. . . "o that at least one R1 contains a halogen, in particular Br, and each R2 is, independently of the others, H, halogen, C1-C4-alkyl, C1-C4-alkoxy, C5-C15-aryl, C5-C15-aryloxy or a radical which contains at least one functional group Y selected from hydroxyl, ether, ester, halogen, amine, amide, thiol, an ethylenically unsaturated double bond, an acetylenically unsaturated triple bond and/or carboxyl" .