Mitigating plastic waste requires a new class of chemically circular materials; ones that deliver the performance of traditional polyolefins but allow for efficient feedstock recovery. While cyclic disulfides have emerged as promising candidates for inherently degradable materials due to the dynamic nature of the S-S bond, prior literature has been limited to embedding these degradable motifs into vinyl copolymers; achieving full closed-loop homopolymerization has remained elusive. Prior attempts using polylipoates suffer from an amorphous polymer that is prone to spontaneous depolymerization. In this work, we report the fast cationic ring-opening homopolymerization of 1,2-dithiepane, a seven-membered cyclic disulfide and access a robust, semi-crystalline material with high molecular weights (Mn > 600 kDa) and low Tg (-64 °C). The resulting stable thermoplastic exhibits mechanical properties that rival flexible polyolefins, including an ultimate tensile strength of 16 MPa, and an exceptional elongation at break of over 570%. Crucially, complete chemical circularity is achieved under mild conditions; in the presence of trichloroacetic acid, dilution shifts the thermodynamic equilibrium to drive rapid, clean depolymerization back to the ring-closed monomer (~95% yield within an hour). The recovered feedstock can be seamlessly repolymerized, allowing poly(1,2-dithiepane) as a viable, zero-waste alternative for traditional plastics.