Ultrasonication-Induced Aqueous Atom Transfer Radical Polymerization

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Cite This: ACS Macro Lett. 2018, 7, 275−280

Ultrasonication-Induced Aqueous Atom Transfer Radical Polymerization Zhenhua Wang,†,‡ Zhanhua Wang,† Xiangcheng Pan,§ Liye Fu,‡ Sushil Lathwal,‡ Mateusz Olszewski,‡ Jiajun Yan,‡ Alan E. Enciso,‡ Zongyu Wang,‡ Hesheng Xia,*,† and Krzysztof Matyjaszewski*,‡ †

The State Key Laboratory of Polymer Materials Engineering, Polymer Research Institute, Sichuan University, Chengdu 610065, China ‡ Department of Chemistry, Carnegie Mellon University, 4400 Fifth Avenue, Pittsburgh, Pennsylvania 15213, United States § The State Key Laboratory of Molecular Engineering of Polymers, Department of Macromolecular Science, Fudan University, Shanghai 200433, China S Supporting Information *

ABSTRACT: A new procedure for ultrasonication-induced atom transfer radical polymerization (sono-ATRP) in aqueous media was developed. Polymerizations of oligo(ethylene oxide) methyl ether methacrylate (OEOMA) and 2-hydroxyethyl acrylate (HEA) in water were successfully carried out in the presence of ppm amounts of CuBr2 catalyst and tris(2pyridylmethyl)amine ligand when exposed to ultrasonication (40 kHz, 110 W) at room temperature. Aqueous sono-ATRP enabled polymerization of watersoluble monomers with excellent control over the molecular weight, dispersity, and high retention of chain-end functionality. Temporal control over the polymer chain growth was demonstrated by switching the ultrasound on/off due to the regeneration of activators by hydroxyl radicals formed by ultrasonication. The synthesis of a well-defined block copolymer and DNA− polymer biohybrid was also successful using this process.

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chemical effects of ultrasound in aqueous solutions have previously been investigated.8 When an ultrasonic wave propagates through aqueous media, hydroxyl radicals are produced because of the acoustic cavitation which provides the primary mechanism for sonochemical effects.9 These radicals can react with a vinyl monomer to form carbon radicals and initiate a radical polymerization.10 Using this strategy, we used an ultrasound probe to in situ initiate the emulsion free radical polymerization of n-butyl acrylate without addition of any conventional initiators.11 We developed initiators for continuous activator regeneration (ICAR) ATRP in which the CuI species can be regenerated by the organic radicals.12 It was anticipated that the hydroxyl radicals generated by cavitation would react with the monomers to form carbon radicals which could reduce CuII species to CuI activators and start aqueous ATRP. Very recently, a successful reversible addition−fragmentation chain transfer (RAFT) polymerization in water was developed, mediated by a highfrequency ultrasound (414 kHz) which generated initiating hydroxyl radicals.13

eversible-deactivation radical polymerization (RDRP) is a versatile synthetic tool that enables preparation of welldefined polymers with precisely controlled molecular weight, low dispersity, and diverse functionality.1 The integration of external stimuli into RDRP allows temporal or spatial control over the composition, architecture, and functionality of polymers.2 As a noninvasive stimulus, ultrasound (20 kHz to 20 MHz) possesses much deeper penetration into tissues and organs and has lower scattering in heterogeneous systems than light.3 Ultrasound is an efficient mechanical stimulus to induce electron transfer from piezoelectric materials to dyes4 or water.5 Coupled with piezoelectric particles, ultrasound was already used to regenerate CuIBr/L and to mediate atom transfer radical polymerization (ATRP) of acrylates.6 Since the reactions were heterogeneous and carried out in organic media, it was desirable to extend the procedure and develop homogeneous ultrasound-induced aqueous ATRP (SonoATRP), which would provide a new platform to synthesize water-soluble polymers for biomedical applications under mild conditions. ATRP is mediated by dynamic equilibrium between activators (CuI) and deactivators (CuII) employing redoxbased transition metal catalyst complexes.7 The key requirement for establishing an aqueous sono-ATRP is the continuous regeneration of CuI species during the ultrasonication. The © 2018 American Chemical Society

Received: January 11, 2018 Accepted: February 9, 2018 Published: February 16, 2018 275

DOI: 10.1021/acsmacrolett.8b00027 ACS Macro Lett. 2018, 7, 275−280

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ACS Macro Letters

Figure 1. Results for aqueous sono-ATRP of OEOMA500 80% (v/v), under initial reaction conditions [OEOMA500]0:[PEG2k-BPA]0:[CuBr2]0: [TPMA]0 = 1:X:4 × 10−4:1.6 × 10−3, 10 mM NaBr, ultrasonic bath (20 °C ± 5 °C, 40 kHz, 110 W). (a) Semilogarithmic kinetic plots; (b) evolution of number-average molecular weight (filled symbols) and molecular weight distribution (Mw/Mn, open symbols) with conversion; and (c) GPC trace evolution of entry 2 in Table 1.

Table 1. Results for Aqueous Sono-ATRP of OEOMA500 under Different Conditions entrya

VEtOH/Vwater

[CuIIBr2]0 (ppm)

t (h)

conversionb

Mn,thc

Mn,GPCd

Mw/Mnd

1 2 3 4 5e 6f 7g 8 9 10 11

0/1 0/1 0/1 0/1 0/1 0/1 0/1 1/3 1/1 1/3 1/3

40 400 2000 4000 400 400 0 400 400 2000 120

2 2 2 2 2 2 2 3 4 4 4

81% 88% 84% 86% 80% 84% 36% 74% 70% 91% 90%

103,450 112,200 107,200 109,700 102,200 107,200 47,200 94,700 89,700 115,950 114,700

72,100 78,200 78,970 78,410 86,670 215,660 314,800 89,800 83,960 103,390 87,810

1.34 1.17 1.17 1.14 1.40 1.24 1.26 1.19 1.19 1.18 1.24

a Reaction conditions: [OEOMA500]0:[PEG2k-BPA]0:[CuBr2]0:[TPMA]0 = 1:1.6 × 10−3:X:4X in 80% (v/v) water, 10 mM NaBr, ultrasonic bath (20 °C ± 5 °C, 40 kHz, 110 W). bConversion determined by 1H NMR. cCalculated on the basis of conversion (i.e., Mn,th = M PEG2k‑BPA + [OEOMA500]0/[ PEG2k-BPA]0 × conversion × Mmonomer). dDetermined by GPC in THF, based on linear PMMA as calibration standard. eWithout NaBr. fWithout macroinitiator PEG2k-BPA. gWithout CuBr2.

polymerization is illustrated in Figure 1 and also summarized in Table 1. Conversion steadily increased as the reaction proceeded. The polymerization with 40 ppm of Cu (Entry 1, Table 1) resulted in formation of a polymer with a molecular weight higher than the theoretical value after 1 h ultrasonication. This was attributed to a slow deactivation and consequently low initiation efficiency since the signal of macroinitiator PEG2kBPA was still observed in the gel permeation chromatography (GPC) traces (Figure S2). Polymerizations with a higher Cu catalyst concentration displayed living characteristics (entries 2−4, Table 1). Mn increased linearly with conversion, while Đ remained low. The reaction with 400 ppm of Cu reached 88% conversion after 2 h ultrasonication, providing a polymer with Mn = 78 200 and Đ = 1.17. The results for control experiments carried out in the absence of either macroinitiator or Cu catalyst showed the characteristics of free radical polymerization (entries 5−6, Table 1), indicating the significance of these reactants for controlling the polymerization. Without NaBr, the reaction provided a poorly controlled polymerization; i.e.,

This communication reports the extension of ultrasoundmediated ATRP to aqueous media using ppm level of Cu/ tris(2-pyridylmethyl)amine (TPMA) catalyst, without any additional conventional initiators or reducing agents. Welldefined polymers with high molecular weight (Mn > 240 000), low dispersity, and high chain-end functionality were prepared. This aqueous sono-ATRP utilizes ppm level catalyst in aqueous media under benign conditions, i.e., using simple agitation in an ultrasonic bath at room temperature. Oligo(ethylene oxide) methyl ether methacrylate, Mn = 500 (OEOMA500), was polymerized in water under sonication (40 kHz, 110 W) using poly(ethylene oxide)−bromophenylacetate (PEO2k-BPA) as a macroinitiator (Figure S1). CuIIBr2/TPMA was selected as the catalyst since it forms a stable complex without a significant disproportionation in aqueous media. The addition of sodium bromide (NaBr) to the solution minimized the dissociation of bromide anions from the deactivators.14 An ultrasonic bath was used to activate the polymerization because of easy scalability and low intensity when targeting high molecular weight. The effect of Cu concentration on the rate of 276


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ACS Macro Letters Table 2. Results of Aqueous Sono-ATRP of OEOMA500 and HEA Targeting Various DPs entry a

1 2a 3a 4e 5e 6e 7e 8f 9f

monomer

DPT

T (h)

conversionb

Mn,thc

Mn,GPCd

Mw/Mnd

OEOMA500 OEOMA500 OEOMA500 OEOMA500 OEOMA500 OEOMA500 OEOMA500 HEA HEA

250 500 800 100 250 500 800 200 400

2 1 2 4 4 3 2 6 6

88% 53% 79% 70% 84% 81% 58% 43% 59%

112,200 134,700 318,100 37,200 107,200 204,700 234,200 12,180 29,580

78,200 87,390 248,270 33,170 92,480 160,530 180,320 12,760g 32,480g

1.17 1.15 1.26 1.21 1.20 1.23 1.18 1.20g 1.23g

Reaction conditions: [OEOMA500]0:[PEG2k-BPA]0:[CuBr2]0:[TPMA]0 = 1:X:4 × 10−4:1.6 × 10−3 in 80% (v/v) water, 10 mM NaBr. Ultrasonic bath (20 °C ± 5 °C, 40 kHz, 110 W). bConversion determined by 1H NMR. cCalculated on the basis of conversion (i.e., Mn,th = MPEG2k‑BPA + [OEOMA500]0/[ PEG2k-BPA]0 × conversion × Mmonomer). dDetermined by GPC in THF, based on linear PMMA as calibration standard. eReaction was performed in an 80% (v/v) ethanol/water hybrid solvent (1/3 v/v). f[HEA]0:[PEG2k-BPA]0:[CuBr2]0:[TPMA]0 = 1:X:4 × 10−4:1.6 × 10−3 in 66.7% (v/v) water, 10 mM NaBr. Ultrasonic bath (20 °C ± 5 °C). gMn was determined by 1H NMR, based on PEG2k-BPA macroinitiator as the internal standard. Mw/Mn was determined by GPC in PBS aqueous solution, based on linear PEG as the calibration standard. a

Figure 2. Temporal control in aqueous sono-ATRP of OEOMA500 under ultrasound agitation through intermittent switching on/off the ultrasound bath: (a) kinetics and (b) molecular weight and dispersity of polymers. Reaction conditions: [OEOMA500]0:[PEG2k-BPA]0:[CuBr2]0:[TPMA]0 = 1:5 × 10−3:5 × 10−4:2 × 10−3 in (v/v) 80% water, 10 mM NaBr, and ultrasonic bath (20 °C ± 5 °C, 40 kHz, 110 W).

molecular weight decreased with increasing conversion (entry 7, Table 1). This can be attributed to the dissociation of the halide anion from the CuBr2/TPMA deactivator, lowering the concentration of deactivators.2e The excess NaBr maintained the sufficient concentration of deactivators, providing a better control over the polymerization. The experimental Mn values were slightly lower than the theoretical value in the aqueous sono-ATRP of OEOMA500. Plausibly, hydroxyl radicals reacted with monomer and not only continuously reduced CuII species but also generated some new polymer chains throughout the reaction. We used terephthalic acid as the scavenger to detect hydroxyl radicals in water under ultrasonication.15 Hydroxyl radicals react with terephthalic acid to produce emissive 2-hydroxyterephthalic acid under UV irradiation. Therefore, we used a fluorescence spectrometer to examine the formation of hydroxyl radicals in water under ultrasonication. As shown in Figure S3, the intensity of the emission peak at 425 nm increased as ultrasonication proceeded. No conversion was attained when using dimethylformamide as solvent instead of water, indicating the significance of hydroxyl radicals (Figure S4). Ethanol can react with hydroxyl radicals to produce carbonbased radicals.16 Thus, after addition of ethanol, the CuII species could be more efficiently reduced by these radicals. Additionally, the introduction of ethanol reduces the water concentration, yielding a lower concentration of hydroxyl radicals. The carbon radicals formed by sonication in an ethanol/water mixture in the absence of monomer can be captured by 4-hydroxy-2,2,6,6-tetramethylpiperidine 1-oxyl

(HO-TEMPO) as shown in Figure S5. Since TEMPO can only capture carbon radicals, but cannot trap hydroxyl radicals, the decrease of absorbance of TEMPO radicals was observed only in the presence of ethanol. Negligible decrease of the TEMPO absorbance was observed when ultrasonication was conducted in water. The effect of volume ratio of ethanol to water on sono-ATRP is summarized in Figure S6. The polymerization performed in ethanol/water showed a better agreement between theoretical and experimental molecular weights than that conducted in water (entries 8−11, Table 1). The polymerization with 400 ppm of Cu in 1/1 ethanol/water showed a linear semilogarithmic kinetic plot, affording a welldefined polymer with Mn = 83 960 and Đ = 1.19 after 4 h ultrasonication (entry 9, Table 1). The effect of Cu concentration was also investigated in the mixed solvent. With 2000 ppm of CuII, the polymerization reached 91% conversion after 4 h ultrasonication, providing a polymer with Đ = 1.18 and Mn = 103 390 (entry 10, Table 1). The experimental molecular weight matched well with the theoretical value. When the Cu concentration was decreased to 120 ppm, the polymerization reached 90% conversion after 4 h ultrasonication, affording a polymer with Đ = 1.25 and Mn = 87 810 (entry 11, Table 1). We detected the hydroxyl radicals using terephthalic acid as the scavenger. The results for aqueous sono-ATRP targeting of various DPs of OEOMA500 and 2-hydroxyethyl acrylate (HEA) are summarized in Table 2. The concentrations of monomer, Cu, and ligand remained constant in all the reactions, while the concentration of macroinitiator was varied with respect to the 277


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Additionally, the use of an ultrasonic bath provides easy scalability and synthesis of well-defined water-soluble homopolymers, block copolymers, and bioconjugates.

target DP. After 1 h ultrasonication, polymerization of OEOMA500 with targeted DPT = 500 reached 53% conversion, affording a polymer with Mn = 87 390 and Đ = 1.15 (entry 2, Table 2). With higher DP targeted, the conversion reached 79% for DPT = 800 after 2 h ultrasonication (entry 3, Table 2). The polymerization of OEOMA500 in EtOH/water was slower because of the lower concentration of water in the medium. All the polymerizations targeted various DPs, resulting in predictable Mn and low dispersity ranging from 1.18 to 1.23 (entries 4−7, Table 2). HEA could be polymerized under standard conditions, providing well-defined polymers. The aqueous sono-ATRP of HEA, with targeted DP = 200, reached 67% conversion after 6 h ultrasonication, providing a polymer with Mn = 12 760 and Đ = 1.20 (entry 8, Table 2). Polymerization of HEA with targeted DP = 400 gave 59% conversion after 6 h ultrasonication and provided a polymer with Mn = 32 480 and Đ = 1.23 (entry 9, Table 2). Since an ultrasound is needed to form hydroxyl radicals, which then react with monomer to produce carbon-based radicals for the regeneration of CuI activators, it was anticipated that the polymerization can be switched on and off by the ultrasound. Thus, ultrasound can temporally control the polymerization rate. As shown in Figure 2a, the chain growth stopped after switching off the ultrasonication and was restarted after reexposure to the ultrasound. An excellent control was achieved over the polymerization and polymers with low dispersities, and predictable molecular weights were attained (Figure 2b). Chain extension of the homopolymer OEOMA500 macroinitiator was carried out with OEOMA300, in order to confirm the retention of chain end functionality and block copolymerization using this aqueous sono-ATRP procedure. The first block polymer of OEOMA500 was synthesized under typical conditions. After 1 h ultrasonication, the polymerization reached 49% conversion, affording a homopolymer with Mn = 50 830 and Đ = 1.23. The synthesis of the second block was performed using the synthesized P(OEOMA500)−Br as the macroinitiator, and polymerization of the OEOMA300 reached 56% conversion after 1 h ultrasonication, providing a block copolymer with Mn = 116 440 and Đ = 1.34 (Figure S7). This procedure was also extended to the synthesis of bioconjugates using a DNA-based macroinitiator prepared according to the previous work.17 The DNA oligonucleotide (SeqAA, 5′-ATC TGA GAC TCA CTG-3′, Mw = 5365 based on MALDI) was prepared with a Cy3 dye at the 3′-end to facilitate the visualization of the hybrid and to confirm that the DNA strand was not cleaved under ultrasonication. The 5′-end included an α-bromoisobutyrate (iBBr) group to initiate the polymerization. The synthesis of the DNA−polymer biohybrid was carried out under highly diluted conditions (Supporting Information). OEOMA500 was used as the monomer to perform the polymerization in aqueous media. SeqAA-bPOEOMA500 bioconjugate was prepared in a well-controlled manner, providing a polymer with Mn = 36 250 and with a dispersity Mw/Mn = 1.41. GPC traces clearly showed that the molecular weight of the DNA−polymer hybrid shifted to a higher range from the original DNA macroinitiator (Figure S8). In summary, a successful ultrasonication-induced aqueous ATRP with ppm level Cu catalyst in an ultrasonic bath enabled polymerization with excellent control, providing polymers with high molecular weight and low dispersity. The low intensity ultrasonic bath was used to continuously regenerate activators and also to prevent chain rupture by shear forces,18 allowing the synthesis of high molecular weight water-soluble polymers.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.8b00027. Experimental details, NMR spectra, UV−vis absorption spectra, and GPC traces (PDF)

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AUTHOR INFORMATION

Corresponding Authors

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

Zhenhua Wang: 0000-0002-9028-6799 Zhanhua Wang: 0000-0003-0493-1905 Xiangcheng Pan: 0000-0003-3344-4639 Jiajun Yan: 0000-0003-3286-3268 Krzysztof Matyjaszewski: 0000-0003-1960-3402 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the NSF (CHE-170749) for financial support. HX acknowledges the National Natural Science Foundation of China (51473094). ZW gratefully acknowledges financial support from China Scholarship Council.

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REFERENCES

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