image: Figure 1. Schematic diagram of the SPN network and its preparation process as well as the control of association and dissociation kinetics by steric hindrance of the cross-linker terminal blockers.
Credit: CCS Chemistry
A team led by Associate Researcher Zhaoming Zhang and Researcher Xuzhou Yan from Shanghai Jiao Tong University introduced pseudorotaxane end groups of varying sizes and utilized their steric hindrance to modulate the dissociation kinetics of supramolecular crosslinks, thereby achieving control over the mechanical properties of supramolecular polymer networks (SPNs). This study systematically elucidated the relationship between supramolecular structure, dissociation kinetics, and macroscopic mechanical properties of the network, providing new insights into the development of high-performance SPNs.
Background:
Supramolecular polymer networks (SPNs) are network structures formed by non-covalent crosslinking of polymer chains. They have attracted widespread attention due to their remarkable dynamic properties, including self-healing, stimulus-responsiveness, and reprocessability. Unlike traditional covalent crosslinking, reversibility is a prominent feature of supramolecular crosslinking, which is governed by both thermodynamic and kinetic factors. Studies have shown that the dissociation kinetics of supramolecular crosslinks have a decisive influence on the viscoelastic and mechanical properties of SPNs, but current approaches to manipulate these kinetics remain limited. A common strategy involves indirectly regulating the association and dissociation rates by varying the strength of supramolecular interactions. However, this approach has two major limitations: first, it is difficult to precisely control kinetic behavior without affecting thermodynamic properties; second, achieving extremely slow dissociation kinetics often requires exceptionally strong supramolecular interactions, which not only requires complex molecular design but is also limited by inherent strength limits. Therefore, developing more efficient and reliable strategies to precisely control the dynamics of supramolecular polymer networks remains an important and pressing issue in this field.
Highlights of this article:
This study modulates supramolecular dissociation kinetics by varying the size of pseudorotaxane end groups, developing a novel kinetic control strategy. This strategy allows for the regulation of dissociation rates over a wide range, reaching speeds as slow as 6.1 × 10−7 s−1. Furthermore, the study reveals the influence of crosslink dissociation rates on various material properties, including linear viscoelasticity, stress-strain behavior, stress relaxation, and creep, providing important insights into the relationship between kinetics and material properties.
The authors monitored the complexation process in acetone, and the complexation of crosslinker 2 reached equilibrium after 20 days (Figure 2a). Based on the slope of the linear fit in Figure 2c, the apparent binding rate constant, ka, for the model reaction system was calculated to be 4.2 × 10−7 s−1. Notably, this apparent constant can be expressed as kapp = k × CB, where CB approximates the initial concentration of dibenzo-24-crown-8 (DB24C8). Therefore, the previously stated "rate constant" is actually an apparent constant that includes a correction factor. Since crosslinker 1's recognition with DB24C8 reaches equilibrium within 5 minutes, its ka is estimated to be approximately 5760 times that of crosslinker 2. Therefore, its apparent binding rate constant, ka, should be greater than 2.4 × 10−3 s−1. Subsequently, the dissociation process was monitored in DMSO, and the dissociation rate constant kdof crosslinker 2 was calculated to be 6.1 × 10−7 s−1 according to Figure 2d (Figure 2b, e), and the kd of crosslinker 1 was > 3.5 × 10−3 s−1.
Figure 3 shows the dynamic viscoelastic behavior of SPN-1 and SPN-2. Strain sweeps reveal that SPN-2 exhibits a broader linear viscoelastic region (strain > 10% ), while the modulus of SPN-1 decreases sharply after strain exceeds 1% (Figure 3a), highlighting the excellent stability of the SPN-2 network due to its large steric end capping. The master curves also reveal that SPN-2 exhibits a wider elastic plateau (2.6 × 10-6 – 6.3 ×102 rad/s), indicating a more stable network. In contrast, SPN-1 exhibits G ″ exceeding Gʹ in the terminal region, indicating rapid network relaxation (Figure 3b, c). During the time scan, the stress of SPN-1 dropped to 9% within 10 seconds at 12% strain, while SPN-2 retained 74.2% of its initial stress (Figure 3d, e). This result further demonstrates that SPN-2 exhibits slower dissociation kinetics and greater network stability due to greater terminal steric hindrance. SRFS testing also provides favorable evidence that SPN- 1 possesses higher energy dissipation capacity due to its faster dissociation (Figure 3f).
To evaluate the macroscopic mechanical properties of two supramolecular polymer networks with different steric hindrances of the end-capping groups, the research team conducted tensile tests. The results showed that SPN-1 and SPN-2 had similar Young's moduli (58.3 vs. 51.2 MPa), but SPN-2 significantly outperformed SPN-1 in terms of breaking stress (14.5 vs. 4.1 MPa), breaking strain (549.5% vs. 273.5%), and toughness (46.7 vs. 9.4 MJ·m⁻³), indicating greater mechanical stability, which is closely related to the slower dissociation kinetics of its crosslinking sites (Figures 4a,b). To further verify the mechanism, the authors exposed both materials to triethylamine (Et₃N) vapor to destroy the recognition sites in the crosslinked network. The results showed that the mechanical properties of SPN-1 decreased significantly (toughness dropped from 9.4 to 2.5 MJ·m⁻³ and fracture stress dropped from 4.1 to 0.6 MPa), while SPN-2 was less affected (toughness dropped from 46.7 to 40.5 MJ·m⁻³ and fracture stress dropped from 14.5 to 10.5 MPa), indicating that the large sterically hindered end groups effectively maintained the integrity of the network structure (Figure 4c–e). Creep and recovery experiments further revealed that SPN-2 exhibited lower creep strain and higher recovery rate, indicating that its slower dissociation kinetics significantly enhanced the network's deformation recovery and structural stability (Figure 4f).
Summary and Outlook:
This study successfully achieved precise control of the dissociation kinetics of supramolecular crosslinks by regulating the steric hindrance of pseudorotaxane end groups, thereby modulating the dynamic behavior and mechanical properties of the supramolecular polymer network. 1H NMR studies revealed that crosslinker 1 with pyrrole end groups exhibited significantly higher association rate constants (> 2.4 × 10-3 s-1) and dissociation rate constants (> 3.5 × × 10-3 s-1), while crosslinker 2 with seven-membered ring end groups exhibited significantly lower rate constants (4.2 × 10-7 and 6.1 × 10-7 s-1). This trend was maintained in the bulk material, with the crosslink dissociation rate of SPN-1 being approximately 550 times higher than that of SPN-2. Rheological studies show that SPN-2 has slower stress relaxation, a more stable network structure, and corresponding mechanical properties are significantly better than SPN-1 , including higher fracture stress (14.5 vs. 4.1 MPa), fracture strain (549.5% vs. 273.5%), toughness (46.7 vs. 9.4 MJ·m -3), and stronger puncture resistance (puncture force: 3.9 vs. 1.7 N, puncture energy: 11.8 vs. 2.5 mJ).
This study proposes a novel strategy for manipulating supramolecular dynamics based on end-group steric hindrance. This strategy not only enables precise control of the mechanical properties of supramolecular polymer materials but also reveals the correlation between molecular dissociation dynamics and macroscopic material properties. This strategy provides new design ideas for developing smart polymer materials with programmable mechanical properties, self-healing properties, and reconfigurable properties.
The research results were published as a research article in CCS Chemistry. Shaolei Qu, a doctoral student at Shanghai Jiao Tong University, is the first author, and Associate Researcher Zhaoming Zhang and Researcher Xuzhou Yan are the corresponding authors. This work was funded by the National Natural Science Foundation of China, the Shanghai Natural Science Foundation, the State Key Laboratory of Supramolecular Structures and Materials , and the China Postdoctoral Science Foundation.
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About the journal: CCS Chemistry is the Chinese Chemical Society’s flagship publication, established to serve as the preeminent international chemistry journal published in China. It is an English language journal that covers all areas of chemistry and the chemical sciences, including groundbreaking concepts, mechanisms, methods, materials, reactions, and applications. All articles are diamond open access, with no fees for authors or readers. More information can be found at https://www.chinesechemsoc.org/journal/ccschem.
About the Chinese Chemical Society: The Chinese Chemical Society (CCS) is an academic organization formed by Chinese chemists of their own accord with the purpose of uniting Chinese chemists at home and abroad to promote the development of chemistry in China. The CCS was founded during a meeting of preeminent chemists in Nanjing on August 4, 1932. It currently has more than 120,000 individual members and 184 organizational members. There are 7 Divisions covering the major areas of chemistry: physical, inorganic, organic, polymer, analytical, applied and chemical education, as well as 31 Commissions, including catalysis, computational chemistry, photochemistry, electrochemistry, organic solid chemistry, environmental chemistry, and many other sub-fields of the chemical sciences. The CCS also has 10 committees, including the Woman’s Chemists Committee and Young Chemists Committee. More information can be found at https://www.chinesechemsoc.org/.
Journal
CCS Chemistry
Method of Research
Experimental study
Subject of Research
Not applicable
Article Title
Supramolecular Dissociation as a Tool for Engineering Material Properties
Article Publication Date
17-Sep-2025
COI Statement
There is no conflict of interest to report.