Aliphatic Isocyanates Research Articles

Effects of Isocyanate Structure on the Properties of Polyurethane: Synthesis, Performance, and Self-Healing Characteristics.

Polyurethane (PU) plays a critical role in elastomers, adhesives, and self-healing materials. We selected the most commonly used aromatic isocyanates, 4,4'-methylene diphenyl diisocyanate (MDI) and tolylene-2,4-diisocyanate (TDI), and the most commonly used aliphatic isocyanates, hexamethylene diisocyanate (HDI), isophorone diisocyanate (IPDI), and dicyclohexylmethane-4,4'-diisocyanate (HMDI), as raw materials, combined with polytetramethylene ether glycol (PTMG) and 1,4-butanediol (BDO) to successfully synthesize five PU materials. The effects of isocyanate structure on polymerization rate, hydrogen bonding, thermal properties, phase separation, wettability, self-healing performance, adhesion, and mechanical properties were systematically investigated. The results show that isocyanates with higher symmetry facilitate hydrogen bonding, but excessive flexibility and crystallinity may inhibit its formation. MDI-based PU exhibits the highest hydrogen bonding index (HBI) of 4.10, along with the most distinct phase separation and the highest tensile strength of 23.4 MPa. HMDI-based PU demonstrates the best adhesion properties, with the highest lap shear strength of 7.9 MPa, and also exhibits excellent scratch healing ability. IPDI-based PU shows good self-healing performance, recovering 88.7% of its original tensile strength and 90.6% of its original lap shear strength after heating at 80 °C for 24 h. Furthermore, all the samples can be reprocessed by melt or solution methods, showing excellent recyclability.

Comparative Analysis of Aliphatic and Aromatic Isocyanates on Soy-Based Polyurethane Films Modified with Schiff Base Diol

Comparative Analysis of Aliphatic and Aromatic Isocyanates on Soy-Based Polyurethane Films Modified with Schiff Base Diol

Improving Water Resistance and Mechanical Properties of Crosslinked Waterborne Polyurethane Using Glycidyl Carbamate

Waterborne polyurethane (WPU) often suffers from poor water resistance and mechanical properties due to hydrophilic emulsifiers. To address these issues, this study introduces glycidyl carbamate (GC) as a crosslinker to improve WPU performance. Three types of GC were synthesized using aliphatic, cycloaliphatic, and aromatic isocyanates, respectively. The crosslinked network was established through a reaction between the epoxide group of GC and the carboxylic acid and amine groups of WPU. Among these, the WPU film utilizing aromatic isocyanate-based GC exhibited the highest crosslink density, modulus, hardness, and water resistance, due to the rigidity of the aromatic molecular structure. However, the film displayed excessive brittleness, resulting in reduced tensile strength, along with yellowing typically associated with aromatic compounds. The WPU crosslinked with cycloaliphatic GC demonstrated the next best mechanical properties and water resistance, with a 2.7-fold increase in tensile strength, a 1.5-fold increase in hardness, and a 66% reduction in the water swelling ratio compared to neat WPU. This study presents a novel and effective strategy to enhance the water resistance and mechanical properties of WPU films, making them suitable for advanced coating applications.

Advancing Food Packaging: Exploring Cyto-Toxicity of Shape Memory Polyurethanes

Cytotoxicity is a critical parameter for materials intended for biological applications, such as food packaging. Shape-memory polyurethanes (SMPUs) have garnered significant interest due to their versatile properties and adaptability in synthesis. However, their suitability for biological applications is limited by the use of aromatic isocyanates, such as methylene diphenyl 4,4′-diisocyanate (MDI) and toluene diisocyanate (TDI), which are commonly used in SMPU synthesis but can generate carcinogenic compounds upon degradation. In this study, thermo-responsive shape-memory polyurethanes (SMPUs) were synthesized using poly(tetramethylene ether) glycol (PTMG) and castor oil (CO) as a chain extender with four different isocyanates—aromatic (MDI and TDI), aliphatic (hexamethylene diisocyanate [HDI] and isophorone diisocyanate [IPDI])—to evaluate their impact on polyurethane cytotoxicity. Cytotoxicity assays were conducted on the synthesized SMPU samples before and after exposure to light-induced degradation. The results showed that prior to degradation, all samples exhibited cell proliferation rates above 90%. However, after degradation, the SMPUs containing aromatic isocyanates demonstrated a drastic reduction in cell proliferation to values below 10%, whereas the samples with aliphatic isocyanates maintained cell proliferation above 70%. Subsequently, the influence of polyol chain length was assessed using PTMG, with molecular weights of 1000, 650, and 250 g·mol−1. The results indicated that the SMPUs with longer chain lengths exhibited higher cell proliferation rates both before and after degradation. The thermal and mechanical properties of the SMPUs were further characterized using thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), and thermomechanical analysis (TMA), providing comprehensive insights into the behavior of these materials.

Low-energy micron dispersion of entangled polymers based on phase inversion composition of emulsion

This article aims to propose a low-energy method for micron dispersion of entangled polymers. We chose Ethylene-Propylene-Diene Monomer (EPDM) as the entangled polymer and acrylic esters as the dispersants to prepare the organic dispersion of the entangled polymer based on the theory that in-situ polymerization of acrylic esters had the higher solubility parameter than the original monomer. The dispersion process of entangled polymer was similar to the phase inversion of emulsion due to the maximum viscosity and minimum interfacial tension at the point of dispersion. Specifically, the average diameter of the dispersed entangled polymer was 5–6 μm. The system could still maintain excellent dispersion stability without additional dispersants. Rheology characterized that the introduction of aliphatic isocyanate derivatives reacting with hydroxyl groups in the polyacrylate to form the cross-linked skeleton could enhance the stability of the system. Excitingly, this organic dispersion allowed flexible printed circuit to fold at any angle and direction when used as the coating, and the coating still maintained excellent dispersibility, flexibility, insulation, and adhesion after aging tests. This low-energy method for micron dispersion of entangled polymers could expand their applications in coatings and other fields while maintaining their own characteristics.

Role of the Soft and Hard Segments Structure in Modifying the Performance of Acrylic Adhesives Modified by Polyurethane Macromonomers.

Acrylic pressure sensitive adhesives, modified by polyurethane (PU), achieve selective optimization through the designability of polyurethanes. In this paper, PU macromonomers were prepared by a two-step synthesis method, using polypropylene glycol or polyethylene glycol with different molecular weights as soft segments and different types of diisocyanates: isophorone diisocyanate, hexamethylene diisocyanate, dicyclohexylmethylmethane diisocyanate (HMDI), 2,4-toluene diisocyanate (TDI), and 4,4'-diphenylmethane diisocyanate (MDI) and chain extenders as hard segments. After being terminated by capping agents, a series of PU macromonomers of different molecular weights and structures were obtained and used to modify the acrylic base adhesives. Compared to unmodified adhesives, acrylic adhesives modified by PU macromonomers have improved adhesion performances and heat resistance and show an increasing trend with the increase of molecular weight of diols. Diols with a molecular weight of 600 have the best effect. Diisocyanates containing benzene rings can better improve the thermal performance of adhesives, where P MDI containing a biphenyl ring is the best, while aliphatic isocyanate groups have a greater improvement in adhesion performance, and the adhesion performance of P HDI with a long carbon chain is the best.

From theory to application: Innovative ice-phobic polyurethane coatings by studying material parameters, mechanical insights, and additives

This investigation introduces new insights into the influence of material parameters on the anti-icing properties of polyurethane (PU) coatings, encompassing chemical characteristics, cross-link densities, mechanical analysis, and additive diversity. To comprehensively address these aspects, a meticulous exploration was conducted to delve into the effects of these parameters on wettability and icephobicity. To fabricate the PU coatings, three distinct acrylic polyols with various hydroxyl content along with two widely employed aliphatic isocyanates boasting diverse %NCO values were utilized. By employing stoichiometric and non-stoichiometric ratios, an array of coatings with different crosslink densities and mechanical properties was fabricated. The principal influence of crosslink density alterations in PU coatings on ice nucleation temperature and ice adhesion strength emerged distinctly, underpinned by the establishment of hydrogen bonds between water molecules and polar functional groups within the coating, coupled with the appearance of a quasi-liquid layer (QLL). The mechanical properties, wettability characteristics, and surface roughness of the coatings were examined and it was clearly demonstrated that Young's modulus and physicochemical properties play significant roles in the characteristics of ice adhesion strength. In particular, lower Young's modulus in the samples was associated with reduced ice adhesion, albeit with compromised durability. In the next step of this investigation, the matrix demonstrating optimal mechanical properties and icephobicity was subjected to surface energy adjustment. This was accomplished by introducing various concentrations of both fluorinated and non-fluorinated additives. This adjustment led to a significant reduction of ice adhesion strength, measuring less than 100 Kpa as an icephobic PU coating. Nevertheless, it was observed that the effectiveness of the fluorinated additives diminished after repetitive icing-deicing cycles, while the non-fluorinated additive maintained its influence on ice adhesion strength even after numerous cycles. The interplay between material attributes, mechanical properties, and additive influences has been meticulously unraveled, charting a course for further advancements in the pursuit of robust icephobic coating.

Exploring structure/property relationships to health and environmental hazards of polymeric polyisocyanate prepolymer substances-3. Aquatic exposure and hazard of aliphatic diisocyanate-based prepolymers.

The water extractability and acute aquatic toxicity of seven aliphatic diisocyanate-based prepolymer substances were investigated to determine if lesser reactivity of the aliphatic isocyanate groups, as well as increased ionization potential of the expected (aliphatic amine-terminated) polymeric hydrolysis products, would influence their aquatic behavior compared to that of previously investigated aromatic diisocyanate-based prepolymers. At loading rates of 100 and 1,000mg/L, only the substances having log Kow ≤9 exhibited more than 1% extractability in water, and a maximum of 66% water extractability was determined for a prepolymer having log Kow = 2.2. For the more hydrophobic prepolymer substances (log Kow values from 18-37), water extractability was negligible. High-resolution mass spectrometric analyses were performed on the water-accommodated fractions (WAF) of the prepolymers, which indicated the occurrence of primary aliphatic amine-terminated polymer species having backbones and functional group equivalent weights aligned to those of the parent prepolymers. Measurements of reduced surface tension and presence of suspended micelles in the WAFs further supported the occurrence of these surface-active cationic polymer species as hydrolysis products of the prepolymers. Despite these characteristics, the water-extractable hydrolysis products were practically non-toxic to Daphnia magna. All of the substances tested exhibited 48-h EL50 values of >1,000mg/L, with one exception of EL50 = 157mg/L. The results from this investigation support a grouping of the aliphatic diisocyanate-based prepolymers as a class of water-reactive polymer substances having predictable aquatic exposure and a uniformly low hazard potential, consistent with that previously demonstrated for the aromatic diisocyanate-based prepolymers.

Microcapsules of Poly(butylene adipate-co-terephthalate) (PBAT) Loaded with Aliphatic Isocyanates for Adhesive Applications.

This work introduces the encapsulation of hexamethylene diisocyanate derivatives (HDI, TriHDI, and PHDI) with the biodegradable polymer poly(butylene adipate-co-terephthalate) (PBAT) through a solvent evaporation method. These microcapsules (MCs) were then employed in adhesive formulations for footwear. Moreover, MCs containing PHDI were produced in a closed vessel, demonstrating the potential for recovering and reusing organic solvents for the first time. The MCs were achieved with an isocyanate payload reaching up to 68 wt %, displaying a spherical shape, a core-shell structure, and thin walls without holes or cracks. The application of MCs as cross-linking agents for adhesives was evaluated following industry standards. The adhesives' strength surpassed the minimum requirement by a significant margin. Creep tests demonstrated that the formulation with MCs exhibits superior thermostability. Furthermore, the formulation with MCs-PHDI presented the best results reported to date for this type of system, as no displacement was observed in the bonded substrates. Environmental assessment indicates that adhesives with MCs have higher global warming potential (+16.2%) and energy consumption (+10.8%) than the standard commercial adhesives, but under alternative realistic scenarios, the differences can be insignificant. Therefore, adhesive formulations incorporating MCs promise to be on par with traditional adhesive systems regarding environmental impacts while providing benefits such as improved and safe handling of isocyanates and excellent bonding effectiveness.

Towards lignin-based polyurethanes: a computational study of the influence of the reactants structure on the formation of urethanes in the presence of tin(II) dicarboxylate as a catalyst

An in-depth computational investigation of the tin-catalysed reaction of an alcohol with an isocyanate forming a urethane has been performed to elucidate the influence of the reactants, most importantly the type of alcohol or phenol, on its kinetics. The model alcohols were chosen based on those alcohol moieties present in lignin and comprised aliphatic, aromatic and benzylic alcohols, and reactions with both aliphatic and aromatic isocyanates were studied by modelling the rate-determining step of the urethane formation based on the mechanism proposed by Devendra et al. The results show that N-coordination is both kinetically and thermodynamically favoured for all reactions under consideration. Aliphatic alcohols react more readily than phenols with an aliphatic isocyanate, while benzylic alcohols react similarly to a bulky aliphatic alcohol. In the case of phenols, the kinetics are not influenced by the presence and/or number of ortho‑methoxy groups, but these will slightly alter the thermodynamics of the reaction. Aliphatic alcohols show slightly improved kinetics when reacting with an aromatic isocyanate, whereas phenols show no such preference. The reactions with the aliphatic isocyanate were also modelled in DMSO, but no significant changes in reaction kinetics were observed.

Photo‐induced catalytic poly‐trimerization of polyisocyanurates

AbstractRadiation curing is an excellent option for the economical implementation of coatings while simultaneously producing technically advanced products. Nevertheless, the number of materials used is mainly limited to pure (meth)acrylate and epoxy systems. Polyisocyanurates could be an excellent addition to this range, especially due to their thermal and chemical stability. In this work, through the use of a transition metal‐based photocatalyst, as well as the use of 1,5‐diazabicyclo(4.3.0)non‐5‐en as a co‐catalyst, formulations are presented, which enable pure polyisocyanurates to be used in radiation‐based coating technology. Trimerization with this system using any primary isocyanates worked and consequently thus opened the possibility for aromatic and aliphatic polyisocyanurates. Using monofunctional isocyanates, the theoretical background of trimerization as well as the theoretical enthalpy is evaluated and the possibilities as well as the limitations for poly‐trimerization of polyisocyanurates are analyzed. Finally, photo‐induced catalytic poly‐trimerization of aliphatic isocyanates is used to obtain samples with a Tg in the range of 100 °C and onset of degradation above 400 °C. Thus, for the first time, a framework for the use of isocyanates in the field of photopolymerization and, in particular, in the field of radiation‐based coating of polyisocyanurates is provided for further development.

Novel (±)-trans-β-lactam ureas: Synthesis, in silico and in vitro biological profiling.

A diastereomeric mixture of racemic 3-phthalimido-b-lactam 2a/2b was synthesized by the Staudinger reaction of carboxylic acid activated with 2-chloro-1-methylpyridinium iodide and imine 1. The amino group at the C3 position of the b-lactam ring was used for further structural upgrade. trans-b-lactam ureas 4a-t were prepared by the condensation reaction of the amino group of b-lactam ring with various aromatic and aliphatic isocyanates. Antimicrobial activity of compounds 4a-t was evaluated in vitro and neither antibacterial nor antifungal activity were observed. Several of the newly synthesized trans-b-lactam ureas 4a-c, 4f, 4h, 4n, 4o, 4p, and 4s were evaluated for in vitro antiproliferative activity against liver hepatocellular carcinoma (HepG2), ovarian carcinoma (A2780), breast adenocarcinoma (MCF7) and untransformed human fibroblasts (HFF1). The b-lactam urea 4o showed the most potent antiproliferative activity against the ovarian carcinoma (A2780) cell line. Compounds 4o and 4p exhibited strong cytotoxic effects against human non-tumor cell line HFF1. The b-lactam ureas 4a-t were estimated to be soluble and membrane permeable, moderately lipophilic molecules (logP 4.6) with a predisposition to be CYP3A4 and P-glycoprotein substrates. The tools PASS and SwissTargetPrediction could not predict biological targets for compounds 4a-t with high probability, pointing to the novelty of their structure. Considering low toxicity risk, molecules 4a and 4f can be selected as the most promising candidates for further structure modifications.

Thermoplastic polyurethane/POSS nanohybrids: Synthesis, morphology, and biological properties.

Recent studies show good osteoinductive properties of polyurethanes modified with polyhedral oligomeric silsesquioxanes (POSS). In this work, three types of POSS; propanediolisobutyl-POSS (PHI-POSS), disilanolisobutyl-POSS (DSI-POSS), and octahydroxybutyl-POSS (OCTA-POSS) were chemically incorporated into linear polyurethane based on an aliphatic isocyanate, hexamethylene diisocyanate (HDI), to obtain new nanohybrid PU-POSS materials. The full conversion of POSS was confirmed by Fourier transform infrared spectroscopy (FTIR-ATR) spectra of the model reactions with pure HDI. The materials obtained were investigated by FTIR, SEM-EDS, and DSC. The DSC studies showed the thermoplasticity of the obtained materials and apparently good recovery. 30-day immersion in SBF (simulated body fluid) revealed an increase in the rate of deposition of hydroxyapatite (HAp) for the highest POSS loadings, resulting in thick layers of hydroxyapatite (~60-40 μm), and the Ca/P ratio 1.67 (even 1.785). The structure and properties of the inorganic layer depend on the type of POSS, the number of hard segments, and those containing POSS, which can be tailored by changing the HDI/poly(tetramethylene glycol) (PTMG) ratio. Furthermore, the obtained composites revealed good biocompatibility, as confirmed by cytotoxicity tests conducted on two cell lines; normal human dermal fibroblasts (NHDF) and primary human osteoblasts (HOB). Adherent cells seeded on the tested materials showed viability even after a 48-h incubation. After this time, the population of viable, and proliferating cells exceeded 90%. Bioimaging studies have shown the fibroblast and osteoblast cells were well attached to the surface of the tested materials.

Simple and Efficient Synthesis of Diamino Derivatives of bis-1,2,4-oxadiazole via Tandem Staudinger/aza-Wittig Reaction.

Two convenient and efficient routes for synthesizing diamino derivatives of bis-1,2,4- oxadiazoles were described. This study provides a simple protocol for the synthesis of bis-1,2,4-oxadiazoles. The two procedures were based on a tandem Staudinger/aza-Wittig reaction from the same starting material of diaziglyoxime, isocyanates and triphenylphosphonium. In synthesis method I, diaziglyoxime 1 was treated with various aromatic or aliphatic isocyanates to give diazioxalimides 2 a high yield. Diazioxalimides 2 reacted with Ph3P to produce the iminophosphoranes 4; the reaction was directly heated from room temperature to 115 ℃ to get the desired diamino derivatives of bis-1,2,4-oxadiazole 4 in 72-92% yields. In synthesis method II, the same target compounds 4 were synthesized in a one-pot reaction by Ph3P and aromatic or aliphatic isocyanates in toluene for 10 h under 115 ℃ in 53-71% yields. The two procedures provide proficient methods of making nitrogen-containing heterocyclic rings. The structures of target compounds 4 were identified by IR, 1HNMR, 13CNMR and HRMS.

Solvent-Free Preparation of Thermally Stable Poly(urethane-imide) Elastomers

Polyurethanes (PUs) are widely used in many applications due to their versatile chemical and physical properties. To meet the increasing demands on PU with respect to intrinsic mechanical and thermal properties, they can be modified with thermally stable aromatic imide structures to form poly(urethane imide) (PUI). However, the preparation of PUI materials has been limited by the need for strong polar solvents, which hinder their industrial scale development. In this work, we prepared imide-containing isocyanate-terminated prepolymers via the reaction of polymeric aromatic or aliphatic isocyanates and pyromellitic dianhydride in completely solvent-free conditions. The subsequent PUI elastomers made from these prepolymers exhibited enhanced char formation and stiffness in comparison to the reference elastomers without imide structures, among which the aromatic PUI elastomer shows improved flame retardancy according to the initial cone calorimetry measurement. This work demonstrates the potential use of imide prepolymers in other PU applications where high thermal stability and flame retardancy are required. The solvent-free synthesis also paves a way for the large-scale production of PUI materials.

3-Cyclohexyl-6-phenyl-1-(p-tolyl)pyrimidine-2,4(1H,3H)-dione

The synthesis of a novel uracil derivative, 3-cyclohexyl-6-phenyl-1-(p-tolyl)pyrimidine-2,4(1H,3H)-dione (4), is reported via a four-component reaction involving an α-chloroketone (1), an aliphatic isocyanate (2), a primary aromatic amine (3) and carbon monoxide. The proposed reaction mechanism involves a Pd-catalyzed carbonylation of 2-chloro-1-phenylethan-1-one (1), leading to a β-ketoacylpalladium key intermediate, and, at the same time, in situ formation of non-symmetrical urea deriving from cyclohexyl isocyanate (2) and p-toluidine (3). After a chemo-selective acylation of the non-symmetrical urea and the subsequent cyclization of the acylated intermediate, 3-cyclohexyl-6-phenyl-1-(p-tolyl)pyrimidine-2,4(1H,3H)-dione (4) is formed. Uracil derivative 4 was isolated in good yield (73%) and fully characterized by 1H, 13C, 2D 1H-13C HSQC and 2D 1H-13C HMBC NMR, FT-IR spectroscopy and GC-MS spectrometry.

THE AVAILABLE CURING AGENTS FOR POLY(3-DIFLUOROAMINOMETHYL-3-METHYLOXETANE/3,3-BIS-AZIDOMTHYLOXETANE) (PDB) OBTAINED THROUGH THE CURING REACTION STUDY OF PDB AND ISOCYANATE

The difluoroamino polymer poly(3-difluoroaminomethy l-3-methyloxetane/3,3-bis-azidomethylox-etane) (PDB) can be an excellent energetic binder in the composite solid propellant, while it has no practical application in curing yet. This study tried isophorone diisocyanate (IPDI), N-100, toluene diisocyanate (TDI), diphenyl methane diisocyanate (MDI), and naphthalene diisocyanate (NDI) to react with PDB, respectively. The ideal curing agent for PDB can form the carbamate structure (-NH-COO-), and the difluoroamino group (-NF<sub>2</sub>) was not destroyed in reactions. Fourier transform infrared, <sup>13</sup>C NMR, and <sup>1</sup>H nuclear magnetic resonance (NMR) was used to analyze the reactions between PDB and isocyanates. The curing reaction kinetics of PDB/TDI, the mechanical performance of PDB/TDI film, and the chemical changes in PDB/TDI reaction were studied separately. The results showed no carbamate structure formed during the reactions with aliphatic isocyanates (IPDI, N-100). In PDB/N-100 reaction, the -NF<sub>2</sub> was destroyed. Aromatic isocyanates (TDI, MDI, and NDI) have higher reactivity, formed carbamate structures in the curing reactions, and no -NF<sub>2</sub> was destroyed. Therefore, TDI, MDI, and NDI can be used as curing agents for PDB. The E<sub>α</sub> of PDB/TDI curing reaction was 42.4189 kJ/mol<sup>-1</sup>, and PDB/TDI films exhibit outstanding mechanical properties.

A robust mixed-charge zwitterionic polyurethane coating integrated with antibacterial and anticoagulant functions for interventional blood-contacting devices.

Antifouling coatings based on zwitterionic polymers have been widely applied for surface modification of interventional blood-contacting devices to combat thrombosis and infection. However, the weak adhesion stability of the zwitterionic coating to the device surface is still the key challenge. In this work, biocompatible mixed-charge zwitterionic polyurethane (MPU) polymers, that bear equal amounts of cationic quaternary amine groups and anionic carboxyl groups, were developed and further uniformly dip-coated onto a thermoplastic polyurethane (TPU) substrate with a commercial aliphatic isocyanate cross-linker (AIC). During the curing process, AIC not only crosslinks MPU chains into a polymer network but also reacts with hydroxyl groups of TPU to interlink the polymer network to the substrate, resulting in a cross-linking reinforced MPU coating (CMPU) with excellent mechanical robustness and adhesion strength. Taking advantage of the mixed-charge feature, the final zwitterionic CMPU coating exhibits both excellent antifouling and antibacterial activities against protein adsorption and bacterial growth, respectively, which is beneficial for effectively inhibiting the occurrence of in vivo infection. Moreover, anticoagulation studies show that CMPU-coated TPU catheters can also prevent the formation of blood clots in ex vivo rabbit blood circuits without anticoagulants. Hence, the designed CMPU coating has immense potential to address thrombosis and infection for interventional blood-contacting devices.

Reaction of OH with Aliphatic and Aromatic Isocyanates

Isocyanates are highly relevant industrial intermediates for polyurethane production. In this work, we used quantum chemistry and transition state theory (TST) to investigate the gas-phase reaction of isocyanates with the OH radical, which is likely one of the most significant chemical sinks for these compounds in the troposphere. para-Tolyl-isocyanate (p-tolyl-NCO) was chosen as a proxy substance for the large-volume aromatic diisocyanate species toluene diisocyanate and methylene diphenyl diisocyanate. Besides p-tolyl-NCO + OH, the model reactions CH3NCO + OH, H2C═CHNCO + OH, C6H5-NCO + OH, C6H5-CH3 + OH, and C6H6 + OH have been studied as well to analyze various substituent effects and to allow for comparison with literature. Quantum chemical computations at the CCSD(T)/cc-pV(T,Q → ∞)Z//M06-2X/def2-TZVP level were used as the basis for tunneling-corrected canonical TST calculations. For CH3NCO + OH, H abstraction by OH at the methyl group is the main reaction channel according to our calculations and predicted to be four orders of magnitude faster than OH addition at the NCO group. The calculated rate coefficient (8.8 × 10-14 cm3 molecule-1 s-1) at 298 K is in good agreement with experimental data from the literature. Likewise, for aromatic isocyanates, OH attack at the isocyanate group was found to be only a minor pathway compared to addition to the aromatic ring. In the OH + p-tolyl-NCO reaction, OH addition at the ortho-position relative to the NCO group has been identified as the main initial reaction channel (branching fraction: 53.2%), with smaller but significant branching fractions for the H abstraction at the methyl group (9.6%) as well as the other ring addition reactions (ipso: 2.3%, meta: 24.5%, para: 10.5%, all relative to the NCO group). By comparing OH addition to the aromatic ring in p-tolyl-NCO with the respective ring addition reactions of phenyl isocyanate and toluene, the site-selective reactivity trends observed for ring addition in the OH + p-tolyl-NCO could be rationalized by a dominating positive mesomeric effect of the NCO group and a positive electron-donating (inductive) effect of the CH3 group. Except for the OH ring adduct formed from OH addition in ipso-position to the NCO group, we estimate the first-generation radical intermediates in the OH + p-tolyl-NCO reaction to have sufficiently long lifetimes to react with O2 under atmospheric conditions and undergo typical oxidative reaction cascades like those known for benzene or toluene.

Chemoselective Hydroboration of Isocyanates Catalyzed by Commercially Available NaH

AbstractA simple, efficient, and economical method for the chemoselective hydroboration of isocyanates is reported. Commercially available NaH, at very low loadings, efficiently catalyzes the selective conversion of isocyanates to N‐boryl formamides, bis(boryl)hemiaminals, and N‐boryl methyl amines. NaH catalyzes isocyanates to controllably open the N=C bond to build an amide bond, and can also remove the C=O bond to obtain N‐boryl methyl amines. Both aliphatic and aromatic isocyanates can be quantitatively converted to the corresponding hydroboration products. Furthermore, there is excellent functional group selectivity over imines, nitriles, and olefins. Additionally, through in situ monitoring, a possible reaction mechanism is proposed. And the chemical intermediates generated by NaH and HBpin are responsible for all the reduction steps.