Diisocyanates dimerization reactions

Isocyanates are capable of co-reacting to form dimers, oligomers and polymers. For example, aromatic isocyanates will readily dimerize when heated, although the presence of a substituent ortho to the -NCO group reduces this tendency. For example, toluene diisocyanate (TDI) is less susceptible to dimer formation than diphenylmethane diisocyanate (MDI). The dimerization reaction is reversible, with dissociation being complete above 200 °C. It is unusual for aliphatic isocyanates to form dimers, but they will readily form trimers, as do aromatic isocyanates. The polymerization of aromatic isocyanates is known, but requires the use of metallic sodium in DMF. 

A drawback to the cast elastomers is limited shelf-life and a need to store them in the absence of moisture. As a result, millable elastomers were developed. These are produced by first forming hydroxy-terminated linear polyurethanes through reactions of linear aliphatic polyesters or polyethers with diisocyanates. The prepolymers are rubber-like gums that can be compounded on rubber mills with other ingredients and crosslinked. Crosslinking is accomplished by adding either more diisocyanates, or sulfur, or peroxides. Diisocyanates dimers that dissociate at about 150 °C are often used ... 

A three-necked flask equipped with a condenser and stirrer was charged with the PET depolymerization product (0.05 mol of BHET and dimer in the ratio of 80 to 20 wt%), 0.05, 0.10, and 0.15 mol of e-caprolactone (in separate experiments), and 0.1 wt% of dibutyltin dilaurate. The reaction mixture was heated at 150°C for 2 h. The resulting co-oligomer (0.01 mol) was dissolved in 500 mL of tetrahydrofuran in a three-necked flask equipped with a condenser and a stirrer. After the temperature was raised to 67°C, a solution of 0.01 mL of hexamethylene diisocyanate in 50 mL of tetrahydrofuran was added dropwise. After heating and stirring the reaction mixture for 12 h, it was cooled and precipitated in ether. The polyurethane precipitate was collected by filtration and dried at 70°C for 12 h. 

Risch et al. have reported on the reaction of l,3-diazetidine-2,4-diones, which are used in the preparation of polymers and advanced materials <1999JPR616>. With the help of special catalysts it was possible to dimerize or trimerize diisocyanate to give l,3-diazetidine-2,4-diones 183 or isocyanurates 184 (Scheme 25). 

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. 

Other aliphatic dllsocyanates that are being used commercially in urethane coatings are 3-(isocyanatomethy1)-3,5,5-trimethyIcyclohexyl isocyanate (Veba-Chemie A.G.) (28). "dimeryl" diisocyanate derived from dimerized linoleic acid (Henkel Corp.) (29). and xylylene diisocyanate (XDI) (Takeda Chemical Co.) (30). It is interesting to note that no catalysts are required for the reaction of XDI with hydroxyl compounds and that its reactivity is similar to that of TDI. 

These compounds are usually listed as uretidinediones, although the term dimer is frequently applied. The practical consequence of this reaction is that some diisocyanates have a limited storage life, losing activity at different rates. 

A routine measurement of diisocyanate content is usually desirable because, as most of the isocyanates are prepared by the phosgenation process, free phosgene may be present if it has not been removed in the final purification step. This will be included in the hydrolyzable chloride test, and partly included in the acidity test, if hydrolysis takes place. Also, if the diisocyanate has been stored under adverse conditions, some isocyanate could be converted by moisture to insoluble urea. Alternatively some dimerization may occur. In both cases isocyanate is consumed and the result is a decrease in active content. Additionally, diisocyanates do show some batch to batch variations in activity hence it is convenient to measure their reactivity against a control, a polyester (or polyether) of previously known activity, by making a small quantity of the polyurethane and measuring its final reaction exotherm temperature, rate at which its exotherm is achieved, pot life and time to achieve gelation. If a more... 

Isocyanates will also undergo dimerization and trimerization reactions, such as those shown in Eqs. (4) and (5). The dimers (7), which can only be formed from aromatic isocyanates, are termed uretidinediones or uretidiones and the trimers (8) are known as isocyanurates. A catalyst is generally needed for these reactions to take place, although certain non-sterically hindered aromatic isocyanates, such as 4,4 -diphenylmethane diisocyanate (MDI) will slowly dimerize at room temperature without a... 

The selective reaction of the p-isocyanato group in 2,4-TDI is used to produce the TDI dimer, which is a higher melting solid diisocyanate. In Table 3 some modified commercial diisocyanates are listed. 

The analogous strategy for synthesizing metal-metal bond-containing polymers also uses difunctional, cyclopentadienyl-substituted metal dimers. A sample polymerization reaction is shown in equation 9, which illustrates the reaction of a metal-metal bonded diol with hexamethylene diisocyanate (HMDl) to form a polyurethane. ... 

Isocyanate, the most commonly used hardeners for polyurethane (diphenylmethane diisocyanate [MDI]) has a low viscosity and undergoes crystallization in chilled conditions under 15°C. At temperatures above 40°C, isocyanate undergoes a chemical reaction (dimerization), which increases its viscosity. In addition, upon exposure to moisture, it reacts with... 

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