Isocyanates dimerisation

Diphenylmethane diisocyanate (MDI) is another important raw material in polyurethane manufacture. It is a solid, melting at 37°C, and has a tendency to dimerise at room temperature. Several other aromatic isocyanates are used in the preparation of polyurethanes but are not widely used. Various aromatic isocyanates are illustrated in Figure 2.14. 

Isocyanates have some important reactions without the participation of active hydrogen compounds. These reactions, of real importance in polyurethane chemistry are dimerisation, trimerisation, formation of carbodiimides and reaction with epoxides and cyclic anhydrides. 

Isocyanates give two types of dimerisation reactions formation of uretidinediones and of carbodiimides [1, 3, 12, 13, 15, 23-25] ... 

Some data are available about catalysis in 1,2-cycloadditions. Tributyl phosphine catalyses dimerisation of phenyl isocyanate to uretidinedione in toluene . The reaction is kinetically of first order with respect to catalyst and overall third order the reverse process is first order with respect to catalyst and overall second order. The mechanism is complex, as revealed by the value of the apparent activation energy of the forward reaction (E= l.l 0.7 kcal.mole" ), which presumably results from the combined temperature dependence of two or more steps, including formation of an isocyanate-phosphine complex (see eqn. (13), p. 113). 

Phenyl isocyanate dimerises at high pressure in the presence of pyridine to give (38) in high yield (94NKK146). 

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