. . . "Compounds of the formula A are obtainable by reacting hydroxyorganoborines of the formula IV or diorganoborinic anhydrides of the formula V with organoaluminum compounds of the formula VI in which R6 is a hydrogen atom or a boron-free C1-C40-hydrocarbon-containing group, such as C1-C20-alkyl, C6-C20-aryl, C7-C40-arylalky or C7-C40-alkylaryl, and in which R1 are identical or different and are a hyd" .