Tertiary amines isocyanate

Catalysis is usually accompHshed through the use of tertiary amines such as triethylenediamine. Other catalysts such as 2,4,6-/m(/V,/V-dimethylaminomethyl)phenol are used in the presence of high levels of cmde MDI to promote trimerization of the isocyanate and thus form isocyanurate ring stmctures. These groups are more thermally stable than the urethane stmcture and hence are desirable for improved flammabiUty resistance (236). Some urethane content is desirable for improved physical properties such as abrasion resistance. 

Tertiary amines have been shown to react with isocyanates ia an analogous fashion to form ureas (41—43). Similarly, a2iridines (three-membered rings containing nitrogen) are found to react with isocyanates to yield cycHc ureas. Tertiary amines have also been shown to form labile dipolar 1 1 adducts with isocyanates reminiscent of salt formation. In contrast, formaldehyde acetal aminals form iasertion products with sulfonyl isocyanates (44,45). 

Commercially, polymeric MDI is trimerized duting the manufacture of rigid foam to provide improved thermal stabiUty and flammabiUty performance. Numerous catalysts are known to promote the reaction. Tertiary amines and alkaU salts of carboxyUc acids are among the most effective. The common step ia all catalyzed trimerizations is the activatioa of the C=N double boad of the isocyanate group. The example (18) highlights the alkoxide assisted formation of the cycHc dimer and the importance of the subsequent iatermediates. Similar oligomerization steps have beea described previously for other catalysts (61). 

In sulfamation, also termed A/-sulfonation, compounds of the general stmcture R2NSO2H are formed as well as their corresponding salts, acid hahdes, and esters. The reagents are sulfamic acid (amido—sulfuric acid), SO —pyridine complex, SO —tertiary amine complexes, ahphatic amine—SO. adducts, and chlorine isocyanate—SO complexes (3). 

The nitrogen of aHphatic and aromatic amines is alkylated rapidly by alkyl sulfates yielding the usual mixtures. Most tertiary amines and nitrogen heterocycles are converted to quaternary ammonium salts, unless the nitrogen is of very low basicity, eg, ia tn phenylamine. The position of dimethyl sulfate-produced methylation of several heterocycles with more than one heteroatom has been examined (22). Acyl cyanamides can be methylated (23). Metal cyanates are converted to methyl isocyanate or ethyl isocyanate ia high yields by heating the mixtures (24,25). 

Some of the chemicals used in the production of polyurethanes, such as the highly reactive isocyanates and tertiary amine catalysts, must be handled with caution. The other polyurethane ingredients, polyols and surfactants, are relatively inert materials having low toxicity. 

H-Bond Acceptor (HBA) Acyl chlorides Acyl fluorides Hetero nitrogen aromatics Hetero oj gen aromatics Tertiary amides Tertiary amines Other nitriles Other nitros Isocyanates Peroxides Aldehydes Anhydrides Cyclo ketones Ahphatic ketones Esters Ethers Aromatic esters Aromatic nitriles Aromatic ethers Sulfones Sulfolanes... 

Formulations for one-shot polyether systems are similar to those used for flexible foams and contain polyether, isocyanate, catalyst, surfactant and water. Trichloroethyl phosphate is also often used as a flame retardant. As with polyesters, diphenylmethane di-isocyanate is usually preferred to TDI because of its lower volatility. Tertiary amines and organo-tin catalysts are used as with the flexible foams but not necessarily in combination. Silicone oil surfactants are again found to be good foam stabilisers. Volatile liquids such as trichlorofluoro-methane have been widely used as supplementary blowing agents and give products of low density and of very low thermal conductivity. 

In actual practice, catalysts are usually employed to catalyze the isocyanate/ alcohol reaction at room temperature. Typical catalysts for this reaction are the tin(IV) salts, e.g., dibutytin dilaurate, or tertiary amines, such as triethylene diamine [2]. 

Initially, the water slowly reacts with the isocyanate. However, the reaction can be catalyzed with an appropriate catalyst, such as dibutyltin dilaurate or a morpholine tertiary amine catalyst. The isocyanate will react with water to form a carbamic acid, which is unstable and splits off carbon dioxide, to produce a terminal amine end group (see p. 76 in [6]). This amine then reacts with more isocyanate-terminated prepolymer, as shown above, to form a polyurea. This process repeats itself, building up molecular weight and curing to become a polyurea-polyurethane adhesive. 

A tertiary amine such as triethylamine is then added to the isocyanate-terminated prepolymer (containing carboxylic acid groups). The tertiary amine reacts with the pendant carboxylic acid groups, forming a carboxylic acid salt. The presence of this salt, together with adequate stirring, allows the dispersion of the prepolymer in water by the so-called melt dispersion process [57]. 

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. 

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

The reaction was carried out at 100°C for about two hours until the theoretical isocyanate content, as determined by the di-n-butylamine titration method (27), was reached. The PU prepolymer with or without tertiary amine nitrogen groups was dissolved in dry MEK to obtain a prepolymer solution of 30-40% solids. It was then mixed with a mixture of 1,4-BD/TMP (4 1 by equiv. ratio) at an NCO/OH = 1.05/1.0 ratio in the presence of T-12 catalyst (0.05% based on total weight). The reaction mixture was cast in a metal mold treated with a release agent at ambient temperature. After standing 3-5 hours at room temperature, the mold was placed in an oven and post-cured at 100°C for 16 hours. The samples were then conditioned in a desiccator for one week before testing. 

The addition of isocyanates to hydroxy compoimds is inhibited by acid compounds (e.g., hydrogen chloride orp-toluenesulfonic acid) on the other hand, it can be accelerated by basic compounds (e.g., tertiary amines like triethylamine, N,hl-dimethylbenzylamine, and especially l,4-diazabicyclo[2.2.2]octane) and by certain metal salts or organometallic compounds (e.g., dibutyltin dilaurate, bismuth nitrate). These catalysts are often effective in amounts of much less than 1 wt%. 

Tertiary Amine Catalytic Activities in the Reaction of Phenyl Isocyanate w th 1-Butanol in Toluene at 39.69°C [11]... 

Initially, water can cause the hydrolysis of the anhydride or the isocyanate, Scheme 28 (reaction 1 and 2), although the isocyanate hydrolysis has been reported to occur much more rapidly [99]. The hydrolyzed isocyanate (car-bamic acid) may then react further with another isocyanate to yield a urea derivative, see Scheme 28 (reaction 3). Either hydrolysis product, carbamic acid or diacid, can then react with isocyanate to form a mixed carbamic carboxylic anhydride, see Scheme 28 (reactions 4 and 5, respectively). The mixed anhydride is believed to represent the major reaction intermediate in addition to the seven-mem bered cyclic intermediate, which upon heating lose C02 to form the desired imide. The formation of the urea derivative, Scheme 28 (reaction 3), does not constitute a molecular weight limiting side-reaction, since it too has been reported to react with anhydride to form imide [100], These reactions, as a whole, would explain the reported reactivity of isocyanates with diesters of tetracarboxylic acids and with mixtures of anhydride as well as tetracarboxylic acid and tetracarboxylic acid diesters [101, 102]. In these cases, tertiary amines are also utilized to catalyze the reaction. Based on these reports, the overall reaction schematic of diisocyanates with tetracarboxylic acid derivatives can thus be illustrated in an idealized fashion as shown in Scheme 29. 

Trimerization of imidates is a valuable route to 1,3,5-triazines. Imidates can be considered as activated nitriles and cyclotrimerize more readily. Most symmetrical 2,4,6-trialkyl-1,3,5-triazines are easily formed, although large alkyl substituents may give rise to steric hindrance (61JOC2778). Symmetrical isocyanurates (525) are readily available from isocyanates, RNCO catalysts include tertiary amines, phosphines and sodium methoxide. Aldehydes RCHO and ammonia give hexahydro-1,3,5-triazines (526), known as aldehyde ammonias (73JOC3288). 

Tertiary amines catalyze the homopolymerization of epoxy resins in the presence of hydroxyl groups, a condition which generally exists since most commercial resins contain varying amounts of hydroxyl functionality (B-68MI11501). The efficiency of the catalyst depends on its basicity and steric requirements (B-67MI11501) in the way already discussed for amine-catalyzed isocyanate reactions. A number of heterocyclic amines have been used as catalytic curatives pyridine, pyrazine, iV,A-dimethylpiperazine, (V-methylmorpholine and DABCO. Mild heat is usually required to achieve optimum performance which, however, is limited due to the low molecular weight polymers obtained by this type of cure. 

Symmetrical isocyanurates are readily synthesized from isocyanates in the presence of a variety of catalysts including tertiary amines, phosphines, sodium acetate and sodium methoxide in anhydrous conditions. The method has been reviewed thoroughly (B-71MI22002,... 

The results show that a number of ruthenium carbonyl complexes are effective for the catalytic carbonylation of secondary cyclic amines at mild conditions. Exclusive formation of N-formylamines occurs, and no isocyanates or coupling products such as ureas or oxamides have been detected. Noncyclic secondary and primary amines and pyridine (a tertiary amine) are not effectively carbonylated. There appears to be a general increase in the reactivity of the amines with increasing basicity (20) pyrrolidine (pKa at 25°C = 11.27 > piperidine (11.12) > hexa-methyleneimine (11.07) > morpholine (8.39). Brackman (13) has stressed the importance of high basicity and the stereochemistry of the amines showing high reactivity in copper-catalyzed systems. The latter factor manifests itself in the reluctance of the amines to occupy more than two coordination sites on the cupric ion. In some of the hydridocar-bonyl systems, low activity must also result in part from the low catalyst solubility (Table I). 

Fully cured polyurethanes present no health hazard they are chemically inert and insoluble in water and most organic solvents. Dust can be generated in fabrication, and inhalation of the dust should be avoided. Polyether-based polyurethanes are not degraded in the human body, and are therefore used in biomedical applications. Some of the chemicals used in the production of polyurethanes, such as the highly reactive isocyanates and tertiary amine catalysts, must be handled with caution. The other polyurethane ingredients, polyols and surfactants, are relatively inert materials having low toxicity. 

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