Circularity of non-isocyanate polyurethanes: Small-molecule recovery from linear and network polyhydroxyurethanes via transcarbamoylation.

We report the first demonstration of small-molecule recovery from polyhydroxyurethane (PHU) harnessing transcarbamoylation dynamic chemistry. We prepared linear and network PHUs by reacting a difunctional amine with a difunctional or trifunctional 5-member cyclic carbonate, respectively, with the cyclic carbonates synthesized from CO2 fixation of the corresponding epoxies. Under a mild condition using a base catalyst and methanol (MeOH), we depolymerized linear and network PHUs, recovering carbamate small molecules by leveraging the exchange reaction between urethane groups on the PHU backbone and MeOH, which is transcarbamoylation. We conducted a screening of depolymerization conditions, with a stronger base catalyst tert-butoxide, a higher temperature of 80 °C, and the use of a binary solvent system tetrahydrofuran/MeOH leading to the highest carbamate yield of 45 mol% from linear PHU. We also explored two greener solvents, water and ethanol (EtOH), as replacements for MeOH, with water leading to virtually no depolymerization and EtOH resulting in less than 5 mol% carbamate yield from linear PHU. For network PHU, the optimal condition led to a 15 mol% carbamate yield. The substantially smaller carbamate yield for the network PHU relative to the linear PHU is consistent with literature results reported for partial small-molecule recovery from polyurethane. Although further improvement on our small-molecule recovery method is desired, this first study highlights the potential of transcarbamoylation, an inherent dynamic chemistry of PHU, to contribute significantly to end-of-life recycling of all PHU materials. [ABSTRACT FROM AUTHOR]