The invention relates to novel polymers containing one or more 4,8-dioxycarbonylalkyl-benzo[1,2-b:4,5-b′]dithiophene repeating units or their thioester derivatives, methods for their preparation and monomers used therein, blends, mixtures and formulations containing them, the use of the polymers, blends, mixtures and formulations as semiconductor in organic electronic (OE) devices, especially in organic photovoltaic (OPV) devices, and to OE and OPV devices comprising these polymers, blends, mixtures or formulations.