Urethanes reaction with isocyanates

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. 

Isocyanates can also undergo a wide variety of primary reactions. For example, they can react with alcohols to form urethanes and react with amines to give ureas. Isocyanates can also undergo secondary reactions reacting with urethanes and ureas to give allophanates and biurets, respectively. Isocyanates can also release carbon dioxide that can appear as small bubbles or micropopping in the clearcoat film. The use of moisture scavengers and other additives can help reduce or completely eliminate these defects in baked systems. 

The fillers used in urethane formulations are similar to those used in sihcones calcium carbonate, talc, clays, and siUca are among the most common. On account of the undesirable reaction of isocyanates with water, fillers used in urethane formulations must be dry. 

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. 

The first urethane reaction in Fig. 1 is used in two major ways in adhesives. In one case, a two-component adhesive usually employs a polyol and polyisocyanate with catalyst. This can react at room temperature to form the polyurethane. The second use of this reaction is to make an isocyanate-terminated prepolymer. Reacting a stoichiometric excess of isocyanate with polyol can produce an isocyanate-terminated prepolymer. A prepolymer is often made with an NCO/OH ratio of 2.0, as shown below, but the isocyanate ratio can range from 1.4 to over 8.0, depending upon the application ... 

In similar fashion, A-substituted-2(3fT)-oxazolones were prepared directly from the hydroxy-ketone by reaction with urethanes in the presence of pyridine and dimethylformamide or by using isocyanates. 

Urethane alkyds These are formed by the reaction of an isocyanate with alkyd, although the curing remains substantially through the oil-oxidation reaction. They have properties similar to alkyds although superior resistance to abrasion is claimed. 

Phenolic-isocyanates (phenolic-urethanes). The binder is supplied in three parts a phenolic resin in an organic solvent (0.8%), methylene diphenyl diisocyanate (MDI) (0.5%), and a liquid amine catalyst. When mixed with sand, the amine causes a reaction between the resin and the MDI, forming urethane bonds, which rapidly set the mixture. The speed of setting is controlled by the type of catalyst. The optimum cure temperature is 25 to 30°C. Compression strength is typically over 4000kPa (600psi). 

Urethane acrylates Urethane acrylates are formed by the reaction of isocyanates with hydroxy-functional acrylate monomers. After UV cure, they produce tough, flexible materials, which exhibit a good abrasion resistance. 

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. 

The reaction of a-naphthyl isocyanate with alcohols has been reported to be a convenient analytical method for the preparation of solid derivatives [16-18]. In addition, the by-product dinaphthylurea is very insoluble in hot ligroin (b.p. 100°-120°C). The urethanes are readily soluble in hot ligroin, and on cooling the solution they recrystallize to sharp-melting solids. It is recommended that two recrystallizations be performed to obtain substances for analysis. Primary alcohols react well without the need for heating the reaction mixture. Secondary alcohols require additional heat, and the yields of urethane oft are smaller than when primary alcohols are used. Tertiary alcohols other than /-butyl [17] or /-amyl [17] were not able to react under the conditions used. Table V lists some representative alcohols and their a-naphthylurethane derivatives. 

Cyclization of side chain nitriles has found extensive use in the synthesis of benzocyclobutenes (70 n = 2),104 the versatile synthons which open on mild thermolysis to give o-quinodimethanes for inter- and intra-molecular [4 + 2] trapping.108 The nitrile group in (70) can be manipulated into a variety of functionalities for appending the dienophile portion. For example, in the synthesis of chelidonine, the nitrile (71) was converted, by hydrolysis followed by Curtius degradation and reaction of the formed isocyanate with benzyl alcohol, to a urethane (72). The latter was then condensed with a benzyl bromide to get the compound (73), which was elaborated further as shown in Scheme 14.109... 

Both chemically and toxicologically, the most significant property of isocyanates is the high chemical reactivity of the isocyanate functional group. Industrially, the most significant such reaction is with alcohols to yield urethane (carbamate) compounds, as shown by reaction 15.8.2. Multiple... 

Because carbamic acids are unstable, normal esterification procedures cannot be used to form urethanes. Urethanes are most commonly made by treating an isocyanate with an alcohol or a phenol. The reaction is highly exothermic, and it gives a quantitative yield of a carbamate ester. 

The next stage is to initiate an exothermic linking of the residual terminal isocyanates with simple diamines. The reaction is again nucleophilic attack on the isocyanate, but the new functional group is now a urea rather than a urethane. Showing just one end of the growing polymer ... 

Reactions of alkoxylated lignin with diisocyanates produce thermoset materials because the lignin polyol is always polyfunctional with a functionality greater than 2. The isocyanate-alcohol reaction produces a urethane linkage that when repeated creates a crosslinked, nonreformable polyurethane. This is shown in Fig. 6. A broad spectrum of lignin-based urethanes have been made and tested. The data show that these materials match if not exceed the properties of synthetic polyurethanes made without lignin [60]. 

Figure 2.19 The reaction of isocyanate with urea and urethane...

Formation of Carbamates. Chiral isocyanates were discussed above as CDAs for the derivatization of amines, but these reagents have also proven useful for hydroxyl compounds. The reaction (Eq. 8) of an isocyanate with an alcohol yields a carbamate (sometimes also referred to as a urethane)... 

The reaction of phenyl isocyanate with methanol in dibutyl ether at 20°C was found to be second order, but the value of the rate coefficient increased slowly with time, indicating catalysis by the product of the reaction. The addition of methyl carbanilate to the initial reaction mixture increased the rate of reaction, confirming catalysis by the urethane, a weak base. The rate coefficient for the initial uncatalysed reaction was 0.28 x lO" 1 mole sec . 

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