Disclosed herein is a copolymer, which may include side chains which may decrease the surface energy of an insulating layer, thereby improving the alignment of a semiconductor material, and side chains including photoreactive functional groups having an increased degree of cross-linking, thereby improving the characteristics of an organic thin film transistor manufactured using the same, an organic insulating layer composition including the copolymer, an organic insulating layer, an organic thin film transistor, an electronic device including the same and methods of fabricating the same. According to the copolymer of example embodiments, the surface energy of an insulating layer may be decreased, so that the alignment of a semiconductor material may be improved, thereby improving the threshold voltage and charge mobility and decreasing the generation of hysteresis at the time of driving the transistor.