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Catalyst-free and reversible step-growth Diels–Alder (DA) polymerization has found a wide range of applications in polymer synthesis and is a promising method to fabricate recyclable thermoplastics. The effectiveness of polymerization and de-polymerization relies on the chemical building blocks, often utilizing furan as the diene and maleimide as the dienophile. Compared to the traditional diene–dienophile or two-component approaches that requires perfect stoichiometry, cyclopentadi-ene (Cp) can serve a dual role via self-dimerization. This internally balanced platform offers a route to access high-molecular-weight polymers and a dynamic handle for polymer recycling, which remains unexplored. Herein, through the reactivity in-vestigation of different telechelic Cp derivatives, the uncontrolled cross-linking of Cp was addressed, revealing the first suc-cessful DA homopolymerization. To demonstrate the generality of our methodology, we synthesized and characterized six Cp homopolymers with backbones derived from common thermoplastics, such as polydimethylsiloxane, hydrogenated poly-butadiene, and ethylene phthalate. Among these materials, the hydrogenated polybutadiene-Cp analog can be thermally de-polymerized (Mn = 68 to 23 kDa) and re-polymerized to the parent polymer (Mn = 68 kDa) under solvent- and catalyst-free conditions. This process was repeated over three cycles without intermediate purification, confirming the efficient thermo-selective recyclability. The varied degradable properties of other four Cp-incorporated thermoplastics were also examined. Overall, this work provides a general methodology to access a new class of reversible homopolymers, potentially expanding the designs and construction of sustainable thermoplastics.