All-Hydrocarbon Polymers with Chemical Recyclability via a Catalyst-Free and Solvent-Free Method

Chemically recyclable polymers are at the forefront of synthetic chemistry and sustainable chemistry, in which the unique recycling process is enabled by the installation of a cleavable C-X (X = heteroatom) unit. Developing chemically recyclable all-hydrocarbon polymers without heteroatoms remains challenging due to the difficulty in the design and installation of a reversible C-C unit. In this study, we utilize the dynamic covalent chemistry of the Diels-Alder cyclopentadienyl (Cp) unit, report the chemically recyclable all-hydrocarbon polymer, and propose the "macromonomer-all-hydrocarbon polymer-macromonomer" recycling loop. Through the design of A₂-type Cp₂-terminated monomers bearing desired alkyl or aryl spacers, all-hydrocarbon homopolymers, random copolymers, and block copolymers are synthesized at 100 °C respectively via an A₂ + A₂ Diels-Alder step-growth polycondensation. These all-hydrocarbon polymers have high molecular weights of up to 245.3 kDa and a wide glass transition temperature range from -20 to 133 °C, offering programmable thermomechanical properties, excellent optical properties, extremely low water absorption, and impressive shape memory function. Depolymerization of an all-hydrocarbon polymer via a retro-Diels-Alder mechanism occurs at 250 °C to generate a Cp₂-terminated oligomer (A₂-type macromonomer), which again repolymerizes at 100 °C to yield the parent all-hydrocarbon polymer with preserved properties. This recycling process is thermo-driven, catalyst-free, solvent-free, and quantitative, indicating a 100% atom economy. These merits enable all-hydrocarbon polymers to be a unique member of an all-carbon-backbone polymer, establishing a new circular polymer.