Introduction

The elegant structure of double-stranded polymers has attracted interest from polymer chemists. The parallel arrangement of chains with restricted mobility results in distinctive topological effects that profoundly influence their physicochemical properties [1]. Double-stranded polymers have been synthesized via elaborate strategies, such as connecting side chains along a conjugated polymer [2], precise assembly through molecular design [3], and repeated cyclization of rigid polymer chains under dilute conditions [4]. However, these methods have allowed for the fabrication of a limited range of polymer backbones and side chains, thereby restricting their diversity and potential applications based on the double-stranded topology.

Metal‒organic frameworks (MOFs) are crystalline network materials consisting of metal ions and organic ligands that offer highly regular and designable nanopores that approximate the molecular size. Numerous porosity-based applications have been developed, including molecular adsorption, separation, and catalysis [5, 6]. Molecular-sized nanochannels of MOFs are also ideal for confining and directing polymerization processes, as these nanochannels realize structural control of the resulting polymers through spatial interactions [7]. Recently, we reported a methodology for synthesizing double-stranded vinyl polymers in which conventional vinyl monomers were copolymerized with crosslinkers within the one-dimensional nanochannels of a MOF [8]. The diameters of the host nanochannels were comparable to the thicknesses of the double strands, thus allowing the formation of two polymer chains with parallel crosslinked structures. While this strategy has enabled the synthesis of double-stranded vinyl polymers composed of commodity monomer units (such as styrene and methyl methacrylate), further development of double-stranded vinyl polymer synthesis with functional side chains is crucial for increasing the utility and diversity of synthetic polymer duplexes.

Post-polymerization modification (PPM) is a method in which monomer units are chemically converted into other structures after polymerization [9, 10]. This technique provides a pathway to polymers that would be difficult or impossible to synthesize through direct polymerization of the corresponding monomers. PPM is also advantageous for the library synthesis of polymers because it introduces pendant groups without altering the main-chain structures. Active esters are often used for this purpose, in which a strong electron-withdrawing substituent makes the ester highly reactive toward nucleophiles [9]. Precursor polymers possessing active esters can be converted into polymers with nucleophile-derived structures via PPM under mild conditions. Since the first report of monomers with an N-hydroxysuccinimide (NHS) ester group [11], polymer chemists have designed active ester monomers to construct structure-controlled polymers with the desired side chains [12,13,14].

In this study, we polymerized an active ester acrylate with crosslinkers in the nanochannels of a MOF to synthesize a double-stranded vinyl polymer with transformable side chains. Among the activated esters, hydrophobic 1,1,1,3,3,3-hexafluoroisopropyl acrylate (HFIPA) [12, 15] was used to prevent unintended hydrolysis during the isolation of double-stranded poly(HFIPA) (DPHFIPA) from the MOF. Subsequent PPM with nucleophiles, such as alcohols and amines, transformed DPHFIPA into double-stranded polyacrylates and polyacrylamides with nucleophile-derived side chains (Fig. 1).

Fig. 1
figure 1

Concept of this work. MOF-templated polymerization for a transformable double-stranded polymer and PPM of active ester acrylate (green sphere) into acrylate or acrylamide units (orange and blue spheres). Crosslinkers are represented by gray dumbbells

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MOF-templated polymerization of HFIPA

To synthesize DPHFIPA, the one-dimensional hexagonal nanopore of [In(OH)bdc]n (1, bdc = 1,4-benzenedicarboxylate, Fig. 2a) [16] was used as a template for crosslinking radical polymerization. Owing to the size of the monomers (HFIPA = 0.7 nm, EDMA = 1.2 nm, Fig. 2a and Supplementary Fig. S1), the hexagonal channels (1.7 nm) are large enough to perform polymerization, whereas the triangular channels (0.5 nm) are too small to encapsulate the reactants. Molecular dynamics (MD) simulations of monomers in hexagonal channels revealed that the monomers could not migrate to adjacent channels on a reasonable timescale (Supplementary Fig. S2). The MD simulation also demonstrated the ability of crosslinking polymerization within the hexagonal channels to form DPHFIPA, thus excluding the possibility of triple-strand formation (Fig. 2b). These results indicate that the hexagonal nanochannels of 1 are suitable hosts for synthesizing DPHFIPA.

Fig. 2
figure 2

MOF-templated polymerization of HFIPA. a Chemical structures of HFIPA, the crosslinker, and 1. b MD structure of DPHFIPA in 1. Atoms: In (brown), O (red), C (gray), F (green), and H (white). c PXRD patterns of 1, 1 ⊃ HFIPA, 1 ⊃ DPHFIPA, and DPHFIPA. d FT-IR spectra of SPHFIPA and DPHFIPA. The absorption bands derived from the HFIPA and EDMA units are highlighted in blue and red, respectively

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MOF-templated polymerization was conducted according to a previously reported method for synthesizing double-stranded polystyrene and polymethyl methacrylate [8]. Vacant 1 was soaked in a mixture containing an HFIPA monomer, a crosslinker (ethylene glycol dimethacrylate, EDMA), and a radical initiator (2,2’-azobisisobutyronitrile, AIBN) to introduce them into the nanochannels of 1. Excess monomers were removed under reduced pressure to give a MOF-monomer composite (1 ⊃ HFIPA), and by heating the composite at 120 °C for 1 day under a nitrogen atmosphere, polymerization proceeded to afford 1 including DPHFIPA in the channels (1 ⊃ DPHFIPA). No changes were observed in the peak positions in the powder X-ray diffraction (PXRD) patterns before and after polymerization (Fig. 2c), indicating that the crystallinity of the host MOF was maintained during polymerization. Discrepancies in peak intensity between the peak patterns of vacant 1 and 1 ⊃ DPHFIPA arose from the difference in electron density distributions due to guest polymers in the nanochannels [17]. Scanning electron microscopy (SEM) images of the composites after polymerization revealed that the crystal morphology of 1 was preserved throughout the polymerization process (Supplementary Fig. S3). The 1H NMR spectra of the digested composites in deuterated DMSO/DCl (9/1) before and after polymerization revealed almost complete monomer consumption (Supplementary Fig. S4). These results indicated that the monomers in the nanochannels were successfully converted into polymers while preserving the crystalline structure of the MOF.

1 ⊃ DPHFIPA was then immersed in a H2O/MeOH solution of Na4EDTA, which is a chelating agent, to liberate DPHFIPA from the host framework. The decomposition of 1 was confirmed by the disappearance of the diffraction peaks in the PXRD patterns (Fig. 2c). The obtained polymer was soluble in good solvents that are suitable for poly(HFIPA) (acetone and tetrahydrofuran), indicating that random crosslinking outside the MOF did not occur. Notably, the active ester in the hydrophobic DPHFIPA remained intact during the isolation process, whereas poly(N-succinimidyl acrylate) underwent rapid hydrolysis under the same conditions. The FT-IR spectrum of DPHFIPA showed strong bands corresponding to the stretching vibration of the C=O bonds in the HFIPA units at 1777 cm–1, with shoulder peaks originating from those in the EDMA units (Fig. 2d). The wavenumber of the main peak was the same as that of the homopolymer, single-stranded PHFIPA (SPHFIPA), and peaks derived from the hydrolyzed structures were not observed (Supplementary Fig. S5). The 1H NMR spectrum of the obtained DPHFIPA showed reasonable signals as a chemical structure in which the PHFIPA chains were crosslinked through EDMA units (Supplementary Fig. S6). The crosslinking ratio of DPHFIPA was calculated to be 6% from the spectrum. Gel permeation chromatography (GPC) of DPHFIPA revealed a unimodal curve for the formation of polymer chains without uncontrolled crosslinking (Supplementary Fig. S7). The number average molecular weight (Mn) and polydispersity (Mw/Mn) were 14,600 and 1.74, respectively.

Post-polymerization modification of DPHFIPA

The transesterification of the active esters in DPHFIPA was conducted using butanol in the presence of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) in tetrahydrofuran (THF) at 50 °C (Fig. 3a) [15]. Despite the potential steric hindrance from the nearby polymer chain due to the duplex structure, the conversion of active esters in DPHFIPA reached 98% after 12 h (Fig. 3b). The resulting polymer, double-stranded poly(butyl acrylate) (DPBA), exhibited signals corresponding to both butyl acrylate and EDMA units in the 1H NMR spectrum (Fig. 3c). The crosslinking ratio estimated from this spectrum was 5%, which was almost the same as that of the polymer before PPM.

Fig. 3
figure 3

Transesterification of DPHFIPA. a Scheme for DPBA via the PPM of DPHFIPA. Conditions [HFIPA unit]/[butanol]/[DBU] = 1/100/1.1 (molar ratio) in THF at 50 °C for 12 h. b 19F NMR spectra of DPHFIPA and the diluted reaction mixture after the PPM (470 MHz, 300 K, Acetone-d6). c 1H NMR spectrum of DPBA (500 MHz, 300 K, CDCl3)

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The size and molecular weight of DPBA were measured via GPC with a light scattering detector. DPBA exhibited a unimodal GPC curve (Supplementary Fig. S7, Mn = 13,500, Mw/Mn = 1.54). Based on the crosslinking density and molecular weight, each polymer was estimated to contain an average of five crosslinking points, thus sufficiently forming a double-stranded structure. The GPC curve without shoulder peaks in the high-molecular-weight region also suggests that random crosslinking outside the MOF did not occur. Figure 4a shows the relationship between the retention time and absolute molecular weight of each fraction. DPBA eluted later than its single-stranded counterpart (SPBA), which has the same molecular weight, reflecting the small hydrodynamic volume of DPBA. This shift in the plot, without an appreciable change in the slope, clearly illustrates the compacted and flexible nature of the double-stranded vinyl polymer [4].

Fig. 4
figure 4

a Plots of log absolute molecular weight vs. retention time in GPC measurements. SPBA (black), DPBA (red). b AFM image of DPBA. The dilute solution in CHCl3 (2 µg/mL) was spin-cast on a mica substrate

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The morphology of DPBA was imaged using atomic force microscopy (AFM). A diluted solution of the polymer in chloroform was spin-cast onto a mica substrate, and an AFM height image was obtained in tapping mode (Fig. 4b). Rope-like meandering polymers were observed, and each chain was thin enough to be regarded as side-by-side adsorption of DPBA on the substrate [8]. This observation confirmed the absence of a three-dimensional network involving multichain stacking, suggesting that the PPM reaction did not induce transesterification of the EDMA units that disrupted the double-stranded topology.

PPM of DPHFIPA with amines was also performed to produce double-stranded polyacrylamides (Fig. 5). Owing to the high reactivity of the amine toward the active esters, the HFIPA units were transformed into the corresponding acrylamides at room temperature in the absence of the DBU catalyst. In the case of PPM with hexylamine, the conversion of the HFIPA ester reached 99% after 4 days of reaction, as determined by 19F NMR. An adsorption band derived from the amide group was observed in the IR spectrum of the purified polymer, and no peaks corresponding to the HFIPA units were observed (Supplementary Fig. S8). In contrast, the peaks corresponding to the EDMA units were still present, indicating that the crosslinked structure remained. In the 1H NMR spectrum of the obtained polymer, peaks derived from poly(hexyl acrylamide) and EDMA units were reasonably observed (Supplementary Fig. S9). The disappearance of the peak for the HFIP side chains at 6.4 ppm indicates that PPM was successful. In addition, double-stranded poly(dodecyl acrylamide) was obtained when dodecylamine was used in PPM (Supplementary Fig. S10). The PXRD pattern of the double-stranded poly(dodecyl acrylamide) after annealing under water vapor at 80 °C for 14 h presented diffraction peaks at q = 1.9, 3.9, and 5.9 nm−1, similar to its single-stranded counterpart (Supplementary Fig. S11). This suggests that the double-stranded poly(dodecyl acrylamide) also formed a lamellar structure similar to that of the homopolymer [18]. As double-stranded poly(hexyl acrylamide) and poly(dodecyl acrylamide) are too large to be encapsulated within the nanochannels of 1, this PPM approach is useful for producing polymers that are otherwise inaccessible, even when MOF scaffolds are used.

Fig. 5
figure 5

Aminolysis of DPHFIPA. a Scheme for double-stranded polyacrylamides. Conditions [HFIPA unit]/[amine] = 1/10 (molar ratio) in THF at room temperature for 4 days (hexyl and dodecyl) or 1 day (dimethyl aminoethyl and diethyl aminoethyl). b Repeating units of the synthesized double-stranded polyacrylamides

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To further demonstrate the advantages of the PPM method, DPHFIPA was converted to double-stranded poly(N,N-dimethylaminoethyl acrylamide) and poly(N,N-diethylaminoethyl acrylamide) [19, 20] using the corresponding amines. The reaction was accomplished within one day, and the IR and 1H NMR spectra of the purified polymers (Supplementary Figs. S8, S12, and S13) indicated the successful synthesis of these double-stranded vinyl polymers via PPM. This result contrasts with the direct polymerization of these amine monomers via MOFs, which suffer from unfavorable MOF degradation during polymerization. Under high-temperature polymerization conditions, the basic side chain of N,N-dimethylaminoethyl acrylamide likely reacts with the metal ions in the framework. As a result, the peaks in the PXRD pattern broadened, and the SEM image revealed cracked particles after polymerization (Supplementary Figs. S14 and S15).

Conclusions

In summary, the combination of MOF-templated polymerization and PPM of active esters provides a synthetic path for preparing double-stranded vinyl polymers with a wide range of side chains. This strategy enabled the rational synthesis of polymer duplexes with functional groups, providing new insights into the development of synthetic polymer duplexes and bundled polymers.