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As predicted, the yield depends on the associated cation. The efficacy of the alkali metal salt is much higher than that of the yttrium salts. 

From the viewpoint of an industrial application, the reaction time and/or the amount of catalyst should be reduced for the alkali metals or the operating conditions should be tightened for the yttrium salts. 

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Study of the Influence of Experimental Parameters 

As the trimerization of the IPDI under the preceding conditions does not give satisfactory results, the operating parameters will thus be modified. 

Influence of the Nature of the Associated Cation (Same Anion as in Example 19) 

The operating conditions are too mild for these associated cations. 

Samarium gives somewhat better results and formation of dimer. 

Neodymium gives similar results to those of yttrium. 

The lanthanides appear to be less effective than the other rare earth metals (Y, Sc). 

The study of this parameter was carried out on the yttrium HMDZ salt at 1 %. 

A rise in temperature thus appears to promote the con› densation of the IPDI monomer. A test with 5% of catalyst at the same temperature (100° C.) made it possible to increase the condensation and to obtain better degrees of conversion (Ov: 26% and T: 16%). 

## EXAMPLE 20 

## EXAMPLE 21 

Influence of the Temperature 

