Reaction of OH with Aliphatic and Aromatic Isocyanates

Abstract

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.

The role and mechanism of temporary blocked isocyanate at interface between polyethylene terephthalate high‐strength fibers and activation oil

“…In Figure 5a, two peaks at 3408 and 3331 cm −1 in the temporary blocked isocyanate spectrum are ascribed to NH 2 23 . The characteristic peak at 1714 cm −1 is contributed by CO 24,25 . However, the infrared characteristic peak of NCO should appear at 2250–2270 cm −1 but could not be observed, indicating that the isocyanate group was blocked 19,26 .…”

“…23 The characteristic peak at 1714 cm À1 is contributed by C O. 24,25 However, the infrared characteristic peak of NCO should appear at 2250-2270 cm À1 but could not be observed, indicating that the isocyanate group was blocked. 19,26 In Figure 5b, three absorption peaks at 1631, 1600, and 1470 cm À1 can be observed in spectrum of activation oil, which are ascribed by stretching vibration of C C in benzene ring skeleton, 27 showing that the activation oil contains benzene ring.…”

Construction of Poly(butylene succinate)-Grafted Graphene Bionanocomposites: Intercalation Structure, Synergistic Thermal–Oxidative Stabilization Effect, and Hydrolytic Behavior

“…Specifically, as shown in Figure h, compared with HDI (aliphatic diisocyanate), −NCO groups of MDI/TDI with an electron-withdrawing aromatic ring are easier to react with nucleophilic reagents. Meanwhile, compared to TDI, two −NCO groups are far apart without substituent groups and steric hindrance around them for MDI, leading to higher activity. − …”

Mechanism of Effect of Non-Reactive Polyhedral Oligomeric Silsesquioxane Nanostructures on HTPB-Based Polyurethane Network Formation: A Rheokinetic Analysis

“…However, it has been observed that the reactivity of different curing agents varies significantly. The reactivity of curing agents can be classified as aromatic > aliphatic [23]. This difference in reactivity can be attributed to the electronic and steric effects of different types of isocyanates.…”