Toluene diisocyanate trimerization

Commonly used isocyanates are toluene diisocyanate, methylene diphenyl isocyanate, and polymeric isocyanates. Polyols used are macroglycols based on either polyester or polyether. The former [polyethylene phthalate) or polyethylene 1,6-hexanedioate)] have hydroxyl groups that are free to react with the isocyanate. Most flexible foam is made from 80/20 toluene diisocyanate (which refers to the ratio of 2,4-toluene diisocyanate to 2,6-toluene diisocyanate). High-resilience foam contains about 80% 80/20 toluene diisocyanate and 20% poly(methylene diphenyl isocyanate), while semi-flexible foam is almost always 100% poly(methylene diphenyl isocyanate). Much of the latter reacts by trimerization to form isocyanurate rings. 

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. 

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. 

U.S. Pat. No. 4,376,144 [60] discloses a thermoplastic composite in which cellulose hber (hardwood pulp, from 1 to 40% by weight) is attached to the PVC matrix with an isocyanate bonding agent (a cyclic trimer of toluene diisocyanate). 

The principal commercial aromatic isocyanates are toluene diisocyanate (TDI), polymeric isocyanate (PMDI), and its coproduct (MDI), but some specialty aromatic isocyanates and some aliphatic diisocyanates are also available. The latter are used in the manufacture of color stable pol5uirethanes. TDI is mainly produced as a 80 20 mixture of the 2,4- and the 2,6-isomers, but a 65 35 mixture of the same isomers, pure 2,4-TDI and a crude partially trimerized product, are also available. PMDI is a mixture of the MDI isomers and their higher oligomers, and MDI is mainly the 4,4 -diisocyanate. The principal commercially available isocyanates are listed in Table 2. 

Diisocyanates are commonly used with either diols or a mixture of diols and polyols. When diols are exclusively used and the trimerization reaction is minimized, the polyurethane is substantially linear, but it becomes increasingly three-dimensional with increasing levels of polyol or with trimerization of the diisocyanate. The trimerization reaction and the use of polyols enable thermoset polyurethanes. Common diisocyanates include toluene diisocyanate (TDl), methylene diphenyl diisocyanate (MDl), naphthalene diisocyanate (NDl), and hexamethylene diisocyanate (HDl). TDI is prepared by the nitration of toluene to give a mixture of mainly 2,4- and 2,6-dinitrotoluene, followed by reduction and reaction with phosgene. The isomers are typically not separated and are used as a mixture. 

The polymer is prepared by crosslinking p-CyD in DMSO with toluene 2,4-diisocyanate (TDI) in the presence of cholesterol as the template. In order to obtain homogeneous samples, the amount of TDI is kept small. Matrix-assisted laser desorption/ionization time-of-flight mass spectra (MALDI-TOFMS) are presented in Fig. 5.5. In the presence of the template (a), both the dimers of P-CyD (mass number (M) = 2000-3500) and its trimers (M = 4000-4500) are efficiently formed. In its absence (b), however, virtually all the products are monomeric P-CyDs (M = 1000-2000). The template enormously accelerates the bridging between two P-CyD molecules. Each of the signals in the spectra corresponds to different amount of substitution by TDI. These analyses clearly show that dimeric P-CyDs (the binding sites for choles-... 

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