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Benefits of Catalyzed Radical Termination: High-Yield Synthesis of Polyacrylate Molecular Bottlebrushes without Gelation Guojun Xie,† Michael R. Martinez,† William F. M. Daniel,‡ Andrew N. Keith,‡ Thomas G. Ribelli,† Marco Fantin,† Sergei S. Sheiko,‡ and Krzysztof Matyjaszewski*,† †

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Department of Chemistry, Center for Macromolecular Engineering, Carnegie Mellon University, 4400 Fifth Avenue, Pittsburgh, Pennsylvania 15213, United States ‡ Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-3290, United States S Supporting Information *

ABSTRACT: Catalyzed radical termination (CRT) in atom transfer radical polymerization (ATRP) of acrylates is usually considered as an unfavorable side reaction, as it accelerates termination and decreases chain-end functionality. CRT proceeds via a L/CuII−Pn organometallic intermediate and results in saturated chain-ends. Thus, CRT can help to suppress gelation in the synthesis of densely grafted poly(nbutyl acrylate) molecular bottlebrushes using the “graftingfrom” method by decreasing the fraction of chains terminated by conventional bimolecular radical combination. Molecular bottlebrushes by ATRP are typically prepared slowly in low yield and to limited monomer conversion to prevent radical combination, cross-linking, and gelation. Under conditions promoting CRT with highly active ATRP catalysts, a relatively high monomer conversion (>70%) was achieved without macroscopic gelation. CRT was favored using conditions that favored the formation of the L/CuII−Pn intermediate such as lower temperature and higher concentration of increasingly more active L/CuI catalysts. These conditions were beneficial for the fast and high-yield synthesis of polyacrylate molecular bottlebrushes, since they reduced the fraction of chains terminated by combination and prevented cross-linking of molecular bottlebrushes. High grafting density (>85%) and wormlike structures of molecular bottlebrushes were confirmed by side-chain cleavage and by molecular imaging via atomic force microscopy (AFM), respectively.

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INTRODUCTION Polymeric side chains in molecular bottlebrushes are densely grafted from a backbone. The steric interaction between these side chains extends the polymer backbone, which results in a wormlike conformation.1 Molecular bottlebrushes have potential applications as anisotropic nanomaterials,2−6 supersoft elastomers,7−9 surfactants,10,11 photonic materials,12−15 biolubricants,16,17 stimuli-responsive materials,18,19 and nanoporous materials.20−24 Three synthetic strategies have been developed for the preparation of molecular bottlebrushes: “grafting-onto” (attaching preformed side chains to the backbone),25,26 “grafting-through” (polymerizing macromonomers),14,15,27−29 and “grafting-from” (growing side chains from the backbone).30,31 Unlike the first two strategies, “grafting-from” generates densely grafted side chains from backbones of any desired length, as long as the side chains can be evenly grown.32,33 To grow uniform side chains, the “grafting from” method is often performed by controlled radical polymerization (CRP) techniques, such as atom transfer radical polymerization (ATRP, Scheme 1a).34,35 One limitation of “grafting from” multifunctional macroinitiators is the possibility of gelation due to intermolecular radical termination (RT) by combination between two growing acrylate brushes (Scheme 1b).36−39 Thus, such © XXXX American Chemical Society

Scheme 1. Mechanism of (a) ATRP and (b) Conventional Radical Termination (RT)

polymers cross-link and the polymerization mixture forms a gel at relatively low conversions.40,41 The resulting networks Received: April 22, 2018 Revised: July 25, 2018

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DOI: 10.1021/acs.macromol.8b00849 Macromolecules XXXX, XXX, XXX−XXX

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RESULTS AND DISCUSSION Synthesis of Bottlebrushes under CRT-Promoting Conditions. More active ATRP catalysts promote faster

trap a large fraction of polymer bottlebrushes, resulting in materials which are difficult to process. To avoid macroscopic gelation, the synthesis of molecular bottlebrushes via the “grafting-from” method is usually taken to low monomer conversion in systems highly diluted with monomer or solvent. Previously, miniemulsion was used to prevent this macroscopic gelation as molecular bottlebrushes were “compartmentalized” inside the droplets’ boundaries.40 Although high monomer conversion was achieved without macroscopic gelation, cross-linking still occurred within the latex particles due to unavoidable radical termination reactions. In addition to conventional radical termination, acrylate radicals in ATRP can undergo copper-catalyzed radical termination (CRT), which proceeds via a L/CuII−Pn organometallic intermediate42 (Scheme 2) that can react with a

Scheme 3. Synthesis of PBA Molecular Bottlebrushes via the “Grafting-From” Approach Using Normal ATRP (BiBEM = 2-Bromoisobutyryloxyethyl Methacrylate)

Scheme 2. Organometallic Radical Polymerization (OMRP) Equilibrium Leading to the Formation of Paramagnetic Organometallic Species, with Subsequent Catalytic Radical Termination Step Leading to Saturated Polymer Species (No Coupling) and Regeneration of L/CuI Species

Table 1. Synthesis of PBA Molecular Bottlebrushesa

hydrogen atom or proton donor to give a saturated dead-chain end.36,43 The equilibrium between Pn• and L/CuII−Pn is defined by the concentration of L/CuI species and the equilibrium constant of organometallic mediated radical polymerization, KOMRP. It was previously proposed that once the L/CuII−Pn species is formed, it quickly terminates with a second radical, resulting in two separate chains with the same molecular weight as the original polymeric radicals.36,42,44 However, by conservation of spin, this would inherently require either combined or disproportionated chains. Previous contributions and current investigations, however, reveal that CRT results in saturated chain-ends.36,43,45 Yamago et al. reported on the likely hydrolysis of the L/CuII−Pn species by CH3OD resulting in deuterated polyacrylate chain-ends (pMA-D). The intimate mechanism of CRT is still being investigated by our laboratory. Nevertheless, under conditions that promote the formation of the L/CuII−Pn species, the contribution of noncatalyzed radical combination is significantly suppressed.46 Strongly reducing catalysts such as Cu/tris[2(dimethylamino)ethyl]amine (Me6TREN) are not only highly active in ATRP due to a high affinity toward alkyl halides but also have higher affinity toward radicals. This increases KOMRP and consequently promotes the CRT pathway in termination.42,46 On one hand, kinetically favored CRT leads to a loss of chain-end functionality, but it can also be exploited to suppress bimolecular radical combination. This could be beneficial toward preventing macroscopic gelation, especially in multifunctional systems such as stars, grafts, or bottlebrushes. In this paper, we report the fast and high-yield synthesis of polyacrylate molecular bottlebrushes under conditions that promote CRT. To better understand the role of CRT in the preparation of bottlebrush polymers, we evaluated the effect of temperature, catalyst concentration, and ligand structure. Sidechain cleavage experiments and molecular imaging by AFM were used to confirm the high grafting density (>85%) and the wormlike structures of bottlebrush products.

reaction entrya

[BA]:[BiBEM]:[CuBr]:[CuBr2]:[L]

time (h)

conv (%)

25Me6T 80Me6T 25Me6Tx3Cu 80Me6Tx5Cu 80TPMA 70bpy

50:1:0.08:0.04:0.14 50:1:0.08:0.04:0.14 50:1:0.25:0.125:0.375 50:1:0.4:0.2:0.63 50:1:0.25:0.04:0.3 50:1:0.5:0.03:1.1

30 30 30 30 30 115

84 90 91 93 90 70

a

Poly(BiBEM) with degree of polymerization 372 was used as macroinitiator (BiBEM = 2-bromoisobutyryloxyethyl methacrylate unit). Ligand: Me6TREN (Me6T), dNbpy (bpy), or TPMA. Solvent content: 16 vol % DMF, 64 vol % anisole; temperature: 80 °C (80Me6T, 80Me6Tx5Cu, 80TPMA), 70 °C (70bpy), or 25 °C (25Me6T, 25Me6Tx5Cu).

CRT and decrease the fraction of chains terminated by combination in the polymerization of acrylates.47 Therefore, we first utilized the highly active [CuI(Me6TREN)]+ catalyst to achieve a fast and high-yield synthesis of molecular bottlebrushes with poly(n-butyl acrylate) (PBA) side chains (Scheme 3).48,49 A mixed solvent system of DMF and anisole was employed to solubilize PBA and the copper complex. The high polarity of DMF increased KATRP and thus accelerated the polymerization.50,51 Anisole was added as a cosolvent to solubilize the bottlebrushes, which are only sparingly soluble in pure DMF. Well-defined PBA bottlebrushes were prepared via normal ATRP at room temperature (25 °C) as described in Table 1, entry 25Me6T ([BA]:[BiBEM]:[CuBr]:[CuBr2]:[Me6TREN] = 50:1:0.08:0.04:0.14), which reached a conversion of 84% in 30 h. This is in contrast to previous reports where conversions of less than 10% were achieved in about 24 h with the less active Cu complex based on 4,4′-dinonyl-2,2′-bipyridyne (dNbpy) ligand at elevated temperature.7 The weight fraction of bottlebrushes coupled during polymerization was determined by deconvolution of the GPC traces (Figure S2). As shown in Figure 1A, at room temperature for the experiment 25Me6T, 21% of the bottlebrushes were coupled via interbrush combination (coupling). No macroscopic or microscopic gelation was detected by dynamic light scattering (DLS) (Figure S3). This reaction is taken as the “reference” standard reaction throughout the rest of this study. Then, the effect of reaction temperature, concentration of Cu catalyst, and ligand structure B


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Figure 2. Comparison between experiments 80Me6T (filled symbols) and 80Me6Tx5Cu (hollow symbols). (a) Kinetics of monomer consumption. (b) Concentration of propagating radicals and weight fraction of coupled chains vs monomer conversion. Reaction conditions: poly(BiBEM) with degree of polymerization 372 was used as macroinitiator (BiBEM = 2-bromoisobutyryloxyethyl methacrylate unit). Solvent content: 16 vol % DMF, 64 vol % anisole. [BA]:[BiBEM]:[CuBr]:[CuBr 2 ]:[Me 6 TREN] = 50:1:0.08:0.04:0.14 (80Me6T) or 50:1:0.4:0.2:0.63 (80Me6Tx5Cu). Temperature: 80 °C.

propagation rate coefficient, kp (experiment 80Me6T, Table 1 and Figure 1). Interestingly, the fraction of intercoupled brushes increased with temperature so that macroscopic gelation occurred at 80 °C after 24 h in the experiment 80Me6T, while only 17% of the brushes coupled at 25 °C after 24 h. The increased coupling can be explained by two concurrent factors: (i) increased temperature leads to higher radical concentration due to increased KATRP, and (ii) the fraction of L/CuII−Pn decreased with increasing temperature. Because ΔGOMRP < 0, increased temperature forces the OMRP equilibrium toward the left and effectively increases the concentration of L/CuI and [Pn•] (Scheme 2).52−54 This causes a greater fraction of chains terminated by conventional radical termination (ratet = kt[R•]2) than by CRT (rateCRT ∼ kappCRT[L/CuII−R]), causing an increased fraction of interbrush combination.47,55 Thus, lower T is beneficial to promote CRT and consequently to reduce the fraction of bottlebrush coupling via conventional radical termination. Effect of Catalyst Concentration. At higher concentration of copper(I) catalyst, more L/CuII−Pn is formed and CRT is favored. At 80 °C, when the total amount of copper was increased by a factor of 5 (80Me6T and 80Me6Tx5Cu in Table 1), the fraction of inter-bottlebrush coupling decreased from 70% to 37%, at a conversion of ∼90%, despite the slightly faster reaction (Figure 1c). A similar effect was observed when the total copper concentration was increased by a factor of 3 compared to the baseline reaction at 25 °C; the rate of polymerization of 25Me6Tx3Cu was slightly faster, and

Figure 1. Evolution of GPC traces during the reaction of (a) 25Me6T, (b) 80Me6T, and (c) 80Me6Tx5Cu. Reaction conditions: poly(BiBEM) with degree of polymerization 372 was used as macroinitiator (BiBEM = 2-bromoisobutyryloxyethyl methacrylate unit). Solvent content: 16 vol % DMF, 64 vol % anisole. [BA]:[BiBEM]: [CuBr]:[CuBr2]:[Me6TREN] = 50:1:0.08:0.04:0.14 (25Me6T), 50:1:0.08:0.04:0.14 (80Me6T), or 50:1:0.4:0.2:0.63 (80Me6Tx5Cu). Temperature: 80 °C (80Me6T and 80Me6Tx5Cu) or 25 °C (25Me6T).

were investigated. The polymerization reactions were encoded by specifying reaction temperature, ligand used, and amount of Cu catalyst used. Thus, the entry 4: 80Me6Tx5Cu indicates reaction conducted at 80 °C in DMF with Me6TREN ligand with 5 times elevated Cu catalyst. Effect of Temperature. The reaction temperature was increased from 25 °C in the standard experiment to 80 °C. At 24 h, monomer conversion increased from 81% at 25 °C to 88% at 80 °C, likely from an increase in both KATRP and C


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Figure 3. Evolution of GPC traces during the synthesis of molecular bottlebrushes grafted from a pBIBEM372 backbone under the conditions: (a) 70bpy:[BA]:[BiBEM]:[CuBr]:[CuBr2]:[dNbpy] = 50:1:0.5:0.03:1.1 in DMF (16%) and anisole (64%) at 70 °C. (b) 80TPMA:[BA]:[BiBEM]: [CuBr]:[CuBr2]:[TPMA] = 50:1:0.25:0.04:0.3 in DMF (16%) and anisole (64%) at 80 °C. (c) Comparison of the GPC traces at ∼70% conversion for the experiments 70bpy, 80TPMA, and 80Me6Tx5Cu.

[L/CuI]:[L/CuII−X] ratio, every termination event decreases this ratio and leads to slower polymerization rate, according to the persistent radical effect.56,57 As shown in Figure 2b, the radical concentration was calculated as [Pn•] = kpapp/kp, using a literature value of kp = 5.0 × 104 M−1 s−1.58 Despite the 5-fold difference in [L/CuI]0, 80Me6T and 80Me6Tx5Cu had similar kinetics but a significantly different increase in interbrush coupling. It is worth noting that the initial concentration of L/CuI activator in 80Me6T and 80Me6Tx5Cu was 8 and 40 mol % vs alkyl halide chainends, respectively. The slightly higher rate in 80Me6Tx5Cu could be due to the fact that the [L/CuI]/[L/CuII] ratio during polymerization was less affected by the persistent radical effect (termination) due to the much higher initial

coupling was decreased compared to the baseline 25Me6T (Figure S4). It should be noted that for the reactions at increased initial copper concentrations the [CuI]/[CuII] ratio was held constant to ensure a similar position of the ATRP equilibrium and thus similar radical concentration and rate of RT. The similar radical concentration was confirmed by the polymerization kinetics of the two reactions at 80 °C, illustrated in Figure 2a. The semilogarithmic kinetic plots were similar and showed downward curvature, indicating a decreasing concentration of propagating radicals with time. This was due to radical terminationeither RT or CRTwith concurrent conversion of L/CuI to L/CuII−X. Because the rate of polymerization in normal ATRP is dependent on the D


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Figure 5. AFM height images of molecular bottlebrushes isolated from 25Me6T deposited on mica surfaces. Reaction condition of 25Me6T: poly(BiBEM) with degree of polymerization 372 was used as macroinitiator (BiBEM = 2-bromoisobutyryloxyethyl methacrylate unit). Solvent content: 16 vol % DMF, 64 vol % anisole. [BA]: [BiBEM]:[CuBr]:[CuBr 2 ]:[Me 6 TREN] = 50:1:0.08:0.04:0.14 (25Me6T).

the synthesis of polymer brushes employed low activity catalysts under dilute conditions to prevent gelation via bimolecular coupling between brushes. For example, ATRP with a dNbpy ligand resulted in a conversion of 70% after 5 days with moderate coupling (70bpy, Figure 3a). The slow reaction rate of 70bpy is attributed to the lower ATRP activity of Cu/dNbpy. Thus, a more active CuI /tris(2-pyridylmethyl)amine (TPMA) catalyst was tested (80TPMA, Table 1 and Figure 3). A similar high Cu concentration was used, but at a lower [L/CuI]/[L/CuII] ratio to compensate for the higher ATRP activity of Cu/TPMA compared to that of Cu/dNbpy. Nevertheless, the reaction with Cu/TPMA was 30 times faster, reaching ∼70% conversion in only 4 h. Both reactions had similar evolution of coupling with conversion, even if the much faster reaction with Cu/TPMA should favor coupling due to a much higher radical concentration. This can be correlated to higher KOMRP, and thus faster CRT, for the more reactive TPMA system compared to the dNbpy system.47 Use of the most active Cu/Me6TREN system at 80 °C and similar high Cu concentration (Figure 2C) resulted in faster reaction and even reduced coupling, with only 14% coupled chains after 4 h at 70% conversion. This results from a more negative redox potential, E1/2, for the Me6TREN-based system and thus a more stable Me6TREN/CuII−Pn species compared to TPMA/ CuII−Pn and dNbpy/CuII−Pn.47,59,60 A comparison between 80Me6Tx5Cu and the less active 70bpy and 80TPMA reactions highlights the importance of catalytic activity in the high yield synthesis of polymer brushes (Figure 3c). All three reactions employed high concentrations of copper; however, 80Me6Tx5Cu benefitted the most from the higher stability of its organometallic intermediate, Me6TREN/CuII−Pn. At a conversion of ca. 70%, the faster 80Me6Tx5Cu had only a 14% weight fraction of coupled brushes, while 80TPMA and 70bpy had a fraction of ca. 30%.

Figure 4. Weight fraction of coupled bottlebrushes vs (a) monomer conversion or (b) time under different initial conditions. Solvent content: 16 vol % DMF, 64 vol % anisole. [BA]:[BiBEM]:[CuBr]: [CuBr 2 ]:[L] = 50:1:0.08:0.04:0.14 (25Me 6 T, 80Me 6 T), 50:1:0.25:0.04:0.3 (80TPMA), 50:1:0.5:0.03:1.1 (70bpy), 50:1:0.25:0.125:0.375 (25Me 6 Tx5Cu), or 50:1:0.4:0.2:0.63 (80Me6Tx5Cu). Temperature: 80 °C (80Me6T, 80Me6Tx5Cu, 80TPMA), 70 °C (70bpy), or 25 °C (25Me6T, 25Me6Tx3Cu).

[L/CuI]. However, the similar kinetic plots indicate that more termination occurred in 80Me6Tx5Cu since, due to the much higher initial [L/CuI], more copper was consumed in this reaction. This is consistent with higher [L/CuI] promoting CRT, as reported in a recent study.42 A similar effect was observed at 25 °C (Figure S4). While polymerization rate decreased during polymerization, the rate of coupling was linear versus time (cf. Figure 4). Therefore, the weight fraction of coupled chains increased faster at the end of the polymerization, when polymerization rate was slowest, and the concentration of radicals was lowest (Figure 2b). At this stage, the [L/CuI] was lowest, therefore decreasing the probability of CRT vs conventional RT. In summary, at higher [L/CuI], the OMRP equilibrium was shifted toward L/CuII−Pn species, and this kinetically promoted CRT. Therefore, 80Me6Tx5Cu had a lower fraction of coupled chains but a higher concentration of terminated chains than 80Me6T due to the enhanced CRT processes. The observed decrease of inter-bottlebrush coupling with increased catalysts concentration confirmed the beneficial effect of increased CRT in the synthesis of bottlebrushes. Effect of Copper Ligand. The use of ligands that give highly active complexes (e.g., Me6TREN vs dNbpy) is important for the high yield synthesis of brushes. Previously, E


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Macromolecules Summary of Effects of Reaction Conditions. The evolution profiles of fraction of coupled bottlebrushes (weight fraction of coupled bottlebrushes vs monomer conversion) under various initial conditions are summarized in Figure 4. The coupling rate was roughly constant during polymerization (Figure 4b), which resulted in an accumulation of coupled chains in the later stage of the reaction (Figure 4a), when polymerization was slowest. Conversely, in the initial stages of the reaction CRT most efficiently suppressed RT, when radical concentration was highest. Coupling and macroscopic gelation in the high-yield synthesis of molecular bottlebrushes were significantly reduced by promoting CRT under the following conditions: (1) lower temperature, (2) high [L/CuI]0, and (3) use of highly active ATRP catalytic systems. Characterization of Products. Atomic force microscopy (AFM) was used to image individual molecular bottlebrushes as synthesized using the 25Me6T reaction (Table 1).61 The molecular bottlebrushes adopted wormlike conformations, characterized by an extended backbone and high persistence length. Statistical analysis of the AFM images shows a narrow distribution of both width and length of the bottlebrush molecules. The number-average contour length (Ln) and width (Wn) were Ln = 97.1 ± 0.8 nm and Wn = 39.2 ± 0.6 nm, respectively. The degree of polymerization (DP) of the polymeric backbone in this study was 372. Assuming that the backbone adapted a fully extended conformation, with the length of the C−C−C monomeric unit equal to 0.25 nm, the contour length of molecular bottlebrush should be 93 nm, which correlates well with the AFM analysis. We also quantified the weight fraction of coupled brushes using AFM images of the product of 80TPMA (Table 1) by measuring the fraction of coupled brushes that could not be separated using linear PBA as the diluent when preparing the sample (Langmuir−Blodgett) film for AFM analysis. However, the estimated weight fraction of coupled brushes (20%) was lower than the value determined by GPC (∼50%) (Figure S9d). High grafting density is an important parameter for molecular bottlebrushes, since their unique properties, such as high persistence length, are governed by steric interactions between side chains.32,62 Thus, efficient initiation is crucial for “grafting-from” methods. To quantify initiation efficiency, the side chains of the molecular bottlebrushes were cleaved via solvolysis and the molecular weights were analyzed via GPC. For the synthesized bottlebrushes shown in Figure 1 (25Me6T, Table 1), 85% initiation efficiency was calculated with a narrow molecular weight distribution Đ = 1.1 (see the Supporting Information for details). This result was comparable to previous reports, as the control experiment with the less active dNbpy-based catalyst resulted in 88% initiation efficiency, but with a 30 times slower reaction (70bpy, Table 1). The similar initiation efficiencies using two different catalytic systems indicated that the conditions presented in this study provided comparable control over the growth and density of the side chains but in a more efficient and faster reaction that required a lower [L/CuI].

(3) more active ATRP catalytic systems. Side-chain cleavage experiments confirmed the high grafting density (>85%) of bottlebrushes, indicating effective polymerization control. Molecular visualization using AFM confirmed the well-defined structure of prepared PBA molecular bottlebrushes. The next challenge will be to apply these CRT-promoting conditions to low-ppm ATRP systems, and to quantify the role of CRT in the high-yield synthesis of molecular bottlebrushes.

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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/acs.macromol.8b00849. Experimental procedures, DLS measurements, and kinetic plots supplementary figures (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (K.M.). ORCID

Sergei S. Sheiko: 0000-0003-3672-1611 Krzysztof Matyjaszewski: 0000-0003-1960-3402 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Support from the National Science Foundation via Grants DMR 1436219 and 1501324 is acknowledged. M.M. acknowledges the 2017 Dr. Konrad M. Weis Fellowship in Chemistry. The authors acknowledge Dr. Ryan Fenyves for useful discussions.

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CONCLUSIONS Molecular bottlebrushes were successfully synthesized in high yield via ATRP to a high monomer conversion (>80%) in ca. 24 h without macroscopic gelation. Cross-linking was avoided by promoting CRT instead of conventional bimolecular radical termination. CRT was favored under the following conditions: (1) lower temperature, (2) higher CuI/L concentration, and F


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