Isocyanurates isocyanate trimerization

Aliphatic isocyanurates (isocyanate trimers) have a strong C=0 band at 1700-1680 cm with a weak shoulder near 1755 cm" Aromatic isocya-nurates have a higher C—0 frequency at 1715-1710 cm with a weak shoulder near 1780 cm" because of the electron withdrawing group on the nitrogen. 

When compound 13 is heated above the melting point, it undergoes rearrangement to give Wethyl-l,2,4-dithia-zolidine-3,5-dione 44 as the main product and triethyl isocyanurate 45, an ethyl isocyanate trimer, as the minor product (Equation 5) <1996JOC6639>. 

Modification of poly(carbodiimide) foams with polyols afford hybride foams containing urethane sections. However, the thermal stabilities of the poly (urethane carbodiimide) foams are lower. Using isocyanate trimerization catalysts, such as l,3,5-tris(3-dimethylaminopropyl)hexahydro-s-triazine, in combination with the phospholene oxide catalyst gives poly(isocyanurate carbodiimide) foams with improved high temperature properties. The cellular poly(carbodiimide) foams derived from PMDI incorporate six-membered ring structures in their network polymer structure. ... 

Another possible cross-linking reaction is isocyanurate, or isocyanate trimer, formation ... 

Because of the high cross-Knk density of polyisocyanurates, the resultant foam tends to be brittle. Consequently, there has been a move toward making polyisocyanurate-polyurethance combinations. For example, the isocyanate trimerization reaction has been carried out with isocyanate end-capped TDI-based prepolymers to make isocyanurate-containing polyurethane foams. Isocyanate trimerization in the... 

The polymerization of the isocyanate groups to high-molecular-weight products occurs only at low temperatures and with specific initiators. At somewhat higher temperatures, reactive isocyanates are dimerized to uret-diones (labile), while at even higher temperatures, less reactive isocyanates trimerize to isocyanurates (stable) ... 

Polyisocyanurate (PIR) n. A polymer containing isocyanurate rings, i.e., isocyanate trimer, and forming foams that have better fire resistance than rigid polyurethanes, but are more brittle, so are... 

In the reaction of the phenyl phosphoramidate anion 53 (R = Ph) with carbon dioxide only triphenyl isocyanurate is obtained because the generated phenyl isocyanate trimerizes under the reaction conditions. Also, from sodium diethyl A -alkoxy-phosphoramidate (R = OR) and carbon dioxide only 1,3,5-trialkoxyisocyanurates are obtained . 

The high selectivity for dimer formation observed with trialkylphosphines can be explained by a stepwise mechanismThe general base catalysis of oligomerization of isocyanates involves the dimeric species as intermediates in the formation of the more stable isocyanate trimers (isocyanurates). Steric effects also play a role in the dimerization of isocyanates because o-tolyl isocyanate does not dimerize. This fact is utilized in the selective dimerization of 2,4-TDI 7, using a polymeric imidazole catalyst. In this reaction only the non-hindered isocyanate group participates to form the [2-1-2] cyclodimer 8. 

The formation of aromatic isocyanate trimers is of economic importance, because rigid insulation foams, having isocyanurate structures built into their network structure, are produced from aromatic diisocyanates. Triphenyl isocyanurates with hydroxyl or carboxyl groups in their p-positions can be obtained on hydrolysis of McsSiO- and McsSiOCO-groups, respectively, with hydrochloric acid °. Such trifunctional compounds are of use in the construction of network polymers. The mechanism of the phenyl isocyanate trimer-ization, using Pd(o) diimide catalysts was elucidated recently. The initial steps of this trimerization reaction involve a chain growth process as encountered in the anionic homopolymerization of isocyanates. 

Commonly used isocyanates are toluene dhsocyanate, methylene diphenyl isocyanate, and polymeric isocyanates. Polyols used are macroglycols based on either polyester or polyether. The former [poly(ethylene phthalate) or poly(ethylene 1,6-hexanedioate)] have hydroxyl groups that are free to react with the isocyanate. Most flexible foam is made from 80/20 toluene dhsocyanate (which refers to the ratio of 2,4-toluene dhsocyanate to 2,6-toluene dhsocyanate). High-resilience foam contains about 80% 80/20 toluene dhsocyanate and 20% poly(methylene diphenyl isocyanate), while semi-flexible foam is almost always 100% poly(methylene diphenyl isocyanate). Much of the latter reacts by trimerization to form isocyanurate rings. 

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. 

Reportedly, simple alkyl isocyanates do not dimerize upon standing. They trimerize to isocyanurates under comparable reaction conditions (57). Aliphatic isocyanate dimers can, however, be synthesized via the phosgenation of A[,A[-disubstituted ureas to yield /V-(ch1orocarhony1)ch1oroformamidine iatermediates which are subsequendy converted by partial hydrolysis and base catalyzed cycUzation. This is also the method of choice for the synthesis of l-alkyl-3-aryl-l,3-diazetidiones (mixed dimers of aromatic and aUphatic isocyanates) (58). 

Both alkyl and aryl isocyanates are found to trimerize upon heating or ia the preseace of catalysts to 1,3,5-trisubstituted hexahydro-j -triaziaetrioaes (18) (isocyanurates) (57). Only highly substituted isocyanates, such as tert-huty isocyanate [7188-38-7] and tert-octy isocyanate, fail to trimerize under these conditions. 

Urethane network polymers are also formed by trimerization of part of the isocyanate groups. This approach is used in the formation of rigid polyurethane-modified isocyanurate (PUIR) foams (3). 

Organic Derivatives. Although numerous mono-, di-, and trisubstituted organic derivatives of cyanuric and isocyanuric acids appear in the hterature, many are not accessible via cyanuric acid. Cyanuric chloride 2,4,6-trichloro-j -triazine [108-77-0], is generally employed as the intermediate to most cyanurates. Trisubstituted isocyanurates can also be produced by trimerization of either aUphatic or aromatic isocyanates with appropriate catalysts (46) (see Isocyanates, organic). Alkylation of CA generally produces trisubstituted isocyanurates even when a deUberate attempt is made to produce mono- or disubstituted derivatives. There are exceptions, as in the production of mono-2-aminoethyl isocyanurate [18503-66-7] in nearly quantitative yield by reaction of CA and azitidine in DMF (47). 

In addition, isocyanates may, under appropriate conditions, react with themselves to give dimers, trimers (isocyanurates) and carbodi-imides. 

The isocyanurate reaction occurs when three equivalents of isocyanate react to form a six-membered ring, as shown in the fifth item of Fig. 1. Isocyanurate linkages are usually more stable than urethane linkages. Model compound studies show no degradation of the trimer of phenyl isocyanate below 270°C [10,11]. Catalysts are usually needed to form the isocyanurate bond. Alkali metals of carboxylic acids, such as potassium acetate, various quaternary ammonium salts, and even potassium or sodium hydroxide, are most commonly used as catalysts for the isocyanurate reaction. However, many others will work as well [12]. 

Trimerization to isocyanurates (Scheme 4.14) is commonly used as a method for modifying the physical properties of both raw materials and polymeric products. For example, trimerization of aliphatic isocyanates is used to increase monomer functionality and reduce volatility (Section 4.2.2). This is especially important in raw materials for coatings applications where higher functionality is needed for crosslinking and decreased volatility is essential to reduce VOCs. Another application is rigid isocyanurate foams for insulation and structural support (Section 4.1.1) where trimerization is utilized to increase thermal stability and reduce combustibility and smoke formation. Effective trimer catalysts include potassium salts of carboxylic acids and quaternary ammonium salts for aliphatic isocyanates and Mannich bases for aromatic isocyanates. 

Trimerization of isocyanate groups to form isocyanurates also occurs and serves as an additional source of branching and crosslinking ... 

Its salts also exist in the form of a trimer, cyanuric acid (III), which is produced on heating the salts of isocyanic acid with acetic acid. Esters of cyanuric acid undergo isomerization when heated and are converted into esters of isocyanuric acid (IV) ... 

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). 

In general, mixed trimerization of isocyanates also gives mixtures (61JOC3334) however, it is possible to prepare isocyanurates of the type (157) from arenesulfonyl isocyanates in the presence of 1,2-dimethylimidazole as catalyst (76JOC3409). The mechanism is similar to that of the trimerization of isocyanates (Scheme 95). 

Reactions with Isocyanates, TYZOR TPT catalyzes the trimerization of isocyanates and polyisocyanates to isocyanurates and polyisocyanurates (38). Titanium alkoxides of the type Cl3TiOR initiate the living polymerization of isocyanates. Polyisocyanates possessing controlled molecular weights and narrow polydispersities can be synthesized using these catalysts (39) ... 

Three isocyanate groups can react to form a trimer or substituted isocyanurate ring. Phosphines or bases such as sodium acetate or sodium formate can catalyze this reaction. The isocyanurate ring is thermally stable, has good chemical resistance, and can enhance the resistance of a urethane adhesive to aggressive environments. 

Physicochemical Reactions with MIC MIC can interact with a large number of molecules as well as with itself Indeed, 21 identified (Varadarajan et al., 1985) and almost ten unidentified chemicals (Sriramachari, 2004) were detected in the culprit MIC Tank E-610. An MIC trimer as well as other metabolites of MIC such as dimethyl isocyanurate and 2,4-dione of methyl isocyanate were identified in autopsy samples from Bhopal victims (Chandra et al, 1991, 1994 Saraf et al, 1995). Reaction of MIC with water is important because this will occur whenever MIC comes into contact with a body or environment, as happened in Bhopal. It is important to note that while excess water can neutralize MIC, a small quantity of water is sufficient to generate heat during the reaction, which would lead to vaporization of MIC, as actually happened in Bhopal. Some important interactions of MIC are enumerated below ... 

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