Cyclic Polycarbonates by N-Heterocyclic Carbene-Mediated Ring-Expansion Polymerization and Their Selective Depolymerization to Monomers

Cyclic polymers exhibit physical properties that are distinct from their linear analogues. However, syntheses of cyclic polymers from macrocyclic structures with chemical recycling to achieve "ring-to-ring" transformations with high selectivities and yields remain a challenge. This is because regeneration of macrocyclic monomers through a ring-closing strategy without using chemical auxiliaries is not thermodynamically favored. Therefore, in this study, we reported the syntheses of an array of cyclic polycarbonates through zwitterionic ring-opening polymerization (ZROP) of macrocyclic carbonates (MCs) having long aliphatic segments mediated by N-heterocyclic carbene (1,3-dimethylimidazol-2-ylidene, Me-NHC) at ambient temperature. A kinetic study revealed that ZROP of all macrocyclic carbonates exhibited slow initiation at low conversion (<20 mol %), while it showed fast propagation at higher conversion (20 mol %). It was found that cyclic polycarbonates with high molecular weights (up to 147.9 kg·mol^-1) depicted significant improvement in thermostabilities (T_d5%= 335-348 °C) in comparison to linear carbonates (T_d5%= 210-338 °C). Moreover, MCs were regenerated through thermal depolymerization from cyclic polycarbonate products at 270-280 °C for 12-24 h showing high selectivities (93-99 mol %) and yields (90%-95%), which helped in achieving a closed loop of a polycarbonate stream.