Catalyst-Free Polyhydroxyurethane Covalent Adaptable Networks Exhibiting Full Cross-Link Density Recovery after Reprocessing: Facilitation by Synthesis with Well-Designed Secondary Amines

Polyhydroxyurethane (PHU) networks are generally synthesized using primary amines and cyclic carbonates (CCs), with or without catalysis. Relative to primary amines, analogous secondary amines are less reactive toward CCs due to increased steric hindrance. Consequently, there is only one literature report of a successful PHU network synthesis using secondary amines and CCs, with no assessment of dynamic character. PHU covalent adaptable networks (CANs) made from primary multifunctional amines require catalysts for reprocessing at moderate temperatures and time frames that minimize undesired side reactions. All prior reports of successful PHU CAN synthesis and reprocessing have involved catalysts. By analogy to hindered-urea-bond-based CANs made from secondary amines, we hypothesized that secondary amines with relatively low steric hindrance enable both catalyst-free synthesis and efficient catalyst-free reprocessing of PHU CANs. Small-molecule studies showed that two monofunctional secondary amines with limited steric hindrance, N-hexylmethylamine (HMA) and piperidine (PiP), reacted with CCs to very high fractional conversion, but reactions involving secondary amines with greater steric hindrance led to much lower fractional conversion. Small-molecule studies also revealed higher catalyst-free bond exchange rates at 140 °C in HMA-based and PiP-based hydroxyurethanes (HUs) compared to a primary-amine-based HU, suggesting that PHU networks made from well-designed secondary amines may yield catalyst-free reprocessability. We synthesized PHU networks from difunctional secondary amines with limited steric hindrance, N,N′-dimethyl-1,6-hexanediamine (DMHDA) and 4,4′-trimethylene dipiperidine (TmPiP). These PHU networks exhibited greater dissociative character and factors of 3- and 4-times faster stress relaxation at 160 °C, respectively, than a primary-amine-based PHU network. Using compression molding at 160 °C for 1.0 h, we could reprocess catalyst-free DMHDA-based and TmPiP-based PHU networks into well-consolidated films but not the catalyst-free primary-amine-based PHU network. The catalyst-free, secondary-amine-based PHU CANs were reprocessable with full recovery of the cross-link density and tensile properties. Thus, this study demonstrates that catalyst-free PHU CANs made from well-designed multifunctional secondary amines can be easily synthesized and exhibit robust reprocessability.