Polymer of formula I wherein A is on each occurrence identically or differently a group selected from the following formulae B is on each occurrence identically or differently a group selected from the following formulae X 1 and X 2 are on each occurrence identically or differently, and independently of each other, O, S or Se, R 1 and R 2 are on each occurrence identically or differently, and independently of each other, -CO-R y, -CO-O-R y, or -O-CO-R y, R 3 and R 4 on each occurrence identically or differently, and independently of each other, denote H or when being different from H, denote primary alkyl, alkoxy or thioalkyl with 1 to 30 C atoms, or when being different from H denote secondary alkyl, alkoxy or thioalkyl with 3 to 30 C atoms, or tertiary alkyl, alkoxy or thioalkyl with 4 to 30 C atoms, wherein in all these groups one or more H atoms are optionally replaced by F, or denote-CO-R y -, -CO-O-R y - or -O-CO-R y, R y is primary alkyl with 1 to 30 C atoms, secondary alkyl with 3 to 30 C atoms or tertiary alkyl with 4 to 30 C atoms, wherein in all these groups one or more H atoms are optionally replaced by F, Ar is on each occurrence identically or differently an aryl or heteroaryl group which has electron acceptor properties and is selected from the group consisting of benzo[2,1,3]thiadiazole-4,7-diyl, 5,6-dialkyl-benzo[2,1,3]thiadiazole-4,7-diyl, 5,6-dialkoxy-benzo[2,1,3]thiadiazole-4,7-diyl, benzo[2,1,3]selena-diazole-4,7-diyl, 5,6-dialkoxy-benzo[2,1,3]selena-diazole-4,7-diyl, benzo[1,2,5]thiadiazole-4,7,diyl, benzo[1,2,5]selenadiazole-4,7,diyl, benzo[2,1,3]oxa-diazole-4,7-diyl, 5,6-dialkoxybenzo[2,1,3]oxadiazole-4,7-diyl, 2H-benzotriazole-4,7-diyl, 2,3-dicyano-1,4-phenylene, 2,5-dicyano-1,4-phenylene, 2,3-difluro-1,4-phenylene, 2,5-difluoro-1,4-phenylene, 2,3,5,6-tetra-fluoro,1,4-phenylene, 3,4-difluorothiophene-2,5-diyl, thieno[3,4-b]pyrazine-2,5-diyl, quinoxaline-5,8-diyl, thieno[3,4-b]thiophene-4,6-diyl, thieno[3,4-b]thiophene-6,4-diyl, 3,6- pyrrolo[3,4-c]pyrrole-1,4-dione, all of which are unsubstituted, mono- or polysubstituted, and wherein "alkyl" means a straight-chain or branched alkyl group with 1 to 30 C atoms and "alkoxy" means a straight-chain or branched alkoxy group with 1 to 30 C atoms, m is on each occurrence identically or differently 0 or 1, wherein in at least one repeating unit m is 1, o is on each occurrence identically or differently 0 or 1, wherein in at least one repeating unit o is 1, i and k are on each occurrence identically or differently, and independently of each other, 0 or 1, wherein in at least one repeating unit at least one of i and k is 1, n is an integer > 1. A is on each occurrence identically or differently a group selected from the following formulae B is on each occurrence identically or differently a group selected from the following formulae X 1 and X 2 are on each occurrence identically or differently, and independently of each other, O, S or Se, R 1 and R 2 are on each occurrence identically or differently, and independently of each other, -CO-R y, -CO-O-R y, or -O-CO-R y, R 3 and R 4 on each occurrence identically or differently, and independently of each other, denote H or when being different from H, denote primary alkyl, alkoxy or thioalkyl with 1 to 30 C atoms, or when being different from H denote secondary alkyl, alkoxy or thioalkyl with 3 to 30 C atoms, or tertiary alkyl, alkoxy or thioalkyl with 4 to 30 C atoms, wherein in all these groups one or more H atoms are optionally replaced by F, or denote-CO-R y -, -CO-O-R y - or -O-CO-R y, R y is primary alkyl with 1 to 30 C atoms, secondary alkyl with 3 to 30 C atoms or tertiary alkyl with 4 to 30 C atoms, wherein in all these groups one or more H atoms are optionally replaced by F, Ar is on each occurrence identically or differently an aryl or heteroaryl group which has electron acceptor properties and is selected from the group consisting of benzo[2,1,3]thiadiazole-4,7-diyl, 5,6-dialkyl-benzo[2,1,3]thiadiazole-4,7-diyl, 5,6-dialkoxy-benzo[2,1,3]thiadiazole-4,7-diyl, benzo[2,1,3]selena-diazole-4,7-diyl, 5,6-dialkoxy-benzo[2,1,3]selena-diazole-4,7-diyl, benzo[1,2,5]thiadiazole-4,7,diyl, benzo[1,2,5]selenadiazole-4,7,diyl, benzo[2,1,3]oxa-diazole-4,7-diyl, 5,6-dialkoxybenzo[2,1,3]oxadiazole-4,7-diyl, 2H-benzotriazole-4,7-diyl, 2,3-dicyano-1,4-phenylene, 2,5-dicyano-1,4-phenylene, 2,3-difluro-1,4-phenylene, 2,5-difluoro-1,4-phenylene, 2,3,5,6-tetra-fluoro,1,4-phenylene, 3,4-difluorothiophene-2,5-diyl, thieno[3,4-b]pyrazine-2,5-diyl, quinoxaline-5,8-diyl, thieno[3,4-b]thiophene-4,6-diyl, thieno[3,4-b]thiophene-6,4-diyl, 3,6- pyrrolo[3,4-c]pyrrole-1,4-dione, all of which are unsubstituted, mono- or polysubstituted, and wherein "alkyl" means a straight-chain or branched alkyl group with 1 to 30 C atoms and "alkoxy" means a straight-chain or branched alkoxy group with 1 to 30 C atoms, m is on each occurrence identically or differently 0 or 1, wherein in at least one repeating unit m is 1, o is on each occurrence identically or differently 0 or 1, wherein in at least one repeating unit o is 1, i and k are on each occurrence identically or differently, and independently of each other, 0 or 1, wherein in at least one repeating unit at least one of i and k is 1, n is an integer > 1.