Synthesis of main chain-type liquid crystalline polyrotaxanes: influence of the wheel components and their mobility on liquid crystalline properties

ABSTRACT Main chain-type liquid crystalline (LC) polyrotaxanes were synthesized via the ene-thiol copolymerization of pseudorotaxane and LC monomers. The effects of the rotaxane moiety and

its content on the LC properties were investigated using differential scanning calorimetry. Introduction of the rotaxane moiety on the LC polymer reduced the stabilization of the LC phase.

Neutralization of the rotaxane moiety also destabilized the LC phase, most likely due to the movable wheel components of the rotaxane on the axle LC polymer chain. SIMILAR CONTENT BEING

VIEWED BY OTHERS CATALYST SWITCH STRATEGY ENABLED A SINGLE POLYMER WITH FIVE DIFFERENT CRYSTALLINE PHASES Article Open access 20 November 2023 COMPARATIVE STUDY ON THE EFFECTS OF

INCORPORATING POLY(D,L-LACTIDE) AND SOLVENT ON STEREOCOMPLEX CRYSTALLIZATION AND HOMOCRYSTALLIZATION IN UNCONSTRAINED AND CONSTRAINED POLY(L-LACTIDE)/POLY(D-LACTIDE) SYSTEMS Article 07

September 2022 INFLUENCES OF THE COMONOMER TYPE ON THE CRYSTALLIZATION KINETICS OF HIGH-DENSITY POLYETHYLENE Article 14 April 2025 INTRODUCTION Interlocked polymers such as polyrotaxanes and

polycatenanes have received great attention not only with respect to the synthesis of these mechanically linked polymers but also because of the unique properties that are derived from

these structures. The synthesis of polyrotaxane was initially reported by Harada _et al._1 Additionally, many studies on the synthesis of polyrotaxanes using cyclodextrin (CD)2, 3, 4, 5, 6,

7 or crown ethers8, 9, 10, 11, 12, 13 as a wheel component have been reported in the last two decades. With respect to CD-based polyrotaxanes, the efficient synthesis of polyrotaxanes has

progressed because of the achievement of effective complexation and end-capping reactions.6, 7 The focus of polyrotaxane synthesis efforts is now shifting to the development of functional

soft materials such as molecular nanotubes,14 molecular wires,15 slide-ring gels,16 polyrotaxane networks17 and multivalent-binding recognition molecules.18 On the other hand, there are many

reports on the synthesis of rotaxanes using a crown ether as a wheel component8, 9, 10, 11, 12, 13 to control the location of the wheel component, that is, to achieve the free movement of

the crown ether along the axle molecule. This active control of the location of the wheel component is called rotaxane switch, and it has been applied to the development of supramolecular

nanotechnologies such as molecular machines,19 molecular elevators20 and molecular shuttles.21, 22, 23 We have recently reported the application of rotaxane switches in a polymer system that

involved the conformational change of a polyacetylene with pendant rotaxane moieties from a one-handed helix to a random helix via control of the location of the crown ether.24, 25 This

result revealed the potential of rotaxanes to be used in stimuli-responsive polymer materials, even though the rotaxane moiety was introduced as a pendant group. To investigate the influence

of the rotaxane moiety existing on the main chain, a main chain-type polyrotaxane was designed in which the liquid crystalline (LC) polymer was used as an axle polymer. There are a few

reports26, 27, 28, 29, 30 on the synthesis of liquid crystals with rotaxane structures, to the best of our knowledge. Aprahamian _et al._26 reported the synthesis of a switchable [2]

rotaxane with LC components to build up the rotaxane moieties in a smectic A phase. A CD-based polyrotaxane with a number of mesogenic side chains was first synthesized by Kidowaki _et

al._27 to explore the effects of movable mesogenic groups. Terao _et al._28 reported on the synthesis of CD-based polyrotaxanes with a conjugated polymer as an axle polymer to develop

insulated molecular wires and found a lyotropic LC phase in concentrated chloroform solution due to the rigidity of the main chain polymer. However, no reports exist regarding investigation

of the effects of rotaxane structures on main chain-type LC polymers. Herein, crown ether-type polyrotaxanes containing a main chain-type LC polymer (PR-LCPs) as the axle polymer were

synthesized via ene-thiol polymerization, and the effects of the rotaxane moiety on the LC properties were investigated. MATERIALS AND METHODS MATERIALS All solvents were dried and distilled

before use according to general purification procedures. Commercially available reagents were used without further purification unless otherwise noted. A radical generator with a low

decomposition temperature, V-70 (2,2′-azobis(4-methoxy-2,4-dimethyl valeronitrile)), was purchased from Wako Pure Chemical Industries, Ltd., Osaka, Japan. All reactions were carried out in

inert atmosphere. Silica gel column chromatography was performed using silica gel N60 (grain size 40–50 μm) (Kanto Chemical Co. Inc., Tokyo, Japan). The axle monomer (1′), LC monomers (2, 3)

and end-capping agents (4, 5) were synthesized according to the literature.31, 32, 33, 34 SYNTHESIS OF LC POLYROTAXANES Scheme 1 shows the synthetic pathway of PR-LCPs. The synthesis of

PR5·PF6 is described here as the typical procedure. To a solution of axle monomer 1′ (82.1 mg, 0.150 mmol) and dibenzo-24-crown-8-ether (DB24C8; 82.4 mg, 0.150 mmol) in dried chloroform (31 

ml) treated with sonication to form the pseudorotaxane complex, LC monomer 2 (538 mg, 1.36 mmol), LC monomer 3 (701 mg, 1.50 mmol) and V-70 (64.4 mg, 0.210 mmol) were added and stirred for

24 h at 30 °C. The end-cap agent 4 (143 mg, 0.940 mmol) and V-70 (32.2 mg, 0.110 mmol) were added to the reaction mixture and stirred for 24 h at 30 °C. In addition, the end-cap agent 5 (765

 mg, 3.70 mmol) and V-70 (127 mg, 0.410 mmol) were added to the reaction mixture and stirred for another 24 h at 30 °C. The reaction mixture was poured into methanol (20 ml). The precipitate

was collected by filtration and dried _in vacuo_ to give PR5·PF6 (94%, 1.32 g) as a white solid. NEUTRALIZATION OF LC POLYROTAXANES To a solution of PR5·PF6 (100 mg) in chloroform (10 ml)

was added tetra(_n_-butyl)ammonium chloride trihydrate (8.34 mg, 0.0300 mmol) at room temperature. After stirring for 6 h, the mixture was washed with water (30 ml) and aqueous Na2CO3 (10

wt%). The organic layer was dried over Na2SO4, filtered and concentrated _in vacuo_ to give PR5 (77%, 77.2 mg) as a white solid. CHARACTERIZATION 1H (400 MHz) and 13C (100 MHz) nuclear

magnetic resonance (NMR) spectra were recorded on a JEOL AL-400 spectrometer (JEOL Ltd., Tokyo, Japan) using CDCl3 as the solvent, calibrated using tetramethylsilane as the internal

standard. NMR chemical shifts were recorded in delta units (δ). IR spectra were recorded on a JASCO FT/IR-230 spectrometer (JASCO Corporation, Tokyo, Japan). Melting points were measured on

Stuart melting point apparatus SMP3 (Stuart Scientific, Bibby Scientific Limited, Staffordshire, UK) and Shimazdu DSC-60 instrument (Shimadzu Corporation, Kyoto, Japan). Size exclusion

chromatography (gel permeation chromatography (GPC)) was performed using JASCO PU-2080 Plus HPLC pump with JASCO UV-1570 UV detector and JASCO RI-1530 RI detector equipped with consecutive

TOSOH TSKgel GMHXL and G5000HXL linear polystyrene gel columns at 30 °C in CHCl3 (1.0 ml min−1). The number average molecular weight (_M_n), weight average molecular weight (_M_w) and

polydispersity index (_M_w/_M_n) of the polymers were calculated on the basis of polystyrene standards. Differential scanning calorimetry (DSC) analysis was carried out with Shimadzu DSC-60

instrument under nitrogen (flow rate of 50 ml min−1), with liquid nitrogen as a refrigerant to determine the phase transition temperature. Elemental analysis was performed with J-Science

JM10 (Kyoto, Japan), Yanaco HSU-20+ICS-1100 (Kyoto, Japan) and Elementar Vario EL. Elementar Analysensysteme GmbH (Hanau, Germany) LC texture was observed with a polarizing optical

microscope (POM) equipped with self-produced hot stage and temperature controller. Wide-angle X-ray powder diffraction (WAXD) measurement was carried out with Bruker AXS D8 DISCOVER μHR

Bruker Corporation (Billerica, MA, USA) equipped with 2D PSPC Hi-STAR, with CuKα radiation at 50 kV and 22 mA. RESULTS AND DISCUSSION SYNTHESIS OF LC POLYROTAXANES The PR-LCPs were

synthesized via the copolymerization of the diolefin-type pseudorotaxane monomer 1 with LC monomers 2 and 3. The pseudorotaxane monomer 1 was prepared via the complexation of the

diolefin-type ammonium hexafluorophosphate (PF6) axle molecule (1′) and DB24C8 in dichloromethane at room temperature (see Supplementary Information) as a result of the favorable

complexation of ammonium PF6 and DB24C8 in less polar solvents (chloroform and dichloromethane) at ambient temperature. The content of the rotaxane moiety in PR-LCPs was controlled by

adjusting the feed ratio of the pseudorotaxane monomer 1 to the LC monomers 2 and 3 (1/2/3) ranging from 5/45/50 to 30/20/50. The polymerization of the mixture proceeded in dichloromethane

at 30 °C for 24 h in an inert atmosphere using V-70 as the radical initiator (half-life at 30 °C=10 h). After the completion of polymerization, the end-capping agents 4 and 5 and additional

V-70 were added sequentially to the reaction mixture and with stirring for 24 h after each addition. As a result, several PR-LCPs were obtained. LC monomers 2 and 3 were chosen to form the

axle polymers of the polyrotaxanes because the resultant polymers were expected to be sufficiently thin to penetrate the cavity of DB24C8. In addition, a PR-LCP without the rotaxane moiety

(PR0) was synthesized as a control sample via the step-growth ene-thiol polymerization using an equivalent amount of diolefin monomer 2 and dithiol monomer 3. PR0 has been reported to show

crystalline–nematic–isotropic phase transitions.35, 36 Table 1 summarizes the feed conditions and the data of the resultant PR-LCPs, which are labeled as PRX·PF6, where X denotes the feed

ratio of the pseudorotaxane monomer 1. The content of the rotaxane moiety in the polymer chain was found to be related to the feed ratio, but the difference between the feed ratio and the

composition gradually increased as the amount of 1 in the feed increased because of its relatively poor solubility in dichloromethane. The polymerization yield was moderate regardless of the

feed ratio, and the molecular weight of the obtained polymers was sufficiently high to allow stable phase transitions independent of the molecular weight. The chemical structure of the

PR-LCPs was determined using 1H NMR, GPC and elemental analyses. Figure 1 shows the 1H NMR and GPC spectra for PR5·PF6 and PR5. In the 1H NMR spectrum of PR5·PF6, specific characteristic

peaks for both the benzyl protons adjacent to the ammonium (b) and the protons in the wheel component (α, γ, φ) were identified and were in accord with the literature for the peaks of

rotaxane structures.23 In addition, the characteristic mesogenic peaks for the neighboring carbonyl group (a) confirmed the existence of LC moieties. The GPC profile of PR5·PF6 shows a

monomodal peak with a rather broad molecular weight distribution. Consequently, we concluded that a main chain-type LC polymer bearing a rotaxane structure in the main chain was successfully

synthesized via the ene-thiol copolymerization of pseudorotaxane and LC monomers. NEUTRALIZATION OF LC POLYROTAXANES Conversion of the ammonium salt of the rotaxane moiety to amine was

performed to remove the strong interaction between the ammonium moiety and DB24C8 and to release the wheel component. The rotaxane moiety of PRX·PF6 was neutralized using a counteranion

exchange method that was recently developed by our group.22, 37, 38, 39, 40 Thus, the polymers PRX·PF6 were treated with tetra-_n_-butylammonium chloride (TBACl) in DMF and then saturated

aqueous NaHCO3 to give the corresponding PR-LCPs with non-ionic rotaxane (PRX) groups. The successful neutralization was confirmed by 1H NMR, as shown in Figure 1a. The signals for the

benzyl protons (b) of PR5·PF6 at 4.4 p.p.m. disappeared or were shifted upfield with the neutralization. In addition, two of the DB24C8 signals (γ, ϕ) were shifted downfield by the removal

of the restraining effect of the DB24C8 group by the ammonium salt group. The GPC profile of PR5 (Figure 1b) implies that no side reactions, such as chain scission, occurred. Based on these

results, it was concluded that the PRX·PF6 polymers were neutralized and converted to PRX along with the release of the wheel components from the interaction with the ammonium moiety. To

confirm the free movement of DB24C8 on the main chain, PR-LCP without end-cap groups was prepared and neutralized. After the neutralization, specific 1H NMR signals for isolated DB24C8 were

observed, indicating slippage from the axle polymer (see Supplementary Information). This result supports the likelihood that the ring size of DB24C8 is large enough to move over the

mesogenic groups. EFFECT OF THE ROTAXANE COMPOSITION ON THE LC PROPERTIES The effect of the rotaxane component on the LC properties of the PR-LCPs was investigated by DSC, POM and WAXD. In

the DSC thermograms of the second heating scan with a heating rate of 10 °C min−1, PR5·PF6 and PR10·PF6 showed two transition peaks corresponding to the crystal-LC and LC-isotropic

transitions as shown in Figure 2, while PR20·PF6 had one broad peak. In the region between the crystal phase and the isotropic phase, PR5·PF6 and PR10·PF6 exhibited LC phases by the POM

observation (see Supplementary Information). Furthermore, the WAXD profiles (see Supplementary Information) showed similar diffraction patterns to PR0 in the same region, indicating the

formation of a nematic phase. The temperatures and enthalpies (Δ_H_) of the transitions are summarized in Table 2. Focusing on the transition peak temperature of the nematic to isotropic

phase (_T_NI), PR5·PF6 showed a lower transition temperature than PR0, implying the destabilization of the LC phase due to the introduction of the rotaxane moiety. PR10·PF6 exhibited a

broader transition peak at a similar peak temperature to PR5·PF6, which had a lower onset temperature of the peak than that of PR5·PF6. Moreover, the transition peak temperature of the

crystal to nematic phase (_T_CrN) increased from 76 to 95 °C for PR5·PF6 and PR10·PF6, making the temperature range of the LC phase narrower. In the case of further introduction of rotaxane

moieties in the LC polymer represented as PR20·PF6, no LC phase was observed. These results suggested that the LC properties were affected by the presence of the rotaxane moiety. As a bulky

group, the rotaxane structure was assumed to destabilize the LC phase in a manner similar to that observed when alkyl substituents were introduced on a mesogenic group.41, 42, 43 EFFECT OF

THE MOBILITY OF THE WHEEL COMPONENTS ON THE LC PROPERTIES The effects of the delocalized wheel components on the LC properties were also investigated. Figure 3 shows the DSC thermograms of

PRX·PF6 and PRX. The phase transition temperatures are summarized in Table 2. Comparing PR5·PF6 with PR5, it was observed that _T_NI decreased from 124 to 120 °C, while _T_CrN increased from

76 to 80 °C after neutralization of the ammonium moiety to make the wheel components movable. In the case of PR10, no significant transition peak indicating the LC phase was detected in the

DSC thermogram, although the LC phase was actually observed for PR10·PF6. No LC phase was observed for both PR20·PF6 and PR20. In summary, it could be a tendency of the LC phase to be

destabilized after neutralization. The reason for this tendency remains unclear, but it may be attributed to the fact that the delocalized wheel components can be positioned at any place on

the polymer chain, even at the alkyl spacer or the mesogenic groups. Therefore, the presence of the wheel components on the mesogenic groups might interfere with their alignment to

destabilize the LC phase. EFFECT OF THE WHEEL COMPONENT ON THE LC PROPERTIES To further explore the effects of the wheel components on the LC properties, a LC polymer bearing ammonium groups

but without any wheel components, AX5·PF6, was synthesized by a similar copolymerization of 1′ instead of 1 (see Supplementary Information). The polymer AX5·PF6 was also neutralized to give

AX5, and the thermal properties of these polymers were investigated using DSC (Figure 4). As mentioned above, the destabilization of the LC phase was observed after the neutralization of

PR5·PF6, with shifts in _T_CrN and _T_NI. In contrast to PR5, the peak temperatures of _T_CrN and _T_NI for AX5 were similar to those of AX5·PF6, as summarized in Table 2. However, the peaks

for AX5 were broader, possibly because of the destabilization of the LC phase by the neutralized dibenzyl amine, which does not behave as a mesogenic group. Taking into account the DSC

thermogram of AX5, the effect of the neutralization on the destabilization of the LC phase might be derived not only from the mobility of the wheel component but also from the chemical

structure change of the ammonium moiety. CONCLUSIONS We have successfully synthesized main chain-type LC polymers bearing rotaxane structures on the main chain, and investigated the effects

of the rotaxane moiety on the transition temperatures. Introduction of the rotaxane moiety on the LC polymer reduced the stability of the LC phase, and neutralization of the ammonium moiety

also destabilized the LC phase, suggesting the effect of the movable wheel components on the axle LC polymer chain. To the best of our knowledge, this is the first example of LC polymers

whose phase transition temperatures can be controlled using a rotaxane switch. However, because other influences of the introduction of the rotaxane moiety on the LC properties, such as the

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from the Ministry of Education, Culture, Sports, Science and Technology, Japan (No. 23245031). AUTHOR INFORMATION AUTHORS AND AFFILIATIONS * Department of Organic and Polymeric Materials,

Tokyo Institute of Technology, Tokyo, Japan Yoko Abe, Hisashi Okamura, Satoshi Uchida & Toshikazu Takata Authors * Yoko Abe View author publications You can also search for this author

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Springer Nature SharedIt content-sharing initiative KEYWORDS * component mobility * ene-thiol polymerization * main chain-type liquid crystalline polymer * polyrotaxane