Cyclic Poly(Thioester Amide)s via Ring-Opening Copolymerization of Aziridines and Phthalic Thioanhydride: Mechanistic Insights and Enhanced Properties for Sustainable Materials

Ring-opening copolymerization renders aziridines promising candidates for constructing N-containing polymers; however, producing polymers with predicted molar masses and defined sequence structures remains challenging. Here, we present a strategy for synthesizing poly(thioester amide)s (PTEAs) via ROCOP of N-alkyl aziridines and phthalic thioanhydride. Remarkably, perfectly alternating copolymerization was achieved by the synergistic catalysis of a phosphazene base and a protic initiator, delivering cyclic PTEAs with a molar mass of up to 188.4 kDa. Model reaction and chain extension experiments supported the rapid S-to-N acyl shift after aziridine ring-opening and spontaneous ring-closure upon applying an external stimulus. Notably, liquid-phase transmission electron microscopy confirmed the linear or cyclic topology before or after the postprocessing by spatially resolving the morphology of transient conformations. The in-chain thioester bonds endow polymers with enhanced thermal and optical properties compared to their poly(ester amide) counterparts, along with desirable degradability and biocompatibility, establishing a robust foundation for developing PTEAs as sustainable and functional biomedical materials.