Introduction

Deep cross-fusion of supramolecular chemistry and traditional polymer science has driven the rapid growth of supramolecular polymers1,2,3,4,5,6,7. Various supramolecular polymers have been constructed by using different recognition motifs, such as hydrogen bonding8,9,10,11,12, π‒π stacking13,14,15,16, host–guest complexations17,18,19,20, ionic attraction21,22,23, and metal–ligand interactions24,25,26, which allow the corresponding materials to be dynamic, reversible, recoverable, and stimuli-responsive. These distinctive advantages make them highly attractive as promising adaptive materials. However, because of the use of relatively weak non-covalent bonds, supramolecular polymers usually display weak mechanical strength compared with covalent polymers, which inevitably restricts their practical applications27,28,29.

To overcome this issue, the integration of covalent polymers and supramolecular polymers could be adopted to fabricate polymeric materials with high robustness30,31,32,33. Synergistic effects of using covalent and non-covalent systems are beneficial to improve the robustness and meanwhile retain the dynamic characteristics of supramolecular polymers34,35,36. Such polymeric networks crosslinked by supramolecular interactions37,38,39,40, dynamic covalent bonds41 or mechanically interlocked bonds42,43,44 were proven to have intriguing properties that cannot be acquired by covalently crosslinked polymers. Owing to the relative weakness of single supramolecular interactions, multiple and strong interactions are preferable in the crosslinkers. Even so, to attain relatively high robustness, a high percentage of supramolecular units (~10 mol% or more) in the networks was generally necessary to ensure strong crosslinking45,46. Nonetheless, high usage of the supramolecular crosslinker units restricts the motion of segments, making the materials more brittle and less stretchable, as well as reducing their practicality. Small usage of supramolecular units is greatly desired to solve this problem.

Over the past decades, coordination-driven self-assembly has attracted much attention, accompanied by the emergence of many supramolecular coordination complexes (SCCs) with well-defined shapes and sizes, including metallacycles and metallacages47,48,49,50,51. These SCCs not only have interesting topological architectures but also exhibit extensive applications in many fields52,53,54,55,56. Coordination-driven self-assembly possesses great advantages, such as high efficiency, wide diversity, and dynamic and reversible characteristics, which enable it as a very powerful tool in supramolecular self-assembly57,58,59,60. The unique advantages of coordination-driven self-assembly have also led it to be used for the construction of linear supramolecular polymers from small supramolecular building blocks, which has also proven it is a strong and tolerant dynamic polymerization method61,62,63,64. Herein, we utilize the self-assembled Pt(II)-based metallacycles to crosslink the linear polymers. With the metallacycles as the strong crosslinkers, we fabricate highly robust ring-opening metathesis polymer networks and unexpectedly find that the presence of only a very small amount of metallacycle crosslinkers is enough to greatly strengthen the mechanical properties of the polymers.

Results

Design, synthesis, and structural characterization

A norbornene monomer (M-1) with a 120° dipyridine substituent was first synthesized in six steps in a 45% overall yield from the phthalimide-derived bromide 1 (Fig. 1). The SN2 substitution reaction of 1 with 3,5-dibromophenol produced compound 2 (Supplementary Figs. 1–3). After Sonogashira cross-coupling of 2 with trimethylsilylacetylene and then deprotection of trimethylsilyl (TMS) groups of 3 (Supplementary Figs. 4–6), the intermediate 4 (Supplementary Figs. 7–9) was achieved. The cross-coupling reaction of 4 with 4-bromopyridine provided 5 (Supplementary Figs. 10–12). Hydrazinolysis of 5 generated 6 (Supplementary Figs. 13–15) and the imidization of 6 with cis-5-norbornene-endo-2,3-dicarboxylic anhydride furnished the monomer M-1 (Supplementary Figs. 16–18). The introduction of the 120° dipyridine ligand on M-1 enables self-assembly to metallacycles.

Fig. 1: Synthesis of monomer M-1.
figure 1

a Synthetic route of M-1. b Cartoon representation of M-1.

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Two-component coordination-driven self-assembly of the monomer M-1 was subsequently examined. Self-assembly of M-1 with the 60° Pt(II)-based acceptor 7 in 1:1 mole ratio afforded a [2 + 2] Pt(II)-coordinated rhomboid (9), while self-assembly of M-1 with the 120° Pt(II)-based acceptor 8 produced a [3 + 3] hexagonal metallacycle (10), in quantitative yields (Fig. 2). The 31P{1H} and 1H NMR spectroscopy of the assemblies indicated the formation of single, discrete species with highly symmetric architectures (Fig. 3). All proton signals were clearly assigned and apparent shifts of the proton peaks were observed in the 1H NMR spectra of 9 (Fig. 3b and Supplementary Fig. 23) and 10 (Fig. 3e and Supplementary Fig. 27), as compared with those of their corresponding building blocks M-1, 7 and 8 (Supplementary Figs. 16, 19, and 21). The 31P{1H} NMR spectra of both 9 (Fig. 3h and Supplementary Fig. 24) and 10 (Fig. 3j and Supplementary Fig. 28) displayed a single peak flanked by two 195Pt satellites, consistent with the homologated Pt‒N coordination environment. Obvious upfield shifts of the 31P{1H} signals were observed after the formation of the two metallacycles (Fig. 3g–j and Supplementary Figs. 20, 22, 24, 28).

Fig. 2: Preparation of the metallacycles and metallacycle-crosslinked polymer networks.
figure 2

Ring-opening metathesis polymerization (ROMP) reaction of the monomer M-1 and M-2 is catalyzed by a pyridine-modified Grubbs II catalyst to produce the polymers CP-n. The cartoon structures represent the rhombic and hexagonal metallacycles and their building blocks. Color code: red = Pt(II) acceptor; blue = dipyridine ligand.

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Fig. 3: Structural characterization of metallacycles.
figure 3

Partial 1H NMR spectra (500 MHz, CD2Cl2, 298 K) of M-1 (a), metallacycle 9 (b), 7 (c), M-1 (d), metallacycle 10 (e) and 8 (f). 31P{1H} spectra (202 MHz, CD2Cl2, 298 K) of 7 (g), 9 (h), 8 (i) and 10 (j). k X-ray crystal structure of metallacycle 9. Counterions, solvents, H atoms, and triethylphosphine ligands on the platinum atoms have been omitted for clarity.

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The structures of metallacycles were further proven by electrospray ionization mass spectrometry (ESI-MS). The main peaks in the ESI-MS of 9 and 10 all supported the formation of [2 + 2] rhomboid 9 (Supplementary Fig. 25) and [3 + 3] hexagon 10 (Supplementary Fig. 29). All of the peaks are isotopically resolved and agree very well with their theoretical distributions (Supplementary Figs. 26 and 30). No peaks were found from the assemblies with other stoichiometries. Fortunately, single crystals of the rhombic metallacycle were obtained by vapor diffusion of hexane into a dichloromethane solution of 9 at room temperature. The single crystal X-ray analysis unambiguously confirmed the formation of rhomboid 9 (Fig. 3k, Supplementary Fig. 49, and Supplementary Table 4).

Copolymerization of M-1 and a diester-derived norbornene monomer M-2 was carried out through ring-opening metathesis polymerization (ROMP). It was found that the copolymerization reaction could not be catalyzed by the usually used Grubbs’ catalysts, including 1st, 2nd, and 3rd generation, but could be catalyzed by a pyridine-modified 2nd generation Grubbs’ catalyst65. By using this modified catalyst, the covalent copolymers (CPs) were synthesized from the mixed monomers with a feed ratio of M-1/M-2 = 1:60 (for CP-1), 1:180 (for CP-2), 1:360 (for CP-3), and 1:600 (for CP-4), respectively. The results of gel permeation chromatography (GPC) indicated that the average molecular weight (Mn) values of copolymers CP-1 (Supplementary Fig. 32), CP-2 (Supplementary Fig. 34), CP-3 (Supplementary Fig. 36), and CP-4 (Supplementary Fig. 38) were 40 kDa, 47 kDa, 50 kDa, and 64 kDa, respectively, with similar polydispersity index (PDI) (Mw/Mn = 1.45 ~ 1.67) using polystyrene (PS) standards. The characteristic peaks of the dipyridine moieties shown in the 1H NMR spectra of the copolymers proved that the 120° dipyridine ligand was introduced into the side chains of CPs (Supplementary Figs. 31, 33, 35, and 37 and Supplementary Table 1). Subsequently, coordination-driven self-assemblies of CPs were performed by mixing the CPs with the 60° Pt(II) acceptor 7 and the 120° acceptor 8, respectively, in tetrahydrofuran (THF) solution to generate the corresponding metallacycle-crosslinked polymer networks (MCPNs). After that, the solution viscosity increased significantly (Supplementary Fig. 39), indicative of the formation of crosslinked polymers through the self-assembly of Pt(II) metallacycles among the linear polymers. Due to the strong crosslinking of metallacycles, the reaction mixture of polymer networks (CP-1R, CP-1H, CP-2R, and CP-2H) made from CP-1 (with 1.64 mol% dipyridine units) and CP-2 (with 0.55 mol% dipyridine units) even changed to gels quickly, while the polymer networks (CP-3R, CP-3H, CP-4R, and CP-4H) from CP-3 (with 0.28 mol% dipyridine units) and CP-4 (with 0.17 mol% dipyridine units) remained soluble but with much higher viscosities. Since the MCPNs made from CP-1 and CP-2 solidified into gels, only the MCPNs from CP-3 and CP-4 were further investigated in the following sections. We also attempted an alternative method, polymerization after the formation of metallacycles, but only obtained solutions with low viscosity, indicating that the polymerization was unsuccessful. This is probably because the presence of these metallacycles impedes the catalytic action of the Ru(II)-based catalyst for polymerization.

Thermal properties can reflect the internal structure of polymeric materials. With the MCPNs in hand, their basic thermal properties were investigated by thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC). The TGA curves demonstrated that the decomposition temperatures (5% weight loss) of the MCPNs were all higher than 250 °C, indicative of their good thermal stability (Supplementary Fig. 40). As shown in Fig. 4a, the DSC studies indicated that all the MCPNs showed obvious glass transition, probably owing to the relaxation of the polynorbornene segments. Comparatively, all the MCPNs possessed higher glass transition temperatures (Tg) (42.9 °C for CP-4H, 45.3 °C for CP-4R, 46.9 °C for CP-3H, and 48.1 °C for CP-3R) than the linear convent polymer CP-3 (41.8 °C), indicating that the crosslinking of Pt(II)-based metallacycles restrained the movement of polynorbornene segments. Noticeably, Tg of CP-3R and CP-3H were higher than those of CP-4R and CP-4H, respectively, implying that the increase in crosslinking density makes the movement of segments more difficult. In addition, CP-3R and CP-4R had higher Tg than CP-3H and CP-4H, respectively, suggesting that the polymers crosslinked by the rhomboid metallacycle are more stable and arranged in better order than those crosslinked by the hexagonal metallacycle. This is probably because the hexagon links to three polymer branches, while the rhomboid links to only two polymer chains. The entanglement of polymer chains makes the [3 + 3] self-assembly more difficult than the [2 + 2] self-assembly.

Fig. 4: Fundamental performance characterizations of MCPNs.
figure 4

a DSC curve of MCPNs and CP-3 recorded in the first heating scan from 0 to 100 °C with a heating rate of 20 °C/min. b Stress‒strain curves of MCPNs and CP-3 recorded with a deformation rate of 100 mm/min. c Young’s moduli and toughness of MCPNs and CP-3 calculated from their stress‒strain curves. d Stress‒strain curves of CP-3R, CP-3H, conCP-3R, and conCP-3H were recorded with a deformation rate of 100 mm/min. e Young’s moduli and toughness of CP-3R, CP-3H, conCP-3R, and conCP-3H calculated from their stress‒strain curves. f Cyclic tensile test curves of CP-3R, CP-3H, conCP-3R, and conCP-3H loaded at a strain of 500% under the deformation rate of 100 mm/min.

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Mechanical properties of MCPNs, CP-3, and controls

To manifest the performance of Pt(II)-based metallacycles as crosslinkers, the mechanical properties of MCPNs and CP-3 were systematically investigated through tensile tests. The stress‒strain curves, the calculated Young’s moduli, and the toughness of MCPNs and CP-3 are illustrated in Fig. 4b, c. Both the MCPNs and CP-3 exhibited yield behavior because all of them were in the glassy state at room temperature. The yield strengths of all MCPNs exceeded 20 MPa, significantly higher than that of CP-3 (about 5 MPa, Supplementary Table 2). The tensile strengths and Young’s moduli of MCPNs exceeded 15 MPa and 350 MPa, respectively, also much higher than those of CP-3. The toughness of CP-3R and CP-3H also remarkably increased from less than 50 MJ/m3 for CP-3 to more than 150 MJ/m3. These results revealed that the mechanical properties of the polymers were greatly improved after crosslinking by the metallacycles, despite the fact that only tiny amounts of crosslinkers were used. Overall, CP-3R and CP-3H possessed more balanced and comprehensive mechanical performance in comparison with CP-4R and CP-4H, so they were selected as representative samples for subsequent investigation. Although their toughness was almost the same, the Young’s modulus of CP-3R was higher than that of CP-3H. The higher Young’s modulus can reflect the better stability of the rhomboid metallacycle crosslinked network, which is consistent with the results of the DSC discussed above. This is probably because the self-assembly of [2 + 2] rhombic metallacycles involves fewer components, while self-assembly of the [3 + 3] hexagonal metallacycles involves more components and thus is relatively slower and more difficult in the polymer solutions. Therefore, more incomplete assemblies (presented in Supplementary Figs. 41 and 42) may exist in CP-3H than in CP-3R. Although some defective assemblies can also act as crosslinkers, they are single-coordinated and weaker than the double-coordinated metallacycles. In other words, the rhomboid-crosslinked polymer networks have relatively fewer soft polymer chains not participating in the self-assembly. For the same reason, the MCPNs crosslinked by the rhomboid metallacycle would result in a more ordered arrangement of the polymer chains, also leading to the higher modulus of CP-3R. Meanwhile, because the partially broken hexagons can still act as crosslinkers after one of the branch chains on the hexagonal metallacycles is broken off, CP-3H has a higher elongation at break.

To better understand the role of the supramolecular Pt(II)-based metallacycle crosslinkers in enhancing mechanical performance, two controls, conCP-3R and conCP-3H were designed and synthesized. Their components are similar to CP-3R and CP-3H, respectively, except that the platinum of the control samples is in the form of a dibromide acceptor which cannot coordinate with the dipyridine ligand (Supplementary Fig. 43). The mechanical performances of the two control were measured by tensile tests and compared with those of CP-3R and CP-3H (Fig. 4d, e). The results indicated that conCP-3R and conCP-3H had mechanical properties similar to CP-3, while Young’s modulus, tensile strength, and toughness of the control samples were much lower than those of CP-3R and CP-3H, respectively. Such significant differences in performance indicated that the crosslinking of polymer chains by the Pt(II)-based metallacycles plays a crucially important role in the enhancement of mechanical properties.

The cyclic tensile tests at a maximum strain of 500% demonstrated that CP-3R and CP-3H possessed higher energy dissipation capacity than the two controls (Fig. 4f). During the stretching process, polymer networks of CP-3R and CP-3H dissociated under external forces until the crosslinking points of the metallacycles were destroyed, which can dissipate a large amount of energy. The intrinsic dynamic features of coordination-driven self-assembly endow the metallacycles with reversible destruction/reformation, which greatly benefits energy dissipation (Fig. 5). The strong coordination interaction and good stability of the metallacycles largely increased the tolerance to an external force. Therefore, CP-3R and CP-3H possess outstanding energy dissipation capability.

Fig. 5: Cartoon representations for destruction and reformation of the metallacycles.
figure 5

a For rhombic metallacycle. b For hexagonal metallacycle.

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Insights into the mechanical performance of CP-3R and CP-3H

As shown by the tensile experiments in the previous section, the mechanical performances of MCPNs underwent extraordinary improvement due to the introduction of robust and meanwhile dynamic metallacycle-based supramolecular crosslinking. It was speculated that the enhanced tensile strength, Young’s moduli, and toughness of MCPNs originated from the supramolecular networks formed during the assembly process. In order to have an in-depth understanding of the improvement in mechanical properties, dynamic mechanical analysis (DMA) experiments were conducted on CP-3, CP-3R, and CP-3H (Fig. 6a–c). For the linear polymer CP-3, the storage modulus (G′) began to decrease as the temperature increased. When it was heated to 30 °C, G′ decreased significantly and then reached a value close to the loss modulus (G″) at about 48 °C, which indicated that the polymer gradually transformed to a rubbery state from a glassy state. During the heating process, CP-3R and CP-3H exhibited dynamic thermal mechanical behaviors similar to CP-3. The difference is that both CP-3R and CP-3H have a storage modulus of over 400 MPa before 32 °C, attributed to the robust metallacyclic crosslinking points in the networks of CP-3R and CP-3H. It is worth noting that CP-3R has a higher storage modulus than CP-3H, indicating that CP-3R has a more stable crosslinking entanglement network (Supplementary Fig. 44), which is consistent with the results of DSC and tensile tests. The value of G′ began to decrease sharply upon further heating, indicative of the destruction of the Pt(II)-coordinated metallacycles which caused rapid collapse of the crosslinked polymer networks. In addition, there is only a slight difference in the value of tanδ among CP-3, CP-3R, and CP-3H, suggesting that they have similar aggregated structures after the metallacycles are disassembled.

Fig. 6: Dynamic thermomechanical properties and micromorphology in the solid state of MCPNs.
figure 6

DMA curves of CP-3 (a), CP-3R (b) and CP-3H (c) were recorded in the heating scan from 10 to 80 °C with a heating rate of 2 °C/min. SEM micrographs of cryofractured surfaces of CP-3 (d), CP-3R (e) and CP-3H (f).

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To further investigate the enhancement of mechanical properties derived from the Pt(II)-coordinated metallacycles, scanning electron micrography (SEM) characterization on the fracture surface was performed (Fig.  6d–f). The cryo-fractured surface of CP-3 is relatively smooth, while those of CP-3R and CP-3H are relatively rough, which can be attributed to the destruction of strong metallacycle networks distributed inside the polymers during the process of fracture. The network gels were also characterized by SEM. It was found that the gel film of CP-3 could be dissolved in 1,2-dichloroethane, but the gel films of CP-3R and CP-3H prepared in situ from their precursors could only be swollen (Supplementary Fig. 45). The swollen films of CP-3R and CP-3H were changed to xerogels by freeze-drying. SEM images of the xerogels of CP-3R and CP-3H showed crosslinked network structures, clearly indicating the formation of crosslinked polymer networks (Supplementary Figs. 46 and 47).

Dynamic properties of CP-3R and CP-3H

The studies in the previous sections demonstrated that the robust metallacycle-based crosslinking is capable of effectively stabilizing the polymer networks to withstand destruction under an external force, thus exhibiting increased strength and toughness of the MCPNs. Meanwhile, the inherent dynamic features of coordination-based supramolecular assembly can endow the MCPNs with attractive dynamic properties. Therefore, we subsequently investigated the dynamic behavior of CP-3R and CP-3H.

First, the energy dissipation capabilities of CP-3R and CP-3H were assessed through cyclic tensile tests at different maximum strains ranging from 100% to 500%. As shown in Fig. 7a, b, when the strain was increased gradually, CP-3R and CP-3H presented large hysteresis loops accompanied by higher residual strains. After the energy dissipations were calculated by using the hysteresis areas, the relationships between the energy dissipation values and the strains were found to be approximately linear (Fig. 7c). It was demonstrated that the damping capacities of CP-3R and CP-3H kept a steadily high value of about 80% at high strains, indicative of their good ability in energy dissipation (Fig. 7c). These results revealed that a small amount of dynamic Pt(II)-based metallacycles was able to provide sacrificial bonds to effectively dissipate the input energy. As it can be seen from the DSC (Fig. 4a) and DMA (Fig. 6b, c) results, the glass transition temperature of our norbornene copolymer is slightly higher than room temperature. Therefore, at room temperature, the polymer segments are in a frozen state. During external stretching, the frozen segments would be unfrozen and undergo movement. Although the Pt(II) metallacycles have been introduced as the crosslinkers, the amount of metallacycles is very small, so the polymer segments can move under external forces. After unloading the external forces, the movement of segments needs to overcome the influence of crosslinking and the rigidity of its own segments. During this process, a large amount of mechanical energy was dissipated in the form of heat.

Fig. 7: Dynamic and stimuli-responsive properties of CP-3R and CP-3H.
figure 7

Cyclic tensile test curves of CP-3R (a) and CP-3H (b) were recorded with increased maximum strains. c Energy dissipation and damping capacity for each circle of the cyclic tensile test curves. d Schematic diagram of reversible stimulus-response of rhomboic (left) and hexagonal (right) metallacycle. e Stress−strain curves of CP-3R, CP-3R + Br−, and CP-3R + Brˉ + Ag+ were recorded with a deformation rate of 100 mm/min. f Stress−strain curves of CP-3H, CP-3H + Brˉ, and CP-3H + Brˉ + Ag+ were recorded with a deformation rate of 100 mm/min. g Young’s moduli and toughness of CP-3R, CP-3R + Brˉ, CP-3R + Brˉ + Ag+, CP-3H, CP-3H + Brˉ, and CP-3H + Brˉ + Ag+ calculated from their stress−strain curves.

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Stimuli-responsive properties of CP-3R and CP-3H

Stimuli-responsiveness has long been a fascinating feature of supramolecular materials. As only a tiny amount of Pt(II)-based metallacycles were used as the dynamic crosslinkers, we were curious about whether the metallacycle-crosslinked polymers CP-3R and CP-3H would exhibit stimuli-responsiveness and reveal changeable macroscopic properties under external stimuli. It is known that the Pt−N coordination bonds in the Pt(II)-based metallacycles are easy to be destroyed by introducing competitive bromide anions66. When tetrabutylammonium bromide (TBABr) in an equimolar ratio with the platinum atoms was added to the crosslinked polymer solid sample suspended in THF, a solution formed because the introduction of bromide anions disrupts the coordination between platinum and pyridine (Supplementary Fig. 48b, e). After that, equimolar silver trifluoromethylsulfonate (AgOTf) was added. It was found that the viscosity of the solution increased and the solution gelled (Supplementary Fig. 48c, f), because the coordination bonds were restored after removal of bromide anions in the form of silver bromide precipitated from solutions. These studies indicate that the Pt(II)-based metallacycles demonstrate the reversible transformation in response to added ions (Fig. 7d).

The mechanical properties also reveal these reversible transformations. After the crosslinked polymer samples were treated with TBABr, mechanical performances decreased; tensile strength (12.13 ± 0.43 MPa for CP-3R + Brˉ and 11.86 ± 0.72 MPa for CP-3H + Brˉ), Young’s moduli (136.17 ± 3.32 MPa for CP-3R + Brˉ and 135.66 ± 5.77 MPa for CP-3H + Brˉ), and toughness (97.14 ± 3.77 MJ/m3 for CP-3R + Brˉ and 100.89 ± 6.88 MJ/m3 for CP-3H + Brˉ) were observed in comparison with the virgin ones (Fig. 7e–g and Supplementary Table 3). Again the addition of TBABr destroyed the Pt−N coordination bonds, and thus led to the partial decomposition of the MCPNs and the dramatic decrease in mechanical properties. The corresponding tensile strength and Young’s moduli were partially restored after the subsequent introduction of AgOTf into the solutions. The tensile strengths of CP-3R + Brˉ + Ag+ and CP-3H + Brˉ + Ag+ were restored to 15.74 ± 3.14 MPa and 13.11 ± 2.72 MPa, respectively. The Young’s moduli were also significantly recovered to be 407.79 ± 4.72 MPa for CP-3R + Brˉ + Ag+ and 290.9 ± 7.25 MPa for CP-3H + Brˉ + Ag+. However, their elongation at break and toughness were lower than those of CP-3R + Brˉ and CP-3H + Brˉ before the addition of AgOTf, perhaps due to the formation of insoluble silver bromide. These results revealed that the Pt(II) metallacycle-based supramolecular crosslinking is subject to modulation by external stimuli, thereby rendering the MCPNs with stimuli-responsiveness.

Discussion

In this work, we have presented the design and construction of tough supramolecular polymer networks crosslinked by a tiny amount of metallacycles. A norbornene substituted with a 120° dipyridine ligand was synthesized and copolymerized with a diester-derived norbornene monomer through ROMP to fabricate the corresponding CPs. The copolymers were then self-assembled with 60° or 120° Pt(II) acceptors to construct metallacycle-crosslinked polymer networks. Although only 0.28 mol% or less pendant dipyridine units were used to form the metallacycle crosslinkers, the mechanical properties of the polymers were significantly improved. After crosslinking, the yield strength, tensile strength, Young’s modulus, and toughness all increased to several times of the original ones. The profound effect of metallacycle crosslinking on the mechanical properties of the polymers is supposed to benefit from the robust, dynamic, and damage-tolerable features of Pt(II)-coordinated metallacycles. Good stimuli-responsiveness of the metallacycle-crosslinked polymer networks was demonstrated by the sequential addition of TBABr and silver triflate. This work provides a strategy to significantly reinforce the mechanical strength of supramolecular polymeric materials by using very small amounts of supramolecular units, meanwhile maintaining their stimuli-responsive features.

Methods

The synthesis and characterization of compounds presented in this work, the experimental details and additional data of tests are described in the Supplementary Information.