Reaction of Isocyanates with Alcohols

The reaction proceeds at ambient temperatures without the use of catalysts. Reactivity is higher for primary alcohols, decreasing for secondary, tertiary, and aromatic alcohols (Fig. 1). 

The reaction between isocyanates and alcohols, the most important reaction involved in polyurethane synthesis, is an exothermic reaction and leads, as mentioned before, to production of urethanes [1-26]  

---

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

The reaction of isocyanates with alcohol is strongly exothermic. A portion of this heat can be removed and better quality control achieved by forming a prepolymer with half of the alcohol and the diisocyanate. The new prepolymer has isocyanate terminals and can be used with the remaining quantities of the difunctional and/or the trifunctional alcohol in the propellant mix. In recent years isocyanate-terminated polymers... 

For the most part, the preparation of monomeric and polymeric carbamates (urethanes), semicarbazides, and ureas consists of condensation reactions of isocyanates with alcohols, hydrazines, or amines. The synthesis of ureas and semicarbazides are described in Chapters 6 and 8, respectively. 

Amines can be linked to polymeric alcohols as carbamates. Carbamate attachment of amines can be achieved by reaction of isocyanates with alcohol linkers, or by treatment of alcohol linkers with phosgene [339,427,428] or a synthetic equivalent thereof, followed by exposure to the amine (Figure 3.26). The reagents most commonly used for the activation of alcohol linkers are 4-nitrophenyl chloroformate [69,429-436] and carbonyl diimidazole [427,437-440], The preparation of support-bound carbamates is discussed in Section 14.6. 

The discovery of the catalytic effect of metal compounds on the reaction of isocyanates with alcohols came relatively recently (37) and led eventually to finding of the most potent catalysts of this kind in certain organometallic derivatives of tin (SS). 

The rate of the triethylamine-catalyzed reaction of phenyl isocyanate with n-butanethiol depends on the solvent used and increases in the order benzene, toluene, dibutyl ether, dioxane, methyl ethyl ketone, and nitrobenzene. This solvent effect increased with the ionizing power of the solvent and was the reverse of that observed in the reaction of isocyanates with alcohol. 

The presently proposed mechanism of synergism implies that ligands other than amine which can coordinate with the tin ion should also synergize its catalytic activity toward alcohol-isocyanate reactions. This apparently is the case for triphenyl phosphine-DBTDL combination. When triphenyl phosphine was added to DBTDL, it accelerated the rate of reaction of isocyanate with alcohol and water (Figure 6). Although triphenyl phosphine is known to dimerize isocyanates, (12), the dimerization... 

PURs include a wide range of materials ranging from thermoplastic elastomers to flexible as well as rigid foams. Polyurethanes are commonly formed upon reaction of isocyanates with alcohols (see Figure 8), in order to form polymeric structures both reaction partners have to contain at least two reactive functions (diisocyanates or diols). 

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. 

Reactions of isocyanates with alcohols are catalyzed by a variety of compounds, including bases (tertiary amines, alkoxides, and carboxylates), metal salts and chelates, organometallic compounds, acids, and urethanes. The most widely used catalysts in coatings are organotin (IV) compounds, most commonly dibutyltin dilaurate (DBTDL) (dibutylbis[(l-oxododecyl)oxy]stannate) [77-58-7] and tertiary amines, commonly diazabicyclo[2.2.2]octane (DABCO). Combinations of DABCO and DBTDL often act synergistically. 

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 ... 

Reaction of isocyanates with different active H-compounds that form the fundamental basis for polyurethane coating systems are shown in Figure 2.60. The order of relative rates for these reactions is -NH -OH > H O > -COOH. Many types of catalysts are available that are used to control these reactions. It should be noted that while urethanes are produced only from reaction of isocyanates with alcohols, other reactions are also effectively controlled and used in different polyurethane coating systems. 

Figure 1 Reaction of isocyanates with alcohols R1 and R2 stand for an aromatic or aryl group.

Alcohols, reaction of isocyanates with, 224-225 Alcoholysis, 69 Alicyclic dianhydrides, 297 Alignment coating, for liquid crystal devices, 269-270 Aliphatic AA-BB-type polyamides, synthesis of, 164-173 Aliphatic AB-type polyamides, 173-180 Aliphatic acids, 60... 

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. 

Reactions of MDI with alcohols proceed via a competitive, consecutive second-order reaction through an intermediate urethano-isocyanate ... 

Figure 2.17 Reactions of isocyanates with (a) the alcohol group (b) the amine group ...

The reaction of isocyanates with hydroxyl compounds is probably the most widely studied field of isocyanate chemistry. The reaction rate in this system depends on the nature of the isocyanate, of the alcohol, the solvent used and, of course, on the catalyst. 

In the following sections first the studies concerned with the reaction of isocyanates and alcohols in the absence of catalysts, i.e., the uncatalyzed or spontaneous reaction, will be discussed. These discussions will then be followed by a section on the catalyzed reaction. 

The reaction of isocyanate with water is more exothermic than the reaction with alcohols and the total heat release per mole of water is about 47 kcal/mol [1-3]. It is evident that one mole of water reacts with two -NCO groups, which is very important in order to calculate the correct quantity of isocyanate needed for polyurethane formulations. 

Aminotetrathiafulvalenes are, as yet, unknown and isocyanates are the only TV-linked substituents of synthetic use. 4,4 (5 )-Diisocyanatotetrathia-fulvalene reacts with diols to afford polycarbamates (76JOC1412), whereas the reaction of 75 with alcohols (84MI1) has been explored in the search for novel D-[Pg.266]

These reactions can be divided into two categories. They are additions to compounds with active hydrogens and self-condensations. The reactions are well described in organic chemistry textbooks, so there is little reason to describe them here. It is noteworthy, though, that the uncatalyzed reactions of isocyanates with various active hydrogen compounds are probably broadly similar. Among them, the most investigated reactions are those of alcohols with isocyanates... 

An interesting general synthesis of carbamates is the direct reaction of isocyanides with alcohols in the presence of thallium(III) nitrate, which offers advantages over the preliminary oxidation of the isocyanide to an isocyanate prior to reaction with the alcohol [583]. 

Isocyanates are very reactive compounds, which react readily with alcohols, amines, water, and thiols or thiolate ions. Reaction of isocyanates with thiols... 

Reductive carbonylation of nitro compounds is catalyzed by various Pd catalysts. Phenyl isocyanate (93) is produced by the PdCl2-catalyzed reductive carbonylation (deoxygenation) of nitrobenzene with CO, probably via nitrene formation. Extensive studies have been carried out to develop the phosgene-free commercial process for phenyl isocyanate production from nitroben-zene[76]. Effects of various additives such as phenanthroline have been stu-died[77-79]. The co-catalysts of montmorillonite-bipyridylpalladium acetate and Ru3(CO) 2 are used for the reductive carbonylation oLnitroarenes[80,81]. Extensive studies on the reaction in alcohol to form the A -phenylurethane 94 have also been carried out[82-87]. Reaction of nitrobenzene with CO in the presence of aniline affords diphenylurea (95)[88]. 

Reaction of the glycol, 70, affords an oxazolidinone rather than the expected carbamate (71) on fusion with urea. It has been postulated that the urea is in fact the first product formed. This compound then undergoes 0 to N migration with loss of carbon dioxide reaction of the amino alcohol with the isocyanic acid known to result from thermal decomposition of urea affords the observed product, mephenoxolone (74) this compound shows activity quite similar to that of the carbamate. An analogous reaction on the glyceryl ether, 75, affords metaxa-lone (76). 

Bonded phases have been prepared by other general methods besides those indicated so far [64-66]. Reaction of silica with an alcohol or isocyanate resulted in the formation of silicate esters (estersils), but these phases were too hydrolytically unstable to be generally useful. Bonded phases with an Sl-R or Sl-NHR structure are more hydrolytically stable than the estersils but... 

Base-catalysed reactions of isocyanates, such as the title compound, with alcohols may be explosively violent in absence of diluting solvents. 

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... 

---