Cylindrical Polymer Brushes by Atom Transfer Radical Polymerization

Dec 4, 2012 - The program ACD NMR version 10.02 was used for data processing ..... Supramolecular, and Nanochemistry (MSN) Program in the Division of ...
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Cylindrical Polymer Brushes by Atom Transfer Radical Polymerization from Cyclodextrin−PEG Polyrotaxanes: Synthesis and Mechanical Stability Christian Teuchert,† Christoph Michel,† Florian Hausen,‡ Doh-Yeon Park,§ Haskell W. Beckham,§ and Gerhard Wenz*,† †

Saarland University, Campus C4.2, D-66123 Saarbrücken, Germany INM − Leibniz-Institute for New Materials, Campus D2 2, 66123 Saarbrücken, Germany § School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, United States ‡

S Supporting Information *

ABSTRACT: α-Cyclodextrin (αCD) was threaded onto 10 kDa poly(ethylene glycol) (PEG), which was then stoppered with bulky end groups (4-methoxynaphthyl or 9-anthracenylmethyl) to give polyrotaxanes containing about 38 αCD rings threaded onto a PEG backbone. The polyrotaxanes were converted into soluble macroinitiators for atom transfer radical polymerization (ATRP) by esterifying the hydroxyl groups of the threaded αCDs with 2-bromoisobutyryl bromide to a degree of substitution (DS) of 8 per αCD. Living ATRP of methyl methacrylate (MMA) from these polyrotaxane macroinitiators led to polymer brushes with molecular weights of up to 1.7 MDa. Polymer brushes were observed by atomic force microscopy. Surprisingly, large amounts of unthreaded αCD star polymer were observed by GPC. The appearance of these unthreaded αCD star polymers was attributed to the shear-induced rupture of the PEG backbone during passage of the brush through the GPC column. Backbone rupture also occurred upon heating the brushes to elevated temperatures. Proof of the bottle-brush structure was further provided without backbone rupture using diffusion ordered NMR spectroscopy.

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INTRODUCTION Polymer chains attached by one end to a scaffold are called a “brush” if packing density is sufficiently high to prevent the attached polymers from forming regular random coils.1 In other words, if the mean distance between two chains is less than their radii of gyration, the chains stretch perpendicular to the scaffold to avoid overlap.2 Because of the dense packing of polymer chains in a brush, the properties of brush polymers are different from relaxed bulk polymers. For instance, protein adsorption is reduced for a polymer brush, which can adapt much less to the geometry of a protein than a regular polymer.3 If the scaffold is a flexible polymer, dense grafting leads also to an extended conformation of the backbone accompanied by a significant increase of its persistence length.4 Depending on the shape of the scaffold, cylindrical (1D), planar (2D), or spherical (3D) brushes can be synthesized by grafting polymer chains to the scaffold (grafting-to approach) or by starting a polymerization from the scaffold (grafting-from approach).5 In general, the grafting-from approach is advantageous because it leads to a higher grafting density; the graftingto approach can suffer from steric hindrance during the coupling step that can limit grafting density. While other polymerizations (e.g., ring-opening of lactones) have been employed, living atom transfer radical polymerization (ATRP) of methacrylates has been especially successful for the synthesis © XXXX American Chemical Society

of polymer brushes because of the very high control of the chain length.6 Both homopolymer and block copolymer brushes7−9 have been synthesized by ATRP. Furthermore, star polymers were already synthesized by ATRP from cyclodextrin 2-bromoisobutyrates.10−12 In this paper, we describe the synthesis of cylindrical polymer brushes, so-called “bottle brushes”, by ATRP of methyl methacrylate (MMA) from an α-cyclodextrin polyrotaxane (αCD-PRx) scaffold. αCD-PRx is obtainable by terminal attachment of stopper groups at poly(ethylene glycol) (PEG) being complexed by αCD.13,14 The star-shaped geometry of the poly(methyl methacrylate) (PMMA) chains attached to threaded cyclodextrin rings, which can freely rotate on PEG backbones, should facilitate the formation of cylindrical superstructures. Cylindrical brushes are a readily available alternative to dendronized polymers.15 These materials are of special interest because they exhibit exceptionally high chain stiffness16 with persistence lengths exceeding 100 nm,17 which allow the assembly of highly ordered films under flow.18 Received: October 23, 2012 Revised: November 22, 2012

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12 mmol) was used as the stoppering agent. Product was isolated as a brown powder (0.72 g, 36% yield based on PEG). 1H NMR (DMSOd6): δ 3.28−3.45 (m, αCD), 3.50 (s, 905 H), 3.57−3.77 (m, αCD), 4.44 (bs, 234 H, OH-6 of αCD), 4.78 (bs, 230 H, H-1 of αCD), 5.50 (bs, 245 H, OH-3 of αCD), 5.66 (bs, 235 H, OH-3 of αCD), 7.45− 7.60 (m, 8 H, anthracenyl), 8.05−8.15 (m, 4 H, anthracenyl), 8.41− 8.49 (m, 4H, anthracenyl), 8.57 (bs, 2H, anthracenyl). Syntheses of the Macroinitiators. The polyrotaxane 1a (0.5 g, 0.7 mmol OH groups of αCD) was dissolved in a solution of LiCl (1.7 g, 40.1 mmol) in anhydrous DMAA (20 mL); pyridine (9 mL, 112 mmol) and 4-(dimethylamino)pyridine (10 mg, 0.08 mmol) were added at 0 °C under N2. After bromoisobutyryl bromide (4.83 g, 21 mmol) was added dropwise, the reaction mixture was stirred for 16 h at 25 °C. The product was precipitated with Et2O and isolated by filtration. It was dissolved again in acetone and precipitated with methanol three times. Finally, the macroinitiator 2a with degree of substitution (DS) of 8 per αCD (0.48 g, 49% based on 1a) was obtained as a brownish powder after drying in vacuum. 1H NMR (DMSO-d6): δ = 1.75−1.90 (bm, 48 H, (CH3)2C−Br), 3.50 (s, O− CH2−CH2−), 3.61−4.45 (m, 41 H, αCD), 5.09 (bs, 6 H, H-1 of αCD), 6.44 (m, naphthyl), 7.15 (m, naphthyl), 7.15 (m, naphthyl) ppm, Mw = 125 kDa. Macroinitiator 2b was synthesized accordingly from polyrotaxane 1b. Syntheses of the Polyrotaxane Brushes by ATRP. A Schlenk flask equipped with a stopcock was loaded with CuBr (11.5 mg, 0.08 mmol), CuBr2 (17.9 mg, 0.08 mmol), 1,1,4,7,10,10-hexamethyltriethylenetetramine, HMTETA (36.9 mg, 0.16 mmol), macroinitiator 2a (50 mg, 0.16 mmol initiator groups), and xylene (850 mg, 8.0 mmol, internal standard), dissolved in DMSO (5 mL) and degassed by repetitive freezing under Ar and thawing under vacuum. Degassed MMA (0.8 g, 8.0 mmol) was added and stirred at 25 °C under Ar. The ATRP was stopped by exposure of the reaction mixture to air after the known half-life of the reaction (3 h). After excess MMA was removed under vacuum, DMSO was removed by lyophilization. The green residue was dissolved in CH2Cl2 (10 mL) three times, extracted with aqueous ethylenediaminetetraacetic acid, EDTA (15 mL of a 5 wt % solution in water, 2.6 mmol), concentrated, and dried in vacuum for 24 h to provide the brush polymer 3a as a white powder (0.42 g, 77%). 1 H NMR (CDCl3): δ = 0.78 (bs, 2 H, CH3C− of PMMA rr), 0.95 (bs, 1 H, CH3C− of PMMA mr), 1.6−2.1 (2 s, 2H, −CH2C− of PMMA mmm), 3.61 (bs, 3 H, −OCHH3 of PMMA), 3.65 (s, 0.135 H, −CH2−CH2−O− of PEG), 4.25 (bm, αCD), 4.90 (bm, αCD), 5.15 (bm, αCD) ppm; Mw shown in Table 1. Synthesis of the αCD Initiator. αCD (1.0 g, 1.03 mmol) was dissolved in anhydrous DMAA (30 mL); pyridine (7 mL, 87 mmol) and DMAP (10 mg, 0.08 mmol) were added at 0 °C under N2. After bromoisobutyryl bromide (4.25 g, 18.5 mmol) was added dropwise, the reaction mixture was stirred for 16 h at 25 °C. The product was

EXPERIMENTAL DETAILS

Materials and Methods. Poly(ethylene glycol) (PEG) (10 kDa, Fluka) was tosylated at its chain ends to give PEG-bis-tosylate according to the method reported by Zhao et al.14 Tetrahydrofuran (THF) for GPC was HPLC grade. Pyridine was freshly distilled from KOH. Methyl methacrylate (Fluka) was filtered over Al2O3 and distilled from CaH2 under reduced pressure. N,N′-Dimethylacetamide (DMAA) was dried over molecular sieves (4 Å). αCD was obtained from Wacker (Munich, Germany). All other reactants and solvents were purchased from commercial suppliers and used without further purification. 1 H and 13C NMR spectra were recorded on a Bruker AVANCE 500 (1H: 500.27 MHz; 13C: 125.79 MHz) or a Bruker Magnet System 400 MHz Ultra shield plus (1H: 400.00 MHz) using CDCl3 or DMSO-d6 at 25 °C. The solvent signals were used as internal standards. The program ACD NMR version 10.02 was used for data processing and deconvolution of the spectra. The following abbreviations were used for multiplicities: s for singlet, d for doublet, dd for double doublet, t for triplet. Overlapping signals and multiplets were labeled with m, broad multiplets with bm, and broad singlets with bs. Diffusion ordered NMR spectroscopy (DOSY) was conducted at 22 °C using 5 mm NMR tubes on a Bruker AMX 400 operating at 400 MHz. Sample concentrations were 0.4% (w/v) in CDCl3. All DOSY spectra were collected using a bipolar pulse pair and longitudinal eddy current delay (BPP-LED) sequence. For the BPP-LED sequence, gradients were applied for 5 ms (δ), and the diffusion time (Δ) was 500 ms. The delay times for gradient recovery and eddy current elimination were 0.2 and 5 ms, respectively. Homospoil gradients were applied for 0.6 ms to remove residual transverse magnetization. The delay between each scan was 10 s, and the number of scans was 16. The gradients were incremented from 0.67 to 32.02 G/cm, which enabled the final intensity of each peak to be less than 3% of its original intensity. Consequently, 32 free induction decays containing 8K data points were collected. The DOSY spectra were constructed based on the assumption that the intensity trends of all chemical shifts over the gradients followed biexponential decays. Gel Permeation Chromatography (GPC). GPC was performed at 25 °C with a standard setup equipped with one column with pore sizes of 105 Å (Polymer Standard Service GmbH, Mainz, Germany), differential refractive index (RI), and multiangle light scattering (LS) detectors. THF was used as the eluent (1 mL/min). The weightaverage molecular weight (Mw) of each polymer peak was calculated from the integrated LS and RI signals according to the Zimm equation by the program WinGPC Unity, Build 5403, using δn/δc = 0.87 for PMMA in THF. Atomic Force Microsocopy (AFM). AFM was performed using an Agilent 5500 AFM in air. A solution of the polyrotaxane brush (100 μg/mL for Figure 6a and 10 μg/mL for Figure 6b) in acetone was cast onto freshly cleaved HOPG (highly ordered pyrolytic graphite) and dried in an argon (Praxair, 5N) flow. Topographic images were recorded in tapping mode using Nanosensor PPP-NCLR cantilevers. Polyrotaxane Synthesis. αCD−PEG Polyrotaxane 1a. Solutions of αCD (55 g, 56.5 mmol) in water (390 mL) and PEG-bis-tosylate (5.0 g, 1.0 mmol tosyl end groups) in water (40 mL) were mixed at 25 °C and stirred for 16 h. The resulting suspension was lyophilized. A part of the white powder (5.0 g) was added in small portions to a solution of 4-methoxynaphthol (2.01 g, 11.5 mmol) and NaH (1.21 g, 30.25 mmol) in anhydrous DMF (20 mL) under N2 and stirred for 16 h. The product was then precipitated in methanol (150 mL), isolated by centrifugation, redissolved in DMSO (25 mL), precipitated in water (150 mL), and isolated by filtration and drying in vacuum (16 h) as a light blue powder (0.85 g, 43% yield based on PEG). 1H NMR (DMSO-d6): δ 3.27−3.45 (m, αCD), 3.51 (s, 905 H), 3.57−3.77 (m, αCD), 3.92 (s, 6 H, CH3-O-naphthyl), 4.41 (bs, 216 H, OH-6 of αCD), 4.79 (bs, 216 H, H-1 of αCD), 5.48 (bs, 216 H, OH-3 of αCD), 5.62 (bs, 216 H, OH-3 of αCD), 6.83 (m, 4 H, naphthyl), 7.53 (m, 4 H, naphthyl), 8.10 (m, 4 H, naphthyl). αCD−PEG Polyrotaxane 1b. 1b was synthesized using the same route described for 1a, except that 9-hydroxymethylanthracene (2.5 g,

Table 1. Characterization of Cylindrical Polymer Brushes by ATRP from αCD−PEG Polyrotaxane Macroinitiatorsa PRTX

N

stopper

DS

Pn

Ptheor n

Mw (MDa)

Mtheor w (MDa)

Xstar (%)

3a 3a′ 3a″ 3b 3b′ 3b″

49 38 28 39 39 39

A A A B B B

8 8 7 7 7 7

44 22 10 50 26 25

50 30 10 48 33 22

1.75 0.76 0.19 1.93 1.62 1.27

2.08 1.00 0.26 1.40 0.99 0.69

53 80 83 77 83 82

a

N = number of threaded rings, stopper: A = 4-methoxynaphthyl and B = 9-anthracenylmethyl, DS = degree of substitution per αCD, Pn = = degree of polymerization of PMMA side chains from 1H NMR, Ptheor n degree of polymerization of PMMA side chains calculated from the conversion and stoichiometry, Mw = molecular weight of polyrotaxane = molecular weight calculated using eq brush by light scattering, Mtheor w 1, Xstar = molar fraction of αCD stars after GPC separation, determined from the RI peak areas of star and brush, respectively. B

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Scheme 1. Syntheses of Cylindrical Polymer Brushes by ATRP from αCD−PEG Polyrotaxane Macroinitiators

precipitated with H2O and isolated by filtration. It was dissolved in acetone and precipitated with H2O again. Finally, the αCD initiator (0.73 g, 50%, DS = 3.4 per αCD) was received as a white powder after drying in vacuum. 1H NMR (DMSO-d6): δ = 1.87−1.95 (bm, 22.6 H, (CH3)2C−Br), 3.35−4.5 (m, αCD), 4.87 (bs, 6 H, H-1 of αCD) ppm. Synthesis of the α-CD Star Polymer by ATRP. A Schlenk flask equipped with a stopcock was loaded with CuBr (72 mg, 0.5 mmol), CuBr2 (110 mg, 0.5 mmol), HMTETA (230.4 mg, 1 mmol), αCD initiator (0.5 mg, 0.35 mmol), and xylene (2.12 g, 20 mmol, internal standard), dissolved in DMSO (10 mL), and degassed by repetitive freezing under Ar and thawing under vacuum. Degassed MMA (13 g, 130 mmol) was added and stirred at 25 °C under Ar. The ATRP was stopped by exposure of the reaction mixture to air after 5 h. After excess MMA was removed under vacuum, DMSO was removed by lyophilization. The green residue was dissolved in CH2Cl2 (30 mL) three times, extracted with aqueous ethylenediaminetetraacetic acid, EDTA (45 mL of a 5 wt % solution in water, 7.7 mmol), concentrated, and dried in vacuum for 24 h to provide the star polymer as a white powder (8.3 g, 74%). 1H NMR (DMSO-d6): δ = 0.75 (bs, 1.85 H, CH3C− of PMMA rr), 0.94 (bs, 1.32 H, CH3C− of PMMA mr), 1.6− 2.1 (2 s, 2.08 H, −CH2C− of PMMA mmm), 3.56 (bs, 3 H, −OCHH3 of PMMA), 4.80 (bm, αCD), 5.59 (bm, αCD) ppm; Mw = 32 kDa (RI detector), 43 kDa (LS detector).

1a and 1b, determined from the integrals of the 1H NMR signals of αCD relative to PEG, were in the range of 28−49 (equivalent to coverages of 25−43% of the polymer), similar to or slightly higher than that reported previously.14 PRxs 1a and 1b were converted to the corresponding macroinitiators 2a and 2b in 51% yield by acylation with 2bromoisobutyryl bromide. A solution of LiCl in N,Ndimethylacetamide (DMAA) had to be used as the reaction medium to ensure complete dissolution of the polyrotaxanes, as already reported by Araki et al. for other acylations.19 After repeated precipitations, macroinitiators 2a and 2b remained without any low molecular weight impurities. The degree of substitution (DS) per αCD was determined to be 7−8 by 1H NMR spectroscopy (see Supporting Information, Figure S2) from the integral of the 2-bromoisobutyrate signal (1.75−1.90 ppm) relative to the αCD H-1 signal (5.08 ppm). Methyl methacrylate (MMA) was polymerized by ATRP from the macroinitiators 2a and 2b activated by CuBr complexed by the ligand 1,1,4,7,10,10-hexamethyltriethylenetetramine, HMTETA (molar ratio 1:1:1, 2-bromoisobutyryl:CuBr:HMTETA), in DMSO at 25 °C. The observed reaction kinetics, shown in Figure 1a, deviated significantly from a first-order rate law. This deviation was attributed to insufficient control of the ATRP, already known for other polymerizations carried out in highly polar solvents like DMSO or water.20 Therefore, CuBr2 was added to increase control, as already proposed by Matyjaszewski et al.21 Indeed, nearly perfect first-order kinetics, shown in Figure 1b, was observed for a molar composition of 1:0.5:0.5:1 (2-bromoisobutyryl:CuBr:CuBr2:HMTETA) indicative of a living polymerization. Consequently, these reaction conditions were consistently employed. Polymerizations were stopped at around 50% conversion to prevent any cross-linking. The degree of polymerization (Pn) of the PMMA side chains was determined by 1H NMR spectroscopy (see Supporting Information, Figure S3) from the ratio of the integrals of the PMMA methoxy group signals relative to the PEG signal. The obtained values for Pn (cf. Table 1) were in accordance with the theoretical ones calculated from the conversion of MMA and the ratio of concentrations of MMA over initiator. The molecular weight distributions of the PRx brushes were determined by gel permeation chromatography coupled with

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RESULTS AND DISCUSSION The synthesis of a polyrotaxane brush 3 was achieved in the three steps depicted in Scheme 1: (1) synthesis of the αCD− PEG polyrotaxane 1, (2) conversion of the polyrotaxane into the macroinitiator 2, and (3) ATRP of MMA from macroinitiator 2. The αCD−PEG polyrotaxanes (PRxs) were synthesized from PEG ditosylate based on a previously published procedure in which 3,5-dimethylphenol was used to displace the tosyl groups and block the chain ends.14 Here, two larger stoppering agents, 4-methoxynaphthol (A) and 9-hydroxymethylanthracene (B), were used in an attempt to avoid dethreading of the αCD rings, which are forced against the stoppers by entropic pressure of the side chains of the brush. PRxs 1a and 1b were obtained in 43% and 40% yields, respectively, after 3-fold reprecipitation to completely remove unbound αCD. The 1H NMR of the αCD−PEG PRxs in DMSO-d6 (see Supporting Information, Figure S1) only showed the broad signals of αCD typical for the threaded state but not the narrow signals known for free αCD. The number of threaded αCD rings (N) in PRxs C

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The second peak was assigned to the αCD−PMMA star 4 by comparing the elution volume with that of the αCD−PMMA star synthesized separately. Since MMA does not show any selfpolymerization at 25 °C, free PMMA could not be the origin of this second peak. The molar fraction of the second peak Xstar (listed in Table 1) was often more than 50% of the sample, which was surprising because free αCD had been carefully removed from polyrotaxanes 1a and 1b and all low molecular weight impurities had been removed again from the corresponding macroinitiators 2a and 2b. Scission of the PEG thread, which is holding together the brush, might be a reasonable explanation for the occurrence of this second peak. Scission might be caused by mechanical forces exerted onto the brush during its passage through the GPC column. Mechanical degradation of high molecular weight polymers in general22,23 and rupture of cylindrical polymer brushes were already described previously by Sheiko et al.18 The rupture of polyrotaxane brushes is phenomenologically different from the rupture of regular bottle brushes because the rupture of just one covalent bond of the backbone thread leads to a complete fragmentation of the brush into the much smaller αCD stars, while the rupture of a regular cylindrical brush still leads to two large pieces. Because the molar fraction of αCD stars Xstar did not depend on the size of the stopper group, slippage of the αCD stars across it could be ruled out as an explanation for their release. The fraction Xstar did not depend much on the length of the PMMA side chains Pn (see Table 1) and did not increase during storage for months at −20 °C. In contrast, storage at 60 °C and especially at 120 °C gave rise to significant increases in the fraction of stars to Xstar = 59% and 77%, respectively, as shown in Figure 3. In all cases, the elution volumes of the two peaks remain constant, which clearly supports the proposed rupture mechanism. The reduced stability of the polyrotaxane brushes at elevated temperatures was attributed to the increased osmotic pressure of the threaded αCDs which challenges the PEG chain.

Figure 1. Kinetics of the polymerization of MMA from macroinitiator 2a in DMSO at 25 °C, initial monomer concentration [M]° over monomer concentration [M] as a function of reaction time t: (○) 2bromoisobutyryl:CuBr:HMTETA:MMA = 1:1:1:100; (●) 2-bromoisobutyryl:CuBr:CuBr2:HMTETA:MMA = 1:0.5:0.5:1:100.

multiangle light scattering (GPC-MALS). Surprisingly, the GPC traces of all brushes revealed two distinct signals (see Figure 2). One signal appeared at the expected low elution

Figure 2. GPC traces (eluent THF, RI detection) of macroinitiator 2a, αCD−PMMA star 4, and polyrotaxane brush 3a.

volume of 6.4 mL, corresponding to a polystyrene molecular weight of Mw ≅ 1.0 MDa, and a second intense peak was observed at a higher elution volume of 11 mL, corresponding to a PS molecular weight of Mw ≅ 40 kDa. The first peak was assigned to the polyrotaxane brush 3 because the experimental molecular weight determined by light scattering (cf. Table 1) agreed well with the theoretical value calculated according to eq 1: M wtheor = N × DS × Pn × 100.1 + 10350

Figure 3. GPC traces of αCD polyrotaxane brush 3a after storage for (a) 14 days at 25 °C, (b) 24 h at 60 °C, and (c) 24 h at 120 °C.

(1) D

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intensities in these two techniques (relaxation times versus pulse delays in NMR and refractive indices in GPC), the signal intensity for the star polymer is still much smaller in NMR than in GPC. Thus, we conclude that shear forces rupture a significant amount of the brush thread as the material passes through the GPC column. Sample 3a was cast as a thin film on a highly oriented pyrolytic graphite (HOPG) surface for observation by tappingmode atomic force microscopy (AFM). The characteristic wormlike structure, already known for other cylindrical brushes,27 was recognized and is shown in Figure 6. A single molecule had a typical length (L) of 125 nm, height of 1.6 nm, and width of 25 nm. The observed length was slightly longer than the theoretical length, Ltheor = LPEG + 2LPMMA = 227× 3 × 0.125 + Pn × 2 ×2 × 0.125 nm = 110 nm, calculated for an alltrans PEG chain in addition to the length of PMMA side chains at both ends. This small discrepancy was attributed to the size of the AFM tip. The appearance of a fully extended PEG conformation is reasonable because the grafted PMMA side chains exert sufficiently high pressure to expand the backbone. Stacking of the threaded αCD rings via hydrogen bonds as reported for unsubstituted αCD PRxs28 was ruled out because of the 6−8 attached substituents, which prevent a close contact between threaded CDs.28 Therefore, an equal distribution of threaded αCD rings along the polymer chain was assumed. The observed width (W) of the brush agreed well with the double length of expanded PMMA side chains plus the diameter of αCD, Wtheor = Pn × 2 × 2 × 0.125 + 1.0 nm = 26 nm, but the size of the tip must be taken into account. Therefore, the actual width is likely smaller than the measured value of 25 nm, which is consistent with slightly coiled PMMA chains. αCD stars 4 were not observed in the AFM images; this was attributed to their low concentration and high mobility on the HOPG surface.

Information on molecular size of specific molecules within mixtures can be provided without chromatographic separation using diffusion-ordered NMR spectroscopy (DOSY), which is based on the measurement of diffusion coefficients (D) using pulsed magnetic field gradients.14,24−26 DOSY spectra are shown in Figure 4 for macroinitiator 2a and PRx brush 3a′. In

Figure 4. DOSY spectra of polyrotaxane macroinitiator 2a (left) and polyrotaxane brush 3a′ (right) in CDCl3.

each spectrum, two sets of peaks are observed (marked with horizontal dashed lines), indicating that each sample consists of two components with characteristic diffusion coefficients. The larger component (smaller D) is the PRx, and the smaller component (larger D) is the αCD star. For both components, the diffusion coefficient decreases from macroinitiator to brush as the grafting reaction leads to larger molecules: log D = −10.35 to −10.6 for the PRx and log D = −9.4 to −9.75 for the αCD star. The DOSY spectrum of the macroinitiator shows 1H peaks due to the CD (3.7−5.15 ppm) and bromoisobutyrate (1.78 ppm) moieties. The DOSY spectrum of the brush reveals only 1H peaks for the PMMA: 0.81, 0.998, 1.78−1.93, and 3.63 ppm. The DOSY spectrum of the macroinitiator shows minor amounts of free αCD initiator even though this is not apparent in the 1D 1H NMR spectrum. These observations indicate that the DOSY technique, in which the NMR signal is spread across a second dimension according to molecular size, is more sensitive than standard 1D 1H NMR for the structural characterization of these materials. The relative amounts of star versus brush polymer are obtained from the slice of the DOSY spectrum at 0.8 ppm (corresponded to the −CH3 group of the PMMA) along the log D axis. Evidently, the NMR signal of the star in sample 3a′ is much smaller (X′star = 20%) than the signal in the corresponding GPC trace, shown for comparison in Figure 5. Even accounting for the parameters that influence peak

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CONCLUSIONS The synthesis of cylindrical polymer brushes by ATRP from αCD−PEG PRx macroinitiators was shown. The structure was proven by GPC-light scattering, DOSY, and AFM. These new polyrotaxane brushes are highly sensitive to mechanical forces, which can cause rupture of the main chain leading to an irreversible collapse of the brush. This mechanically triggered release of CD rings might be applied for the construction of nanocapsules for the controlled delivery of pharmaceutical drugs29 or for the construction of self-healing materials.30 Further investigations of these approaches are in progress.

Figure 5. Comparison of (a) GPC trace and (b) DOSY trace at 0.8 ppm for polyrotaxane brush 3a′. E

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Figure 6. AFM tapping mode images of polyrotaxane brush 3a on HOPG; profiles along (red) amd perpendicular (green) to the main direction of the brush are shown to the left. The brush had aligned along a dislocation of the HOPG surface.

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(11) Plamper, F. A.; Becker, H.; Lanzendorfer, M.; Patel, M.; Wittemann, A.; Ballauf, M.; Müller, A. H. E. Macromol. Chem. Phys. 2005, 206, 1813−1825. (12) Busche, B. J.; Tonelli, A. E.; Balik, C. M. Polymer 2010, 51, 454− 462. (13) Harada, A.; Li, J.; Kamachi, M. Nature 1992, 356, 325−327. (14) Zhao, T. J.; Beckham, H. W. Macromolecules 2003, 36, 9859− 9865. (15) Zhang, A.; Shu, L.; Bo, Z.; Schlüter, A. D. Macromol. Chem. Phys. 2003, 204, 328−339. (16) Feuz, L.; Leermakers, F. A. M.; Textor, M.; Borisov, O. Macromolecules 2005, 38, 8891−8901. (17) Sheiko, S. S.; Gerle, M.; Fischer, K.; Schmidt, M.; Möller, M. Langmuir 1997, 13, 5368−5372. (18) Sheiko, S. S.; Sumerlin, B. S.; Matyjaszewski, K. Prog. Polym. Sci. 2008, 33, 759−785. (19) Araki, J.; Ito, K. J. Polym. Sci., Part A: Polym. Chem. 2005, 44, 532−538. (20) Haddleton, D.; Monge, S.; Darcos, V. J. Polym. Sci., Part A: Polym. Chem. 2004, 42, 6299−6308. (21) Tang, W.; Matyjaszewski, K. Macromol. Theory Simul. 2008, 17, 359−375. (22) McIntyre, D.; Fetters, L. J.; Slagowski, E. Science 1972, 176, 1041−1043. (23) Caruso, M. M.; Davis, D. A.; Shen, Q.; Odom, S. A.; Sottos, N. R.; White, S. R.; Moore, J. S. Chem. Rev. 2009, 109, 5755−5798. (24) Zhao, T.; Beckham, H. W.; Gibson, H. W. Macromolecules 2003, 36, 4833−4837. (25) Cohen, Y.; Avram, L.; Frish, L. Angew. Chem., Int. Ed. 2005, 44, 520−554. (26) Zhao, T. J.; Beckham, H. W.; Ricks, H. L.; Bunz, U. H. F. Polymer 2005, 46, 4839−4844. (27) Sheiko, S. S.; da Silva, M.; Shirvaniants, D.; LaRue, I.; Prokhorova, S.; Moeller, M.; Beers, K.; Matyjaszewski, K. J. Am. Chem. Soc. 2003, 125, 6725−6728. (28) Jarroux, N.; Guegan, P.; Cheradame, H.; Auvray, L. J. Phys. Chem. B 2005, 109, 23816−23822. (29) Brøndsted, H.; Andersen, C.; Hovgaard, L. J. Controlled Release 1998, 53, 7−13. (30) Esser-Kahn, A. P.; Odom, S. A.; Sottos, N. R.; White, S. R.; Moore, J. S. Macromolecules 2011, 44, 5539−5553.

ASSOCIATED CONTENT

* Supporting Information S

H NMR spectra of the αCD polyrotaxane 1a, the macroinitiator 2a, and the polyrotaxane brush 3a′. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support of this work was provided through the NSF Macromolecular, Supramolecular, and Nanochemistry (MSN) Program in the Division of Chemistry with an International Collaboration in Chemistry (ICC) grant (CHE-1124719).

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REFERENCES

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dx.doi.org/10.1021/ma302204a | Macromolecules XXXX, XXX, XXX−XXX