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

Cyclic Polycarbonates by N-Heterocyclic Carbene-Mediated Ring-Expansion Polymerization and Their Selective Depolymerization to Monomers
来源：黄金副高级个人网站 2024-02-26

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 (Td5% = 335–348 °C) in comparison to linear carbonates (Td5% = 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.
[来源：中国聚合物网]