Acid-Triggered Cyclization Enables Selective Depolymerization of Robust Polyamides for Recovery of Monomer Derivatives

Abstract

The development of polymeric materials that combine practical performance with selective degradability is an important goal in sustainable materials research. Selective degradation into defined small molecules offers a promising strategy for the deconstruction and potential chemical recycling of polymeric materials. However, extending this concept to mechanically robust polyamides remains challenging because of the high stability of their amide backbones. In this study, we used microbially produced poly(ε-lysine) (PLys) as a renewable feedstock to construct biobased polyamides that undergo rapid acid-mediated depolymerization. The amino groups at the α-carbons of PLys were modified with isothiocyanates bearing different side groups to afford thiourea-functionalized PLys derivatives. These thiourea moieties were selectively cleaved through an acid-mediated cyclization reaction, yielding the corresponding cyclic lysine derivatives as discrete depolymerization products. Structural analyses revealed that the thiourea-modified PLys underwent complete depolymerization in the presence of acids such as hydrochloric acid, and that the cyclic lysine derivatives could be recovered without specialized purification. Moreover, thiourea modification converted PLys into amorphous polyamides that formed robust films and exhibited a high storage modulus at ambient temperature. The polyamides could also be cross-linked using multifunctional isothiocyanates, and the resulting network materials exhibited shape-memory behavior together with a high storage modulus. Overall, this study establishes a molecular design strategy for mechanically robust biobased polyamides that undergo rapid acid-triggered depolymerization into defined cyclic lysine derivatives, opening a route toward selectively deconstructable polyamides for sustainable materials applications.