High Molar Mass Polycarbonates as Closed-Loop Recyclable Thermoplastics

Using carbon dioxide (CO2) to make recyclable thermoplastics could reduce greenhouse gas emissions associated with polymer manufacturing. CO2/cyclic epoxide ring-opening copolymerization (ROCOP) allows for >30 wt% of the polycarbonate to derive from CO2; so far, the field has largely focused on oligocarbonates. In contrast, efficient catalysts for high molar mass polycarbonates are under-investigated and the resulting thermoplastic structure-property relationships, processing and recycling need to be elucidated. This work describes a new organometallic Mg(II)Co(II) catalyst that combines high productivity, low loading tolerance, the highest polymerization control and yields polycarbonates with Mn values from 4-130 kg mol-1, with narrow, monomodal distributions. It is used in the ROCOP of CO2 with bicyclic epoxides to produce a series of samples, each with Mn>100 kg mol-1, of poly(cyclohexene carbonate) (PCHC), poly(vinyl-cyclohexene carbonate) (PvCHC), poly(ethyl-cyclohexene carbonate) (PeCHC, by hydrogenation of PvCHC) and poly(cyclopentene carbonate) (PCPC). All these materials are amorphous thermoplastics, with high glass transition temperatures (85 < Tg < 126 °C, by DSC) and high thermal stability (Td > 260 °C). The cyclic ring substituents mediate the materials’ chain entanglements, viscosity, and glass transition temperatures. Specifically, PCPC was found to have 10x lower entanglement Mn and 100x lower zero-shear viscosity compared to PCHC, showing potential as a future thermoplastic. All these high molecular weight polymers are fully recyclable, either by re-processing or by using the Mg(II)Co(II) catalyst for highly selective depolymerizations to epoxides and CO2. PCPC shows the fastest depolymerization rates, achieving an activity of 2500 h-1 and >99% selectivity for cyclopentene oxide and CO2.