Isocyanate reaction

Industrially, polyurethane flexible foam manufacturers combine a version of the carbamate-forming reaction and the amine—isocyanate reaction to provide both density reduction and elastic modulus increases. The overall scheme involves the reaction of one mole of water with one mole of isocyanate to produce a carbamic acid intermediate. The carbamic acid intermediate spontaneously loses carbon dioxide to yield a primary amine which reacts with a second mole of isocyanate to yield a substituted urea. 

Primary cycloaUphatic amines react with phosgene to form isocyanates. Reaction of isocyanates with primary and secondary amines forms ureas. Dehydration of ureas or dehydrosulfuri2ation of thioureas results in carhodiimides. The nucleophilicity that deterrnines rapid amine reactivity with acid chlorides and isocyanates also promotes epoxide ring opening to form hydroxyalkyl- and dihydroxyalkylaniines. Michael addition to acrylonitrile yields stable cyanoethylcycloalkylarnines. 

The reaction of water with isocyanate is shown in the third item of Fig. 1 [5]. The water/isocyanate reaction is the major curing mechanism for the one-component urethane adhesives. Most one-component urethanes are based on an isocyanate-terminated prepolymer (I). Usually, the moisture in the air is used to cure the adhesive, but in some instances, a fine mist of water may be introduced on top of the adhesive before the bond is closed, in order to facilitate cure ... 

For all its benefits, the water/isocyanate reaction can be troublesome as well. All raw materials that go into a one-component moisture-cured adhesive must have very low moisture content, usually less than 0.05% water. If higher water levels are present, the adhesive can start curing in the reactor, causing an increase in viscosity, or, in extreme cases, gelation. 

The most common catalyst used in urethane adhesives is a tin(lV) salt, dibutyltin dilaurate. Tin(IV) salts are known to catalyze degradation reactions at high temperatures [30J. Tin(II) salts, such as stannous octoate, are excellent urethane catalysts but can hydrolyze easily in the presence of water and deactivate. More recently, bismuth carboxylates, such as bismuth neodecanoate, have been found to be active urethane catalysts with good selectivity toward the hydroxyl/isocyanate reaction, as opposed to catalyzing the water/isocyanate reaction, which, in turn, could cause foaming in an adhesive bond line [31]. 

Open times of two-component urethanes can vary widely, depending on the level of catalyst. Reaction times can vary from 90 s to over 8 h. Dibutyltin dilaurate is the most common catalyst employed to catalyze the urethane reaction. This is normally added to the polyol side. A tertiary amine may also be added in small amounts. Tin catalysts do not catalyze the amine/isocyanate reaction very well. Acids, such as 2-ethyl hexanoic acid, may be employed to catalyze the amine/isocyanate reaction where needed. 

Catalytic Effect in Isocyanate Reactions A. Farkas and G. A. Mills... 

Isocyanates react with carboxylic acids to form amides, ureas, anhydrides, and carbon dioxide, depending on reaction conditions and the structure of the starting materials (Scheme 4.13). Aliphatic isocyanates more readily give amides. Aromatic isocyanates tend to react with carboxylic acids to first generate anhydrides and ureas, which at elevated temperatures (ca. 160°C) may further react to give amides. In practice, the isocyanate reaction with carboxylic acid is rarely utilized deliberately but can be an unwanted side reaction resulting from residual C02H functionality in polyester polyols. 

I. S. Bechara, The Mechanism of Tin-Amine Synergism in the Catalysis of Isocyanate Reaction with Alcohols, in Urethane Chemistry and Applications, ACS Symposium Series 172, K. N. Edwards, (Ed.), American Chemical Society, Washington, DC, 1981. 

The acrylic polyol component is then mixed with the water contaminated isocyanate component and allowed to cure via the standard polyol isocyanate reaction ... 

The formation of Intermediate compounds (e.g. carbamlc acid) Is not described in the model. So, the formation of urethane (reaction 1) and the hydrolysis of the isocyanate (reaction 2) are the rate-determining steps. 

Good yields of imidazoline derivatives have been obtained in the cocyclooligomerization of phenylacetylene with isocyanates and carbodiimides (Scheme 100).166 It has been demonstrated166 by labeling studies in the isocyanate reaction that the hydrogen shift is intramolecular and a mechanism accommodating this feature is illustrated in Scheme 101.166 The final step (85 - 86) in the proposed166 mechanism (Scheme 101) probably occurs via a coordinated acetylene complex and it is notable that related complexes... 

Ethyl formate, condensation with 2-methylcydohexanone, 48, 41 purification of, 48, 42 Ethyl isocyanate, reaction with N,N-dimethy 1-1,3-propanediamine,... 

The reviewer did the reaction on loose resin (because MicroTubes are no longer available) and washed it extensively in the isocyanate reaction with DMF to remove the unwanted symmetrical urea. 

Recently, poly(vinyl alcohol)-isocyanate reactions have been run successfully in dimethylsulfoxide solutions by several lab- oratories, including ours. While there are reports of solvent derived products from the reactions of isocyanates in DMSO (60), these reactions do not seem to pose a problem in the reaction with poly(vinyl alcohol). This solvent has been used to react PVA with methoxymethyl isocyanate (61.62) and a series of isocyanates including methyl, ethyl, isopropyl, phenyl and 1-naphthyl (63). This reaction is illustrated in Equation 17 where R is... 

A controlled release herbicide based on Metribuzin attached to poly(vinyl alcohol) has been reported via an isocyanate reaction as shown in Equation 18. Data on the release rate has been published (54). 

Catalytic decomposition, of formic acid, 14 35 Catalytic effects, in isocyanate reactions, 13 393... 

Isocamphane, 20 281 Isocyanate reactions, 13 393 species, spillover of, 34 41 2 formation in catalytic exhaust-gas reaction, 34 41... 

It should be noted that isocyanates react with DMF to form bicychc ureas or isocyanurates (Chadwick and Cleveland, 1981). DMF is an unsuitable solvent for wood isocyanate reactions because of these side reactions. 

West, H. and Banks W.B. (1986). Topochemistry of the wood isocyanate reaction. An analysis of reaction profiles. Journal of Wood Chemistry and Technology, 6(3), 411-425. 

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