Facile Preparation of a Poly[2]Catenane Network Using Self‐Assembled [2]Catenane Unit

ABSTRACT

Polycatenanes are an emerging class of polymeric materials linked through interlocked rings, possessing a higher degree of conformational freedom and mobility. However, the number of its studies is far lower than that of other dynamic polymers. This may reflect the fact that the preparation of polycatenanes involves the complicated synthesis of a covalent catenation (ring closing) step. Here we report a self‐assembly approach that allows the facile preparation of a poly[2]catenane network with good material properties. First, a clip‐type self‐associated dimer is generated through a spontaneous self‐assembly process driven by hydrogen bonding interactions between two individual isophthaloyl bis(diamine) subunits modified with olefin groups. Next, polymerization via a thiol‐alkene click reaction is used to produce an H‐bond‐based polymer network. Finally, ring closure involving the formation of dynamic imine bonds was used to create a series of poly[2]catenane‐containing networks. The resulting polymer networks not only demonstrated excellent mechanical features and stimuli‐responsiveness but also proved ready degradability. We believe that this work will significantly accelerate the development of polycatenane materials.

Polycatenanes show a higher degree of conformational freedom and mobility compared to covalent polymers. However, the number of its studies is far lower than that of other dynamic polymers. This may reflect the fact that the preparation of polycatenanes involves the complicated synthesis of covalent catenation. Here we report a self‐assembly approach that allows the facile preparation of a poly[2]catenane network with good material properties.

1. Introduction

Polycatenanes are polymers containing mechanically interlocked [1, 2] catenane subunits [3, 4, 5]. The constituent catenanes are generally able to support a variety of relative motions, including rotation, elongation, and twisting of the individual rings [6, 7]. As a consequence, polycatenanes generally possess a higher degree of conformational freedom and mobility than conventional covalent polymers. This, in turn, endows them with unique mechanical properties [8, 9]. However, since the first report of polycatenanes in 1995 [10], the number of studies on polycatenanes is far lower than that of other dynamic polymers, such as polyrotaxanes [11], dynamic covalent polymers [12], and supramolecular polymers [13]. This may reflect the fact that the preparation of polycatenanes involves complicated synthesis of covalent catenation (ring closing) steps (Scheme 1a) [3], which greatly limits the output of catenane units. However, for polymer materials, large amounts of catenane units (gram‐scale) are needed due to the large sample demand in mechanical testing. Therefore, to promote the development of polycatenanes, advances in synthetic methodologies will be required. This will facilitatethe preparation of key catenane units and further allow access to significant amounts of polycatenanes [7].

SCHEME 1.

(a) Schematic overview of previous polycatenanes prepared by covalent [2]catenane units. (b) Schematic overview and chemical structures of polycatenanes prepared by self‐assembled [2]catenane units. (c) Mechanical properties, stimulus responsiveness, and degradability of PCN‐n.

The self‐assembly approach exhibits clear advantages over those formed by the covalent way: mild reaction conditions, higher yield, dynamic error correction, and facile functionalization [14]. To date, considerable effort has been devoted to the preparation of polymers containing other macrocyclic and cage‐type subunits by self‐assembly. For instance, Huang, Stang, and coworkers have reported macrocycle‐based supramolecular polymers constructed via self‐assembly [15, 16, 17, 18, 19, 20] processes, where metal‐ligand bond‐based monomers with cyclic rhomboid and cyclic hexagon structures were converted into linear supramolecular polyrhomboid and cross‐linked three‐dimensional supramolecular polymeric networks, respectively [21]. These researchers also reported macrocycle‐based polymers with dendronized structures [22]. Johnson's group applied the principles of cooperative self‐assembly to construct polymer cages based on metal coordination interactions [23], while Zhang's group prepared an elastic metallacage‐cored polymeric material by means of self‐assembly [24]. Against this background, as detailed below, we have now developed a self‐assembly‐based strategy that allows for facile preparation of a poly[2]catenane network at a relatively large scale (Scheme 1b) and also shows good material properties (Scheme 1c).

The key dynamic catenation step underlying the present approach is based on a report by Hunter that detailed a covalently synthetic strategy for constructing [2]catenanes through a covalent ring‐closure reaction between the –NH₂ groups in isophthaloyl bis(diamine) and –COCl groups in isophthaloyl dichloride [25]. We postulated that replacing the isophthaloyl dichloride –COCl groups with –CHO moieties would permit reaction with the –NH₂ groups in isophthaloyl bis(diamine) under dynamic covalent bond‐forming conditions. This change was expected to allow for so‐called error correction, resulting in an improved yield for the dynamic ring‐closure reaction. On the other hand, the change from Cl in the original Hunter acid chloride to formyl in the present imine‐containing analog was not expected to affect the size and spatial configuration of the [2]catenanes formed under dynamic conditions.

As shown in Scheme 1b, isophthaloyl bis(diamine) modified with an olefin group (M₁) underwent a spontaneous self‐assembly process mediated by presumed hydrogen‐bonding interactions to form the corresponding clip‐like dimer (2M₁). Upon adding 2,6‐pyridinedicarboxaldehyde (M₂), the two aldehyde groups in M₂ and the two amino groups in M₁ react under dynamic imine bond‐forming conditions to produce [2]catenane (M) characterized by the presence of two interlocked rings [26, 27]. Presumably as the result of the error‐correcting nature of the ring closure process [28, 29], purification of M proved unnecessary. It was thus thought that a similar clean catenation process could be implemented after subjecting 2M₁ to polymerization. To test this proposition, 2M₁, pentaerythritol tetrakis(3‐mercaptopropionate) (TMP), and poly(ethylene glycol) diacrylate (PEG‐DA, M_w  = 700) were mixed with 2,2‐dimethoxy‐2‐phenylacet (DMPA) as an initiator. Upon UV irradiation, hydrogen bond‐based polymer networks (HPN‐n) were obtained as the result of the induced thiol‐alkene click reaction. Upon adding M₂, dynamic covalent imine bond formation occurred in analogy to what was seen for the mixture of 2M₁ and M₂ discussed above. The resulting ring closure produces a series of poly[2]catenane networks (PCN‐n) with a higher yield according to the ¹H NMR results. As discussed below, PCN‐n demonstrated excellent mechanical properties and favorable stimuli‐responsiveness (Scheme 1c). Moreover, the self‐assembled poly[2]catenane network generated in this way was found to degrade upon treatment with hydrochloric acid in marked contrast to what was observed in the case of polycatenane networks based on covalent bonds [30, 31].

2. Results and Discussion

The synthesis and characterization of monomer M₁ are presented in the Supporting Information (Figures S1–S7). M₂ and M₄ can be purchased and used directly, while the synthesis of M₃ relies on a previously reported literature preparation [32]. The ¹H NMR spectrum of M₃ is shown in Figure S8 in the Supporting Information. NMR spectroscopy was used to monitor the self‐assembly process and confirm the formation of the expected [2]catenane. Solutions of M₁ at different concentrations ranging from 10 to 300 mM in CDCl₃ were subjected to ¹H NMR spectroscopic analysis. As shown in Figure 1a and Figure S9, the proton signals of H_a attached to the benzene ring and H_b attributed to the amide both underwent downfield shifts from 7.95 to 8.14 ppm and 7.41 to 7.89 ppm, respectively, as the concentration was increased. This observation is ascribed to the formation of NH···O hydrogen bonds as would be expected during self‐assembly [33]. Next, solutions of M₁, M₂, and M₁ and M₂ were mixed for different times and analyzed by ¹H NMR spectroscopy (Figure 1b and Figure S10). Upon mixing, the –CHO proton peak at 10.16 ppm gradually diminished in intensity, while a new proton signal ascribed to the –CH═N proton was observed at around 8.41 ppm. These results are taken as evidence that M₁ and M₂ become linked through CH═N connections [34] as the result of an imine‐forming condensation between the –NH₂ and –CHO moieties. Additionally, based on NMR integration, the yield of the reaction between the aldehyde and amino groups was calculated to be 75% (Figure S11). In most cases, the reaction between aldehydes and ammonia proceeds in quantitative yield. However, when the amine is a poor nucleophile or the electrophilicity of the aldehyde is relatively weak, the yield of the imine product can fall below 100% [35, 36]. Monomer M₁ comprises an aniline‐type nucleophile. Delocalization of the nitrogen lone pairs into the π system of the attached benzene ring results in decreased electron density at nitrogen and reduced nucleophilicity. Additionally, the two methyl groups adjacent to the ‐NH₂ group exert a hindering steric effect. These two factors cause a decrease in the equilibrium constant for the formation of the imine from the aldehyde of M₂ and the –NH₂ group of M₁. Consequently, the yield of this particular imine‐forming reaction is less than 100%.

FIGURE 1.

(a) Partial concentration‐dependent ¹H NMR spectrum of M₁. (b) Partial ¹H NMR spectrum of M₁, M₂, and a mixture of M₁ and M₂ recorded after the indicated mixing times. Partial (c) NOESY, (d) COSY spectra, and (e) MALDI‐TOF‐MS of a mixture of M₁ and M₂ recorded 24 h after mixing.

Additional experiments were carried out to confirm the formation of the self‐assembled [2]catenane (M). Nuclear Overhauser effect spectroscopy (NOESY) and correlated spectroscopy (COSY) were used to study the interactions within the mixture of M₁ and M₂. Peaks ascribable to correlations between H_f and H_d‐e were observed in the NOESY spectrum; these were taken as evidence for the spatial interactions expected for a mechanically interlocked product. Interactions between H_c and H_d‐e were also seen in accord with what would be predicted for a ring‐closure reaction (Figure 1c and Figure S12) [37]. In addition, correlation peaks between H_c and H_d‐e were present in the COSY spectrum. This is as expected for a system characterized by chemical bonds between H_c of the CH═N moiety and the –CH₃ H_d‐e protons (Figure 1d and Figure S13) [38].

Matrix‐assisted laser desorption/ionization time‐of‐flight mass spectrometry (MALDI‐TOF‐MS) analyses were also carried out and served to confirm the formation of a [2]catenane (M). In particular, peaks corresponding to the addition or loss of different molecules and protons from M were seen, which matched well with the theoretical predictions (Figure 1e and Figure S14). For example, peaks at m/z = 2000.159, 2018.174, and 2082.093, respectively, were observed that correspond to [M + H]⁺, [M + H₂O + H]⁺, and [M + 2MeOH + H₂O + H]⁺ (Note: MeOH was used in the purification of M₁). Additionally, peaks at m/z = 1901.156 and 1964.037 were observed that were readily assigned to [M − M₂ + H]⁺, [M − M₂ + 2MeOH]⁺.

Once the self‐assembled [2]catenane (M) was successfully prepared, an effort was made to carry it forward to construct a polymer material. However, the formation of a polymer network via simply mixing 2M₁, TMP, PEG‐DA, M₂, and the initiator DMPA followed by UV irradiation was not anticipated to be successful because the reaction of the thiol groups in TMP and the aldehyde groups in M₂ was expected to inhibit the polymerization process (Figure S15) [39, 40]. Therefore, the order of the reaction steps was modified such that polymerization was carried out before ring‐closure. The preparation of the polymer network PCN‐n (Table S1) and the control polymer network (polymerization without M₁) are detailed in the Supporting Information. Note: n = 1, 2, 3, 4 are used to denote molar ratios of M₁ to PEG‐DA of 0.5:4, 1:4, 2:4, and 3:4, respectively. Fourier transform infrared (FT‐IR) spectroscopic analyses confirmed that polymerization had occurred (Figure 2a). In accord with our expectations, no signal corresponding to a S‐H stretching mode at 2571 cm⁻¹ was seen, either in PCN‐n or the control polymer network [41, 42]. Meanwhile, the N‐H vibration at 3468 cm⁻¹ in M₁ weakened after conversion to PCN‐n [43]. In the control polymer network, a CH═N vibration at 1645 cm⁻¹ was not observed [44, 45, 46]. In contrast, this band was seen in PCN‐n. The intensity of the CH═N vibration was found to increase as the molar ratio of M₁ increased. These findings are consistent with the formation of a [2]catenane containing product.

FIGURE 2.

FT‐IR spectra of (a) TMP, PEG‐DA, M₁, M₂, control, and PCN‐n. (b) TGA curves of control and PCN‐n. (c) DSC curves of control and PCN‐n.

Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were used to characterize the thermal property of the control polymer network and PCN‐n (Figure 2b,c). The decomposition temperature of these samples was in all cases about 300°C (Figure 2b), leading us to suggest that they display excellent thermal properties. The DSC curves also revealed that the glass transition temperatures (T _g) of the control polymer network and the PCN‐1 to PCN‐4 samples were −42.1°C, −37.6°C, −32.9°C, −28.9°C, −24.8°C, respectively, thus increasing as the [2]catenane content increased (Figure 2c). On this basis, we suggest that the increase in T _g values reflects increasing restrictions on the motion of the [2]catenane and a tighter arrangement within the polymer chain [47].

To investigate whether the [2]catenane content would influence the mechanical properties of the resulting polymer networks, a series of preparative and characterization studies were carried out. First, 2M₁, TMP, and PEG‐DA were used to form a set of [2]catenane‐free polymer networks, termed HPN‐n. This allowed the effect of 2M₁ content on the mechanical properties to be tested. As shown in Figure 3a,b, a fracture strain analysis revealed an increase from 40% in the control polymer network to 344% in HPN‐4, while the fracture stress only showed a slight elevation from 0.3 to 0.57 MPa as the 2M₁ content in HPN‐n was increased. This was rationalized in terms of a greater level of hydrogen bonding in the main chain of HPN‐n providing for an increase in the elongation ability of the chain segments, thus enhancing the fracture strain. A similar trend was observed in the case of the toughness, which increased from 0.066 to 1.035 MJ/m³. Conversely, the Young's modulus decreased from 0.8 to 0.3 MPa as the 2M₁ content was increased (Figure 3c) [48]. This latter finding was interpreted in terms of an increase in the number of hydrogen bonds in the polymeric network, which serves to lower the stiffness while providing for a greater level of energy dissipation. As a consequence, the Young's modulus decreases while the toughness increases.

FIGURE 3.

(a) Stress‐strain curves, (b) summarized fracture stress and strain, and (c) calculated Young's modulus and toughness of HPN‐n (n refers to the molar ratio of M₁ and PEG‐DA; see text for details). (d) Stress‐strain curves, (e) summarized fracture stress and strain, (f) calculated Young's modulus and toughness of PCN‐n. (g) Stress‐strain curves, (h) summarized fracture stress and strain, (i) calculated Young's modulus, toughness of control, HPN‐3, PCN‐3, DCN, and reduced PCN‐3. (j) Cyclic tensile test curves at different tensile strains from 6% to 30%. (k) Energy dissipation and damping capacity of PCN‐3 calculated from cyclic tensile tests. (l) Continuous cyclic tensile tests of PCN‐3 under a strain of 30%.

Following these control studies, the effect of [2]catenane content on the mechanical properties was analyzed. For these experiments, M₂ was added to HPN‐n to close the rings and form the corresponding [2]catenane‐based networks, referred to as PCN‐n. This ring closure led to an increase in the fracture stress to 8.4 MPa; however, relative to the control polymer network PCN‐4, the fracture strain showed a slight increase (Figure 3d,e). This was interpreted in terms of the presence of more dynamic covalent bonds with enhanced bond energies, making it more difficult for the network to be disrupted. This was expected to be reflected in enhanced fracture stress, while the slight increase in fracture strain is ascribed to the space‐restricted elongation provided by the [2]catenane structure. Both the toughness and the Young's modulus also demonstrated an increasing trend compared to 2.719 MJ/m³ and 16.35 MPa as the [2]catenane content is raised (Figure 3f).

Since PCN‐4 was prone to crack when prepared on a larger scale, PCN‐3 was chosen for more detailed studies. The mechanical properties of HPN‐3, PCN‐3, and the control polymer networks were compared so as to assess the effect of ring‐closure on the mechanical properties. As shown in Figure 3g,h, HPN‐3 (0.515 MPa, 242%) and PCN‐3 (4.5 MPa, 57%) displayed higher fracture stress levels than the control polymer network. Relative to this latter control, almost equal fracture stress was seen for HPN‐3 and almost equal fracture strain was observed for PCN‐3. The higher fracture strain of HPN‐3 relative to PCN‐3 is ascribed to HPN‐3 undergoing chain extension more readily as the result of creating and destroying the constituent hydrogen bonds, while the presence of the [2]catenanes in PCN‐3 restricts extension of chain segments. Meanwhile, the higher energy required to cleave imine bonds compared to hydrogen bonds accounts for the higher fracture stress seen for PCN‐3 relative to HPN‐3. At the same time, the toughness of PCN‐3 (1.435 MJ/m³) proved to be almost twice that of HPN‐3 (0.665 MJ/m³), while both proved far tougher than the control sample. We thus suggest that both hydrogen bonds and dynamic imine bonds contribute to the toughness of PCN‐3. Notably, the Young's modulus of PCN‐3 (8.63 MPa) was found to be far higher than that of HPN‐3 (0.31 MPa) and the control sample (Figure 3i). Additionally, following the procedure used to prepare PCN‐3, replacing M₁ with M₃ and M₂ with terephthalaldehyde M₄ afforded the dynamic covalent network DCN (Figure S16). The structure of DCN was confirmed by infrared spectroscopy (Figure S17). The imine bonds of PCN‐3 were reduced using NaBH(OAc)₃ to obtain Red. PCN‐3 (Figure S18) [49]. The structure of Red. PCN‐3 was confirmed by infrared spectroscopy (Figure S19). Relative to PCN‐3, the stress and strain of DCN (0.9 MPa, 22%) are reduced, and the Young's modulus (5.04 MPa) and toughness (0.06 MJ/m³) are also reduced (Figure 3g–i). This is taken as evidence that the hydrogen bonds and mechanical bonds in PCN‐3 can dissipate energy effectively [50], thereby exhibiting better mechanical strength. The stress of Red. PCN‐3 (1.59 MPa) was lower than that of PCN‐3, while the strain remained almost unchanged. The Young's modulus (4.98 MPa) and toughness (0.31 MJ/m³) were also considerably reduced. This is attributed to the reduction of C═N to C–N, as the bond energy of C‐N is lower than that of C═N [51]. Consequently, the mechanical properties of Red. PCN‐3 deteriorated.

Cyclic tensile tests at different tensile strain levels from 6% to 30% were also carried out. The corresponding damping ability (dissipated energy divided by input energy) was calculated as an indicator of effective energy dissipation. As shown in Figure 3j, the loop areas of PCN‐3 increased with increasing strain. This is consistent with the increase in energy dissipation shown in Figure 3k. On the other hand, the damping ability remains almost unchanged at around 25%. These findings are rationalized in terms of the hydrogen and imine bonds acting as sacrificial structures that can make and break in the [2]catenane‐containing materials, thus providing for an increased energy dissipation ability [52]. Continuous cyclic tensile tests were carried out to study the recovery property of PCN‐3. The stress only decreased from 2.29 to 1.85 MPa at 30% strain, with little change in the loading‐unloading curves being observed. This leads us to propose that PCN‐3 possess excellent fatigue resistance and mechanical stability (Figure 3l).

Under the influence of external stimuli, such as temperature, the hydrogen bonds within the rings in the [2]catenane subunits can be disrupted. This was expected to allow the rings to move freely, thereby affecting the performance of the poly[2]catenane networks. To test this proposition, a series of dynamic mechanical analysis (DMA) and rheological tests were carried out to study the influence of the [2]catenene content on the viscoelasticity properties of the polymer network. Stress relaxation experiments were carried out at 10% strain and at 40°C in the control polymer network and PCN‐n (n = 1–3). As shown in Figure 4a, the stress of the control sample was essentially constant, whereas a continuous drop in stress was observed in the case of PCN‐n (n = 1–3). The stress relaxation was found to occur more rapidly as the [2]catenane content increases. This could be interpreted in terms of the hydrogen bond content provided by the [2]catenane moieties providing more cleavable sites that allowed for enhanced motion, resulting in lower stress [53]. Linear oscillatory sweep tests were performed on PCN‐3 to assess its rheological properties (Figure S20). The G' (storage modulus) was found to be greater than G″ (loss modulus). Such a finding is consistent with the formation of a polymer network [54]. Moreover, both G' and G″ gradually increased with increasing [2]catenane content, as would be expected for a tightened polymer network, and align with the DSC tests [55]. Additionally, rheological tests were conducted on HPN‐3, DCN, and reduced PCN‐3. The resulting data are presented in Figure S21 of the Supporting Information.

FIGURE 4.

(a) Stress relaxation experiments involving the control polymer network and PCN‐n (n = 1–3). DMA temperature sweeps of (b) control polymer network, (c) PCN‐1, (d) PCN‐2, (e) PCN‐3 from −60°C to 90°C. (f) The temperature corresponding to the maximum tan δ value of the control group and PCN‐n. (g) Photographs, (h) stress‐strain curves, (i) summarized fracture stress and strain studies of PCN‐3 immersed in DMSO for different times from 0 to 6 h. (j) Stress relaxation curves of PCN‐3 before and after immersion in DMSO for 6 h at 40°C and 10% strain.

To investigate further the effect of [2]catenane content on the physical properties of PCN‐n, temperature scans of DMA tests were carried out. As can be seen from inspection of Figure 4b–e, the storage modulus E' remained higher than the loss modulus E″; this was true not only for the control polymer network but also for PCN‐n (n = 1–3). This leads us to suggest that these networks possess an ability to dissipate energy effectively through vibrations. Notably, the E' values for the PCN‐n (n = 1–3) samples were all higher than those of the control polymer network. This finding is interpreted in terms of restriction in the chain segments arising from the hydrogen bond interactions supported by the [2]catenane subunits [56]. The peaks of the tan δ‐temperature curves were also found to widen (Figure 4b–e, red curves) as the [2]catenane content increased. This was ascribed to two factors: First, a higher [2]catenane content provides for increased hydrogen bonding interaction, which leads to a higher energy dissipation ability. Secondly, an increased number of [2]catenane subunits permit a greater degree of motion after the majority of the hydrogen bonds are broken during the viscoelasticity tests. In a similar vein, the temperature corresponding to the maximum tan δ value (−27°C, −25°C, −13°C, −1°C, for the control polymer network and PCN‐n (n = 1–3), respectively) was found to increase as the number of presumed hydrogen bonding interactions increased as the [2]catenane levels were raised (Figure 4f) [50]. Taken in concert, these results reveal that PCN‐3 possesses the best viscoelasticity features.

In addition to temperature, reagents that can break up the internal hydrogen bonds were expected to affect the dynamic features of PCN‐3. To test this hypothesis, an H‐bond acceptor, DMSO, was selected as a potential hydrogen bonding competitor [57]. In terms of experiment, a sample of PCN‐3 was cut into pieces of dimensions 25 mm × 6.5 mm × 0.25 mm (length × width × height) and immersed in 20 mL of DMSO. As can be observed by eye, PCN‐3 was seen to retain its form upon placing in DMSO, with the sample size remaining nearly constant over the course of a 6 h immersion time (Figure 4g). On this basis, we conclude that DMSO does not destroy the [2]catenane structure and that the overall polymeric structure is maintained. On the other hand, the final fracture stress and strain decreased from 4.5 to 0.45 MPa and from 57% to 14%, respectively, over the course of 6 h (Figure 4h,i). This is interpreted in terms of the number of hydrogen bonds being reduced gradually during extended immersion in DMSO, leading to a reduced force resistance capacity. Stress relaxation experiments were also carried out on a sample of PCN‐3 before and after immersion in DMSO for 6 h at 40°C and at 10% strain. As can be seen in Figure 4j, the internal stress of PCN‐3 before and after immersion in DMSO underwent a continuous decrease over time, with a rapid stress relaxation being seen within 100 s and nearly complete relaxation being observed after 1000 s. Notably, the stress relaxation for PCN‐3 after immersion in DMSO proved to be much slower than what was seen for the as‐prepared sample of PCN‐3. On a molecular scale, stress relaxation could be ascribed to the dissociation of hydrogen bonds, while faster relaxation of PCN‐3 could be interpreted in terms of a greater number of hydrogen bond interactions needing to be broken to achieve a given level of motion within the [2]catenane structure.

Immersion experiments involving PCN‐3 were conducted to investigate its tolerance to various solvents. PCN‐3 was cut into strips with dimensions of 5 × 5 × 0.25 mm and then immersed in the organic solvents n‐hexane, ethyl acetate (EA), dichloromethane (DCM), acetonitrile (ACN), toluene, EtOH, and tetrahydrofuran (THF). After 24 h, it was found that PCN‐3 did not dissolve in any of the organic solvents (Figure S22a), demonstrating good organic solvent tolerance. Regarding its swelling behavior, PCN‐3 exhibited almost no swelling in n‐hexane and EtOH, while it swelled by 30%, 100%, 20%, 20%, and 40% in EA, DCM, ACN, toluene, and THF, respectively (Figure S22b,c).

Since PCN‐3 was prepared through a self‐assembly process that involved the formation of dynamic imine bonds to create the constituent [2]catenane substructures, it was expected to display degradability [4]. To test this proposition, 12 M hydrochloric acid was selected as the treatment medium since it was expected to break up hydrogen bonds and hydrolyze both imine and ester bonds [44, 58, 59, 60, 61]. Upon immersing PCN‐3 (25 mm × 6.5 mm × 0.25 mm) in hydrochloric acid, the sample gradually swelled and an obvious increase in volume was observed (Figure 5a). After 30 min, the suspended sample almost disappeared, leaving only yellow sol on the bottom of the vessel. Additionally, after 40 min of immersion followed by shaking, the yellow sol was seen to disperse throughout the solution. We propose that upon HCl immersion, PCN‐3 degrades into five limiting fragments as depicted in Figure 5b due to the breakup of the hydrogen bonds, the imine bonds in the [2]catenane units, and the ester bonds in the TMP crosslinker. ¹H NMR and Fourier transform infrared (FT‐IR) spectroscopic analyses and GPC tests were carried out to test this proposition. As can be seen in Figure 5c, after being treated with HCl, the signals in the ¹H NMR spectrum of PCN‐3 at 6.92 and 6.85 ppm proved concordant with that of the benzene proton in M₁. This was taken as evidence for the production of degradation fragment 1. Meanwhile, the proton signals at 8.10, 8.03, and 10.10 ppm proved consistent with those of the pyridine and aldehyde groups in M₂, as would be expected for the production of fragment 2. In addition, the proton signals at 2.72 and 2.56 ppm were essentially the same as those of the methylene subunits in TMP, indicating the formation of fragments 3. Furthermore, the proton signals at 4.18 ppm were consistent with those of the TMP methylene protons, indicating the formation of fragment 4. Moreover, the proton signals at 3.7‐3.5 ppm proved analogous to those of the methylene groups in PEG‐DA, indicating the formation of fragment 5. Besides, a proton signal is seen at 5.51 ppm that is attributed to the hydroxyl moieties of fragments 3 or 4. Corresponding Fourier transform infrared (FT‐IR) spectroscopic studies (Figure 5d) revealed that the C═N vibration peak at 1645 cm⁻¹ gradually disappeared upon exposure to HCl, while an N‐H vibration at 3478 cm⁻¹ appeared. These changes are as expected if, as proposed, the imine bonds are undergoing hydrolysis. Supporting GPC analyses revealed a highest molecular weight peak of 970 Da (Figure 5e), a value that matches well the calculated molecular weight of the largest fragment 1, namely, 1008 Da. Considered in concert, these results provide support for the design expectation that treatment of PCN‐3 with hydrochloric acid leads to first the swelling and then the degradation of the as‐prepared poly[2]catenane networks. Additionally, we investigated the degradation of PCN‐3 in moderate‐concentration hydrochloric acid (2 M). The experimental results showed that the half‐life for degradation of PCN‐3 in 2 M hydrochloric acid was roughly 48 h (Figure S23). In contrast, the degradation of PCN‐3 in 12 M hydrochloric acid required only 30 min, indicating that higher concentrations of hydrochloric acid accelerate the hydrolysis of imine and ester bonds, leading to faster network degradation.

FIGURE 5.

(a) Photographs of PCN‐3 immersed in 12 M HCl for varying times from 0 to 40 min. (b) Possible degradation products of PCN‐3. (c) ¹H NMR spectra of (i) PCN‐3 after treatment with 12 M HCl, (ii) TMP, (iii) PEG‐DA, (iv) M₂, (v) M₁. (d) FT‐IR curves of PCN‐3 immersed in 12 M HCl for varying times from 0 to 40 min. (e) GPC curve of presumed PCN‐3 degradation products, using THF as the eluent and polystyrene as the standard.

3. Conclusion

In conclusion, a poly[2]catenane network is described that is easily prepared via a self‐assembly process. A combination of ¹H NMR, COSY, and NOESY spectroscopic studies and MALDI‐TOF analyses served to confirm the formation of the constituent [2]catenane that proceeds as the result of hydrogen bond and dynamic imine bond formation. UV irradiation‐initiated polymerization of a clip‐like, self‐associated two‐component monomer could be carried out in the presence of other polymer precursors as confirmed by FT‐IR spectroscopy. The influence of [2]catenane content on the material property of the polymer networks was studied. It was found that one representative polymer network (PCN‐3) displayed excellent mechanical properties, stimuli‐responsiveness, and favorable degradability. This study provides support for the proposition that polycatenane with useful material properties, including both robustness and ease of degradation, can be accessed facilely via the judicious use of self‐assembly. This work solved the primary challenge, synthetic methodologies, in the field of polycatenanes. As such, we believe that this work will accelerate the development of polycatenane materials and also new polymeric materials with a sustainability focus.

Author Contributions

Jinsa Li: investigation. Rui Hu: investigation. Yuxin Fu: investigation. Xiaohe Zhou: investigation. Jonathan L. Sessler: conceptualization, writing – review and editing, funding acquisition. Xiaofan Ji: conceptualization, funding acquisition, writing – original draft, project administration.

Conflicts of Interest

The authors declare no conflicts of interest.

Supporting information

Supporting File 1: The authors have cited additional reference within the Supporting Information [62–64].

Data Availability Statement

The data that support the findings of this study are openly available in the Supporting Information.