Polymer containing 1 to 30 mol% of structural units of the following formula: characterised in that at least one representative from Ar and Ar' contains an aldehyde group, where the symbols and indices used have the following meanings: Ar and Ar' represent, independently of one another, a substituted or unsubstituted, mono- or polycyclic, aromatic or heteroaromatic ring system, where a substituted or unsubstituted, mono- or polycyclic, aromatic ring system denotes a ring system having 6 to 60 aromatic ring atoms, which does not necessarily contain only aromatic groups, but instead in which, in addition, a plurality of aromatic units may be interrupted by a short non-aromatic unit selected from an sp 3 -hybridised C atom, or an O or N atom, or a CO group, and where a substituted or unsubstituted, mono- or polycyclic, heteroaromatic ring system denotes a ring system having 5 to 60 aromatic ring atoms, where one or more of these atoms is/are a heteroatom, which does not necessarily contain only aromatic groups, but instead in which, in addition, a plurality of aromatic groups may be interrupted by a short non-aromatic unit selected from an sp 3 -hybridised C atom, or an O or N atom, or a CO group; X represents a single covalent bond or a straight-chain, branched or cyclic C 1-10 -alkylene, C 1-10 -alkenylene or C 1-10 -alkynylene group, in which one or more H atoms may be replaced by F and in which one or more CH 2 groups may be replaced by O, NH or S; and n is 1, 2, 3 or 4; and the dashed lines represent bonds to the next structural unit of the polymer, and further structural units which are different from the structural unit of the formula, selected from - units which are typically used as backbone and additionally - units which increase the hole-injection and/or hole-transport properties of the polymers and/or - units which increase the electron-injection and/or electron-transport properties of the polymers, where the structural units which have hole-injection and/or hole-transport properties are selected from triarylamine, benzidine, tetraaryl-para-phenylenediamine, triarylphosphine, phenothiazine, phenoxazine, dihydrophenazine, thianthrene, dibenzo-para-dioxin, phenoxathiyne, carbazole, azulene, thiophene, pyrrole and furan derivatives; where the structural units which have electron-injection and/or electron-transport properties are selected from pyridine, pyrimidine, pyridazine, pyrazine, oxadiazole, quinoline, quinoxaline, anthracene, benzanthracene, pyrene, perylene, benzimidazole, triazine, ketone, phosphine oxide and phenazine derivatives, and triarylboranes; and where the structural units which are typically used as polymer backbone are selected from 4,5-dihydropyrene derivatives, 4,5,9,10-tetrahydropyrene derivatives, fluorene derivatives, 9,9'-spirobifluorene derivatives, phenanthrene derivatives, 9,10-dihydrophenanthrene derivatives, 5,7-dihydrodibenzoxepine derivatives and cis- and trans-indenofluorene derivatives. Ar and Ar' represent, independently of one another, a substituted or unsubstituted, mono- or polycyclic, aromatic or heteroaromatic ring system, where a substituted or unsubstituted, mono- or polycyclic, aromatic ring system denotes a ring system having 6 to 60 aromatic ring atoms, which does not necessarily contain only aromatic groups, but instead in which, in addition, a plurality of aromatic units may be interrupted by a short non-aromatic unit selected from an sp 3 -hybridised C atom, or an O or N atom, or a CO group, and where a substituted or unsubstituted, mono- or polycyclic, heteroaromatic ring system denotes a ring system having 5 to 60 aromatic ring atoms, where one or more of these atoms is/are a heteroatom, which does not necessarily contain only aromatic groups, but instead in which, in addition, a plurality of aromatic groups may be interrupted by a short non-aromatic unit selected from an sp 3 -hybridised C atom, or an O or N atom, or a CO group; X represents a single covalent bond or a straight-chain, branched or cyclic C 1-10 -alkylene, C 1-10 -alkenylene or C 1-10 -alkynylene group, in which one or more H atoms may be replaced by F and in which one or more CH 2 groups may be replaced by O, NH or S; and n is 1, 2, 3 or 4; and - units which are typically used as backbone and additionally - units which increase the hole-injection and/or hole-transport properties of the polymers and/or - units which increase the electron-injection and/or electron-transport properties of the polymers,