This article presents a new organometallic Mg(II)Co(II) catalyst for the synthesis of high molar mass polycarbonates through CO₂/cyclic epoxide ring-opening copolymerization (ROCOP). The catalyst enables the production of polycarbonates with number average molecular weights (Mn) ranging from 4 to 130 kg/mol, with narrow, monomodal distributions. The resulting materials are amorphous thermoplastics with high glass transition temperatures (Tg, 85–126 °C) and high thermal stability (Td > 260 °C). The cyclic ring substituents influence chain entanglements, viscosity, and Tg. Polycyclohexene carbonate (PCHC) and poly(vinyl-cyclohexene carbonate) (PvCHC) were synthesized, with PCHC showing high brittleness and low elongation at break. In contrast, poly(cyclopentene carbonate) (PCPC) exhibited lower entanglement molecular weight (Me,n) and zero-shear viscosity, making it a promising thermoplastic. All materials are fully recyclable, either through mechanical reprocessing or chemical depolymerization using the Mg(II)Co(II) catalyst, which achieves high selectivity for epoxides and CO₂. PCPC showed the fastest depolymerization rates, with an activity of 2500 h⁻¹ and >99% selectivity for cyclopentene oxide and CO₂. The study highlights the potential of high molar mass polycarbonates as sustainable thermoplastics, with PCPC being particularly promising due to its favorable thermal-mechanical properties and high CO₂ utilization. The Mg(II)Co(II) catalyst is effective for both polymer synthesis and depolymerization, offering a pathway for efficient recycling of CO₂-based plastics.This article presents a new organometallic Mg(II)Co(II) catalyst for the synthesis of high molar mass polycarbonates through CO₂/cyclic epoxide ring-opening copolymerization (ROCOP). The catalyst enables the production of polycarbonates with number average molecular weights (Mn) ranging from 4 to 130 kg/mol, with narrow, monomodal distributions. The resulting materials are amorphous thermoplastics with high glass transition temperatures (Tg, 85–126 °C) and high thermal stability (Td > 260 °C). The cyclic ring substituents influence chain entanglements, viscosity, and Tg. Polycyclohexene carbonate (PCHC) and poly(vinyl-cyclohexene carbonate) (PvCHC) were synthesized, with PCHC showing high brittleness and low elongation at break. In contrast, poly(cyclopentene carbonate) (PCPC) exhibited lower entanglement molecular weight (Me,n) and zero-shear viscosity, making it a promising thermoplastic. All materials are fully recyclable, either through mechanical reprocessing or chemical depolymerization using the Mg(II)Co(II) catalyst, which achieves high selectivity for epoxides and CO₂. PCPC showed the fastest depolymerization rates, with an activity of 2500 h⁻¹ and >99% selectivity for cyclopentene oxide and CO₂. The study highlights the potential of high molar mass polycarbonates as sustainable thermoplastics, with PCPC being particularly promising due to its favorable thermal-mechanical properties and high CO₂ utilization. The Mg(II)Co(II) catalyst is effective for both polymer synthesis and depolymerization, offering a pathway for efficient recycling of CO₂-based plastics.