Polymer which contains 1 to 100 mol% of at least one structural unit selected from the following formulae (I-1a), (I-1b), (I-1c) and (I-1d): where Ar 1 to Ar 5 are on each occurrence, in each case identically or differently, a mono- or polycyclic, aromatic ring system having 6 to 24 aromatic ring atoms or a mono- or polycyclic heteroaromatic ring system having 5 to 24 aromatic ring atoms, where one or more of these atoms is/are a heteroatom, which may be substituted by one or more radicals R; R is on each occurrence, identically or differently, H, a straightchain alkyl or alkoxy group having 1 to 10 C atoms or an alkenyl or alkynyl group having 2 to 10 C atoms or a branched or cyclic alkyl or alkoxy group having 3 to 10 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, C=O, C=NR 1, NR 1, O or CONR 1, or an aromatic or heteroaromatic ring system having 5 to 20 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 20 aromatic ring atoms, which may be substituted by one or more radicals R 1, or an aralkyl or heteroaralkyl group having 5 to 20 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 20 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 benzo-fused ring system with one another; R 1 is on each occurrence, identically or differently, H or an aliphatic hydrocarbon radical having 1 to 10 C atoms, an aromatic and/or a heteroaromatic hydrocarbon radical having 5 to 10 C atoms; m = 0, 1, 2, 3 or 4, n = 0, 1, 2 or 3, X = CR 2, O or S, p and q are in each case 0 or 1, where the sum (p + q) = 1 or 2; and r = 0 or 1; where the dashed lines represent bonds to adjacent structural units in the polymer, characterised in that Ar 2 and/or Ar 4 is in each case substituted by Ar 6 in at least one of the two ortho positions, where Ar 6 is a mono- or polycyclic, aromatic ring system having 6 to 24 aromatic ring atoms or a mono- or polycyclic heteroaromatic ring system having 5 to 24 aromatic ring atoms, where one or more of these atoms is/are a heteroatom, which may be substituted by one or more radicals R and where Ar 2 and/or Ar 4, if substituted by Ar 6, is/are a mono- or polycyclic, aromatic ring system having 5 to 60 aromatic ring atoms, which may be substituted by one or more radicals R. Ar 1 to Ar 5 are on each occurrence, in each case identically or differently, a mono- or polycyclic, aromatic ring system having 6 to 24 aromatic ring atoms or a mono- or polycyclic heteroaromatic ring system having 5 to 24 aromatic ring atoms, where one or more of these atoms is/are a heteroatom, which may be substituted by one or more radicals R; R is on each occurrence, identically or differently, H, a straightchain alkyl or alkoxy group having 1 to 10 C atoms or an alkenyl or alkynyl group having 2 to 10 C atoms or a branched or cyclic alkyl or alkoxy group having 3 to 10 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, C=O, C=NR 1, NR 1, O or CONR 1, or an aromatic or heteroaromatic ring system having 5 to 20 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 20 aromatic ring atoms, which may be substituted by one or more radicals R 1, or an aralkyl or heteroaralkyl group having 5 to 20 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 20 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 benzo-fused ring system with one another; R 1 is on each occurrence, identically or differently, H or an aliphatic hydrocarbon radical having 1 to 10 C atoms, an aromatic and/or a heteroaromatic hydrocarbon radical having 5 to 10 C atoms; m = 0, 1, 2, 3 or 4, n = 0, 1, 2 or 3, X = CR 2, O or S, p and q are in each case 0 or 1, where the sum (p + q) = 1 or 2; and r = 0 or 1; where the dashed lines represent bonds to adjacent structural units in the polymer,