Generally, polymers based on dynamic chemistry possess excellent self-healing, recyclability and reprocessability, which can effectively alleviate the pressure of environmental pollution and resource shortage, leading to broad applications in the context of carbon neutrality strategy.Boronic ester bond, as one of the common dynamic bonds, has been widely studied and used in the design and synthesis of dynamic covalent crosslinked materials in biomedicine, flexible electronics, optoelectronic materials and other fields. However, polymers based on boronic esters still exist some problems, such as the water sensitivity and difficult self-healing conditions. In recent years, the group of Prof. Cheng-Hui Li has conducted a series of research and exploration on boronic ester-based dynamic polymers. They not only promoted the stability and dynamics of boronic esters, but also synthesized a series of self-healing/ recyclable polymers with satisfied mechanical properties and stabilities. They also explored and discussed the functional applications in different fields of these dynamic polymers.
Firstly, a novel boronic ester (3OH-BO) containing an ortho hydroxyl group was developed based on the structural design of boronic ester molecule. Experiments on model small molecules proved that the incorporation of ortho-electronic hydroxy group can not only effectively promote the dynamic exchange between boronic esters, but also endow it with unique intramolecular dynamic transformation.Through introducing 3OH-BO as the linkages into the model polymers based on polydimethylsiloxane (PDMS), a series of boronic ester covalent polymers (3OH-PBM) with different crosslinking densities were obtained.Compared with the control polymers 2OH-PBM with the same crosslinking densities, 3OH-PBM exhibited excellent self-healing properties at room temperature. After being damaged and self-healed for 12 h at room temperature, the healing efficiencies of the mechanical properties reached more than 80%.The study of polymer dynamics showed that the boronic ester linkages containing ortho hydroxyl could significantly accelerate the dynamic exchange and topological rearrangement processes in polymer networks, resulting in the self-healing property of polymers at room temperature (Figure 1, ACS Materials Letters. 2021, 3, 1328-1338).
Figure 1. Boronic esters and the dynamic polymers based on the neighboring hydroxyl group promotion mechanism
In order to solve the problems of water sensitivity and hydrolysis of boronic esters, a six-membered cyclic boronic ester (BN-6) with intramolecular B–N coordination was designed and synthesized via the synergetic strategy of B–N coordination bond and ring tension.The model small molecule hydrolysis experiments via NMR spectra illustrated that BN-6 with lower ring tension exhibited higher hydrolytic stability. Moreover, based on the thermal reversible of B–N coordination, the six-membered ring of BN-6 turned into ten-membered ring with higher ring tension under heating, resulting in more active dynamic exchange of BN-6. Furthermore, a boronic ester covalent adhesive with strong adhesive strength (4.21 MPa) and recyclability was successfully prepared via utilizing BN-6 as crosslinking points. The adhesive demonstrated excellent stability in water and other harsh environments such as strong acid and alkali. The adhesion strengths remained above 4 MPa after long-term immersion. This work has effectively broken through the bottleneck of underwater applications of boronic ester-based adhesives and provided a novel strategy for the design of underwater adhesive materials with high stability (Figure 2, Advanced Functional Materials. 2022, 32, 220159).
Figure 2. The six-membered boronic ester containing internal B–N coordination and the derived adhesive.
Recently, by combining the six-membered cyclic boronic esters with intramolecular B–N coordination with commercial epoxy resins, this group successfully synthesized a recyclable room-temperature phosphorescent (RTP) material PTBN6-GER with high toughness, high stability and shape memory property. The internal weak B–N coordination bonds in polymer network can dissociate reversibly upon loading, which can effectively dissipate the external energy. Therefore, compared with existing epoxy resins, the toughness of the epoxy polymer was greatly improved by the coordination interaction (12.26 MJ m-3) (Figure 3). In addition, because the highly cross-linked polymer network can effectively inhibit the non-radiative decay of the triplet excitons and resist the invasion of external factors (such as oxygen, water, etc.), PTBN6-GER showed an ultralong RTP lifetime (540.4 ms), and its blue-green afterglow could maintain more than 8 s at room temperature. The RTP properties of polymers were perfectly maintained even after prolonged immersion in water and various organic solvents (Figure 4).
Figure 3. (a) Synthetic route and schematic illustration of the covalent crosslinking networks for PTBN6-GER. (b) The stress-strain curves of polymers. (c) The supposed toughening mechanism of PTBN6-GER with internal B–N coordination bonds.
Figure 4. The RTP property and the multiple application displays of PTBN6-GER.
The presence of a large number of dynamic boronic ester bonds, B-N coordination bonds, and hydrogen bonds in the PTBN6-GER polymer network endow the material with excellent shape memory, reprocessability and degradability. After multiple cycles of reprocessing, the mechanical and RTP properties of the sample showed no significant degradation. This work further cleverly combined the shape memory of polymers with the fluorescence and phosphorescence properties, resulting in lots of practical applications in biomimetic simulation, information anti-counterfeiting, multiple encryption, pattern erasure, and other fields (Figure 5). In conclusion, this new design strategy and synthesis method based on dynamic chemistry provides a new approach for the subsequent development of multifunctional RTP polymer materials.
Figure 5. The shape memory property of PTBN6-GER and illustration of the combination between shape memory and RTP properties.
The related paper entitled “Tough,Reprocessable and Recyclable Dynamic Covalent Polymers with Ultrastable Long-Lived Room-Temperature Phosphorescence” has been published in Angewandte Chemie International Edition (DOI:10.1002/anie.202301993). Prof. Cheng-Hui Li is the corresponding author and doctoral student Zi-Han Zhao is the first author. Prof. Jing-Lin Zuo and Prof. You-Xuan Zheng participated in this work.