Urethanes, basic reactions

Most useful polyurethanes are cross-linked. Those commonly used in foams start with a diisocyanate like toluene diisocyanate (TDI) and a low molecular weight polyether such as poly(propylene glycol). Recall that the basic reaction of an isocyanate plus an alcohol gives the urethane functionality. 

Figure 22.1 Basic reactions of isoq nate with different reactants (a) hydroxyl compound to give urethanes, (b) amines to give ureas, (c) water to form an unstable compound, carbamic acid, which decomposes and produces an amine, (d) amines and excess isocyanate produces a urea linkage, (e) urea linkage and one molecule of isocyanate produces biuret, (f) urethane and asocyanate produce allophanates. Secondary reactions of isoc3canate self-condensation produce (g) uretidione ring (dimer-), (h) isocyanurate (trimer-), (j) carbodiimide (reprinted with permission of Chattopadh3ray et al., 2007, Elsevier [15]).

The basic reaction scheme for formulating urethane-based pseudo poly(amino acids) requires a reaction of a diisocyanate with a diol in the presence of stannous oc-toate as a catalyst. Depending on their properties, either the diol or the diisocyanate could serve as the hard segment (h) or soft segment (s) as reported in Table 12.4. Several poly(urethanes) also include a diol- or diamine-based chain extender to yield a higher molecular weight product. 

The basic reaction involved in the formulation of PU is between hydroxy compounds and isocyanates. The first record of this reaction was made by Wurtz and Hoffmann in 1848. Commercially significant advances did not occur until 1937, when 0. Bayer of Germany developed diisocyanate. Bayer and co-workers continued the development of polyester-based urethane polymers as nylon substitutes in the 1940-45 time period. Polyester toluene-diisocyanate (TDI) foams became commercially available in the United States in 1952-53. Two further developments occurred that allowed the use of PU foam to reach the present level. The first of these was the development of polyether polyol and the subsequent "one-shot" foaming process. Rigid polyether-based foams were available in 1957. The second development was the incorporation of halocarbons as blowing agents in 1958. 

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 ... 

The applications of the strong affinity to oxygen of organotin compounds are as catalysts. Organotin compounds are used for the polymerization of polyurethanes, polymerization of silicones and esterifications as the catalysts [62,63,65]. The catalysts for reactions of phenylisocyanate with butanol as a basic reaction of polyurethane formation reaction are shown in Table 10.5 [78]. The reaction rate with dialkyItin compounds are 30000-80000 times faster compared with the reaction without a catalyst. The urethane is considered to be formed via the following four center mechanism [79]. 

The basic reaction used in the polymerisation reaction to form polyurethanes is the addition of an hydroxyl group to an isocyanate group to give a urethane linkage... 

The basic reactions are similar to the Inside-out methodology. However, the reaction sequence starts with reaction of HEA (the outside of the polymer) with the diisocyanate instead of the polyol (the inside). The reaction vessel is charged with diisocyanate, catalyst, and stabilizer to which the HEA is added, resulting in an acrylate-functional isocyanate. Next the polyol is added, and a urethane acrylate can be obtained. 

Scheme 7.1 Basic reaction scheme for urethane 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. 

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. 

FIGURE 7.34 Decomposition of the symmetrical anhydride of A-methoxycarbonyl-valine (R1 = CH3) in basic media.2 (A) The anhydride is in equilibrium with the acid anion and the 2-alkoxy-5(4//)-oxazolone. (B) The anhydride undergoes intramolecular acyl transfer to the urethane nitrogen, producing thelV.AT-fcwmethoxycarbonyldipeptide. (A) and (B) are initiated by proton abstraction. Double insertion of glycine can be explained by aminolysis of the AA -diprotected peptide that is activated by conversion to anhydride Moc-Gly-(Moc)Gly-0-Gly-Moc by reaction with the oxazolone. (C) The A,A -diacylated peptide eventually cyclizes to the IV.AT-disubstituted hydantoin as it ejects methoxy anion or (D) releases methoxycarbonyl from the peptide bond leading to formation of the -substituted dipeptide ester. 

Olsen and co-workers used a solution of nitronium tetrafluoroborate in acetonitrile for the V-nitration of acetamides and urethanes at —30°C. The following nitramides were obtained by this method V-nitroacetamide (13 %), V-nitro-2-chloroacetamide (55 %), V-nitro-n-butylacetamide (40 %), V-nitrobenzamide (53 %), ethyl V-nitro-n-butylcarbamate (91 %) and V-nitrosuccinimide (43 %). The low yield of V-nitroacetamide, a primary nitramide, is attributed to competing hydrolysis due to the release of tetrafluoroboric acid as the reaction progresses. The scope of the reaction is improved by moving to more basic solvents like ethyl acetate, 1,4-dioxane and trimethyl phosphate. ... 

The instability of the 3-hydroxymethylindoles over a wide pH range results in the lack of success in acetylation of the hydroxy compound and also in the failure to hydrolyze the acetoxymethylindole without conversion into the bis(3-indolyl)methane (79HC(25-3)l). In contrast with the 3-isomer, 2-hydroxymethylindoles are stable to bases, but are polymerized by acids (79HC(25-3)l). Similarly, it is possible to convert 3-hydroxymethylpyrroles into their acetates and methyl ethers under basic conditions, and reaction with isocyanates yields the expected urethanes (79JMC977). Under acidic conditions, however, they produce the bis(3-pyrrolyl)methanes (B-77MI30504). 

The kinetics and mechanism of the phosphorus-catalysed dimerization of acrylonitrile to give 1,4-dicyanobut-l-ene and 2,4-dicyanobut-l-ene have been studied.114 The reactions of aryhminodimagnesium (138) with //-substituted p-cyanobenzophenones, l-cyano-9-fluorenenone, o-, m-, and p-dicyanobcnzcnes, and o-, m-, and p-nitrobenzonitriles have been examined.115 The effect of pressure on the reaction of 3 -methyl- l-(4-tolyl)triazene (139) and benzoic acid in chloroform and acetonitrile has been studied.116 The effect of acids on the rate of urethane formation from alcohols and isocyanates in the presence of alkyltin carboxylates has been examined.117 A Hammett a value has been reported for the amidine group N=CHNMe2 and used for the prediction of the basicity of sites in bifunctional amidines.118... 

Thermal degradation of foams is not different from that of the solid polymer, except in that the foam structure imparts superior thermal insulation properties, so that the decomposition of the foam will be slower than that of the solid polymer. Almost every plastic can be produced with a foam structure, but only a few are commercially significant. Of these flexible and rigid polyurethane (PU) foams, those which have urethane links in the polymer chain are the most important. The thermal decomposition products of PU will depend on its composition that can be chemically complex due to the wide range of starting materials and combinations, which can be used to produce them and their required properties. Basically, these involve the reaction between isocyanates, such as toluene 2,4- and 2,6-diisocyanate (TDI) or diphenylmethane 4,3-diisocyanate (MDI), and polyols. If the requirement is for greater heat stability and reduced brittleness, then MDI is favored over TDI. 

Propylene Oxide. Propylene oxide is another basic chemical used in manufacturing intermediates for urethane foams (cushioning and insulation), coatings, brake fluids, hydraulic fluids, quenchants, and many other end uses.23 The classic industrial synthesis of this chemical has been the reaction of chlorine with propylene to produce the chloro-hydrin followed by dehydrochlorination with caustic to produce the alkylene oxide, propylene oxide, plus salt. 

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