Computational Study of Catalytic Urethane Formation

Polyurethanes (PUs) are widely used in different applications, and thus various synthetic procedures including one or more catalysts are applied to prepare them. For PU foams, the most important catalysts are nitrogen-containing compounds. Therefore, in this work, the catalytic effect of eight different nitrogen-containing catalysts on urethane formation will be examined. The reactions of phenyl isocyanate (PhNCO) and methanol without and in the presence of catalysts have been studied and discussed using the G3MP2BHandHLYP composite method. The solvent effects have also been considered by applying the SMD implicit solvent model. A general urethane formation mechanism has been proposed without and in the presence of the studied catalysts. The proton affinities (PA) were also examined. The barrier height of the reaction significantly decreased (∆E0 > 100 kJ/mol) in the presence of the studied catalysts, which proves the important effect they have on urethane formation. The achieved results can be applied in catalyst design and development in the near future.

SYNTHESIS AND PROPERTIES OF OLIGODIURETANEDIOLS BASED ON A MIXTURE OF (2,4–2,6) TOLUILENDIISOCYANATE

The paper describes the synthesis, the reaction of a mixture of isomers (2,4–2,6) of toluilendiisocyanate with a double molar excess of aliphatic individual or oligomeric diols, a number of previously unknown oligodiuretanediols and their physicochemical constants. It is shown that with an increase in the synthesis temperature from 50 to 70 °C, the reaction time to complete depletion in the mixture of free NCO-groups decreases from 8–9 hours to 3–4 hours. The reaction temperature of 70–2 °С should be considered optimal, because at higher temperatures side reactions of free NCO-groups with already formed urethane ones are possible. Because the presence of even a small amount of moisture in the diols can provoke side effects during the urethane formation reaction, all of the above diols were dried from the adsorbed moisture by azeotropic distillation with toluene before use in the reaction. Since the final products are even at the synthesis temperature (68–70 °C) viscous liquids, and there are difficulties with the homogenization of the reaction mass during synthesis, and when unloading the finished product from the reaction plant, in all cases, the synthesis was performed in solution cyclohexanone by 50 % by weight of the final product. Control of the reaction was performed by changing the % wt. free NCO-groups in time. The reaction was considered complete if the measured % wt. free NCO-groups in the reaction mixture for at least one hour twice showed zero. The isolated oligodiuretanediols range from solid at room temperature to very viscous products, which significantly depends on the molecular weight of the diol used in the reaction (ie the concentration of urethane groups formed). They are homogeneous, transparent compounds that are readily soluble in esters, ethers, aromatic and halide-containing, aprotic solvents, ketones, poorly or completely insoluble in aliphatic saturated hydrocarbons. The structure of the synthesized oligomeric products is confirmed by functional analysis, IR–spectra. In the IR-spectra of each of the synthesized oligodiuretanediols there are no absorption bands in the region of 2270 cm-1, which confirms the complete completion of the reaction of urethane formation according to the scheme. At the same time, the absorption bands in the region of 3450 cm-1, 1720 cm-1, 1540 cm-1 are fixed, which are characteristic of the presence of urethane groups in the structure of the target products. As the chain length of the diol component –R– increases in the target product (which synchronously leads to an increase in molecular weight), the intensity of these absorption bands decreases, which is associated with a decrease in the concentration of formed urethane groups in the structure of oligodiuretanediols. The refractive index also decreases synchronously. Synthesized series of oligodiuretanediols can be used for synthesis on its basis of other classes of oligomers with the simultaneous presence in the structure of urethane groups. The ability of such compounds to be soluble in solvents of different nature has been studied, which provides information for the directions of their further use (varnishes, enamels, primers).

Urethane Formation with an Excess of Isocyanate or Alcohol: Experimental and Ab Initio Study

A kinetic and mechanistic investigation of the alcoholysis of phenyl isocyanate using 1-propanol as the alcohol was undertaken. A molecular mechanism of urethane formation in both alcohol and isocyanate excess is explored using a combination of an accurate fourth generation Gaussian thermochemistry (G4MP2) with the Solvent Model Density (SMD) implicit solvent model. These mechanisms were analyzed from an energetic point of view. According to the newly proposed two-step mechanism for isocyanate excess, allophanate is an intermediate towards urethane formation via six-centered transition state (TS) with a reaction barrier of 62.6 kJ/mol in the THF model. In the next step, synchronous 1,3-H shift between the nitrogens of allophanate and the cleavage of the C–N bond resulted in the release of the isocyanate and the formation of a urethane bond via a low-lying TS with 49.0 kJ/mol energy relative to the reactants. Arrhenius activation energies of the stoichiometric, alcohol excess and the isocyanate excess reactions were experimentally determined by means of HPLC technique. The activation energies for both the alcohol (measured in our recent work) and the isocyanate excess reactions were lower compared to that of the stoichiometric ratio, in agreement with the theoretical calculations.