Harnessing β-Hydroxyl Groups in Poly(β-Amino Esters) toward Robust and Fast Reprocessing Covalent Adaptable Networks

Abstract

Poly(β-amino esters) (PBAEs), which include tertiary amines at the β-position of ester linkages, are promising in biomaterials due to their biodegradability and pH responsiveness. Such characteristics in the molecular structure are also appealing for designing catalyst-free covalent adaptable networks (CANs), but this has rarely been explored in the literature. Herein, we synthesize a series of PBAE-based CANs by aza-Michael addition, using diacrylate monomers with and without β-hydroxyl groups, and a triamine crosslinker. By leveraging hydrogen bonding, the thermal and mechanical properties of these PBAE-based CANs are effectively tuned through the monomer composition. Owing to the numerous tertiary amines serving as internal catalysts, these CANs undergo catalyst-free network exchange through a dynamic aza-Michael reaction. Interestingly, increasing the amount of βhydroxyl groups accelerates overall stress relaxation from the synergistic effects of transesterification (associative type) at lower temperatures and dynamic aza-Michael reaction (dissociative type) at higher temperatures. Based on these features, we successfully demonstrate the reprocessing and healing at elevated temperatures under mild pressure, as well as shape memory and shape reconfiguration. Thus, controlling the β-hydroxyl group concentration in PBAE-based CANs is a useful strategy for enhancing both the mechanical strength and reprocessing rate.

Fully Recyclable Covalent Adaptable Network Composite with Segregated Hexagonal Boron Nitride Structure for Efficient Heat Dissipation

“…It is noteworthy that recent studies have reported that CAN bearing β-amino esters could undergo a retro-aza Michael reaction, favoring a dissociated state at higher temperature. 34,38 A dissociative retro-aza Michael reaction was also observed in the PBAE CAN here, as confirmed by the decrease in storage modulus (or rubbery plateau) above 150 °C (Figure S3 in the Supporting Information). All the aforementioned evidence proved that the PBAE without the catalyst exhibited CAN characteristics.…”

“…Notably, no discernible changes in the mechanical properties, glass transition temperature ( T g ), and chemical structure were observed over three cycles of reprocessing, as evidenced by the universal testing machine, DSC, and ATR-FTIR measurements performed after each reprocessing cycle, as shown in Figure b–d, respectively. It is noteworthy that recent studies have reported that CAN bearing β-amino esters could undergo a retro-aza Michael reaction, favoring a dissociated state at higher temperature. , A dissociative retro-aza Michael reaction was also observed in the PBAE CAN here, as confirmed by the decrease in storage modulus (or rubbery plateau) above 150 °C (Figure S3 in the Supporting Information). All the aforementioned evidence proved that the PBAE without the catalyst exhibited CAN characteristics.…”

Enhanced Viscosity Control in Thermosets Derived from Epoxy and Acrylate Monomers Based on Thermoreversible Aza-Michael Chemistry

“…The very high swelling degree results in relatively high soluble fractions for the β-hydroxyamine-cured networks but not for the normal amine-cured networks, which is in line with previous observations in BAE networks. 38,39,41 The "neighboring group effect" of the hydroxyl can indeed affect the kinetics and thermodynamics of the expected associative and dissociative network rearrangement reactions and can also give rise to new pathways. One particular option here is a dissociative transesterification pathway via the reversible formation of a seven-membered lactone (Scheme 3).…”

“…39,40 Further, Lee et al incorporated this dynamic chemistry in their materials by utilizing BPA-based acrylates and a triamine cross-linker to obtain materials with shape memory and healing properties. 41 While it is clear that BAE chemistry has been picked up rapidly, its mechanistic understanding and the control over the reactivity of the bonds should be improved. Several exchange pathways are conceivable, and further mechanistic investigations are required for their rational applications in material design.…”

Chemically Recyclable Supramolecular Thermosets toward Strong and Reusable Hot-Melt Adhesives

“…Plastics are indispensable in our daily life and most technology fields, which drive the mass production of plastic products in the past decades. − As one of the most used plastics, thermosetting plastics have accounted for 18% of the total polymers and the worldwide annual production of more than 65 million tons during 2010–2015. − However, only a small fraction of thermosetting plastics are disposed in terms of circular economy manner after their service life due to their permanent covalently cross-linking structures. ,,, Therefore, it is of critical urgency to develop new sustainable thermosetting plastics. Chemically recyclable thermosets as a new kind of alternative have shown great potential due to their capability to be degraded to their corresponding precursors or building blocks. − Compared with traditional thermosets, the disposal method of chemically recyclable thermosets is environmentally friendly. ,,,,,, To create chemically recyclable thermosets, various cleavable or dynamic covalent bonds such as dynamic imine bonds, ,− ,,, transesterification, ,, and boroxines ,, have been decorated on the polymer backbones as cleavable units. For example, Mecking and co-workers reported that polyethylene-like polymers containing the in-chain functional groups as break points can be chemically recycled under heating in a basic ethanol solution at 120 °C .…”