Molecular Bottlebrushes Featuring Brush-on-Brush Architecture | ACS

Letter Cite This: ACS Macro Lett. 2019, 8, 749−753

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Molecular Bottlebrushes Featuring Brush-on-Brush Architecture Yi Chen, Ziyang Sun, Huaan Li, Yunkai Dai, Zhitao Hu, Huahua Huang, Yi Shi, Yuanchao Li,* and Yongming Chen* School of Materials Science and Engineering, Key Laboratory for Polymeric Composite and Functional Materials of Ministry of Education, Research Center for Functional Biomaterials Engineering and Technology Guangdong, Sun Yat-Sen University, Guangzhou 510275, China

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S Supporting Information *

ABSTRACT: Molecular bottlebrushes featuring brush-onbrush (BoB) architecture were prepared by combining azide− alkyne click chemistry, ring-opening polymerization (ROP), and atom transfer radical polymerization (ATRP). Primary side chains of diblock copolymers with a poly(ε-caprolactone) (PCL) block and a poly(α-bromo-ε-caprolactone) (P(CLBr)) block were synthesized by ROP and then grafted onto PCL backbone by the click reaction. Then the secondary side chains of poly(oligo(ethylene glycol) acrylate) (POEGA) were grafted from the P(CL-Br) block by ATRP, yielding an amphiphilic core/shell structure. Imaging of individual macromolecules by atomic force microscopy (AFM) demonstrated dramatically thickened wormlike formation with distinct hairy side chains. Interestingly, for the BoB molecular bottlebrushes with enough long primary and secondary side chains, sufficient tension can be generated along the backbone and thus lead to its cleavage.

M

reaction, ring-opening polymerization (ROP), and atom transfer radical polymerization (ATRP). Most synthetic molecular bottlebrushes are of C−C backbones, the intrinsic grafting density is usually reduced when other chemical bonds or groups are introduced into the backbones, which may result in loosely grafted bottlebrushes and therefore affect their conformation and morphology. However, BoB molecular bottlebrush features bulky bottlebrush side chains that allow stretch of the backbone even at a low grafting density and may offer more control over its chemistry and dimension. Unlike the molecular assemblies with BoB architectures via interpolyelectrolyte complexation reported by Müllner and co-workers,35 the BoB bottlebrushes presented in this work are individual macromolecules with well-defined structures via covalent bonding. The skeleton of the BoB molecular bottlebrushes based on poly(ε-caprolactone) (PCL) was designed for two reasons. On one hand, PCL has received a great deal of attention for its use as biocompatible and biodegradable materials.36 On the other hand, PCL has seven backbone bonds per repeating unit, leading to a lower grafting density of side chains than poly(meth)acrylates with one side chain per two backbone bonds, which allows more space for the growth of secondary side chains. The synthetic route, shown in Scheme 1, was used for the preparation of PCLn-g(PCLx-b-(P(CL-Br)y-g-POEGAm)) bottlebrushes with BoB architecture, where n, x, y, and m denote the DPs of PCL backbone, PCL block of the primary side chain, poly(α-bromo-

olecular bottlebrushes are a special class of graft copolymers with side chains covalently attached to a linear polymer backbone at a high grafting density.1−3 Due to steric repulsion between densely grafted side chains, the backbone is extended and therefore molecular bottlebrushes exhibit a wormlike morphology.4 The length and width of a molecular bottlebrush can be tuned by controlling the degree of polymerization (DP) of its backbone and side chains, as well as the grafting density. This unusual and tunable architecture gives molecular bottlebrushes a number of unique and interesting properties (e.g., low entanglement, dense functionality, and ability to self-assemble into highly ordered nanostructures), which allow them to demonstrate many potential applications, such as supersoft elastomers,5−7 drug delivery,8−10 photonic crystals,11,12 lithographic patterning,13−16 molecular tensile machines,17−19 porous materials,20−22 and so on. Synthetically, a variety of approaches has been adopted to prepare molecular bottlebrushes, including “grafting through” (polymerizing macromonomers), “grafting onto” (attaching side chains to the backbone), and “grafting from” (growing side chains from the backbone).1,23 Moreover, molecular bottlebrushes with various complex structures have been synthesized by one or more of the above-mentioned approaches, such as molecular stars with bottlebrush arms,24,25 molecular bottlebrushes with multiple side chains per repeating backbone unit,15,26,27 molecular bottlebrushes with dendritic28,29 or cyclic30,31 side chains, and molecular bottlebrushes with linear polymer tails.32−34 In this communication, we demonstrate the synthesis and characterization of molecular bottlebrushes featuring brush-onbrush (BoB) architecture by combining azide−alkyne click © XXXX American Chemical Society

Received: May 26, 2019 Accepted: June 4, 2019

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DOI: 10.1021/acsmacrolett.9b00399 ACS Macro Lett. 2019, 8, 749−753

Letter

ACS Macro Letters Scheme 1. Synthesis of Molecular Bottlebrushes with Brush-on-Brush Architecture

Table 1. Characterization of PCLn-g-(PCLx-b-(P(CL-Br)y-g-POEGAm)) BoB Molecular Bottlebrushes BoB-1 BoB-2 BoB-3 BoB-4 BoB-5

na

xa

ya

mb

grafting efficiencyc (%)

Mnd (×106 g/mol)

Đe

Lf (nm)

Wg (nm)

160 160 160 160 64

16 16 16 16 18

5 5 5 14 9

16 31 66 9 22

98 98 98 89 99

4.20 8.24 16.34 12.48 1.68

1.23 1.33 1.42 1.30 1.28

122 128 124 84 71

48 61 86 59 64

a Degree of polymerization determined by NMR (Figure S1 in Supporting Information). bDegree of polymerization of POEGA determined by gravimetry. cPercentage of PCLx-b-P(CL-Br)y conversion of grafting. dNumber-average molecular weight determined by SEC with multiangle light scattering detector using dn/dc = 0.0418 mL·g−1 for POEGA in dimethylformamide (DMF) reported elsewhere.37 eDispersity (Mw/Mn) determined by SEC. fNumber-average length (L) of molecular bottlebrushes measured by AFM for an ensemble of ∼100 individual molecules. g Width (W) of molecular bottlebrushes measured by AFM.

ε-caprolactone) (P(CL-Br)) block of the primary side chain, and the secondary side chain of poly(oligo(ethylene glycol) acrylate) (POEGA), respectively. These BoB bottlebrushes with an amphiphilic core/shell structure may be used as unimolecular micelles for encapsulation of small molecules. In addition, this type of BoB bottlebrushes with backbones containing chemical bonds (e.g., ester bond) other than C−C bond may have potential applications as molecular tensile machines for varied mechanochemistries. The backbone was synthesized via ROP of α-bromo-εcaprolactone (CL-Br, which was synthesized following a previously published procedure38) using benzyl alcohol (BnOH) as the initiator and stannous octoate (Sn(Oct)2) as the catalyst. The polymerization was carried out at 110 °C. By

changing the ratio of monomer to initiator, two backbones with varied DPs (n) of 64 and 160, as determined by NMR, were prepared. The P(CL-Br)n backbones with pendant bromines were then reacted with sodium azide to yield P(CL-N3)n backbones. The alkyne-terminated side chains with a PCL block and a P(CL-Br) block were synthesized by sequential ROP of CL and CL-Br using propargyl alcohol as the initiator. Three different primary side chains were prepared with varied (x, y) values of (16, 5), (16, 14), and (18, 9), respectively. The dispersities (Đ) of all backbones and primary side chains are below 1.15, indicating good control of the ROP. The primary side chains were then grafted onto the backbone by Cu-catalyzed azide−alkyne cycloaddition (CuAAC) coupling reaction at feed ratio of 1:1.05 (azide: 750


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ACS Macro Letters alkyne), yielding PCLn-g-(PCLx-b-P(CL-Br)y) molecular bottlebrushes. The SEC (size-exclusion chromatography) trace after the click reaction shifted toward the high molecular weight region, and the peak became slightly broader, while the peak from the P(CL-N3)160 backbone mostly disappeared (Figure S4 in Supporting Information), indicating that the primary side chains (PCL16-b-P(CL-Br)5) were successfully grafted onto the backbone with a pretty high conversion. The grafting efficiency for shorter primary side chains (PCL16-b(P(CL-Br)5) or shorter backbones (P(CL-N3)64) was close to 100%, which is higher than that for longer primary side chains (PCL16-b-(P(CL-Br)14) with longer backbones (P(CL-N3)160), 89%. This is because longer chains have a larger coil size in solution and therefore it is more difficult for them to react with other polymer chains. Due to the low grafting density (about one side chain per seven backbone bonds), these molecular bottlebrushes exhibit thin and flexible wormlike structures with an average width of about 10 nm, as revealed by atomic force microscopy (AFM; Figure S4, inset). The secondary side chains of POEGA were grown from the P(CL-Br) block of the primary side chains by ATRP. The polymerization was conducted at 60 °C and stopped at low monomer conversions, and controlled molecular weights and low dispersities were observed. Five molecular bottlebrushes with BoB architecture were prepared, as summarized in Table 1. As an example shown in Figure 1, after growing POEGA

Figure 2. AFM height micrographs of the BoB molecular bottlebrushes spin-cast from dilute dichloromethane solutions on freshly cleaved mica substrates. The scale bar is 100 nm.

about the same (∼125 nm), which is in good agreement with the contour length of the PCL backbone with a DP of 160. It is noticed that BoB-3 has a halo of diffuse side chains, which is similar to that observed for molecular bottlebrushes with bimodal length distribution of side chains.39 For BoB-4, the DP of P(CL-Br) block of the primary side chains is higher than that for BoB-1, -2, and -3, and its width (∼59 nm) is as high as that of BoB-2 (∼61 nm), even though its DP of POEGA secondary side chains is lower. Particularly, BoB-5 exhibits starlike morphology because the DP of its backbone is so small that the length of the bottlebrush is close to its width. These demonstrate that the length and width of BoB molecular bottlebrushes can be tuned by controlling the DPs of the primary and secondary side chains as well as the backbone. To correlate the width of the BoB bottlebrush with its molecular parameters (e.g., x, y, and m), we assume that a CL monomeric unit is approximately equivalent of 3.5 acrylate monomeric units in terms of contour length and take into consideration the end-caps formed by POEGA secondary side chains, then the width (W) can be estimated as

Figure 1. SEC traces in DMF eluent: red line, PCL160-g-(PCL16-bP(CL-Br) 5 ); green line, PCL 160 -g-(PCL 16 -b-(P(CL-Br) 5 -gPOEGA 16 )); pink line, PCL 1 60 -g-(PCL 16 -b-(P(CL-Br) 5 -gPOEGA 31 )); purple line, PCL 160 -g-(PCL 16 -b-(P(CL-Br) 5 -gPOEGA66)).

W /2 ≅ l0 × N where l0 ≅ 0.25 nm is the acrylic monomer contour length, N = 3.5(x + y) + m. To verify our estimation, we plot the measured width by AFM as a function of N. As shown in Figure 3, the linear fit gives a slope of 0.36 nm that is substantially larger than l0, which may be attributed to the facts that CL monomeric units contain ester bonds with different

from PCL160-g-(PCL16-b-P(CL-Br)5), the SEC traces shifted toward the high molecular weight region, suggesting successful grafting of the secondary side chains from the primary side chains. AFM was also employed to explore the structure of these BoB molecular bottlebrushes spin-cast from dilute dichloromethane solutions on freshly cleaved mica substrates. As shown in Figure 2, BoB-1, -2, and -3 exhibit thick and extended wormlike structures with hairy side chains, as revealed by AFM imaging of individual macromolecules, in contrast to their precursor PCL160-g-(PCL16-b-P(CL-Br)5) that exhibits a thin and flexible wormlike structure (Figure S4, inset), indicating successful grafting of the POEGA secondary side chains. The width of BoB-1, -2, and -3 increases from 48 to 86 nm as the DP of POEGA side chains increases from 16 to 66, which is significantly larger than that of their precursor (∼10 nm) with coiled side chains due to low grafting density. This suggests that the grafting of secondary side chains can extend the primary side chains and thus lead to thickening of the BoB bottlebrushes. The average lengths of BoB-1, -2, and -3 are

Figure 3. Half width of the BoB bottlebrushes as a function of N. The solid line represents a linear fit to the data points, yielding a slope of 0.36 with adjusted R-square of 0.90. 751


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ACS Macro Letters

been confirmed by SEC and AFM. Although the grafting density of the primary side chains is low (about one side chain per seven backbone bonds), the growth of the secondary side chains allows the extension of the main backbone, and the resulting molecular bottlebrushes exhibit thick and hairy wormlike structures, as revealed by AFM imaging. Surprisingly, sufficient tension could be generated along the backbone if the BoB molecular bottlebrushes have enough long primary and secondary side chains (e.g., BoB-4) and, therefore, lead to scission of the backbone. This implies that BoB bottlebrushes may have potential applications as molecular tensile machines and offer more options for varied mechanochemistries.

bond lengths and angles from C−C bonds and that among the dispersed side chains longer ones prefer to adsorb on the substrate.39 Yet, interestingly, the average length (∼84 nm) of BoB-4 is substantially smaller than that of BoB-1, -2, and -3, which were synthesized from the same backbone. This could be due to mechanochemically induced scission of the backbone. The growth of POEGA secondary side chains results in the increase in steric repulsion between the side chains consisting of a linear block and a bottlebrush block as a whole and, thus, extension of the backbone, which also generates tension along the backbone and mechanically activates its chemical bonds.17,40,41 Therefore, the ester groups within the backbone could become more reactive and vulnerable to hydrolysis under tension in an ambient (humid) environment, which resulted in the scission of the BoB molecular bottlebrushes, as shown in Figure 4.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.9b00399. Materials, characterization methods, synthesis, and supporting figures (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Yi Shi: 0000-0003-2943-5465 Yuanchao Li: 0000-0002-8005-947X Yongming Chen: 0000-0003-2843-5543

Figure 4. (Left) AFM height micrograph of BoB-4 molecular bottlebrushes spin-cast on freshly cleaved mica substrate from dilute dichloromethane solution. The red arrows indicate possible scission points where the molecular bottlebrushes broke apart. (Right) Length distributions of BoB-3 and BoB-4 spin-cast on freshly cleaved mica substrates for an ensemble of ∼100 individual molecules, respectively.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the financial support from the Natural Science Foundation of China (Nos. 51533009 and 21805317), the Guangdong Innovative and Entrepreneurial Research Team Program (No. 2013S086), and the Natural Science Foundation of Guangdong Province (No. 2014A030312018).

Because of the interaction between the adsorbed side chains and the substrate, some of the cleaved BoB-4 molecular bottlebrushes did not have enough time to slide away from each other thoroughly upon cleavage, as indicated by the red arrows. However, we did not observe scission of other BoB molecular bottlebrushes, even including BoB-3 that has the highest DP of POEGA side chains and the largest brush width. This is probably because the bottlebrush block of the primary side chain of BoB-3 is too short to generate along the backbone a large enough tension that is required to break the backbone bonds mechanically or mechanochemically. We also tested whether the BoB-4 molecular bottlebrushes could be cleaved on the water surface using a Langmuir−Blodgett (LB) trough. After adsorption on the water surface for 24 h, its average contour length was 130 nm and did not decrease compared to that for 2 min (Figure S7 in Supporting Information), suggesting that backbone scission did not occur. This is probably because scission of molecular bottlebrush backbones is highly sensitive to substrate surface energy.42 The surface energy of water is ∼72 mN/m at room temperature, which is smaller than that of a freshly cleaved mica substrate (130−170 mN/m),43 and thus, the selfgenerated backbone tension was insufficient to cause scission on the experimental time scale. In conclusion, we have prepared molecular bottlebrushes featuring a BoB architecture with a core−shell structure by combining azide−alkyne click reaction, ROP, and ATRP. Successful synthesis of these bottlebrush macromolecules has

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