Isocyanates tertiary amine catalysis

Two main routes have been described for the preparation of hydantoins cyclization of urea-derivatized amino acids and of carbamate-bound amino acid carboxamides (Figure 15.10) [95, 96]. In the first route, a bound amino acid is treated with an isocyanate to obtain the urea that is subsequently cyclized and released by acid (HC1) or base (tertiary amines) catalysis upon warming (60-100 °C) [95, 97]. The second route employs a carbamate linkage to attach the amino acid through the amino... 

As stated, tertiary amines catalyze both the hydroxyl/isocyanate and the water/isocyanate reactions. One-shot foams utilizing primary hydroxyl-terminated polyesters as well as all types of prepolymer foams require tertiary amine catalysis only. Polypropylene ether one-shot foam formulations based on triols, in part, because of their low viscosity (about 300 cP versus 10000-30000 cP for polyesters or prepolymers) require the use of tertiary amine-metal catalyst combinations, even if the percentage of primary hydroxyl groups in the polyether is increased by capping with ethylene oxide. This is because of the relatively low polypropylene glycol activity. 

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. 

Naegeli et al. [177] reported mild catalysis by tertiary amines and carboxylic acids, but not by water, inorganic acids, salts or bases. In contrast. Craven [179] found that the typical tertiary amines and acids had little catalytic effect in the systems he studied. Certain substituted ureas appeared to catalyse the reaction to a greater extent than many tertiary amines. Ten mole % of butyric acid reduced the half-life of the reaction between phenyl isocyanate and o-toluidine by 57%, and 10 mole % of N-phenyl-N -o-tolylurea reduced it by 38%. Arnold et Jil. [178] and... 

Entelis assumes the formation of an activated alcohol-isocyanate binary complex during the catalysis of the methanol-phenyl isocyanate reaction by dibutyltin dilaurate (DBTDL) (3, 5) Activated alcohol-isocyanate-catalyst ternary complexes have also been proposed by others. However, significant differences can be noted in the structures of either the postulated one (2, 4, 6, 7) or two (8) coordination positions of the isocyanate to the metal. To explain the synergistic effects observed when tertiary amine and organometallic compounds are combined, several authors suggest the formation of an activated quaternary complex I, II or III (2, 6, 9, 10, 11, 27). 

Tertiary amines and tin carboxylates are important catalysts in the production of polyurethane foams from polyisocyanates and and polyhydroxy compounds. Many articles have been written on the mechanism of the catalysis of the isocyanate-alcohol reactions by such compounds. Farkas and Mills (1), Entelis and Nesterov (2), Frisch and Rumao (3)and Petrus (4) have written excellent reviews on this subject. Wolf (5) has shown that these catalysts are synergistic to each other. 

The action of the tin catalyst was found to be quite different from the action of the tertiary amine catalyst. In the presence of the amine catalyst the reactivity of the phenol and benzyl alcohol was approximately equal (see Table IV). In the case of DBTDL, the reactivity ratio was similar to that of the non-catalyzed reaction, which indicates that the polarization of the isocyanate by the tin catalyst due to complex formation presumably played an important role in the reaction catalysis (see Table VI). 

Catalysis plays an important role in the deblocking or thermal dissociation of the blocked isocyanates. Notably organometallie compounds and tertiary amines are capable of lowering both the deblocking temperature and time as compared to the uncatalyzed systems. Wicks (61) has pointed out that since most of the deblocking reactions are carried out in the presence of hydroxyl... 

A great number of both tertiary amines and metal catalysts are available, and a detailed discussion of these as well as their mechanisms is beyond the scope of this paper. The reader is referred to several reviews on the subject of catalysis in isocyanate reactions (118-120). 

Catalysts exert strong influence on the rates of reactions of isocyanates with active hydrogens compounds. Those most widely used are tertiary amines and metal salts, particularly tin compounds. The mechanism of catalysis by tertiary amines is believed to proceed according to the following scheme ... 

Various catalysts are used to prepare polyurethane at a relatively low temperature and with a much faster rate of polymerisation than would be the case with an uncatalysed reaction. Catalysts may be classified into two broad categories namely, amine (basic) compounds and organometalhc complex compounds. Tertiary amine is stiU one of the most frequently used urethane catalysts. Commonly used amine catalysts are triethylenedi-amine (TEDA), l,4-diazabicyclo[2.2.2]octane (DABCO), triethylamine (TEA), dimethylethanolamine (DMEA) and dimethylcyclohexylamine (DMCHA). The catalysis mechanism of tertiary amine catalysed urethane reaction involves complexation of the amine with isocyanate groups, followed by reaction of the complex with alcohol to produce polyurethane. A list of catalysts used in polyurethane preparation is given in Table 6.4. 

Two general classes are very well known. The first is composed of tertiary amines, of which DABCO, tertiary alkylamines, substituted morpholines, piperazines, guanidines, and substituted hydroxy amines are representative types. A wide range of activities is represented in this class, but stronger catalysts are sometimes needed to promote the reaction of secondary hydroxyl groups with isocyanates. Stronger catalysis is supplied by catalysts of the second class, organotin compounds like dibutyltin dilaurate... 

The reaction of isocyanates with alcohols and with water can be catalyzed by amines and by organometallic compounds. Tertiary amines, such as l,4-diazo-[2.2.2]-bicyclooctane (DABCO) or triethylamine, are particularly effective in promoting the isocyanate-water reaction, while organometallic complexes, such as dibutyltin dilaurate or stannous octoate, are very useful for catalyzing isocyanate-alcohol reactions. Numerous articles have been written on various aspects of the catalysis of isocyanate reactions and representative examples are cited in refs. 8-10. 

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). 

Huynh-Ba, G., and Jerome, R. (1981). Catalysis of isocyanate reactions with protonic substrates a new concept for the catalysis of polyurethane formation via tertiary amines and organometaUic compounds. In Urethane Chemistry and Applications (Edwards, D. N., ed.), ACS, Washington D.C. 

Reactions between isocyanates and active hydrogen compounds are also extremely susceptible to catalysis and many commercial applications of isocyanates utilize catalyzed reactions. The most widely used catalysts are tertiary amines and certain metal compounds, particularly tin compounds. The mechanisms through which these catalysts operate are probably similar to that shown above. Thus tertiary amine (R aN) catalysis is thought to proceed as follows ... 

The catalysis of isocyanate reactions has been extensively studied because of its critical importance in many of these processes. Noncatalyzed (or rather, self-catalyzed) reactions may sometimes be fast enough in practice isocyanate reactions with amines are so fast that only recent studies using stopped-flow methods could lead to useful data [255, 256], metallic or tertiary amine catalysts being ineffective in this case. 

A similar substitution on anilines causes the reverse effect. Nitro groups in ortho position either in the isocyanate or the aniline lower the reactivity by steric hindrance. These authors also reported that the reaction is subject to catalysis by pyridine, tertiary bases, and certain carboxylic acids but is unaffected by water, inorganic acids, bases, or salts. Relative rates for the reactions of some primary aliphatic amines with phenyl isocyanates have been determined by Davis and Ebersole (52). 

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