Introduction

Topologically crosslinked polymers with rotaxane structures at the crosslinking points (rotaxane-crosslinked polymers, RCPs) have attracted great interest experimentally and theoretically1, 2, 3, 4, 5, 6, 7 because RCPs exhibit unique properties and functions that are attributed to the movable crosslinking points. RCPs are also known as polyrotaxane networks or slide-ring gels. RCP synthesis methods include seven different approaches (Figure 1). Methods 1 and 2 are statistical methods based on the accidental penetration of polymer termini into a macrocycle. Gibson et al. first reported that the polycondensation of a diamine with a 32-membered macrocyclic dicarboxylic acid formed an RCP gelled product (method 1).8, 9, 10, 11, 12, 13 Zilkha et al.,14, 15, 16, 17 Tezuka et al.,18 and Kubo et al.19, 20, 21 also reported the statistical synthesis of RCPs via the radical copolymerization of vinyl monomers with large macrocycles (method 2). In 1999, de Gennes22 theoretically predicted that RCPs possess unique properties because of easy polymer chain sliding. Ito et al.23, 24, 25, 26, 27 and Yui et al.28, 29, 30, 31 synthesized RCPs by directly linking Harada’s cyclodextrin-based polyrotaxanes at the wheels (method 3).32, 33 The physical properties of RCPs were experimentally elucidated by Ito et al.23, 24, 25, 26, 27 They observed that RCPs undergo significant swelling in several solvents, have high elasticity and can relieve stress effectively. These observations directly supported the validity of de Gennes’s predictions. In addition, Yui et al.28, 29, 30, 31 demonstrated that RCP polymer frameworks act as unique biodegradable systems. To take advantage of these very important features of RCPs, we developed four other synthesis approaches: crosslinking macrocycle-containing polymers using strategic rotaxanation as the crosslinking reaction (method 4) and crosslinking vinyl polymers using a [3]rotaxane (method 5), pseudorotaxane-crosslinked polymer (method 6) or bismacrocycle (method 7) as the crosslinker. Methods 4 and 5 are useful for not only creating new types of RCPs but also inserting special structures and functions into the crosslinking points. We successfully prepared unique RCPs with stimuli-responsiveness and photodegradability. Methods 6 and 7 were developed as practical RCP synthetic methods that allow RCPs containing vinyl trunk polymers to be easily prepared. The resulting vinylic RCPs exhibit unique properties that are attributed to polymer chain sliding. These seven RCP synthetic methods can be distinguished by the (i) need for pre-synthesizing the (poly)rotaxane precursor, (ii) controllability of the crosslinking degree and (iii) polymer versatility. Table 1 classifies these RCP synthetic strategies and indicates that the features of these methods are orthogonal. This review describes recent progress on the synthesis of RCPs via methods 4−7, which were developed by Takata et al.,34, 35, 36, 37 and their applications. Related research topics and future perspectives on RCPs are also described.

Figure 1
figure 1

Synthetic strategies for preparing rotaxane-crosslinked polymers (RCPs).

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Table 1 Classification of RCP synthetic strategies
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Stimuli-degradable rotaxane-crosslinked polymers

Stimuli-responsive smart materials have attracted considerable attention because they not only represent progress in fundamental chemistry but are also advanced, sophisticated materials. Stimuli-degradable gels comprise one family of smart materials that are mainly employed in intelligent devices based on sol−gel transitions, drug delivery systems, dynamic pattern formation, and so on. Most stimuli-degradable systems are based on cleaving weak noncovalent bonds, such as hydrogen bonds, π–π interactions and hydrophobic interactions. The properties of gels are attributed to the presence of these noncovalent bonds, and thus, they are typically chemically and thermally unstable. In addition, soaking a gel in an excess of an appropriate solvent past the gel point leads to the decomposition of the gel. Therefore, more robust, stimuli-responsive materials are needed. To develop stable, stimuli-degradable covalently crosslinked polymers, we hypothesized that rotaxanes with stimuli-responsive deslipping behavior could serve as crucial crosslinking structures. Thus, two stimuli-degradable systems of covalently crosslinked polymers were developed via methods 4 and 5.

In method 4,34 the axle component serves as the crosslinker and has an end group size that is complementary to that of the macrocyclic cavity on the trunk polymer.38, 39, 40, 41 The end groups provide an energy barrier to slow dissociation, thereby kinetically stabilizing the rotaxane skeleton. The RCP normally stabilizes the network structure, but it can be de-crosslinked under certain conditions, such as exposure to chemostimuli that accelerate the dissociation of the rotaxane skeleton. Furthermore, the stability and de-crosslinking capability of RCPs can be adjusted by changing the size of the axle end groups. We first investigated the size-complementary effect using crown ether/ammonium salt [2] rotaxanes (Scheme 1). Rotaxanes with suitable end groups (R=cyclohexyl (1a), tert-butyl (1b), and 4-tert-butylphenyl (1c)) were sufficiently stable and maintained their rotaxane structures because of the bulky end groups and hydrogen bonds between the dibenzo-24-crown-8-ether (DB24C8) and ammonium groups. However, these rotaxanes dissociated into two components when the hydrogen bonds were disrupted by an appropriate stimulus. When dissolved in dimethylsulfoxide (DMSO), for example, only 1a underwent deslipping, and the half-life (τ1/2) was estimated to be 5.8 h. On the other hand, neutralizing the rotaxanes by adding Et3N caused the prompt deslipping of 1a (τ1/2=2.6 h) and slow deslipping of 1b (τ1/2=18 d), while 1c barely underwent deslipping. We also examined exchanging the counter anion of the ammonium salt with TBAF.42 Based on the HSAB principle, fluoride should strongly interact with the ammonium moiety and replace the crown ether. In fact, the addition of TBAF led to the rapid deslipping of all three rotaxanes. The half-life of 1a was too rapid to measure, whereas those of 1b and 1c were 0.70 and 88 h, respectively. Thus, this model clearly indicated that the deslipping rates of size-complementary rotaxanes depend on both the end group size and type of external stimulus.

We next examined the de-crosslinking of RCPs with size-complementary end groups at the crosslinking points. Poly(crown ether) (Mn 4000, PDI 1.35) was treated with sec-ammonium salts bearing suitable end groups (R=cyclohexyl, tert-butyl, and 4-tert-butylphenyl) in CHCl3 to afford the corresponding pseudopolyrotaxanes, which were then crosslinked with a diisocyanate (MDI) to obtain a quantitative RCP yield (Scheme 2). The degradabilities of these RCPs were examined using three methods: solvation in DMF, neutralization with Et3N and counter anion exchange with TBAF. The cyclohexyl-terminated RCP was gradually de-crosslinked in DMF at room temperature, giving a homogeneous solution after 90 min. The tert-butyl- and tert-butylphenyl-terminated RCPs were insoluble in all solvents and only swelled, indicating that these gels had high chemical stability due to the kinetic stability of the rotaxane-crosslinking points. On the other hand, treatment of the cyclohexyl- and tert-butyl-terminated RCPs with Et3N resulted in slow de-crosslinking, and homogeneous solutions were formed after 60 and 360 min, respectively. In contrast, the tert-butylphenyl-terminated RCP retained its shape under these conditions without any degradation. Furthermore, all the RCPs were smoothly de-crosslinked by TBAF, giving homogeneous solutions after 20 min (cyclohexyl), 75 min (tert-butyl) and 96 h (tert-butylphenyl). Notably, the degradation behavior of these RCPs was consistent with the dissociation behavior of the model [2]rotaxanes. The recyclability of the RCPs was also evaluated by recovering the poly(crown ether) in 98% yield from a homogeneous solution of the cyclohexyl-terminated RCP in DMF. The 1H NMR spectrum and size-exclusion chromatography profile of the recovered poly(crown ether) matched those of the starting polymer, clearly indicating that the rotaxane skeleton was selectively degraded, while the polymer backbone remained intact.

We also developed an alternative approach for preparing RCPs with excellent photodegradability (method 5).35 In this method, a vinylic [3]rotaxane with a photoreactive aromatic disulfide on the axle and polymerizable vinyl moieties on the wheels was utilized to crosslink a vinyl polymer. In fact, [3]rotaxanes can be efficiently synthesized via the reversible cleavage of a disulfide linkage in the presence of a catalytic thiol (Scheme 3),43, 44, 45, 46, 47, 48, 49 and aromatic disulfide linkages react more rapidly than their aliphatic disulfide counterparts. UV irradiation of aromatic disulfide-containing [3]rotaxanes in DMF results in the cleavage of the aromatic S–S bond and subsequent deslipping to afford the original components.

Building on these model studies, a vinylic [3]rotaxane was designed, prepared and employed as the crosslinker in the radical polymerization of MMA in the presence of AIBN (Scheme 4). The radical polymerization was performed at 60 °C for 19 h to afford a gelled product. To evaluate its chemical stability, the gel was soaked in excess DMF for 1 d to produce a swollen organogel. In contrast to unstable non-covalently crosslinked polymers soaked in an appropriate solvent, the swollen gel maintained its shape in DMF for an additional day without any degradation.

The photodegradation behavior of the swollen gel in DMF was then investigated. UV irradiation of the gel at room temperature caused the rapid and selective degradation of the irradiated RCP section within 30 min, implying photoresponsive de-crosslinking occurred. This behavior is attributed to the homolysis of the aromatic disulfide S–S linkage of the rotaxane moiety and subsequent deslipping of the axle component from the wheel, which led to the decomposition of the RCP network structure. The de-threading of the axle component, which was stabilized by intramolecular hydrogen bonding in highly polar DMF, is consistent with a previous report on the solvent polarity-dependent complex formation of sec-ammonium/crown ether-type rotaxanes.43, 44, 45, 46, 47, 48, 49 The systems were not susceptible to thermal degradation because of their stability.

The remaining DMF-soluble product was then precipitated in MeOH, and the white precipitate was collected by filtration. The 1H NMR spectrum of the filtrate indicated that the polymer was mainly composed of PMMA, suggesting that the RCP was selectively photochemically degraded due to the photodissociation of the aromatic disulfide linkage and structural properties of the interlocked linkage.

Movable rotaxane crosslinks in vinyl polymers

To facilitate the synthesis of RCPs, we recently developed a very simple, effective protocol for directly introducing rotaxane crosslinks into vinyl polymers via the radical polymerization of the corresponding vinyl monomers with a crosslinker (method 6).36 This protocol is the first approach for preparing RCPs that does not require the pre-synthesis or crosslinking of the polyrotaxane before transforming the crosslinker structure. The crosslinker was a CD-based vinylic supramolecular crosslinker (VSC) consisting of an oligocyclodextrin (OCD) and terminal, bulky end-tethering, polyethylene glycol-type methacrylate (TBM). An OCD and TBM mixture in 0.1 M aq. NaOH was sonicated for 5 min at room temperature to produce a white viscous gel (Scheme 5). As the OCD feed ratio increased, the viscosity increased remarkably and reached a maximum of ∼40 Pa s−1 at OCD concentrations >100 g l−1. The change in the VSC viscosity indicated that OCD formed a network structure with TBM, which penetrated into the OCD cavities. Direct UV irradiation of the resultant VSC in the presence of a photoinitiator (Irgacure 500) for 3 min produced a self-standing gelled film (RCPVSC), clearly demonstrating the VSC threaded structure and polymerizability of the VSC vinyl groups.

Next, the usefulness of VSCs as crosslinkers was examined using a vinyl monomer. A mixture of N,N-dimethylacrylamide (DMAAm), a VSC, and a photoinitiator was UV-irradiated in water at room temperature for 3 min to afford the corresponding transparent RCPDMAAm film in 84% yield (Scheme 6). In addition to photopolymerization (method A), thermal polymerization using a redox-type radical initiator (K2S2O8 and tetramethylethylenediamine, method B) was also effective (97%). The mechanical properties of the resulting RCPDMAAm samples were compared with those of covalently crosslinked polymers. The RCPDMAAm films exhibited larger elongation and Young’s modulus values than the covalently crosslinked DMAAm films prepared using a polyethylene glycol-based diacrylate as the crosslinker instead of OCD in the radical polymerization of DMAAm. As the elongation and Young’s modulus values are typically inversely proportional, our results suggested that the rotaxane crosslink has a unique effect on the mechanical properties.

We also employed a Pd-templated bismacrocycle as a topological crosslinker to enable the efficient penetration of the propagating end into the macrocyclic cavity during radical polymerization (method 7)37 for the following reasons: (1) the coordination bond between the Pd complex and ligand-containing polymer axle should be highly stable compared with other intermolecular interactions, such as hydrophobic interactions, hydrogen bonding and electrostatic interactions, and thus should maintain the interlocked structure even under polymerization conditions; (2) the Pd complex method should be applicable to macrocycles with cavities larger than the vinyl polymers, leading to novel RCPs; and (3) the distance of the macrocycle along the polymer axle should be easily controlled by introducing bulky groups into the axle in varying concentrations to act as stopper groups. The inherent sliding of RCPs can also be studied by controlling the distance of the macrocycle along the polymer axle.

Thus, a Pd-templated bismacrocycle was prepared in good yield using a slightly modified procedure based on our previously established protocol. As we previously found that rotaxanes were formed efficiently from a Pd-templated macrocycle and pyridine-containing axle,50, 51, 52 4-vinylpyridine was selected as the vinyl monomer. Before the RCP synthesis, the coordination behavior of 4-vinylpyridine with the Pd-templated bismacrocycle was evaluated (Scheme 7), and it was determined via 1H NMR analysis that a coordination complex was formed, strongly indicating that the bismacrocycle is a suitable topological crosslinker for poly(4-vinylpyridine).

Next, RCPs were synthesized via ligand exchange on the Pd bismacrocycle with 4-vinylpyridine during radical polymerization. The radical polymerization of 4-vinylpyridine in the presence of the Pd-templated bismacrocycle (0.5 mol%) and AIBN (1.0 mol%) was performed in DMF at 60 °C to afford a gelled product in quantitative yield (Scheme 8a). However, diluting the gel with a large amount of DMF led to its prompt dissociation to poly(4-vinylpyridine) and the bismacrocycle. This behavior is typical of conventional crosslinked polymers formed via metal–ligand interactions, indicating that the gel included only topological crosslinking points without any covalent interactions. To prepare a stable RCP, 4-tert-butylstyrene was added to the reaction mixture as a vinyl comonomer. It was assumed that the bulky monomer would sufficiently prevent the dissociation of the RCP components. The radical polymerization of a 1:1 mixture of 4-vinylpyridine and 4-tert-butylstyrene afforded an RCP that was stable even when highly diluted in DMF (Scheme 8b). This result indicated that the 4-tert-butylbenzene moieties in the gel trunk polymer acted as stopper moieties and prevented the polymer axle from deslipping from the macrocyclic components. To elucidate the role of the rotaxane crosslinks and the significance of the topological crosslinker, the polymerization was performed in the presence of a bis pincer-type Pd complex (Scheme 8c). A gel was obtained in quantitative yield, and after treatment with a large amount of DMF, it immediately transformed into a sol containing the trunk polymer. These results confirmed that the macrocyclic structure of the crosslinker is important for the formation of stable gelled products.

Conclusions and outlook

This paper summarizes four strategic approaches for synthesizing RCPs that have been recently developed by Takata et al.34, 35, 36, 37 and are described in Figure 1. The fabrication of novel RCPs is expected to be achieved using suitable rotaxane components, including crown ethers/ammonium salts, cyclodextrins/hydrophobic guests and metal-coordinated macrocycles/ligands, in these synthetic approaches. These approaches could provide rational and efficient pathways for inserting rotaxane-crosslinking points into polymers. They are also promising methods for programming network polymers with macroscopic transformation systems that are linked to the dynamic behavior of rotaxane architectures with unique switching capabilities. The development of this type of system could, in turn, result in the creation of new stimuli-responsive materials based on versatile rotaxane chemistry (for a recent report concerning the synthesis of stimuli-responsive RCPs via method 4, see Iijima et al.53). We are currently developing stimuli-responsive fluorescent switching materials that exploit a fluorescence rotaxane switching system.

scheme 1

Deslippage of model [2]rotaxanes. A full color version of this scheme is available at Polymer Journal online.

scheme 2

Synthesis of RCPs with chemo-responsive degradability via method 4 and their de-crosslinking reactions. A full color version of this scheme is available at Polymer Journal online.

scheme 3

Formation and dissociation of [3]rotaxanes utilizing reversible disulfide linkages. A full color version of this scheme is available at Polymer Journal online.

scheme 4

Synthesis of a photodegradable RCP via method 5 using a [3]rotaxane crosslinker and its site-selective photoresponsive de-crosslinking reaction.

scheme 5

Synthesis of RCPVSC (VSC, vinylic supramolecular crosslinker). A full color version of this scheme is available at Polymer Journal online.

scheme 6

Synthesis of RCPDMAAm via the radical polymerization of DMAAm in the presence of VSC (method 6).

scheme 7

Coordination of 4-vinylpyridine to a Pd-templated bismacrocycle. A full color version of this scheme is available at Polymer Journal online.

scheme 8

Radical polymerization of 4-vinylpyridine in the presence of a Pd complex: (a) formation and dissociation of a pseudo-RCP, (b) synthesis of chemically stable RCPVP using a bismacrocycle crosslinker via method 7 and (c) formation and dissociation of a Pd complex gel using a bispincer-type Pd complex. A full color version of this scheme is available at Polymer Journal online.