The reaction of isocyanates with amines

The reactions of monoisocyanates with amines or water follow the general principles outlined in Section 4.1, and are closely analogous to the reactions with alcohols (see Section 7). 

Half-lives of the reaction between ArNCO and ArNH  

Isocyanate Amine Dissociation constant of amine Reaction half-life (min) 

The very slow rate of reaction between an orfho-substituted isocyanate and an ortho-substituted amine in the cure of polyurea-urethanes based on tolylene diisocyanate has been considered to be of significant importance in the final cure of such polymers [180]. 

Naegeli et al. [177] reported mild catalysis by tertiary amines and carboxylic acids, but not by water, inorganic acids, salts or bases. In contrast. Craven [179] found that the typical tertiary amines and acids had little catalytic effect in the systems he studied. Certain substituted ureas appeared to catalyse the reaction to a greater extent than many tertiary amines. Ten mole % of butyric acid reduced the half-life of the reaction between phenyl isocyanate and o-toluidine by 57%, and 10 mole % of N-phenyl-N -o-tolylurea reduced it by 38%. Arnold et Jil. [178] and 

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The reactions of isocyanates with amines and with water have been reviewed in detail [116], as have the preparations of many polymers... 

Isocyanates will react with all compounds containing hydrogen atoms attached to a nitrogen atom [2]. There are four basic reactions chemists employ to make polyurethanes. The reaction of isocyanates with hydroxyl groups to produce urethane is the primary reaction. The reaction of isocyanates with amines yields urea and the reactions of isocyanates with urea and urethane produce biurets and allophanates, respectively. 

The reaction of isocyanates with alcohols to form carbamates is catalyzed by amines and a variety of organometaHic compounds. 

Conversely, the rate of reaction of isocyanates with amines to yield ureas is both rapid and quantitative. Much has been written concerning the reaction... 

In the manufacture of highly resident flexible foams and thermoset RIM elastomers, graft or polymer polyols are used. Graft polyols are dispersions of free-radical-polymerized mixtures of acrylonitrile and styrene partially grafted to a polyol. Polymer polyols are available from BASF, Dow, and Union Carbide. In situ polyaddition reaction of isocyanates with amines in a polyol substrate produces PHD (polyhamstoff dispersion) polyols, which are marketed by Bayer (21). In addition, blending of polyether polyols with diethanolamine, followed by reaction with TDI, also affords a urethane/urea dispersion. The polymer or PHD-type polyols increase the load bearing properties and stiffness of flexible foams. Interreactive dispersion polyols are also used in RIM appHcations where elastomers of high modulus, low thermal coefficient of expansion, and improved paintabiUty are needed. 

For the reaction of isocyanate with active hydrogen compounds, tertiary amines or tin compounds act on... 

The first methanes involved the reaction of isocyanate with simple alcohols and amines. They were of sufficient economic value to foster the development of a number of isocyanates, including the aromatics that play dominant roles in modem polyurethanes. An isocyanate is made by reacting an amine with phosgene. 

The prime function of tertiary amines is to catalyze the reaction of isocyanate with water to form COj gas for blowing the polymer. 

This occurrence significantly affects the stoichiometry, not only from consumption of the isocyanate functionality in the process, but also by production of an amine product with two active hydrogens, which adds reactive functional groups to the glycol component of the polyurethane reaction. For these reasons care has to be taken to exclude moisture for the polyurethane applications described so far. However, for the production of flexible and rigid polyurethane foams the reaction of isocyanate with water is exploited to produce bubbles. 

The polyurethanes, although not used in large volumes, have specific applications of value for aerospace. Accordingly, a brief discussion of the chemistry and the material applications is in order. There are three chemical reactions of importance the reaction of hydroxyl groups with isocyanates to form urethanes (Reaction 1), the reaction of isocyanates with water to form amines (Reaction 2), and the reaction of amines with isocyanates to form ureas (Reaction 3). 

Both types of reaction are carried out in apolar organic solvents, such as benzene, toluene or chlorinated hydrocarbons. For the enhancement of the reaction of isocyanates with hydroxy compounds, metal salts or tertiary amines are used as catalyst. In the case of reaction (h) the hydrochloric acid liberated is bound with a tertiary amine (pyridine, triethylamine). 

Additionally, reactions of isocyanate with amines provide another cross-linking approach to make nanogels. Cross-linked micelles with pH-responsive features were obtained by the addition of excess amounts of 1,8-diaminooctane to micellar aggregates consisting of poly (poly(ethylene glycol) methyl ether methacrylate)-b/oc -poly(methyl methacrylate-co-poly (3-isopropenyl-a,a-dimethylbenzyl isocyanate)) [PPEGMEMA-b-P(MMA-co-TMI)] (Figure 54.9). 

Catalysts. To make the final polyurethane, common catalysts are tertiary amines and organotin compounds, often used in mixtures. Amine catalysts favor the reaction of isocyanate with water, producing urea linkages and CO2, which acts as a blowing agent. On the other hand, organotin catalysts favor the isocyanate/hydroxyl reaction. Additional catalysts and catalyst decomposition products may be present if graft polyols are part of the mixture. The most common of these catalysts are free radical initiators. Phosphorus compounds may be present in the case of carbodiimide-modi-fied MDI. 

The synthetic route to isocyanates that is used exclusively in industry is the reaction of phosgene with amines. The reaction is carried out in two stages to suppress the formation of polyurea. In the first stage (cold phos-genization), a solution or suspension of amine at 0°C is converted with excess phosgene. This produces a mixture of hydrochloride and free carbamyl chloride ... 

Isocyanate plastic n. A plastic based on polymers made by the polycondensation of organic isocyanates with other compounds. Reaction of isocyanates with hydroxyl-containing compounds produces polyurethanes having the urethane group -NHC (=0)0-. Reaction of isocyanates with amine-containing compounds produces... 

The reaction of isocyanates with alcohols and with water can be catalyzed by amines and by organometallic compounds. Tertiary amines, such as l,4-diazo-[2.2.2]-bicyclooctane (DABCO) or triethylamine, are particularly effective in promoting the isocyanate-water reaction, while organometallic complexes, such as dibutyltin dilaurate or stannous octoate, are very useful for catalyzing isocyanate-alcohol reactions. Numerous articles have been written on various aspects of the catalysis of isocyanate reactions and representative examples are cited in refs. 8-10. 

Isocyanates react with alcohols and phenols to form urethanes. In general, rates of urethane formation decrease in the following order primary alcohols > secondary alcohols > 2-alkoxyethanols > l-alkoxy-2-propanols. Isocyanates can react with urethanes to form allophanates. This reaction is much slower than the reaction of isocyanate with alcohol. Isocyanates react rapidly with primary and secondary amines to form ureas. The reaction is much faster than the reaction of isocyanates with alcohols. Isocyanates can react with ureas to form biurets. Biuret formation is slower than urethane formation, but faster than allophanate formation. Isocyanates react with water to form imstable carbamic acids, which dissociate into carbon dioxide and an amine. The amine is so much more reactive that it reacts with another isocyanate (in preference to water) to form mea. The reactivity of water with isocyanates is somewhat slower than that of secondary alcohols, but much more rapid than that of imcatalyzed reaction with methanes or ureas. 

An alternative stable interactive organic dispersion in polyol has recently been introduced. This consists of particles of polyurea dispersed in polyol, formed by the in situ polyaddition reaction of isocyanates with amines in a polyol medium, and offers similar advantages to the polymer/polyols. 

Carbamates are usually prepared by the addition of an alcohol to an isocyanate. The reaction of isocyanates with alcohols is fast and quantitative (indeed, it is used to characterize alcohols). The reaction with phenols is slower, but can be catalyzed by tertiary amines. 

The reaction of isocyanate with alcohols is strongly exothermic (170-190 kJ/mol). One of the basic reactions in the urethane foam technology is the reaction of isocyanate with water with evolution of carbon dioxide and amine formation ... 

This section of the chapter will present the basic reactions found in the chemistry of polyurethane compounds, such as the reaction of isocyanates with polyols, water, and amines. The reactions of isocyanates with urethanes, ureas, and amides are also of significant importance in poljrurethane chemistry as they will lead to an increase in materials choice. 

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 addition of phenylisocyanate to aldehyde-derived enamines resulted in the formation of aminobutyrolactams (438,439). As aminal derivatives these produets can be hydrolyzed to the linear aldehyde amides and thus furnish a route to derivatives of the synthetically valuable malonaldehyde-acid system. With this class of reactions, a second acylation on nitrogen becomes possible and the six-membered cyclization products have been reported (440). Closely related to the reactions of enamines with isocyanates is the condensation of cyclohexanone with urea in base (441). 

A hydantoin structure can be built by the reaction of a primary amine with CDI to an isocyanate (Section 8.1), followed by conversion with an amino acid [146]... 

The reaction of CDI with primary phosphines was expected to lead first to an azolide ImCOPHR, analogous to imidazole-N-carboxamide as the reaction product of primary amines and CDI. In fact, reaction of phenylphosphine with CDI leads directly to imidazole, carbonmonoxide, and tetraphenylcyclotetraphosphine (THF, reflux, 5h). In analogy to the dissociation of imidazole-AT-carboxamide into isocyanates and imidazole, this can be explained by the assumption that the first-formed ImCOPHC6H5 dissociates into an isocyanate analogue, C6H5P=C=0, which is unstable and decomposes into carbon monoxide and phenylphosphene (C6H5P) which tetramerizes. However, the intermediate formation of phenylphosphene has not yet been definitely proved. 

Other microwave-assisted SPOS processes reported in the literature are summarized in Scheme 12.8. The addition of isocyanates to amines bound to Wang resin, for example, was studied both under conventional conditions at room temperature and under the action of microwave irradiation in open vessels by use of a monomode instrument. By monitoring the progress of the addition by on-bead FTIR it was demonstrated that the microwave procedure proceeded significantly faster than the reaction at room temperature (12 compared with 210 min) [38], The temperature during the microwave irradiation experiment was not determined, however, so it is unclear if any nonthermal microwave effects were responsible for the observed rate-enhancements (Scheme 12.8a) [38]. 

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