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 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 molar mass values from 4-130 kg mol-1, with narrow, monomodal distributions. Used in the ROCOP of CO2 with cyclohexene oxide (CHO), vinyl-cyclohexene oxide (vCHO) or cyclopentene oxide (CPO) 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, produced 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 the glass transition temperatures. Specifically, PCPC shows an entanglement molar mass (Me = 4-5 kg mmol-1) which is 10x lower than PCHC (Me ~ 50 kg mol-1) and has a zero-shear viscosity (0.78 MPa s) which is 100x lower than PCHC (90 MPa s). PCPC also shows higher tensile strength (59 MPa) and toughness (3.3 MJ m-3) compared with PCHC; its properties are competitive with thermoplastics like polystyrene and improve upon the high temperature application window for poly(L-lactide). All the new polymers are fully recyclable, either by re-processing using compression molding 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 CPO and CO2 (1:5000, [catalyst]:[PCPC], neat films, 140 °C). In future, CO2/epoxide-thermoplastic research should prioritize alkyl-substituted cyclohexenes or cyclopentenes since these show the highest chain entanglements, optimal processing and properties. Furthermore, the highly controlled, organometallic Mg(II)Co(II) catalyst should access other polycarbonates, terpolymers or block copolymers from CO2.