Synthesis and Thermo-Selective Recycling of Diels-Alder Cyclopentadiene Thermoplastics.

Catalyst-free and reversible step-growth Diels-Alder (DA) polymerization has a wide range of applications in polymer synthesis and is a promising method for fabricating recyclable thermoplastics. The effectiveness of polymerization and depolymerization 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 approach that requires precise stoichiometry, cyclopentadiene (Cp) can serve dual roles via self-dimerization. This internally balanced platform offers a route to access high-molecular-weight polymers and a dynamic handle for polymer recycling, which has yet to be explored. Herein, through a reactivity investigation of different telechelic Cp derivatives, the uncontrolled cross-linking of Cp was addressed, revealing the first successful DA homopolymerization. To demonstrate the generality of our methodology, we synthesized and characterized six Cp homopolymers with backbones derived from common thermoplastics, such as poly(dimethylsiloxane), hydrogenated polybutadiene, and ethylene phthalate. Among these materials, the hydrogenated polybutadiene-Cp analog can be thermally depolymerized ( M _n = 68 to 23 kDa) and repolymerized to the parent polymer ( M _n = 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 the other four Cp-incorporated thermoplastics were also examined. Overall, this work provides a general methodology for accessing a new class of reversible homopolymers, potentially expanding the design and construction of sustainable thermoplastics.