Synthesis of Basic Urethane Building Blocks

Only the diisocyanates are of interest for urethane polymer manufacture and relatively few of these are employed commercially. The most important ones used in elastomer manufacture are the 2,4- and 2,6-toluene diisocyanates (TDl) 4,4 -diphenylmethane diisocyanates (MDI) and its aliphatic analogue 4,4 -dicyclohexylmethane diisocyanate (Hi2 MDI) 1,5-naphthalene diisocyanate (NDI) 1,6-hexamethylenediisocyanate (HDI) xylyene diisocyanate (XDI), isophorone diisocyanate (IPDI) and 3-isocyanatomethyl-3,5,5-trimethylcyclohexyl isocyanate (TMDI). Also manufactured commercially are various monoisocyanates, including methyl, /7-propyl, /7-butyl, cyclohexyl, phenyl, and 4-chloro- and 3,4-dichlorophenyl isocyanates which are used for substituted ureas and carbamates important as herbicides and crop protection agents. 

Some of the basic diisocyanates are converted on a commercial scale into derived products containing residual isocyanate groups such as polymerized diisocyanates of polyfunctional isocyanates to give low to nonvolatile products. 

Isocyanates can be made in many ways. The chemical laboratory routes. 

The use of azides in the Curtius reaction is hazardous and the utility of the Hoffman and Lossen rearrangements is limited to preparation of aliphatic isocyanates, as aqueous media are employed (aromatic isocyanates react readily with water to form substituted ureas). Tertiary butyl hypochlorite can be used for non-aqueous Hoffman rearrangements but is costly. In practice mostly phosgenation of a primary amine is commercially important  

This route, first used by Hentschel (1884), enables a wide range of aliphatic and aromatic isocyanates to be obtained from the corresponding amines. The laboratory preparation of numerous examples by this method has been described by Siefken (1949). 

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