Mixture obtained by reacting polyol and anhydride and its use in polyisocyanates for making polyisocyanurates

Isocyanate-reactive mixture comprising a trimerization catalyst and obtained by reacting phthalic anhydride and/or trimellitic anhydride with a polyol having an average equivalent weight of 100-2500 and an average nominal hydroxyl functionality of 2-8, in such amounts that the molar amount of the anhydride ranges from 0.1 to 99% of the hydroxyl equivalent amount of the polyol, wherein the ratio of the number of carboxylic acid groups to the number of ester groups, both formed in the reaction between the anhydride groups and the polyol, is 0.9-1.1 to 1 and wherein at least 60% of the anhydride groups has been converted.