Polyurethane reprocessing as covalent adaptable networks

Carbamate formation and exchange catalysts enable efficient polyurethane (PU) manufacturing as well as emerging recycling and reprocessing methods for PU thermosets. Zirconium β-diketonate complexes, such as Zr acetylacetonate [Zr(acac)₄], are effective alternatives to toxic organotin catalysts that have been used for PU reprocessing. I will demonstrate that Zr(acac)₄ undergoes a thermally activated process in the PU network during reprocessing that transforms it into a more active urethane exchange catalyst. This process is associated with the irreversible loss of acetylacetonate ligands and is not observed for the more sterically hindered Zr 2,2,6,6-tetramethyl-3,5-heptanedione [Zr(tmhd)₄] complex. Thermal gravimetric analysis suggested that this process is associated with the loss of one protonated acac ligand. Stress relaxation analysis of PU thermosets indicated a distinct change in the characteristic relaxation time associated with thermal activation of Zr(acac)₄ at temperatures above 140 °C; no such change was observed for samples reprocessed using Zr(tmhd)₄. Density functional theory and molecular experiments suggest that irreversible ligand exchange of acac with alkoxide or carbamate reduces the activation energy for urethane formation and reversion. Furthermore, the Zr(acac)₄ catalyst activated in the presence of a PU’s polyol precursor provided more porous and less dense PU foams compared to those made using the unactivated Zr(acac)₄ catalyst. These findings are important for developing improved PU synthesis and recycling processes. Thermally activating a catalyst during reprocessing may provide more nuanced control of the in-use and reprocessing characteristics of PU thermosets.