Catalyst-Free Polyhydroxyurethane Covalent Adaptable Network Exhibiting Full Cross-Link Density Recovery after Reprocessing: Facilitation by Synthesis with Sterically Hindered Secondary Amines

Polyhydroxyurethane (PHU) covalent adaptable networks (CANs) made from primary multifunctional amines require catalysts for reprocessing by compression molding at moderate temperatures and within time frames that minimize undesired side reactions. We developed a catalyst-free PHU CAN using a sterically hindered secondary diamine. Our choice of 4,4'-trimethylene dipiperidine (TmPiP) was informed by small-molecule studies showing that hydroxyurethane (HU) could be produced at high fractional conversion by reacting propylene carbonate (PC), a five-membered cyclic carbonate, with not only primary amines but also with piperidine, a sterically hindered secondary amine. In contrast, the synthesis of HU from PC with other secondary amines led to much lower fractional conversion. Our studies also revealed a higher catalyst-free bond exchange rate at 140 degrees C in the piperidine-based HU than in primary amine-based HU, suggesting that piperidine-based PHU networks may allow for easier reprocessing. Our catalyst-free TmPiP-based PHU network and hexamethylenediamine (HMDA, a primary amine)-based PHU network had rubbery plateau tensile moduli of 1.0 MPa, indicating substantial cross-link density. However, the TmPiP-based PHU network exhibited four times faster stress relaxation at 160 degrees C. Using compression molding at 160 degrees C for 1.0 h, we could reprocess the catalyst-free TmPiP-based PHU network into consolidated films but not the catalyst-free HMDA-based PHU network. The catalyst-free TmPiP-based PHU CAN was reprocessable multiple times with full recovery of cross-link density and tensile properties. Thus, with well-designed molecular structure, inherent PHU chemistry can lead to catalyst-free PHU CANs with robust reprocessability.