Achieving Exceptional Thermal and Hydrolytic Resistance in Chemically Circular Polyesters via In‐Chain 1,3‐Cyclobutane Rings

Abstract Polyesters, a highly promising class of circular polymers for achieving a closed‐loop sustainable plastic economy, inherently exhibit material stability defects, especially in thermal and hydrolytic instability. Here, we introduce a class of polyesters, P(4 R ‐BL) (R=Ph, Bu), featuring conformationally rigid 1,3 ‐ cyclobutane rings in the backbone. These polyesters not only exhibit superior thermostability ( T d,5% =376–380 °C) but also demonstrate exceptional hydrolytic resistance with good integrity even after 1 year in basic and acidic aqueous solutions, distinguishing themselves from typical counterparts. Tailoring the flexibility of the side group R enables the controlled thermal and mechanical performance of P(4 Ph ‐BL) and P(4 Bu ‐BL) to rival durable syndiotactic polystyrene (SPS) and low‐density polyethylene (LDPE), respectively. Significantly, despite their high stability, both polyesters can be effectively depolymerized into pristine monomers, establishing a circular life cycle.