Unexpected Two-Step Crystallization in Polyethylene with Isolated In-Chain Functional Groups

Polyethylene (PE) functionalization is a promising route to expand material performance while preserving the favorable properties of PE, such as mechanical robustness. However, the influence of isolated in-chain functional groups embedded in the PE backbone on the crystallization of PE remains poorly understood. Herein, we investigate the crystallization behavior of polyethylene bearing in-chain carbonyl (PE-CO) and amide (PE-CONH) groups that are embedded randomly in the PE backbone in a highly isolated manner. PE-CONH was obtained from PE-CO via main-chain editing, ensuring closely matched in-chain group distributions. Strikingly, PE-CO and PE-CONH showed a distinct two-step evolution of crystallinity during cooling at a constant rate. Such a two-step crystallization phenomenon was found in random copolymers for the first time. A comprehensive multiscale analysis revealed the molecular mechanism of the two-step crystallization: the in-chain groups were rejected from the PE crystalline phase in the first step but were later incorporated into the crystal lattice in the second step. Amide groups induced a more pronounced delay in the onset of the second step than carbonyl groups did. The discovery of the two-step crystallization offers a new opportunity for designing advanced PE-derived materials through precise main-chain modification.