Catalyst and process for preparing low-viscosity and color-reduced polyisocyanates containing isocyanurate groups

A process for preparing low-viscosity polyisocyanates and polyisocyanates of reduced color containing isocyanurate groups, which comprises, partially trimerizing aliphatic and/or cycloaliphatic diisocyanates in the presence of 0.02 to 2% by weight, based on the weight of the diisocyanate starting material, of at least one trimerization catalyst of formula (I):

• and wherein substituents A, B, C, D, and E simultaneously or independently of one another are hydrogen, chloro, C1-C4-alkyl, C1-C4-alkoxy, C1-C4-alkoxycarbonyl, hydroxyl, (R5)3SiO—, (R5)2 N—,—COOH, (R5)2N—CH2- or phenyl, it being possible for any two adjacent radicals selected from the group A, B, C, D and E to form a conjoint 5- or 6-membered saturated or unsaturated ring which may also include nitrogen, sulfur or oxygen heteroatoms;
• F is hydrogen, methyl or fluoro;
• G is hydrogen, methyl or fluoro;
• R2 and R3 simultaneously or independently of one another are C1-C18-alkyl or R1;
• R4 is hydrogen, methyl, C2-C18-alkyl, C3-C8-cycloalkyl or C2-C12-alkoxy;
• R5 is C1-C18-alkyl;
• Y−is R6COO−;
• R6 is hydrogen or a branched or unbranched aliphatic or araliphatic C1-C12-alkyl radical, and then removing excess diisocyanate from the reaction medium.