Isocyanates dimers

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). 

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). 

Asymmetric aryl isocyanate dimers, ia which the C=0 group of oae molecule reacts with the C=N group of another, have been postulated as labile iatermediates ia the formation of carbodiioiides (17) upoa heating isocyanates. 

Heating of isocyanates above 150 °C slowly produces carbodiimides. For example, heating of hexamethylene diisocyanate at 189-195 °C for 20 hr produced 4-6 % of oligomeric isocyanate terminated carbodiimides, but in addition 18-20 % of isocyanate terminated isocyanurates were formed. The reaction is facilitated if a slow stream of nitrogen is passed through the boiling isocyanate. The unsymmetrical isocyanate dimer 47 was proposed as an intermediate in this transformation. 

Scheme 126 A peculiar isocyanate dimer from phosgenation of substituted isoxa-zolamines.

In this volume, we have already examined some alternatives to the synthesis in situ and/or to the use of methyl isocyanate. We thought that 1,3-dimethyl diazetidine dione (XII) (methyl isocyanate dimer) should be a suitable methyl isocyanate precursor which can release methyl isocyanate in safe conditions. This dimer was prepared from N-chlorocarbonyl-N-methyl N -methyl urea (XI) obtained by phosgenation of N,N -dimethyl urea as previously described in scheme 156, section 3-3-4. Cyclization of (XI) in a suitable solvent and in presence of a base such as DABCO afforded the expected dimer (XII) as depicted in scheme 220. 

Isocyanate dimers are solids having a fairly high melting point. For example, the dimer of phenyl isocyanate melts at 175°. As expected, diisocyanates can polymerize to form resins. Much attention has been given to the physical properties of dimers including mixed dimers (2, 17), but such properties are not of primary concern here. 

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 ... 

The same compound was obtained when phenyl isocyanate dimer was treated with COF under similar conditions. 

Isocyanates can also react with other isocyanate molecules to form oligomers (Fig. 4). This polymerization is more likely to occur in the presence of basic catalysts.Isocyanate dimers, also called uretidine-diones, can only be formed by aromatic isocyanates, and uretidinedione formation is inhibited by ortho substituents. Hence, only 4,4-diphenylmethanediiso-cyanate (MDI) dimerizes at room temperature, and its rate of formation is quite low. At higher temperatures, MDI would form an insoluble polymeric mate-rial. Trimers of isocyanates are also possible these structures are called isocyanurates. Isocyanurates are formed by both aliphatic and aromatic isocyanates, and the resulting structure is highly stable to a temperature of approximately Isocyanurates give... 

Isocyanate Dimers and Trimers, Aromatic isocyanates have a tendency to dimerize readily... 

Isocyanate dimerization is an equilibrium reaction. Dissociation of the dimer occurs only at elevated temperatures (42). In the absence of catalysts, temperatures as high as 175 C are required to completely dissociate the dimer of 2,4-TDI, although initial dissociation was observed to occur at 150 C (43). The preparation of aliphatic isocyanate dimers has also been reported (44). 

Alkoxythiocarbonyl isothiocyanates (172 X = S) are also useful 47t components and with imines at room temperature furnish 6-alkoxy-2//-l,3,5-thiadiazine-4(3//)-thiones (173) albeit in variable yields <83CB2044>. In a similar manner ethoxythiocarbonyl isocyanate (172 R = Et, X = O) and benzylideneaniline produce 2,3-diphenyl-6-ethoxy-2/f-l,3,5-thiadiazin-4(3//)-one (174) <82CB1252>. In warm toluene the ethoxythiocarbonyl isocyanate dimerizes to give the 0-ethyl ester (175) of 6-ethoxy-3,4-dihydro-2,4-dioxo-2//-l,3,5-thiadiazine-3-thiocarboxylic acid (Scheme 24). 

Uretdione groups (isocyanate dimers traces originate from the as-received Desmodur CD component) are subject to similar effects. In films on copper, uretdion bands (1780 cm ) vanish during post-curing, independently of fihn thickness (Figs. 6.7 and 6.9). 

Dihydro-2//-l,3,5-thiadiazines containing a ring carbonyl or thiocarbonyl group are synthesized from reaction of thioamides with phenoxycarbonyl isocyanate, dimerization of thiocarbamoylisothiocyanates, or dimerization of carbamoyl isocyanates. The latter two [4-1-2] cycloaddition reactions complement the tabulated list of dienes and dienophiles presented in Table 2 (Section 9.09.9.2). 3,6-Disubstituted-3,4-dihydro-2//-l,3,5-thiadiazines are synthesized by treatment of N-substituted A, Wbis(l//-l,2,3-benzotriazol-l-ylmethyl)-amines with thioamides and zinc bromide (Section 9.09.9.1.3). 

The reaction of the difluoro compound CXXI with KOCN yields the isocyanate dimer CXXII and l,2-bis(dialkylamino)-l,2-diisocyanatoethylene (CXXIII) ( ). 

Phenyl-3,3-diethyl urea Diphenyl urea 1,2-Bis(2 -phenylureido) ethane Phenyl isocyanate dimer 3312 3442, 3425, 3325 (shoulder) 3449, 3329, 3305 (max.) 3447, 3329 3440 (max.) 3300 (shoulder)... 

Trimeric isocyanates also contain the CO—N—CO system. AUcyl derivatives show their main band around 1700—1680 cm" with a weaker shoulder at 1715—1710 cm". Aromatics have a higher frequency with the main band near 1715 cm" and the weaker shoulder near 1780 cm" This is parallel to the raised carbonyl frequencies of anilides as compared with alkyl amides. Isocyanate dimers are of some theoretical interest in that the carbonyl groups are in direct opposition so the symmetric mode is forbidden in the infra-red. Only a single carbonyl band is therefore seen, and this is near 1780 cm"... 

The formation o carbodiimide group as indicated in IR spectrum at 2200 cm may come from the isocyanate generated from the same mechanism as shown in Equation (8). Consequently, the isocyanate dimerizes and releases CO under the catalyzation of chemical species in the decomposition product. 

---