Forming freestanding 2D crystals with intrinsically disordered polymer materials is an important and challenging goal in various fields. Here, we show a convenient method to prepare freestanding polymer crystalline layers with a single polymer in one step. We find that the cyclo-polymethacrylate with bis-dimethylsiloxane side groups (pbMA) forms a layer structure spontaneously on a water surface and the bis-dimethylsiloxane side groups form a 2D hexagonal crystal (a = b = 5.4 angstrom, gamma = 120 degrees) in the layers. Because of the significance of silicon materials in microelectronics, we anticipate that the method has potential applications in fields such as microelectronics and high-performance sensors.