(569fm) Use of Mass Transport Limitations for Selective Depolymerization of Thermoplastic Polyurethane

Polyurethane is the 6^th most used synthetic polymer with an annual global production of 26 MMT in 2022. Thermoplastic polyurethane (TPU), a class of polyurethanes that can be melted and reshaped, are segmented structures composed of hard and soft segments. The hard segments (HS) consist of diisocyanates and chain extenders (e.g. short-chain diols) which form highly stable crystalline structures, while the soft segments (SS) are amorphous structures comprised of a polyol. While chemical depolymerization of TPU for circular re-use is highly attractive, only recovery of the polyol has been demonstrated in the literature; depolymerization of TPU with recovery of all constituent components for truly circular reuse has been elusive to-date.

We are proposing the targeted depolymerization of TPU at the linkage between HS and SS in order to separate and recover both hard and soft segments based on their different physical properties for repolymerization in a subsequent processing step. This poses a chemical selectivity challenge as the bonds between the chain extender and isocyanate within the HS, and the bond between the polyol and isocyanate at the HS-SS linkage are chemically identical. We therefore explored a novel approach which utilizes – usually undesired - mass transport limitations that arise due to the crystallinity of the HS to induce selectivity in the depolymerization reaction. We synthesized model SS, HS, and TPU and performed a thorough solvent screening to identify a solvent that can further enhance selective depolymerization by selectively penetrating the SS. Using this solvent, depolymerization experiments were conducted using an iron acetylacetonate catalyst and benzyl alcohol as capping agent and monitored via H-NMR and GPC. Our results indicate successful depolymerization of SS and TPU, while no reaction was observed for the HS, supporting the basic hypothesis of the approach.