Dimer diisocyanate

Starting from DDA the corresponding dimer diisocyanate (DDl, OCN-R-NCO) can be manufactured by phosgenation or, alternatively, DDl can be manufactured via a phosgene-free route by reacting dimer acid chloride with sodium azide in acetonitrile. ... 

DDI has low toxicity and high resistance to moisture it reacts with diamines to form polyurea coatings that are internally plasticized and have excellent resistance to moisture and to yellowing by ultraviolet light. Besides coatings, important application fields for dimer diisocyanates, are polyurethane implants like stents or heart-assist devices and PUR rocket propellant liners or sealants. 

Other dimer acid markets include intermediates for nitriles, amines and diisocyanates. Dimers are also used in polyurethanes, in corrosion inhibition uses other than for downweU equipment, as a "mildness" additive for metal-working lubricants, and in fiber glass manufacture. 

In certain cases, even dimers of certain isocyanates, such as toluene diisocyanate or hexamethylene diisocyanate, can act as blocking agents, thermally reversing to regenerate the isocyanate [16,17]. 

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. 

Tris(hydroxymethyl)propane (TMP) TNT-based condensation monomers, 297 Toluene diamine (TDA), 222 Toluene diisocyanate (TDI), 200, 219 dimer of, 240... 

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. 

Asymmetric diisocyanates such as 2,4-TDI are more complex because the initial reactivity of the two isocyanate groups is not equivalent and the substitution effect amplifies the difference. The 4-NCO is about 10-20 times more reactive than the 2-NCO, but the reactivity ratio also depends on temperature (see Chapter 5). This difference also explains why the TDI dimer can be prepared quantitatively (Eq. 2.28). 

Dow Chemicals (2001) gives the following storage life data regarding Isonate 125M, which is a pure MDI (Table 3.1). A prepolymer prepared with pure MDI does not have the dimerization problems of the diisocyanate. 

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). 

Risch et al. reported the dimerization of diisocyanates in the presence of a catalyst to give only l,3-diazetidine-2,4-dione (uretidiones) (Scheme 25) (Section 2.13.7.3) <1999JPR616>. 

Fig. 15. Viscoelastic behavior of isocyanurate-oxazoiidone resins51). DEN 431 (polyglycidyl ether of phenol-formaldehyde novolac Dow Chemical Co.), EP 4080 (2,2-bis[p-(2,3-epoxypropyloxy)cyclo-hexane]propane Asahi Denka, Ltd.), EP 871 (diglycidyl ester of linoleic dimer acid Shell Chemical Co.), and L-MDI (modified diphenylmethane-4,4 -diisocyanate Desmodur CD Bater AG)...

These dimers can be dissociated to regenerate the original isocyanates with heat, and some of the dimers, such as the dimer of toluene diisocyanate, have become commercially important. 

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. 

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. 

Dimers, in contrast to trimers, are in dynamic equilibrium with monomer. Toluene diisocyanate dimerizes to a greater extent the lower the temperature, 90% at 10° compared to 73% at 25° (19). In the absence of a catalyst. 

The structure of the isocyanate affects its tendency to polymerize. While diisocyanates can polymerize to form resins due to cross-linking, the dimer of toluene 2,4-diisocyanate is readily formed as a separate compound. This is because the isocyanate group ortho to the methyl is relatively much less active than the para-isocyanate group ... 

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