensive mechanistic studies of cyclotrimerization.

Aluminium-catalysed isocyanate trimerization, enhanced by exploiting a dynamic coordination sphere

Main-group metals are inherently labile, hindering their use in catalysis. We exploit this lability in the synthesis of isocyanurates. For the first time we report a highly active catalyst that trimerizes alkyl, allyl and aryl isocyanates, and di-isocyanates, with low catalyst loadings under mild conditions, using a hemi-labile aluminium-pyridyl-bis(iminophenolate) complex.

Low-coordinate first-row transition metal complexes in catalysis and small molecule activation

In this Perspective, we will highlight selected examples of transition metal complexes with low coordination numbers whose high reactivity has been exploited in catalysis and the activation of small molecules featuring strong bonds (N₂, CO₂, and CO).

Yttrium dialkyl supported by a silaamidinate ligand: synthesis, structure and catalysis on cyclotrimerization of isocyanates

A four-coordinate yttrium dialkyl complex with a sterically demanding silaamidinate ligand exhibited high activity and excellent functional group tolerance for the catalysis of isocyanate cyclotrimerization.

Cyclotrimerisation of isocyanates catalysed by low-coordinate Mn(ii) and Fe(ii) m-terphenyl complexes

Isocyanurates can be synthesised through the cyclotrimerisation of primary isocyanates catalysed by low-coordinate manganese(ii) and iron(ii) m-terphenyl complexes under mild conditions.

Enhanced Carbon Dioxide Capture from Landfill Gas Using Bifunctionalized Benzimidazole-Linked Polymers

Tuning the binding affinity of small gases and their selective uptake by porous adsorbents are vital for effective CO2 removal from gas mixtures for environmental protection and fuel upgrading. In this study, an amine-functionalized benzimidazole-linked polymer (BILP-6-NH2) was synthesized by a combination of pre- and postsynthetic modification techniques in two steps. Presynthetic incorporation of nitro groups resulted in stoichiometric functionalization (1 nitro/phenyl) in addition to noninvasive functionalization, where more than 80% of the surface area maintained compared to BILP-6. Experimental studies presented enhanced CO2 uptake and CO2/CH4 selectivity in BILP-6-NH2 compared to BILP-6, which are governed by the synergetic effect of benzimidazole and amine moieties. DFT calculations were used to understand the interaction modes of CO2 with BILP-6-NH2 and confirmed the efficacy of amine groups. Encouraged by the enhanced uptake and selectivity in BILP-6-NH2, we have evaluated its performance in landfill gas separation under vacuum swing adsorption (VSA) settings, which resulted in very promising working capacity and sorbent selection parameters outperforming most of the best solid adsorbent in the literature.