Aliphatic isocyanate oligomers

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. 

Oligomers of phosgene, such as diphosgene [503-38-8], (COCl, have found use in the laboratory preparation of isocyanates. Carbamoyl chlorides, N,N-disubstituted ureas, dimethyl- and diphenylcarbonates, and arylsulfonyl isocyanates have also been used to convert amines into urea intermediates, which are subsequently pyrolyzed to yield isocyanates. These methods have found applications for preparation of low boiling point aliphatic isocyanates... 

Michael addition reactions are particularly useful when linear aliphatic bis-nitramines are used because the products contain two terminal functional groups like in the diester (182). The terminal functionality of such products can be used, or modified by simple functional group conversion, to provide oligomers for the synthesis of energetic polymers such oligomers often use terminal alcohol, isocyanate or carboxy functionality for this purpose. 

Ellwood PA, Hardy HL, Walker RF. 1981. Aliphatic and aromatic isocyanates and their oligomers were determined in air by high-performance thin-layer chromatography. Analyst(London) 106(1258) 85-93. 

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... 

NIPU networks are created by the reaction between polycyclic carbonate oligomers and aliphatic or cycloaliphatic polyamines with primary amino groups [4], This forms a cross-linked polymer with p-hydroxy urethane groups of a different structure—polyhydroxyurethane polymer. Since NIPU is obtained without using highly toxic isocyanates, the process of synthesis is relatively safe for both humans and the environment in comparison to the production of conventional polyurethanes. Moreover, NIPU is not sensitive to moisture in the surrounding environment. 

Besides aliphatic diisocyanates, an aromatic isocyanate was also used to prepare polyurethanes. A new renewable copolymer (Scheme 17) was synthesized from reactive bis-hydroxylated poly(3-hydroxybutyrate-co-3-hydroxyvalerate) oligomers (PHBHV-diol), isosorbide, and 1,4-phenylene diisocyanate. The molar number-average molecular weights (M of most of the copolyesters were about... 

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. 

---