NOVELTY - Polymer comprising at least one structural unit (I) which comprises the aldehyde groups, is new.    USE - The polymer comprising the structural unit (I) is useful in preparing crosslinkable and/or a crosslinked polymers. The crosslinked polymers are useful in electronic devices, preferably organic electronic device comprising organic or polymeric organic electroluminescent devices, organic integrated circuits, organic field-effect transistors, organic thin film transistors, organic solar cells, organic laser diodes, organic photovoltaic elements or devices or organic photoreceptors (all claimed).    ADVANTAGE - The polymer itself does not exhibits the crosslinkable groups but have the groups that can be easily and quantitatively converted into crosslinkable groups and thus avoids the unwanted side reactions of crosslinkable groups during the polymerization of monomers and thus improves the performance and/or capacity of the resulting devices. The crosslinked polymer obtained by the polymer comprising the structural unit (I) exhibits high degree of crosslinking.    DETAILED DESCRIPTION - Polymer comprising at least one structural unit of formula Z-Ar(-X-Ar1)n-Z (I) which comprises the aldehyde groups, is new.    Ar, Ar1 = optionally substituted, mono- or polycyclic aromatic or heteroaromatic ring system (where Ar and Ar1 exhibits an aldehyde group);    X = covalent single bond or optionally cyclic 1-10C alkylene, 1-10C alkenylene or 1-10C alkynylene (where at least one H is optionally replaced by F and at least one CH2 is optionally replaced by O, NH or S);    n = 1-4; and    Z = bond to the next structural unit of polymer.    INDEPENDENT CLAIMS are also included for:    (1) a mixture comprising at least one polymer(s) comprising the structural unit (I) and an additional polymeric-, oligomeric-, dendritic- and/or low molecular substances;    (2) a formulation comprising at least one polymer comprising the structural unit (I) or the mixture;    (3) preparing a crosslinked polymer, comprising: providing the polymer comprising the structural unit (I) and an aldehyde group, (b) converting the aldehyde group into vinyl group or alkenyl group, and crosslinking the polymer; and    (4) crosslinked polymer obtained by the above method.