Trimerization of Isocyanates to Isocyanurates

Triaryl isocyanurates are useful as activators for the continuous anionic polymerization and postpolymerization of c-caprolactam to nylon-6 possessing a low unreacted monomer content and a highly stable melt viscosity [93]. Although a wide variety of catalysts for the trimerization of aryl isocyanates to triaryl isocyanurates are known, purity of trimerized product (a requirement for nylon of good quafity) is problematic and purification results in product 

Pro-azaphosphatranes lb [93], Ic [67a], and Id [67a] are superior catalysts for the trimerization of isocyanates at room temperature in nearly quantitative yield, with Id reacting considerably faster than lb which was much faster than Ic. By contrast, P(NMe2)3 produces only small yields of the dimer, which is an undesirable impurity in activators. 

---

Hofmann [14] in 1858 was the first to report the trimerization of isocyanates to isocyanurates. Hofinann used triethylphosphine as a catalyst to trimerize phenylisocyanate to triphenylisocyanurate. 

---