Isocyanate, solution reaction

Amides. Because amides are less basic, they chlorinate less rapidly than amines. A/-Halamides are converted to amines in basic solution via intermediate formation of an isocyanate (Hofmann reaction) (91). 

Barbitone (barbital, 282) was irradiated in aqueous solution at pH 10 with light of 254 nm wavelength to give the ureide (289) in 68% yield. In ethanol, the derivative (290) was formed (62%). With A-methylbarbitone (283) in buffer at pH 10, a mixture of the ureide (291) and the imidazole (292) was obtained. It was suggested that the ring-opened compounds were formed via isocyanates (288). Reaction with water would then give an acid which would spontaneously decarboxylate, but in ethanol a stable urethane (290) would form [173]. 

In diluted aprotic solutions of isocyanates, the reaction rate between isocyanates and stabilizing water of cyclic sulfonium zwitterions is negligible but becomes important at high concentrations of isocyanates or high reaction temperatures. 

Several reaction intermediates can be observed by varying the reaction conditions. If equimolar amounts ofbromine and hydroxide are added to acetamide (26, R = Me, Scheme 2.11), the product is N -bromoacetamide (27, R = Me). Further reaction with base gives unstable salts (28), which rearrange to isocyanates (29) reaction with water and an excess of hydroxide finally leads to the amine product (30), but in alcoholic solutions urethanes (31) are formed. 

Isocyanate-epoxy reactions have been investigated bulk and in solution in the presence of DBU and other bases (85MI1). 

An ingeniously simple screening method was used by Britain and Gemeinhardt [146] to evaluate catalysts for the isocyanate/hydroxyl reaction. To approximate as closely as possible actual polymerization conditions, the 80 20 ratio of 2,4- and 2,6-tolylene diisocyanate (80 20 TDI) isomers and a polyether triol of 3000 molecular weight were mixed at NCO OH ratio of 1.0. A 10% solution of catalyst in dry dioxane was added, the final catalyst concentration being 1% of the weight of polyether. The time for the mixture to gel at 70°C was noted as an indication of catalytic strength. This technique used the same reactants employed in one-shot flexible polyether-based foam systems, almost completely eliminated solvent, and was used to screen quickly hundreds of possible catalysts. 

Certain organometallic compounds have been found to be strong catalysts for the isocyanate/water reaction in dilute solution and relatively hi catalyst concentrations [145]. In dioxane solution, at 70°C, with 0.25M phenyl isocyanate, 0.125M water and catalyst concentration at or extrapolated to 0.025M, the relative results shown in Table 21 were obtained. Somewhat in contrast to these results, Wolfe [147] found that stannous octoate and dibutyltin dilaurate were milder catalysts than was triethylamine. 

One-shot polyether foams were studied, using a variety of catalysts. The formula contained 100 parts by weight of poly(oxypropylene)triol of 3000 M.W., 38 parts of 80 20-TDI, 2.9 of water, 0.3 of 4-dimethyl-aminopyridine, 0.5 of lV,iV-dimethylbenzylamine, varying amounts of metal catalysts, and 0.1 part of X-520 siloxaneoxyalkylene copolymer. All of the gas was evolved from these systems within 60 sec after mixing. Viscosity measurements were not satisfactory due to fracture of the polymeric phase. Analysis of the reaction mixture at the end of 55 sec reaction time indicated the relative rate of formation of various products, as indicated in Table 22. The importance of selecting the proper catalyst to avoid undesirable side reactions is readily apparent. The results shown in Table 22 indicate that both tin catalysts promote the isocyanate/water reaction more than the isocyanate/hydroxyl reaction in the system studied. This is unusual, since other reports, though often of dilute solution studies, have shown the tin catalysts to promote the isocyanate/ hydroxyl reaction more [145,147,196]. 

The technique of measuring maximum heat rise during reaction has been utilized as a measure of catalytic activity. In contrast to many kinetic measurements made in dilute solution, relatively concentrated solutions can be employed which more nearly represent practical foam manufacture. The isocyanate-water and isocyanate-hydroxyl reaction can be studied separately. This has been used extensively under the designation of the Wolfe test (74). The results of these tests demonstrate the very high activity of triethylenediamine for the isocyanate-water reaction. Likewise, this catalyst is the most active amine catalyst for the isocyanate-hydroxyl reaction, although less active than tin compounds. This test can also be used effectively for testing mixtures of catalysts. This is in accord with present commercial practice of using an amine—usually triethylenediamine —and a tin compound to achieve optimum results. 

Kinetic Experiments. Stock solutions were prepared by weighing out the correct amount of material and diluting to the proper level, using volumetric flasks. Reaction mixtures were prepared by adding the desired amount of catalyst and active hydrogen compound to a volumetric flask, diluting with solvent short of the calibration mark, and adding the proper amount of phenyl isocyanate solution and then solvent to the mark. 

Typical procedure. Valylalanine methyl ester isocyanate [313] Reaction of valylalanine methyl ester hydrochloride (123 mg, 0.515 mmol) with triphosgene (a solution of 112 mg in 1 mL of dichloromethane, 0.38 mmol) yielded 112 mg (95%) of the crude isocyanate 454 as a colorless oily solid. NMR analysis of the product revealed the presence of 8% of hydantoin 455 and 15% of an additional impurity. 

A typical procedure followed in the preparation of siloxane-urethane segmented copolymers is as follows Calculated amounts of HMDI and PDMS oligomer are introduced into the reaction flask, stirred and heated up to about 60°C. This mixture is not miscible. However, when 0,5 mL of 1.0% DBTDL solution in toluene is added, the mixture turns clear in about one minute indicating a fairly fast reaction between PDMS and HMDI. There is also a dramatic increase in the reaction temperature from 60 to about 90°C, typical for very exothermic urethane formation reaction. Prepolymer formation is followed by FTIR, monitoring the disappearance of the broad hycfroxyl peak centered around 3300 cm Prepolymer obtained is then diluted with THF to about 50% solids and heated to reflux temperature of 64.5 C. Chain extender, BD, is dissolved in THF and added dropwise into the reaction mixture. As the system became viscous as a result of the chain extension reaction, it is diluted with THF and DMF to prevent the premature precipitation of the copolymer formed, which is indicated by the formation of a cloudy solution. Reaction was continued until the complete disappearance of sharp isocyanate peak around 2250 cm in the FTIR... 

D) Phenylurea Derivatives. This reaction usually proceeds readily when cold solutions of the dried amine and of phenyl isocyanate, each in petroleum (b.p. 100-120 ), are mixed if no reaction is obvious, heat under reflux for 30 minutes. Care in using the isocyanate, p. 336.)... 

Liquid carbon suboxide added to a solution of 2-ethylaminothiarole in anhydrous ether at 0°C gives immediate formation of a white precipitate of the mesoionic xanthine (121) (R=H) (130), otherwise prepared by reaction between 2-ethylaminothiazole and phenoxycarbonyl isocyanate (see p. 65 and Ref. 304),... 

Polyurethane adhesives are known for excellent adhesion, flexibihty, toughness, high cohesive strength, and fast cure rates. Polyurethane adhesives rely on the curing of multifunctional isocyanate-terrninated prepolymers with moisture or on the reaction with the substrate, eg, wood and ceUulosic fibers. Two-component adhesives consist of an isocyanate prepolymer, which is cured with low equivalent weight diols, polyols, diamines, or polyamines. Such systems can be used neat or as solution. The two components are kept separately before apphcation. Two-component polyurethane systems are also used as hot-melt adhesives. 

Thiocarbamates, formed by reaction of a thiol with an isocyanate, are stable in acidic and neutral solutions and are readily cleaved by basic hydrolysis. The... 

Reaction of these polyols with an excess of isocyanate yields isocyanate terminated materials which are then chain extended by an amine such as hydrazine (NH2NH2) or ethylenediamine. The fibre is usually spun from solution in dimethylformamide. 

The reaction mixture is not cooled during the addition of the last two or three portions of phenyl isocyanate, so that the final temperature is near 25° this procedure prevents separation of the sodium salt of cyanophenylurea, which crystallizes readily at low temperatures. For the same reason, the filtered solution of the salt is not precooled, but rather is cooled during the precipitation of the free cyanourea. 

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