posted on 2024-03-14, 19:41authored byGloria Rosetto, Fernando Vidal, Thomas M. McGuire, Ryan W. F. Kerr, Charlotte K. Williams
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 underinvestigated,
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, and the highest polymerization control to yield
polycarbonates with number average molecular weight (Mn) values from 4 to 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 differential
scanning calorimetry) 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 10× lower
entanglement molecular weight (Me)n and 100× lower zero-shear viscosity compared to those
of PCHC, showing potential as a future thermoplastic. All these high
molecular weight polymers are fully recyclable, either by reprocessing
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.