27  US 6,946,538 B2  28  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.  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).  Influence of the Temperature  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%).  Trimer  Entities present  Appearance of  bands  and their amounts  Tin°  C.  the RM  in cm- 1  (GPC/Ff!R analysis)  DC  60  100  Yellowish, not  Weak  Biuret  +  Urea  Carbamate  !POI  very viscous,  trimer  liquid  Yellowish, not  Yes  Biuret  +  Urea  Carbamate  !POI  Ov: 14%  very viscous,  trimer  5%  53%  T  +  B: 4%  liquid  2.2%  EXAMPLE  21  EXAMPLE 20  Metal  center  y  Sm  Trimer  Appearance  bands  Entities present and their  of the RM  in cm- 1  amounts (GPC/Ff!R analysis)  DC  Yellowish,  Weak  Trimer+  Urea  Carbamate  !POI  not very  biuret  viscous, liquid  Yellowish,  Weak  Trimer+  Urea  Dimer  !POI  Ov: 0.1%  not very  biuret  4.3%  0.4%  76%  viscous, liquid  1.5  Catalyst  Appearance  of the RM  Trimer  bands  in cm- 1  Entities present  and their amounts  (GPC/Ff!R analysis)  Y(N(SiMe$_{3}$ ) 2 ) 3  Yellowish  Not very  Heavy  Biuret  +  Urea  (1% by mass)  liquid  significant  products  trimer  NaN(SiMe$_{3}$) 2  Yellowish  1692  Heavy  Tris- Bis- (1% by mass)  paste  1446.5  products  trimer  trimer  Carbamate  !POI  Trimer  !POI 