Photoredox-controlled alternating copolymerization enables highly crystalline structures and block copolymers from thermoplastic to elastomer.

Ethylene-based crystalline copolymers are important materials across broad applications. However, precise control over their primary structures remains a critical challenge in advancing functional materials, limited by the intrinsic reactivity difference between ethylene and comonomers. In this work, we report the development of a light-driven organocatalyzed reversible-deactivation radical copolymerization to access well-defined ethylene-chlorotrifluoroethylene copolymer (ECTFE) under mild conditions (<5 atm, 25 °C). The rational design of a three-armed phenothiazine catalyst in combination with a fluorinated dithiocarbamate furnishes good chain-growth control in the photoredox-mediated copolymerization, yielding ECTFE of excellent alternating sequence with minimized chain defects, which resulted in high crystallinity and superior melting points (up to 263.8 °C). Importantly, the obtained ECTFE exhibits outstanding chain-end activity/fidelity, enabling chain-extension (co)polymerization to access a variety of unprecedented ECTFE-based block copolymers upon visible-light exposure, which has successfully integrated rigid and soft blocks in single chains. The ease of synthesizing such block copolymers creates a versatile and convenient platform to largely tune mechanical properties, affording polymeric materials spanning from thermoplastics to elastomers via structural tailoring.