Polymer which contains at least one structural unit of the following formula (Ia): where Ar 1 and Ar 2 are on each occurrence, in each case identically or differently, a mono- or polycyclic, aromatic or heteroaromatic ring system having 5 to 60 aromatic ring atoms, which may be substituted by one or more radicals R; Ar 3 is a mono- or polycyclic, aromatic ring system having 5 to 60 aromatic ring atoms, which may be substituted by one or more radicals R; R is on each occurrence, identically or differently, H, D, F, Cl, Br, I, N(R 1) 2, CN, NO 2, Si(R 1) 3, B(OR 1) 2, C(=O)R 1, P(=O)(R 1) 2, S(=O)R 1, S(=O) 2 R 1, OSO 2 R 1, a straight-chain alkyl, alkoxy or thioalkoxy group having 1 to 40 C atoms or a branched or cyclic alkyl, alkoxy or thioalkoxy group having 3 to 40 C atoms, which may in each case be substituted by one or more radicals R 1, where one or more non-adjacent CH 2 groups may be replaced by R 1 C=CR 1, C≡C, Si(R 1) 2, C=O, C=S, C=NR 1, P(=O)(R'), SO, SO 2, NR 1, O, S or CONR 1 and where one or more H atoms may be replaced by D, F, Cl, Br, I or CN, or a mono- or polycyclic, aromatic or heteroaromatic ring system having 5 to 60 aromatic ring atoms, which may in each case be substituted by one or more radicals R 1, or an aryloxy or heteroaryloxy group having 5 to 60 aromatic ring atoms, which may be substituted by one or more radicals R 1, or an aralkyl or heteroaralkyl group having 5 to 60 aromatic ring atoms, which may be substituted by one or more radicals R 1, or a diarylamino group, diheteroarylamino group or arylheteroarylamino group having 10 to 40 aromatic ring atoms, which may be substituted by one or more radicals R 1, or a crosslinkable group Q, where two or more radicals R may also form a mono- or polycyclic, aliphatic, aromatic and/or benzofused ring system with one another; R 1 is on each occurrence, identically or differently, H, D, F or an aliphatic hydrocarbon radical having 1 to 20 C atoms, an aromatic and/or a heteroaromatic hydrocarbon radical having 5 to 20 C atoms, in which, in addition, one or more H atoms may be replaced by F; where two or more substituents R 1 may also form a mono- or polycyclic, aliphatic or aromatic ring system with one another; Q is 0, 1, 2, 3, 4, 5 or 6; X is CR 2, SiR 2, O, S, C=O or P=O; R is 0 or 1; and the dashed lines represent bonds to adjacent structural units in the polymer, characterised in that Ar 3 is substituted by Ar 4 in at least one of the two ortho positions, where Ar 4 is a mono- or polycyclic, aromatic or heteroaromatic ring system having 5 to 60 aromatic ring atoms, which may be substituted by one or more radicals R, with the proviso that the polymers P19a, P19b, P20a, P20b, P22a, P22b, P33a, P33b, P41a and P41b from WO 2013/156125 A1 are excluded. Ar 1 and Ar 2 are on each occurrence, in each case identically or differently, a mono- or polycyclic, aromatic or heteroaromatic ring system having 5 to 60 aromatic ring atoms, which may be substituted by one or more radicals R; Ar 3 is a mono- or polycyclic, aromatic ring system having 5 to 60 aromatic ring atoms, which may be substituted by one or more radicals R; R is on each occurrence, identically or differently, H, D, F, Cl, Br, I, N(R 1) 2, CN, NO 2, Si(R 1) 3, B(OR 1) 2, C(=O)R 1, P(=O)(R 1) 2, S(=O)R 1, S(=O) 2 R 1, OSO 2 R 1, a straight-chain alkyl, alkoxy or thioalkoxy group having 1 to 40 C atoms or a branched or cyclic alkyl, alkoxy or thioalkoxy group having 3 to 40 C atoms, which may in each case be substituted by one or more radicals R 1, where one or more non-adjacent CH 2 groups may be replaced by R 1 C=CR 1, C≡C, Si(R 1) 2, C=O, C=S, C=NR 1, P(=O)(R'), SO, SO 2, NR 1, O, S or CONR 1 and where one or more H atoms may be replaced by D, F, Cl, Br, I or CN, or a mono- or polycyclic, aromatic or heteroaromatic ring system having 5 to 60 aromatic ring atoms, which may in each case be substituted by one or more radicals R 1, or an aryloxy or heteroaryloxy group having 5 to 60 aromatic ring atoms, which may be substituted by one or more radicals R 1, or an aralkyl or heteroaralkyl group having 5 to 60 aromatic ring atoms, which may be substituted by one or more radicals R 1, or a diarylamino group, diheteroarylamino group or arylheteroarylamino group having 10 to 40 aromatic ring atoms, which may be substituted by one or more radicals R 1, or a crosslinkable group Q, where two or more radicals R may also form a mono- or polycyclic, aliphatic, aromatic and/or benzofused ring system with one another; R 1 is on each occurrence, identically or differently, H, D, F or an aliphatic hydrocarbon radical having 1 to 20 C atoms, an aromatic and/or a heteroaromatic hydrocarbon radical having 5 to 20 C atoms, in which, in addition, one or more H atoms may be replaced by F; where two or more substituents R 1 may also form a mono- or polycyclic, aliphatic or aromatic ring system with one another; Q is 0, 1, 2, 3, 4, 5 or 6; X is CR 2, SiR 2, O, S, C=O or P=O; R is 0 or 1; and characterised in that Ar 3 is substituted by Ar 4 in at least one of the two ortho positions, where Ar 4 is a mono- or polycyclic, aromatic or heteroaromatic ring system having 5 to 60 aromatic ring atoms, which may be substituted by one or more radicals R, with the proviso that the polymers P19a, P19b, P20a, P20b, P22a, P22b, P33a, P33b, P41a and P41b from WO 2013/156125 A1 are excluded.