Circular Polymer Designed by Regulating Entropy: Spiro-Valerolactone-Based Polyesters with High Gas Barriers and Adhesion Strength.

Enthalpy is often the focal point when designing monomers for polymer circularity, but much less is explored on how entropy can be exploited to create polymers with synergistic circularity and properties. Here, we design a series of spiro-lactones (SLs) with closed-chain cycloalk(en)yl substituents at the α,α-position of δ-valerolactone (δVL), which, when combined with the parent δVL and -α,α-dialkyl-substituted δVL with open-chain alkyl groups, provide a desired platform for exploring the circular polymer design by focusing on the entropy change of polymerization. These SLs exhibit finely balanced (de)polymerizability that is regulated chiefly by entropy differentiation, allowing both the facile synthesis of polyester PSLs ( up to 1000 kg mol) in a living fashion and selective depolymerization of the PSLs to completely recover monomers under mild conditions (using a recyclable catalyst at 100 °C). One such PSL is semicrystalline ( = 134 °C), strong (ultimate strength = 43 MPa), hard (modulus = 1.85 GPa), and modestly flexible. These notable mechanical properties are bolstered by its superior barriers to oxygen and moisture permeation compared to common packaging materials. In addition, this PSL can be postfunctionalized to a recyclable OH-containing PSL that shows higher adhesion strength than the comparative commercial adhesives.