Side-chain functionalized supramolecular helical brush copolymers

Abstract

We report poly(isocyanide)-based random copolymers (co-PIC) featuring alkoxycarbonyl-based side-chains synthesized via the metal-catalyzed controlled polymerization of chiral and achiral isocyanide monomers. The pyridine-functionalized achiral monomer provides functional sites while the chiral monomer drives the formation of a one-handed preferred helix. The side-chain functionalized helical polymer undergoes self-assembly with palladated pincer ligands, as evidenced by ¹H NMR and UV-Vis spectroscopies. Circular dichroism (CD) spectroscopy confirms that the side-chain self-assembly does not affect the backbone helicity. We construct supramolecular helical brush copolymers via the metal coordination of the co-PIC backbone with telechelic poly(styrene)s. ¹H NMR and UV-Vis spectroscopies confirm the metal coordination, and CD measurements suggest that the backbone retains its helical conformation. Additionally, viscometry measurements verify the formation of high molecular weight polymers while dynamic light scattering confirms the increasing hydrodynamic radii of the resulting supramolecular brush copolymers. Our methodology constructs complex 3D materials with fully synthetic, secondary structure containing building blocks. We view this as a platform for building architecturally controlled 3D supramolecular materials with high degrees of complexity.

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Introduction

Synthetic polymers that mimic the structure and functionality of biological macromolecules have led to remarkable achievements in polymer science.^1–3 These synthetic systems simulate the folding, chain collapse, and inter/intra-chain interactions of biomacromolecules to form various topologically diverse structures,^4–6 and induce enzymatic functions including catalysis, recognition, and long-range ordered molecular machines.^7–9 Despite these advances in structural complexity, the scope of artificial building blocks to build highly ordered polymeric structures remains limited. Compared to amino acid-based foldamers, non-amide polymeric backbones enable the utilization of more diverse sets of monomers and polymerization methods.^10–13 We have introduced the directed assembly of non-amide based polymers bearing secondary structures, including helices, sheets, and coils.^14–17 This approach uses multiple molecular recognition pairs, allowing for precise spatial control over building blocks thereby expanding our capability to achieve complexity and may lead to the fabrication of functional materials.¹⁸

The incorporation of secondary structure elements into three-dimensional polymeric ensembles displays intriguing material properties that are not achievable through the 2D linear assembly. Maeda and co-workers reported the “sergeants and soldiers effect” of helical brush copolymers with poly(phenylacetylene) backbones and poly(phenyl isocyanide) pendants, showing long-range chiral amplifications.^19,20 Baumgartner et al. reported a polypeptide-based brush copolymer where the spatial arrangements of helical side-chains catalyzed their own formations.²¹ The incorporation of helical segments in complex polymeric architectures has found various applications including size separation, catalysis, circularly polarized luminescence and chiral recognition.^22–27 Elacqua et al. brought β-sheet mimicking poly(p-phenylenevinylene) block copolymers to a new level of complexity.²⁸ The stacked antiparallel sheets with up to five turns demonstrated that the increasing folds of the polymers induced a higher degree of order. These examples prove that 3D spatial assembly will be the key to fabricating the next generation of functional materials.

Compared to building 3D complex materials via covalent linkages, fabricating 3D architectures through noncovalent interactions remains challenging. Unlike head-to-tail linear assemblies, 3D spatial control requires polymers with multiple molecular recognition units functionalized on the side-chains.²⁹ This class of polymers possess unique properties including self-healing capabilities,³⁰ enhanced mechanical properties,³¹ and the ability to build dynamic polymer networks.³² Using a single strand collapse strategy, Barner-Kowollik and Meijer produced block copolymers with multiple recognition units. Under high dilution, these polymers fold into single-chain nanoparticles (SCNPs) driven by interchain helical stacking and noncovalent cross-linking.³³ The SCNP strategy demonstrates the feasibility of 3D assembly through noncovalent interactions, yet it lacks precise control over diverse secondary structure building elements. A universal platform is required to build 3D noncovalent polymeric architectures with well-defined local secondary and tertiary structural elements.

In this contribution, we report a side-chain functionalized helical polymer as a key building block for the formation of complex 3D materials, in particular supramolecular brush copolymers. Poly(isocyanide) (PIC) based on chiral menthyl-ester aryl isocyanides is known to fold into static helical polymers with preferred handedness and high helix inversion barriers due to the presence of lone pairs and bulky side groups.³⁴ The helix sense can be retained when mixing with achiral aryl isocyanides with alkoxylcarbonyl groups at the p-positions.³⁵ Therefore, the use of metal-catalyzed isocyanide polymerization with both chiral and achiral isocyanides should generate helical random copolymers with full control over basic polymer properties, including dispersity and molecular weights.^36,37 We synthesize helical PIC copolymers (co-PIC) with dispersed pyridine functionalities along the side-chains by combining the chiral menthyl-ester isocyanide with an achiral pyridine functionalized isocyanide. The pyridinyl groups are capable of coordinating with palladated sulfur-carbon-sulfur (SCS) pincer complexes, allowing directional assembly of the polymer blocks. While several noncovalent interactions on helical side-chains have been reported, including host–guest interactions,^38,39 the use of metal coordination to derive complex structures from the side-chains of a helical PIC is yet to be explored. To demonstrate the potential of our strategy, we describe the formation of a supramolecular helical brush copolymer featuring coil-like poly(styrene) (PS) pendant chains end-functionalized with Pd^II-pincer complexes.

Results and discussion

Synthesis of side-chain functionalized helical polymer

Scheme 1 depicts the synthesis of the pyridine functionalized aryl isocyanide, 5, in three steps starting from the formation of aryl formamide (2) according to published protocols.⁴⁰ 4-Pyridinepropanol, 3, is coupled to the formamide by Steglich esterification to produce 4. Visualizing a triplet at δ ∼4.0 ppm (ester-CH₂) in the ¹H NMR spectrum confirms the new ester formation. The short alkyl chain between the pyridine and the ester group provides spatial flexibility between the functional group and the polymer backbone. Then, 4 is converted to the corresponding isocyanide using phosphoryl chloride and triethylamine. Purification via column chromatography and recrystallization affords the final aryl isocyanide monomer that is stored in solution at 0 °C to minimize self-polymerization. The monomer is characterized by NMR and IR spectroscopies and high-resolution mass spectrometry (ESI†). A strong absorption band at 2120 cm⁻¹ in the FT-IR spectrum corresponds to the formation of the isocyanide.

Scheme 1 Synthetic route toward the pyridine functionalized, achiral aryl isocyanide monomer.

Controlled polymerization of the isocyanides is mediated by a palladium-anisole initiator prepared according to published protocols.⁴¹ We target a degree of polymerization of 30 with a ratio of chiral to achiral monomers of 20 : 10 (Fig. 1a). While the circular dichroism (CD) signal intensities change linearly with the percentage of chiral isocyanides,^36,37 this feed ratio ensures a stable helical backbone with a high percentage of functional groups for further functionalization via self-assembly. Polymerizations are performed in THF at 55 °C for 16 hours. The disappearance of the isocyanide ν_CN band in the FT-IR spectra confirms the complete consumption of the isocyanide monomers (Fig. S4†). The resulting co-PIC is purified by precipitation in cold methanol for at least three times. The polymer displays monomodal distribution and a dispersity (Đ) of 1.44, as determined by size-exclusion chromatography (SEC) in DMF (Fig. 1b). The ¹H NMR spectrum of the polymer contains all respective polymer resonances (Fig. S5†) including a broad signal at δ 8.40 ppm that can be assigned to the ortho protons of the pyridine.

Fig. 1 (a) Structure of the side-chain functionalized helical co-PIC with dots highlighting backbone signals of interest in the ¹³C NMR spectrum. (b) SEC curve of co-PIC showing number average molecular weight (M_n) of 7200 and dispersity (Đ) of 1.44. (c) Partial ¹³C NMR spectrum (600 MHz, CD₂Cl₂) of co-PIC showing backbone signals from two types of monomers. Integration of these signals confirms that the ratio of chiral to achiral component is 2 : 1.

To gain quantitative insights of the polymer composition, ¹³C NMR spectroscopy with inverse-gated-decoupling and prolonged relaxation delay has proven useful.^42,43 The menthyl-ester isocyanide has a distinctive signal at δ 75–76 ppm, corresponding to the menthyl carbon adjacent to the carbonyl group (Fig. S3†). Similarly, in the ¹³C NMR spectrum, the signal of the carbon next to the carbonyl group in the pyridine-containing 5 is located at δ 64–65 ppm (Fig. S2†). We select these two distinctive signals for quantitative analysis since there is no-overlay with neighboring peaks. A partial ¹³C NMR spectrum with inverse-gated-decoupling, recorded in deuterated dichloromethane (CD₂Cl₂) is shown in Fig. 1c, where the two backbone signals of interest are present at δ 75.1 and 64.0 ppm. Integration of the two signals reveals a ratio of 1.98 : 1 that is very close to the 20 : 10 feed ratio, confirming the chemical composition of co-PIC.

Supramolecular assembly of co-PIC with Pd^II-pincer small molecule analog

To investigate the metal coordination along the polymer side-chains, we first assemble a small molecule analog of the complimentary recognition unit, a Pd^II-SCS-pincer ligand Pd-Pin (Fig. 2a). The t-butyldimethylsilyl (TBS) protected pincer ligand is prepared according to published methods.⁴⁴ The use of the TBS protecting group prevents self-coordination between the metal center and the hydroxy group.⁴⁵ Upon metalation with Pd(MeCN)₂Cl₂, the signal of the hydrogen para to the aryl ring completely disappears in the ¹H NMR spectrum, confirming complete metalation of the ligand (Fig. S8†). We mix co-PIC with Pd-Pin in CD₂Cl₂ with a molar ratio of 1 : 10 (1 : 1/pyridine : Pd^II pincer), followed by the addition of AgBF₄ to the physical mixture. AgBF₄ is an activating agent that triggers metal coordination by removing the Cl⁻ from the metal center and exposing the coordination site to pyridine molecules. After 30 minutes of stirring, we remove the insoluble AgCl by passing the solution through a syringe filter.

Fig. 2 (a) Schematic representation of the side-chain supramolecular assembly of co-PIC and Pd-Pin complex. (b) Partial ¹H NMR (600 MHz) spectroscopic overlay of (top to bottom) co-PIC, physical mixture of co-PIC and Pd-Pin, and metal coordination assembly after AgBF₄ addition.

The metal coordination is tracked by ¹H NMR spectroscopy. As shown in Fig. 2b, we observe no shifts in the ¹H NMR spectra of the physical mixture, suggesting no interactions between the two species. Upon addition of AgBF₄, the resulting ¹H NMR spectrum displays the signature downfield shift of the α-pyridyl signal from δ 8.40 to 8.65 ppm. Furthermore, the aromatic proton signals on the pincer complex (β and γ in Fig. 2b) shift from 7.85 and 7.41 ppm, to 7.79 and 7.48 ppm, respectively. These signals are also broadened and lower in intensities. These diagnostic changes indicate metal coordination between co-PIC and Pd^II pincer complex.

UV-Vis spectroscopy is another powerful tool to track metal coordination owing to the signature metal–ligand charge transfer (MLCT) band of Pd^II pincer.⁴⁶ UV-Vis traces of the samples in solution are displayed in Fig. 3, where the MLCT band of Pd-Pin is located at 335 nm. The trace of the physical mixture is approximately the sum of two species and the MLCT band remains at 335 nm indicating no interaction between the two components. After AgBF₄ treatment, the band blueshifts to 310 nm, suggesting metal coordination. To prove that this blueshift is induced solely by the coordination of Pd^II pincer with the pyridyl end-groups, we perform control experiments of Pd-Pin pyridine and AgBF₄ (Fig. S14†). While there is no MLCT band change in the physical mixture of Pd-Pin and pyridine, mixing Pd-Pin and AgBF₄ results in a blueshift of the band to 325 nm. The blueshift to 310 nm is only observed in the assembly of Pd-Pin and pyridine in the presence of AgBF₄. Hence, the blueshift of MLCT band on UV-Vis spectroscopy suggests the metal coordination between pincer ligands and pyridine, thereby proving the assembly of co-PIC with Pd-Pin.

Fig. 3 Spectroscopic analysis of co-PIC assembly with Pd-Pin. (top) UV-Vis traces of (green) Pd-Pin, (red) co-PIC, (orange) physical mixture, and (blue) metal coordination after AgBF₄ addition, recorded in DCE at 25 °C with [co-PIC] = 7.5 μM. (bottom) CD traces of (red) co-PIC and (blue) metal coordination after AgBF₄ addition, recorded in DCE at 25 °C with [co-PIC] = 7.5 μM.

We then investigate the helicity of co-PIC using circular dichroism (CD) spectroscopy. The CD spectra are recorded in 1,2-dichloroethane (DCE) at 7.5 μM with respect to the helical polymer. Co-PIC displays a broad band at 360 nm correlating to the n–π* transition of the C=N backbone, as well as a sharper peak at 250 nm corresponding to the π–π* transitions of the aryl rings.³⁴ Consistent with other examples,^35,37 the signal of co-PIC has an identical pattern but lower intensity compared to the PIC of menthyl-ester aryl isocyanides under the same concentrations. As shown in Fig. 3b, the CD traces before and after metal coordination are identical confirming that the side-chain assembly of co-PIC and Pd-Pin does not affect the helicity of the backbone.

Assembly of supramolecular brush copolymer with telechelic poly(styrene) side-chains

Knowing that co-PIC is capable of side-chain metal coordination, we explore the formation of supramolecular brush copolymers via self-assembly by using a coil-like poly(styrene) functionalized with Pd^II pincer (Pd-Pin-PS). Pd-Pin-PS is prepared by atom-transfer radical polymerization (ATRP) with a pincer-functionalized initiator according to published procedures.⁴⁷ SEC analysis shows a dispersity of 1.23 with M_n of 4000 (Fig. S11†). We perform the assembly by mixing co-PIC and Pd-Pin-PS so that the molar ratio of pyridine and Pd^II pincer moieties is 1 : 1. The addition of excess AgBF₄ triggers metal coordination, resulting in the helical brush copolymer, co-PIC-brush-Pd-Pin-PS.

The metallosupramolecular assembly to form the brush copolymer is examined using ¹H NMR and UV-Vis spectroscopies. Fig. 4a shows the ¹H NMR spectra of a 1 : 10 physical mixture of co-PIC and Pd-Pin-PS (1 : 1 pyridine/Pd^II pincer) and the 1 : 10 mixture after the addition of AgBF₄. Consistent with observations made for the small molecule analogs, the α-pyridyl signal, originally appearing at δ 8.30–8.60 ppm, broadens and shifts to δ 8.38–8.82 ppm. The aromatic thioether proton signals on Pd-Pin-PS, broaden and shift from δ 7.84 and 7.43 ppm, to δ 7.78 and 7.47 ppm, respectively. The UV-Vis spectra of the physical mixture and assembled co-PIC-brush-Pd-Pin-PS are shown in Fig. 4b. We observe the blueshift of the MLCT band of Pd-Pin-PS from 335 nm to 310 nm. These spectroscopic results provide evidence for metal coordination between the helical backbone and PS side-chains.

Fig. 4 (a) ¹H NMR spectroscopic overlay of (top) physical mixture of co-PIC and Pd-Pin-PS, and (bottom) assembled co-PIC-brush-Pd-Pin-PS after AgBF₄ addition. (b) UV-Vis traces of (purple) Pd-Pin-PS, (brown) physical mixture of co-PIC and Pd-Pin-PS, and (black) assembled co-PIC-brush-Pd-Pin-PS. (DCE, 25 °C).

We use CD spectroscopy again to investigate the effect of brush copolymer formation on the helical conformation. As shown in Fig. 5, co-PIC-brush-Pd-Pin-PS displays signature Cotton effects of helical PIC at 360 and 250 nm, although there is an intensity decrease of the peak at 250 nm. Kanbayashi et al. reported that the electrostatic repulsion and bulky substituents on the side-chain can cause partial collapse of the helical structure.³⁸ In our case, we eliminate the contribution of electrostatic repulsion, owing to the fact that the assembly with small-molecule analogs does not change the helical conformation. Thus, we hypothesize that the decrease in CD intensity is due to the presence of the bulky polymer side-chain. The helical backbone has to partially extend in order to accommodate the incoming polymeric brushes, resulting in decreased signal intensity at 250 nm. Despite this, the assembled structure still displays strong Cotton effects, suggesting that the PIC backbone maintains its helical conformation. We are actively investigating whether the steric effect can be eliminated by introducing longer spacers between the pyridine functionalities and the aryl groups.

Fig. 5 CD spectrum of co-PIC-brush-Pd-Pin-PS. (DCE, 25 °C).

Unlike a physical mixture of individual polymer blocks, the brush copolymer assembled species should possess higher molecular weights, thereby inducing unique solution properties such as increased viscosity. The assembly via metal coordination can be characterized by comparing the solution viscosities before and after triggering the pincer complexation.⁴⁸ Viscosities of individual polymer blocks and the assembled brush copolymer are obtained in chloroform at room temperature with a Cannon-Ubbelohde viscometer. The specific viscosity (η_sp) of co-PIC-brush-Pd-Pin-PS increases in comparison to the individual blocks (Fig. 6, top). Compared to the viscosity measurements for the linear block copolymer assemblies, we observe larger specific viscosity changes.^16,17 This correlates to a significant increase in molecular weight, suggesting the presence of the supramolecular brush copolymer.

Fig. 6 Characterizing the brush copolymer assembly by measuring (top) specific viscosity (η_sp) in CHCl₃ and (bottom) hydrodynamic radii (R_h) in DMSO; (red) co-PIC, (purple) Pd-Pin-PS, and (black) supramolecular brush assembly.

The bulk structure of the supramolecular assembly can be further analyzed by tracking the hydrodynamic radii (R_h) changes through dynamic light scattering (DLS). DLS data is obtained from an LS spectrometer with CONTIN fitting algorithm.^49,50 The bottom of Fig. 6 depicts the size distributions of homopolymers and assembled brush structures. The mean R_h of Pd-Pin-PS is 2 nm, which is smaller than the results from PS standard samples at similar molecular weight (Fig. S17†). The solvent and the presence of telechelic pincer complex could have caused this minor discrepancy. The co-PIC displays a slightly larger R_h (2.2 nm) due to its higher molecular weight yet more compact conformation. The assembled brush copolymer records a dramatic increase in R_h (∼60 nm) with a positively skewed distribution. Similar size increases have been reported for covalent grafting-onto and grafting-from linear brush copolymers.^51,52 Owing to the high local concentration of tethered chains, the grafted chains are forced to extend away around the polymer backbone, resulting in larger R_h and wider size distributions.

The significant increase in hydrodynamic volume of the brush copolymers suggests the presence of microphase separation between the polymer blocks.^53–55 This leads us to investigate the solid state properties of the metallosupramolecular brush assembly. The behavior is visualized by atomic force microscopy (AFM), where a lamellar structure with alternating bright and dark bands is observed in the brush copolymer assembly (Fig. S20†). This pattern is not captured in the AFM images of individual polymer blocks (Fig. S21 and 22†) and is most likely induced by the supramolecular assembly. Thus, in addition to ¹H NMR and UV-Vis spectroscopies, viscometry, DLS and AFM measurements support the formation of a supramolecular brush copolymer.

We further evaluate the solid-state assemblies using wide-angle X-ray scattering (WAXS). Co-PIC, Pd-Pin-PS and brush copolymer samples are mounted on silicon wafer as thin films for measurement. As shown in Fig. 7, the patterns of the homoblocks display broad bands, indicative of amorphous film formation. A bright peak appears in the wide-angle region of the brush copolymer assembly. The Bragg diffraction at 2θ = 38.2° correlates well to the palladium-ligand bond in the pincer complex (2.36 Å).^45,56 The phase segregation between the backbone and side-chains can induce the transformation from amorphous, disordered homoblocks to the semi-crystalline morphology.⁵⁷ Therefore, the solid state studies strongly suggest the formation of an ordered arrangement of the metallosupramolecular brush species.

Fig. 7 Two-dimensional diffraction patterns of (left to right) co-PIC, Pd-Pin-PS, and brush assembly. The bright peak at wide-angle area in the brush assembly suggests the formation of long-range ordered structures.

Conclusions

In summary, we report a methodology for the synthesis of side-chain functionalized helical polymers. A pyridinyl isocyanide is copolymerized with a chiral menthyl-ester isocyanide to form one-handed preferred helical random copolymers containing pyridine groups in the side-chains. The pyridinyl functional groups can participate in metal coordination with Pd^II-pincer complexes, as evidenced by ¹H NMR and UV-Vis spectroscopies. Using Pd^II-pincer functionalized telechelic poly(styrene), our methodology allows for the assembly of a supramolecular brush copolymer with a helical backbone. ¹H NMR and UV-Vis spectroscopies confirm the metal coordination. Viscometry, DLS, and solid-state characterization techniques verify the formation of a large polymeric ensemble in solution, with the presence of microphase separation, proving the successful preparation of a supramolecular brush copolymer. Our methodology leaps beyond main-chain supramolecular assembly, enabling the design of complex 3D materials. We envision this strategy as an expansion to the platform of building diverse architectures and fabricating functional materials.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors acknowledge financial support from the National Science Foundation under award number CHE-1902917. We thank Dr Joel Tang for the quantitative ¹³C NMR spectroscopy analysis. We thank Hengyu Zhou and Professor Michael D. Ward for help in obtaining the AFM images and Dr Chunhua Hu for his contribution to the X-ray measurements. The authors acknowledge the use of shared facilities provided through the Materials Research Science and Engineering Center (MRSEC) program of the National Science Foundation under Award Number DMR-1420073.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d1py00373a

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