In a process for reducing the formation of polymers during side-chain alkylation or side-chain alkenylation of alkylaromatic compounds by reaction with olefins or diolefins in the presence of an alkali metal catalyst followed by distillation in order to isolate the alkylated or alkenylated compound, the alkali metal is present in the catalyst on an inorganic support, and the catalyst is mechanically separated from the reaction mixture after the reaction and before the distillation.