Isocyanates trimerization catalysts

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. ... 

A comprehensive review of trimerization catalysts was prepared by Zhitankina et al (116). Isocyanate trimerization catalysts are shown in Table 11. 2-Oxazolidone catalysts and carbodiimide catalysts are shown in Tables 12 and 13 respectively. 

In the case of urethane-modification, two kinds of catalysts, i.e., urethane-formation catalysts and isocyanate-trimerization catalysts, are usually used. Major trimerization catalysts are listed in Table 11 and major urethane catalysts are listed in Tables 8 through 10. 

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 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). 

A special technique of trimerization has been described by Kogon 24, 25). Phenyl isocyanate reacts with ethyl alcohol to form a urethane (ethyl carbanilate). At 125° a substantial yield of ethyl a,7-diphenyl allophanate is observed as well as a small amount of phenyl isocyanate dimer. However, when A-methyhnorpholine (NMM) is added as a catalyst, the reaction is altered and the product is triphenylisocyanurate (isocyanate trimer) in high yield. The reaction sequence is believed to be ... 

As mentioned earlier, metal carboxylates are trimerization catalysts 23, 24). They also catalyze the reaction of aryl isocyanates and substituted ethyl carbanilates to give ethyl a,7-diarylallophanates even at room temperature (24). 

Nicholas and Gmitter (58) studied the relative effectiveness of a number of trimerization catalysts (see Table I). Isocyanurate coatings based on TDI and HDI have been reported by Kubitza (59). In addition, Sandler (60) prepared polyisocyanurate adhesives from isocyanate-terminated prepolymers employing calcium naphthenate as a trimerization catalyst. 

In their extensive efforts to devise a new strong nonionic base, Verkade and coworkers found that a highly basic dendrimer containing a PAPT base fragment could act as an efficient catalyst for Michael addition reactions, nitroaldol (Henry) reactions and aryl isocyanate trimerization reactions [42] (Figure 6.3). In view of the characteristic nature of this dendrimer, which has sixteen catalytic sites per molecule, the attachment of other superbase functionalities might also be attractive. 

The formation of isocyanurates in the presence of polyols occurs via intermediate allophanate formation, ie, the urethane group acts as a cocatalyst in the trimerization reaction. By combining cyclotrimerization with polyurethane formation, processibility is improved, and the friability of the derived foams is reduced. The trimerization reaction proceeds best at 90-100°C. These temperatures can be achieved using a heated conveyor or a RIM machine. The key to the formation of PUIR foams is catalysis. Strong bases, such as potassium acetate, potassium 2-ethylhexoate, and tertiary amine combinations, are the most useful trimerization catalyst. A review on the trimerization of isocyanates is available (104). 

Isocyanates undergo dimerization reactions by a [2+2] cycloaddition across their C=N bonds to give diazetidinediones 3. The isomeric unsymmetrical dimers have never been isolated but they are postulated to be intermediates in the formation of carbodiimides from isocyanates. The isocyanate dimers usually dissociate back to the monomers on heating. Therefore, they are considered to be masked isocyanates. The dimerization of isocyanates requires the use of a base or a Lewis acid as a catalyst, and often isocyanate trimers are formed as coproducts. 

Aliphatic isocyanate dimers are not common, and usually low yields are obtained in their dimerization reactions. An exception is the use of 1,2-dimethylimidazol as a catalyst for the dimerization of benzyl isocyanates, which provides the cyclodimers in good yields (see Table 3.1). In the benzyl isocyanate dimerization benzyl isocyanate trimers are also formed as coproducts, and when the reaction is conducted for more than 16 h at room temperature the dimers are slowly converted to trimers. The structure of the catalyst is of importance, as shown in Table 3.1. A slight change in the substituents of the heterocyclic carbene catalyst affords either a 64 % yield of cyclohexyl isocyanate dimer or a 100 % yield... 

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. 

A comprehensive review of trimerization catalysts appeared in 1985 . The need to synthesize high purity triphenyl isocyanurate as an activator for the continuous anionic polymerization of caprolactam to nylon-6 prompted the development of new catalysts, which trimerize aryl isocyanates at a fast rate at room temperature, achieving virtually quantitative yields. Examples of these new supercatalysts are tetrabutylammonium fluoride (TBAF), cesium fluoride and [P(MeNCH2CH2)3N]. The latter catalysts gives a 97 % yield of triphenyl isocyanurate after 3.8 min at room temperature (catalyst concentration, 0.33 mol%). Likewise, TBAF affords 98.8 % of triphenyl isocyanurate after 5 min at room temperature. A similar fast trimerization can be achieved using potassium t-butoxide as the catalyst . 

Since some catalysts initiate dimerization and trimerization under specific conditions, equilibria between monomeric isocyanates/catalyst, dimer/catalyst and trimer/catalyst exist. For example, interrupting the trimerization of phenyl isocyanate with a penta-substituted guanidine catalyst after short periods of time leads to the formation of the phenyl isocyanate dimer. ... 

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. 

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. 

Oligomerization and Polymerization Reactions. One special feature of isocyanates is their propensity to dimerize and trimerize. Aromatic isocyanates, especially, are known to undergo these reactions in the absence of a catalyst. The dimerization product bears a strong dependency on both the reactivity and stmcture of the starting isocyanate. For example, aryl isocyanates dimerize, in the presence of phosphoms-based catalysts, by a crosswise addition to the C=N bond of the NCO group to yield a symmetrical dimer (15). 

Dimerization is reportedly catalyzed by pyridine [110-86-1] and phosphines. Trialkylphosphines have been shown to catalyze the conversion of dimer iato trimer upon prolonged standing (2,57). Pyridines and other basic catalysts are less selective because the required iacrease ia temperature causes trimerization to compete with dimerization. The gradual conversion of dimer to trimer ia the catalyzed dimerization reaction can be explained by the assumption of equiUbria between dimer and polar catalyst—dimer iatermediates. The polar iatermediates react with excess isocyanate to yield trimer. Factors, such as charge stabilization ia the polar iatermediate and its lifetime or steric requirement, are reported to be important. For these reasons, it is not currently feasible to predict the efficiency of dimer formation given a particular catalyst. 

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. 

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). 

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). 

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]. 

The effect of relative permitivity D on the cyclo-trimerization of phenyl isocyanate was studied in the solvent system acetonitrile (AN) - ethylacetate (EA) using cyclic sulfonium zwitterion VI as catalyst. It was found that the rate constant increased with the increase of relative permitivity D of the solvent system, the experimental data correlate well with the Kirkwood equation ... 

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). 

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