Thermodynamic and Kinetic Control of Ring–Chain Interconversion in Entropy‐Driven Ring‐Opening Polymerization of a Macrocyclic Di(Ester‐thioester)

ABSTRACT Entropy‐driven ring‐opening polymerization (ED‐ROP) of macrocyclic (thio)esters offers a promising platform for chemically recyclable polymers through intrinsically reversible ring–chain equilibria, yet its broader development remains constrained by low monomer reactivity, highly dilute depolymerization conditions, and the limited repolymerizability of recycled macrocyclic oligomers (MCOs). Here we report a 14‐membered sulfur‐containing macrocyclic di(ester‐thioester), 14‐DET, whose thioester linkages enhance monomer reactivity and enable controllable macrocycle‐mediated recycling. Using efficient salen‐ or salan‐based lanthanum catalysts, 14‐DET undergoes rapid ED‐ROP at ambient temperature to afford poly(ester‐ alt ‐thioester), P(14‐DET), with number‐average molar mass ( M n ) up to 163 kDa and high sequence alternation ( P AB H up to 0.97). Modulation of temperature, concentration, and time directs the reversible ring–chain system toward high‐molar‐mass polymer, MCOs, or the 28‐membered cyclic dimer (14‐DET) 2 , revealing distinct thermodynamic and kinetic regimes, including transient kinetically controlled polymer formation at 0°C before equilibration toward the precipitated cyclic dimer. Notably, both recovered (14‐DET) 2 and MCOs generated during monomer synthesis can be directly repolymerized to regenerate high‐molar‐mass P(14‐DET), providing feedstock flexibility and improving overall material utilization. These results establish a closed‐loop “monomer–dimer–polymer” cycle and demonstrate a macrocycle‐mediated recycling paradigm in which the cyclic dimers and MCOs serve as valuable polymerizable feedstocks.