A process for the preparation of a reaction product (A), which comprises the following stage (i): (i) Reaction, under free radical conditions, of a reaction mixture comprising at least one monomer (a), capable of free radical reaction in the presence of at least one free radical initiator and a compound of the formula (I)  where R1 to R4, independently of one another, are each hydrogen, an alkyl radical, cycloalkyl radical or aralkyl radical, each of which is unsubstituted or substituted, an unsubstituted or substituted aromatic hydrocarbon radical, with the proviso that at least two of the radicals R1 to R4 are an unsubstituted or substituted aromatic hydrocarbon radical, or R1 and R2 or R3 and R4, in each case in pairs, are a substituted or unsubstituted aromatic hydrocarbon having 6 to 18 carbon atoms and a functional group which, in conjugation with the C—C double bond in the formula I, has a multiple bond between a carbon atom and a heteroatom, 10% by weight or less of water being present in the reaction mixture during the reaction.