Supramolecular Polymerization and Morphological Transformation of Dynamic Helical Poly(Phenylacetylene)s via Chain‐End Host–Guest Interactions

ABSTRACT

Integration of dynamic helical polymers with supramolecular polymerization offers a promising strategy for constructing adaptive hierarchical chiral materials. However, the transformation of molecular‐level conformational changes in dynamic helical polymers into structural transformations at the supramolecular level remains unexplored. This study demonstrated the synthesis of supramolecular polymers composed of poly(phenylacetylene) (PPA) chains bearing a pillar[5]arene host unit at one chain end and complementary guest unit at the other. The telechelic PPAs were synthesized using a living polymerization method that enables precise end‐functionalization and subsequently assembled into supramolecular polymers through intermolecular host–guest interactions. To impart stimuli‐responsive behavior, alanine‐derived pendants, capable of forming intramolecular hydrogen bonds, were incorporated into the PPA chains. The resulting supramolecular polymers formed stable assemblies that persisted even after dilution below the concentration required for their initial formation. Solvent‐dependent switching of intramolecular hydrogen bonding induced simultaneous helicity inversion and modulation of the backbone rigidity, leading to pronounced morphological transformations of the supramolecular polymers between cyclic and rigid‐rod architectures, exhibiting viscosities up to 2000 times higher than the control polymer. These results demonstrate that molecular‐level conformational changes in dynamic helical polymers can be translated into higher‐order supramolecular structural transformations, providing a general strategy for designing adaptive hierarchical chiral materials.

A helical polymer whose chain ends are connected through pillar[5]arene‐based host–guest interactions translates molecular helicity into supramolecular morphology. Solvent‐triggered helicity inversion accompanied by changes in chain rigidity switches the supramolecular architecture between cyclic and rigid‐rod nanostructures.

Supramolecular polymers [1, 2, 3, 4, 5, 6, 7, 8, 9, 10], constructed through reversible noncovalent interactions such as coordination, hydrogen bonding, and host–guest recognition, have emerged as a class of versatile, adaptive materials. Owing to their dynamic nature, these polymers exhibit unique properties, including stimuli responsiveness, self‐healing, and structural adaptability, which are difficult to achieve with conventional covalently linked polymers [9]. Recent advances in molecular design and the engineering of intermolecular interactions have expanded the scope and functionality of supramolecular polymer systems [6, 7, 8, 9].

Synthetic helical polymers have attracted considerable interest as artificial analogues of biological macromolecules because they can generate sophisticated functionalities from well‐defined helical structures [10, 11, 12, 13, 14, 15, 16, 17]. Among them, poly(phenylacetylene)s (PPAs) are particularly notable owing to their dynamic helical structures, which enable helicity induction through noncovalent chiral interactions, as well as helicity memory, helicity inversion, and asymmetric amplification [10, 12, 13, 18, 19]. These unique characteristics establish PPAs as a promising platform for stimuli‐responsive chiral materials [10, 13, 19].

Several elegant studies have demonstrated that one‐handed helical polymers can serve as macromolecular building blocks for supramolecular polymers through chain‐end host–guest interactions, enabling the construction of higher‐order helical architectures [20, 21]. However, the extension of this concept to dynamic helical poly(phenylacetylene)s, whose helical conformation and morphology respond to external stimuli, has not yet been explored [6, 7, 8, 9]. Constructing supramolecular polymers from such dynamic helical polymer segments is particularly attractive because it offers the possibility of coupling molecular‐scale conformational dynamics with higher‐order supramolecular organization, enabling the development of hierarchical chiral materials with emergent structures and functions [6, 7, 8, 9]. Nevertheless, the synthesis of helical polymer building blocks equipped with well‐defined supramolecular recognition sites at both chain ends remains challenging. Accordingly, strategies capable of translating molecular‐scale conformational information into controllable supramolecular topology and morphology remain scarce [6, 7, 8, 9, 22].

Previously, we established a facile and versatile living polymerization method for the synthesis of end‐functionalized cis‐stereoregular PPAs with controlled molecular weights [23]. This approach employs a multicomponent catalytic system comprising a rhodium complex, a boronic acid derivative, diphenylacetylene, triphenylphosphine, and aqueous KOH [24]. A key feature is the quantitative introduction of an aryl group derived from the aryl boronic acid at the initiating chain end. Additionally, a variety of functional groups can be installed at the terminal chain end through appropriate terminating reagents [25]. This strategy provides versatile access to structurally well‐defined PPAs bearing distinct functionalities at both termini.

Yashima et al. reported that PPAs bearing alanine‐derived pendants connected through amide linkages exhibit exceptional solvent‐responsive behavior [26]. Depending on the solvent polarity, intramolecular hydrogen bonding between the pendant amide groups can be switched ON and OFF, leading to simultaneous inversion of the helix sense and modulation of the backbone rigidity. Such cooperative conformational changes at the molecular level provide an attractive platform for constructing higher‐order stimuli‐responsive architectures.

Motivated by these findings, this study aimed to extend this solvent‐responsive helical behavior to supramolecular polymer systems. To this end, PPAs bearing a pillar[5]arene (P[5]A) host unit at the initiating end and a complementary guest unit at the terminal end were synthesized, facilitating chain extension through intermolecular host–guest interactions (Figure 1). P[5]A was selected as the host motif because of its facile functionalization and strong binding affinity toward suitable guest molecules in common organic solvents [27, 28]. The resulting telechelic helical PPAs spontaneously assembled into supramolecular polymers in concentrated solutions, and these structures persisted even after subsequent dilution. Furthermore, solvent‐controlled ON/OFF switching of intramolecular hydrogen bonding in the alanine‐containing PPA segments induced not only helix‐sense inversion but also pronounced morphological transformations of the supramolecular polymers between cyclic and rigid‐rod architectures. This behavior represents coupled structural changes across multiple hierarchical levels, spanning molecular helicity, backbone rigidity, supramolecular topology, and morphology. In this manner, the present study establishes a conceptual framework for hierarchical structural transduction, in which molecular‐scale conformational information encoded in helix sense and backbone rigidity is translated into supramolecular topology and morphology. This strategy provides a general platform for programming structural transformations across multiple length scales and for designing adaptive hierarchical chiral materials based on dynamic helical polymers.

FIGURE 1.

Schematic illustration of supramolecular polymer formation from PPAs bearing host–guest interaction sites at both chain ends and their solvent‐controlled morphological transformation via ON/OFF switching of intramolecular hydrogen bonding between the pendant amide groups.

The association constant between the guest residue containing a triazole unit (G‐1, Table 1) and P[5]A in CHCl₃ was estimated by ¹H NMR titration of the terminating reagent T‐1 (Table 1), which serves as a low‐molecular‐weight model compound for G‐1, with P[5]A. Upon addition of P[5]A, the signals of methylene protons adjacent to the ester oxygen atom and those adjacent to the triazole ring in T‐1 shifted upfield, indicating host–guest interactions between T‐1 and P[5]A (Figure S1). Analysis of the chemical‐shift changes afforded an association constant (K _a) of approximately 514 M–¹. During the titration, noticeable signal broadening was observed, suggesting a dynamic host–guest exchange process in the solution. In contrast, the association constant between the guest residue containing a pyridinium salt moiety (G‐2, Table 1) and P[5]A could not be estimated using the same method. This is because T‐2, which serves as the low‐molecular‐weight model compound of G‐2, is insoluble in chloroform, tetrahydrofuran (THF), and toluene. Nevertheless, previous studies have demonstrated favorable complexation between pyridinium guests and pillar[5]arenes [28, 29, 30], supporting the feasibility of the proposed chain‐end host–guest interaction.

TABLE 1.

Synthesis of PPAs with and without host–guest interaction sites at the chain ends.

Entry	Aryl boronic acid	Terminating reagent	Polymer
			Code	X	Y	Yield(%) a	M n b	M w/M n b
1	I‐1	T‐1	poly‐1	P[5]A	G‐1	84	7100	1.06
2	I‐1	T‐3	poly‐2	P[5]A	G‐3	85	7500	1.05
3	I‐2	CH3CO2H	poly‐3	CH3	H	90	5100	1.04
4	I‐1	CH3CO2H	poly‐4	P[5]A	H	92	6100	1.04

^a MeOH insoluble part.

^b Determined by SEC based on polystyrene standard.

To introduce a P[5]A residue at the initiating chain end of PPA, we synthesized P[5]A‐based aryl boronic acid (I‐1) (see Supporting Information) [31]. Using the Rh‐based multicomponent catalytic system consisting of [Rh(nbd)Cl]₂, I‐1, diphenylacetylene, PPh₃, and 50% KOH aq. [23, 25], living polymerization of phenylacetylene (PA) was performed, thereby introducing a P[5]A residue derived from I‐1 at the initiating chain end. The polymerization was terminated using T‐1 and T‐3 as terminating reagents [25], affording end‐functionalized poly‐1 and poly‐2, respectively (Entries 1 and 2 in Table 1). Direct introduction of the pyridinium‐containing guest residue using the corresponding terminating reagent T‐2 was not feasible because T‐2 was insoluble in THF, which was employed for the termination reaction. Therefore, poly‐2 bearing a terminal iodo group was first prepared using T‐3 and subsequently converted into poly‐2′ through nucleophilic substitution with pyridine, thereby introducing a pyridinium salt residue (G‐2) at the chain terminus (Table 1; see Supporting Information). For comparison, we synthesized poly‐3 and poly‐4, which lack host–guest interaction sites at both chain ends or at one chain end, respectively (Entries 3 and 4 in Table 1). In all cases, the desired cis‐stereoregular PPAs with controlled molecular weights and low dispersities were obtained in high yields (Table 1).

A concentrated THF solution (300 mg/mL) of poly‐1 and poly‐2′, each bearing a P[5]A residue at the initiating end and a guest residue (G‐1 or G‐2) at the terminating end, exhibited a gradual increase in apparent viscosity over time and gelation after 12 h. In contrast, concentrated THF solutions (300 mg/mL) of poly‐3 and poly‐4, which lack host–guest interaction sites at both chain ends or at one chain end, respectively, showed minimal change in apparent viscosity even after 12 h and did not undergo gelation. These observations suggest that PPAs possessing both P[5]A and guest residues at the chain ends form supramolecular polymers via host–guest interactions between P[5]A and the guest residues, eventually leading to gelation (Figure 2).

FIGURE 2.

Photographs of concentrated THF solutions (300 mg/mL) of the synthesized PPAs (poly‐1–poly‐4) immediately after preparation and after standing at 25°C for 12 h. Only poly‐1 and poly‐2′, bearing host–guest interaction sites at the chain ends, formed gels after 12 h.

The viscosities of poly‐2′ and poly‐3 in THF (80 mg/mL), prepared by diluting concentrated THF solutions (300 mg/mL) that had been allowed to stand for 12 h, were measured using a rotational viscometer. The viscosity of poly‐2′, bearing host–guest interaction sites, was considerably higher than that of poly‐3, which lacked such sites (Figure S2). In the absence of applied shear force, the viscosity of poly‐2′ was approximately 2,000 times that of poly‐3. Even at a shear rate of 100 s⁻¹, the viscosity of poly‐2′ was approximately 300 times that of poly‐3. THF and poly‐3 exhibited flow behaviors characteristic of a Newtonian liquid, consistent with the absence of significant intermolecular association [32]. In contrast, poly‐2′ exhibited a considerable decrease in viscosity with increasing shear rate (i.e., shear‐thinning behavior), characteristic of a non‐Newtonian liquid. This implies the presence of supramolecular interactions that extend throughout the material [32]. Although the association constant of the model host–guest pair is only moderate (K _a = 514 M^{− 1}), the polymeric nature of the telechelic system, together with multiple weak intermolecular interactions, is likely to stabilize the supramolecular assemblies, resulting in gelation and markedly enhanced viscosity.

To investigate the hydrodynamic dimensions of the assemblies formed by poly‐2′, dynamic light scattering (DLS) measurements were performed at 25°C using THF solutions of poly‐2′, poly‐3, and poly‐4 (5.0 and 0.1 mg/mL) prepared by diluting concentrated THF solutions that had been allowed to stand for 12 h. Because DLS provides hydrodynamic dimensions rather than the exact morphology of supramolecular assemblies, the obtained hydrodynamic diameters were used only as supporting evidence for solvent‐dependent changes in aggregate dimensions. The hydrodynamic diameter of poly‐2′ was approximately 100 times larger than those of poly‐3 and poly‐4 (Figure 3a). These results, together with the viscosity measurements and gelation behavior, support the formation of larger supramolecular assemblies through chain‐end host–guest interactions between the terminal P[5]A and guest residues. The corresponding hydrodynamic diameters and polydispersity index (PDI) values are summarized in Figure S3. In the concentrated THF solution (5.0 mg/mL), the hydrodynamic diameter of poly‐2′ remained nearly unchanged even after 24 h. However, in the diluted THF solution (0.1 mg/mL), although the hydrodynamic diameter of poly‐2′ remained almost unchanged for approximately 2 h, the fraction of smaller species with hydrodynamic diameters comparable to those of poly‐3 and poly‐4 gradually increased over time, and the peak corresponding to the larger assemblies disappeared after 24 h (Figure 3b). These results suggest that larger assemblies formed under high‐concentration conditions through host–guest interactions at the chain ends are maintained for several hours even after dilution below the concentration required for their initial formation. This observation suggests the presence of a kinetically stabilized assembled state. Although the origin of this kinetic stability remains unclear, dissociation of the supramolecular polymers may be slowed by the cooperative nature of the chain‐end host–guest interactions, resulting in a delayed return to equilibrium under dilute conditions.

FIGURE 3.

(a) DLS charts of poly‐2′, poly‐3, and poly‐4 measured in THF ([polymer] = 0.1 mg/mL) at 25°C immediately after dilution with THF after being allowed to stand at 25°C for 12 h at [polymer] = 300 mg/mL. (b) Time‐dependent DLS charts of poly‐2′ measured in THF ([polymer] = 0.1 mg/mL) at 25°C upon dilution with THF after being allowed to stand at 25°C for 12 h at [polymer] = 300 mg/mL.

Studies have shown that poly‐LAlaC₁₀, a chiral PPA wherein an L‐alanine‐n‐decyl ester substituent is introduced into the side chain via an amide bond, forms a one‐handed helical structure and exhibits remarkable stimuli‐responsive behavior [26]. This behavior includes helix‐sense inversion and notable changes in main‐chain rigidity based on solvent‐polarity‐dependent ON/OFF switching of intramolecular hydrogen bonding between the pendant amide groups. To construct a supramolecular polymer leveraging such a stimuli‐responsive one‐handed helical PPA, we synthesized chiral PPAs with host–guest interaction sites at the chain ends (Figure 4). To suppress undesired intramolecular host–guest inclusion of the terminal guest moiety into the P[5]A cavity, a chiral phenylacetylene derivative M1 was designed (Figure 4a), in which the n‐decyl group of poly‐LAlaC₁₀ was substituted with branched alkyl chains. Although this modification introduces an additional stereogenic center, it is racemic and located remotely from both the polymer backbone and the terminal host–guest recognition site. Therefore, it is not expected to influence the folding behavior or chiroptical properties of the resulting polymer. M1 was polymerized using a Rh‐based multicomponent catalytic system with I‐1 as the aryl boronic acid component, yielding the corresponding chiral PPA (poly‐5) with host–guest interaction sites at both chain ends (Figure 4b and Supporting Information). A control polymer without host–guest interaction sites (poly‐6) was also synthesized (Figure 4c and Supporting Information). Notably, DLS measurements showed that poly‐5 exhibited a particle size comparable to that of poly‐6, indicating that supramolecular polymer formation did not occur. Molecular modeling suggested that the terminal guest residue in poly‐5 is sterically shielded by the bulky pendant alkyl groups, preventing efficient host–guest complexation with the P[5]A unit at the initiating end (Figure S4).

FIGURE 4.

(a–d) Chemical structures of (a) M1, (b) poly‐5, (c) poly‐6, and (d) poly‐7. (e) DLS charts of poly‐5, poly‐6, and poly‐7 measured in THF ([polymer] = 0.1 mg/mL) at 25°C immediately after dilution with THF after being allowed to stand at 25°C for 12 h at [polymer] = 300 mg/mL.

Taking advantage of the living polymerization strategy [23, 24, 25], a short PA block (m ≈ 5) was inserted at the terminal end by sequential monomer addition to alleviate this steric hindrance, affording poly‐7 (Figure 4d and Supporting Information). DLS measurements revealed a pronounced increase in the particle size of poly‐7 compared with those of poly‐5 and poly‐6, consistent with the formation of supramolecular polymers. These results indicate that insertion of the short PA spacer enhances the accessibility of the terminal guest residue, thereby facilitating intermolecular host–guest interactions and subsequent supramolecular polymer formation.

Poly‐7, which formed a supramolecular polymer, exhibited a strong positive Cotton effect in the absorption region of the polymer main chain in THF, suggesting the formation of a right‐handed helical structure. In contrast, in CCl₄ and toluene, a negative Cotton effect with a nearly mirror‐imaged shape was observed, indicating the formation of a left‐handed helical structure (Figures 5 and S5). These findings indicate that, similar to the previously reported poly‐LAlaC₁₀ [26], the helix sense of poly‐7 undergoes inversion owing to the solvent‐polarity‐dependent ON/OFF switching of the intramolecular hydrogen bonds between the pendant amide groups. Poly‐7 exhibited a CD intensity nearly identical to that of poly‐6, which did not form a supramolecular polymer, indicating that supramolecular polymer formation in poly‐7 did not enhance the CD intensity. Notably, poly‐6 exhibited solvent‐dependent CD spectral changes essentially identical to those previously reported for poly‐LAlaC₁₀. This result indicates that the additional stereocenters introduced into the branched alkyl substituents, which are present as a racemic mixture and located remote from the polymer backbone, have little influence on the solvent‐responsive helical conformational behavior of the polymer.

FIGURE 5.

CD and absorption spectra of poly‐7 in THF and toluene (2 mM) at –10°C immediately upon dilution with THF and toluene, respectively after allowing the mother solution (800 mg/mL) in THF to stand at 25°C for 12 h.

Atomic force microscopy (AFM) measurements revealed that the morphology of the supramolecular polymer formed by poly‐7 depended strongly on solvent polarity (Figures 6 and S6). In THF, where intramolecular hydrogen bonding between the pendant amide groups was disrupted (OFF), flexible ring‐shaped supramolecular polymers with an average diameter of 40 nm were observed in AFM topographical images (Figure 6a). In contrast, in toluene, where intramolecular hydrogen bonding was active (ON), rigid rod‐like assemblies with an average width of 2 nm and a length exceeding 500 nm were observed (Figure 6b). Representative height profiles collected from multiple independent regions are shown in Figure S7. Occasional protrusions were observed along the rigid rod‐like assemblies. These features may originate from the bulky pillar[5]arene units or from locally overlapped polymer chains, although their origin cannot be assigned unambiguously. Nevertheless, the assignment of these structures as rod‐like supramolecular assemblies is supported by the overall morphology observed across multiple independent regions and further corroborated by the solvent‐dependent DLS results. The DLS results of poly‐7 obtained in THF and toluene were consistent with the AFM observations. Poly‐7 exhibited a slightly smaller hydrodynamic diameter in THF than in toluene (Figure S8). These DLS results support solvent‐dependent differences in aggregate size, while the assignment of compact cyclic structures and rod‐like assemblies is based primarily on the AFM observations. The observed morphological transformation is likely associated with the solvent‐dependent change in backbone rigidity [26]. In THF, disruption of the intramolecular hydrogen bonds increases the flexibility of the helical PPA backbone, facilitating intramolecular cyclization through chain‐end host–guest interactions and yielding cyclic supramolecular polymers. In contrast, activation of intramolecular hydrogen bonding in toluene increases backbone rigidity, suppressing cyclization and favoring intermolecular chain extension, thereby generating rigid rod‐like supramolecular assemblies. These results demonstrate a process of hierarchical structural transduction, in which solvent‐triggered molecular‐level conformational changes are translated into supramolecular topology and morphology. This hierarchical response illustrates how local conformational changes in dynamic helical polymers can be amplified into large‐scale morphological transformations through supramolecular organization, providing a general strategy for the design of adaptive hierarchical materials.

FIGURE 6.

AFM images of poly‐7 spin‐coated onto HOPG from (a) THF and (b) toluene solutions. The solutions were diluted from the mother solution (800 mg/mL) that had been allowed to stand at 25°C for 12 h immediately before spin‐coating.

In summary, end‐functionalized PPAs bearing a cyclic host P[5]A residue at the initiating end and a complementary guest residue at the terminating end were successfully synthesized via living polymerization using a Rh‐based multicomponent catalytic system. Intermolecular host–guest interactions between the terminal P[5]A and guest residues promoted the formation of supramolecular polymers, as evidenced by gelation, enhanced viscosity, and increased particle sizes. AFM observations revealed that the morphology of supramolecular polymers composed of PPA chains bearing alanine pendants connected through amide linkages was governed by solvent polarity and transformed from flexible cyclic structures in THF to rigid rod‐like structures in toluene. This morphological transformation originates from the ON/OFF switching of intramolecular hydrogen bonds between the pendant amide groups, which modulates the rigidity of the helical polymer backbone. These findings exemplify hierarchical structural transduction in dynamic helical polymer systems. This study establishes a general strategy for programming structural transformations across multiple length scales and provides a platform for the design of adaptive hierarchical chiral materials based on dynamic helical polymers.

Author Contributions

Tatsuya Nishimura: writing – original draft, writing – review and editing, funding acquisition, conceptualization, methodology, supervision, project administration, investigation, formal analysis, validation, data curation, visualization, resources. Haru Nozue: investigation, validation, visualization, data curation, formal analysis. Feng Li: writing – original draft, conceptualization, investigation, validation. Tomoki Ogoshi: writing – review and editing, conceptualization. Katsuhiro Maeda: writing – review and editing, writing – original draft, project administration, supervision, conceptualization, funding acquisition.

Conflicts of Interest

The authors declare no conflicts of interest.

Supporting information

The authors have cited additional references within the Supporting Information.Supporting File: anie74190‐sup‐0001‐SuppMat.pdf.

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.