Chapter 14
Reversible Deactivation Radical Polymerization: Mechanisms and Synthetic Methodologies Downloaded from pubs.acs.org by UNIV OF ADELAIDE on 10/14/18. For personal use only.
Hydroxyl Radical Activated RAFT Polymerization Thomas G. McKenzie, Amin Reyhani, Mitchell D. Nothling, and Greg G. Qiao* Department of Chemical Engineering, Polymer Science Group, The University of Melbourne, Parkville 3010, VIC, Australia *E-mail:
[email protected].
In this chapter, we review some recent examples of RAFT polymerization initiated by highly reactive hydroxyl radicals. A number of approaches to generate hydroxyl radicals to initiate polymerization are described, including the Fenton reaction, photocatalytic processes, enzymatic cascade reactions, and the direct sonolysis of water. Although hydroxyl radicals are thought to be indiscriminate due to their highly reactive and short-lived nature, the results outlined here for initiating RAFT polymerizations are promising, with excellent control over the resultant polymer dispersities and “livingness” observed in most cases.
Introduction Reversible addition-fragmentation chain transfer (RAFT) polymerization (1) has emerged as one of the most versatile methods of reversible deactivation radical polymerization (RDRP), affording access to polymers of controlled molecular weights with narrow dispersities, as well as predefined macromolecular architectures (2–4). RAFT polymerization allows excellent deterministic precision in the physical/material properties of the resultant polymer products. Advances in the concepts and synthetic execution of precision polymers is forecast to provide significant value across a wide range of applications over the next decade (5). © 2018 American Chemical Society
RAFT operates via a mechanism of efficient chain transfer using distinctive thiocarbonylthio-containing compounds, referred to as ‘RAFT agents’. Although several novel uses for RAFT agents have recently appeared in the literature (6), a ‘traditional’ RAFT polymerization reaction requires the addition of a radical initiator to activate the polymerization process. A wide variety of radical sources have been explored as initiators for RAFT polymerization (see Figure 1), and the appropriate selection of an initiator/initiation system can be profoundly important for achieving optimal results (7). For example, where spatial and/or temporal control over the polymerization is desired, a photo-active radical initiator may be selected. Alternatively, RAFT polymerization conducted in the presence of biomolecules (such as grafting-from polymerization from a protein-conjugated RAFT agent) frequently require an aqueous initiation system, active under ambient conditions so as not to decompose the biomolecule. In this case, a redox initiation system may be the most suitable choice. Finally, where a RAFT-derived polymer is required quickly and in a straightforward manner, then, a thermally-activated initiator such as an azo-containing compound (e.g. AIBN) may be sufficient. The broad tolerance of RAFT toward different sources of initiation confers a significant advantage over other RDRPs in terms of its generalizability. In many cases carbon-centered radicals have been chosen as radical initiators for RAFT reactions, as they show excellent addition towards unsaturated (vinyl) double bonds. However, several examples have emerged more recently where hydroxyl radicals – ubiquitous in biology as powerful oxidising agents – are employed as radical initiators for RAFT polymerization. Hydroxyl radicals are encountered frequently in nature (10), constituting one of the most highly reactive oxidising agents known, with a standard reduction potential of Eo (•OH/H2O) = 2.80 V/SHE. In comparison, H2O2 alone is a considerably weaker oxidising agent (Eo (H2O2/H2O) = 1.76 V/SHE in acidic media, Eo (H2O2/OH-) = 0.88 V/SHE in alkaline conditions) (11). The biochemical reactions of hydroxyl radicals in nature are the subject of intense ongoing investigation, having been linked to a range of deleterious biological processes, from cancer to the effects of aging (12). In an industrial context, they have long been employed as oxidising agents for the degradation of organic contaminants in wastewater streams (13), and as initiators for the polymerization of several vinyl compounds (14). Hydroxyl radicals are known to chemically attack organic molecules through three primary mechanisms: 1) abstraction of hydrogen to yield water and a substrate-derived radical; 2) “hydroxylation”, or electrophilic addition of •OH to an unsaturated carbon-carbon bond, and 3) (re)combination to yield hydrogen peroxide (Figure 2) (11). Their use as radical initiators in controlled polymerization – particularly in RDRP – has been reported only recently, and there remains a significant amount yet to be investigated. In particular, the potential for undesired oxidative degradation of other components of the polymerization reaction mixture (15), as well as other uncontrolled side reactions, requires further understanding. However, these unique and highly reactive species appear to be very effective in their capacity for initiating polymerization. In this chapter, we present some examples of hydroxyl radical initiation of RAFT reactions. While commenting on the associated benefits of such initiation systems and highlighting some specific 308
examples where hydroxyl-radical initiation has been particularly effective, we also identify aspects where future research in this field may be directed.
Figure 1. Some examples of commonly used initiation systems for RAFT polymerization; including thermally-, redox-, and photo-activated radical formation. Note: this outline is not exhaustive, and other types of activation (e.g. acid-activation (8)), or even non-radical methods (9) have been omitted for brevity.
Fenton-RAFT Polymerization In the late 19th century, H.J.H Fenton reported the facile oxidation of malic acid in the presence of hydrogen peroxide (H2O2) and ferrous ions (Fe2+) – a combination that would come to be known as “Fenton’s reagent (16)”. Some years later, it was shown that hydroxyl radicals are the active oxidizing agents in this system, and that their formation is the result of the catalytic decomposition 309
of H2O2 in the presence of Fe2+ via a complex reaction sequence, with the key reactions presented below (17, 18):
Fenton chemistry has gone on to find widespread application in the oxidation of organic compounds, particularly as a strategy for water remediation (13). Due to the low cost and ready availability of the required reagents, the Fenton reaction has also been used extensively as a redox initiation system for radical polymerization (14, 19, 20). Recently, our group has harnessed the power of Fenton chemistry for the initiation of a RAFT process (Figure 3a), termed Fenton-RAFT polymerization (21). At ambient temperature, an ultrafast aqueous RAFT polymerization of different water-soluble monomers (N,N-dimethylacrylamide (DMA), N-acryloylmorpholine (NAM), and 2-hydroxyethyl acrylate (HEA)), together with a symmetrical trithiocarbonate chain transfer agent (CTA) (Figure 3b), produced well-defined (Ð < 1.1) linear homopolymers with high conversions (> 70%) within 1 minute. The optimal concentrations of the Fenton reagents were found to be: [H2O2]0 = 11 mM and [Fe2+]0 = 3.3 mM, i.e. a peroxide to iron ratio of approximately 3 to 1, which agrees well with other observations in the literature (11). It is generally accepted that both the ratio [H2O2]/[Fe2+], and [Fe3+]/[Fe2+] play an important role to ensure the efficient production of hydroxyl radicals, while minimizing “wasting” reactions (21). Near-quantitative monomer conversions were also achieved within 15 minutes when higher concentrations of H2O2 were employed. In all cases, obtained experimental number-averaged molecular weights (Mn) were in close agreement with theoretical values.
Figure 2. Common reactions involving hydroxyl radicals (•OH) in the domains of biology and chemistry. 310
Figure 3. a) Fenton initiation system for RAFT polymerization; b) Monomers and chain transfer agents (CTA or RAFT agents) employed.
To test the living character of the synthesized polymers, a chain extension was performed on an isolated macro-RAFT poly(DMA) (Figure 4a), while different chain lengths were readily accessible by tuning the ratio of monomer to CTA. A comparison between an analogous metal-free redox system of ascorbic acid (AA) and H2O2 and the Fenton system demonstrated a much higher efficiency of hydroxyl radical generation early in the reaction when iron is present, giving almost instantaneous polymer growth (Figure 4b). The Fenton-RAFT system was also shown to be relatively stable towards polymerization in air (i.e., without traditional degassing); however, multiple injections of H2O2 were required, and the resulting GPC chromatograms were broadened compared with the degassed case (21). Nevertheless, Fenton-RAFT represents an exciting new technique for the rapid synthesis of controlled, “living” polymers using cheap and relatively benign reagents. The aqueous conditions and rapid reaction times make this approach of particular interest for the development of machine-programmable, or high-throughput, polymer synthesis for biomedical applications (22–24). 311
Figure 4. a) The Fenton-RAFT polymerization of an isolated macro-CTA provides a well-defined pseudo-block copolymer in only 1 minute (GPC-DRI chromatograms shown); b) Comparison of the reaction kinetics for a Fe2+/H2O2 (Fenton) redox pair vs. the metal-free ascorbic acid (AA)/H2O2 initiation system. Reproduced with permission from ref. (21). Copyright 2017 John Wiley and Sons. 312
Photocatalyst-based Polymerization The use of an AA/H2O2 redox pair for the generation of hydroxyl radicals has also been studied in a photo-induced electron/energy-transfer (PET) RAFT reaction system. Initially employed to engender oxygen tolerance (25), ascorbic acid was later found to enable the continuation of chain growth after the light source has been turned off (26). This so-called “dark” polymerization system is thought to proceed via a distinctly different mechanism from the light-activated PET-RAFT (Figure 5a), although it should be noted that the light source is still required to generate H2O2 in situ, allowing for a degree of temporal control over when the reaction begins. Interestingly, even with an irradiation period of only 5 minutes, the polymerization continued in the dark up to high monomer conversions (> 70%). This indicates that enough H2O2 had been formed during the period of illumination to sustain the full polymerization. Reducing the irradiation period further to 1 min, or 30 s, did not decrease the apparent rate significantly, although an extended induction period became evident (up to 90 min). This was proposed to be due to the incomplete removal of dissolved oxygen at short irradiation times. Both Rose Bengal, and Eosin Y (Figure 5b) were shown to be effective photocatalysts for enabling the production of singlet oxygen (1O2) under visible light irradiation. The versatility of this technique was demonstrated for the polymerization of acrylamide, acrylate, and methacrylate-type monomers, as well as the use of both trithiocarbonate and dithiobenzoate RAFT agents. This versatility is an important observation for hydroxyl radical-initiated RAFT polymerizations, as other studies have often been limited to water-soluble monomers and RAFT agents, in some cases reporting difficulties polymerizing methacrylates (21). This indicates that the lack of control observed in these alternate cases likely stems from the choice of RAFT agent, rather than the nature of hydroxyl radical initiation. Mechanistic studies to verify the role of hydroxyl radicals as the key initiating species in the dark polymerizations included investigations into the amount of oxygen present (through controlled additions of different volumes of air), UV-vis monitoring of the photocatalysts, and direct detection of hydroxyl radicals via a fluorescent dye assay. These studies inferred that the amount of oxygen initially present dictates the amount of H2O2 (and eventually hydroxyl radicals) formed. Additionally, as ascorbic acid is partially consumed during the reduction of the photocatalyst in its excited state (generating the reduced, non-active, Leuco-dye form), the observation that the rate of polymerization decreases with increasing catalyst loading is reasoned to be due to a smaller amount of residual ascorbic acid being left to react with the H2O2 during the non-illuminated, polymerization phase. This led to the interesting behaviour in that the polymerization could be re-activated via the introduction of a small volume of air, combined with a brief period of irradiation. This directly contrasts with traditional radical polymerization, in which exposure to air is often used to quench the polymerization. Lastly, the stability of the trithiocarbonate RAFT agents toward potential H2O2-mediated radical oxidation (15, 27) was investigated via UV-vis spectroscopy. No degradation of the C=S bond was observed over 8 313
hours, indicating good stability of the employed RAFT agents towards unwanted oxidation under the given conditions.
Figure 5. a) Proposed mechanism of PET-RAFT with “dark” polymerization via a redox reaction that generates hydroxyl radicals from in situ generated hydrogen peroxide, and b) the structures of the photocatalyst dyes employed.
This type of “energy-storage” polymerization system may find use in applications where temporal control is still desired, but continuous irradiation may not be achievable or practical – e.g., dental curing, adhesives, or specialized coatings where light penetration is problematic.
Enzyme-Assisted Initiation Systems Biological catalysts are becoming increasingly utilized in polymer chemistry for both polymer synthesis (28–32) and post-polymerization modification (33) due to their ability to perform well under biologically-relevant conditions (i.e., low temperature, atmospheric pressures, aqueous environments, etc). Glucose oxidase (GOx) in particular has been used extensively (34–36) as a deoxygenating agent 314
that can catalyse the oxidation of β-D-glucose (DG) to D-gluconic acid (DGA) in the presence of oxygen, generating H2O2 as a by-product. While most studies to date have ignored the generated H2O2 as an unimportant by-product, An et al. designed a redox initiation system that utilizes this in situ generated H2O2 via pairing it with added ascorbic acid (AA), leading to the generation of hydroxyl radicals for the initiation of RAFT polymerization (Figure 6) (37).
Figure 6. a) An “enzyme-assisted” initiation system to generate reactive hydroxyl radicals. Glucose oxidase (GOx) is employed for the in situ conversion of dissolved oxygen to hydrogen peroxide in the presence of β-D-glucose. H2O2 can then be coupled into a redox pair to generate the hydroxyl radicals necessary to initiate a RAFT polymerization; b) Monomers and chain transfer agent reported in ref. (37).
Well-defined polymers were obtained in both open and sealed vessels without prior deoxygenation, with linear increases in the molecular weight with conversion and pseudo-first order kinetics observed for both cases following a brief (ca. 0.5 h) induction period (37). The effects of temperature, stirring 315
speed, and pre-polymerization incubation time were investigated, along with optimization of the reagent concentrations. The key findings from this work include: 1) that 5 minutes of pre-polymerization incubation was sufficient to almost completely remove dissolved oxygen from the reaction mixture via the GOx pathway; 2) a mild stirring speed is favourable, as too vigorous stirring can introduce excess oxygen (particularly in open vessels), which acts to slow down or terminate the polymerization; and 3) for DG concentrations < 100 mM, increasing the initial DG concentration increases the observed polymerization rate, while concentrations above this threshold show minimal further enhancement, probably due to enzyme saturation kinetics. The initiation pathway reported in this work is different from those reported by Stevens and co-workers (35, 38), but provides a novel method for metal-free redox initiation utilizing enzymatic-degassing.
Sonochemical Initiation Systems Ultrasound has been used extensively in polymer chemistry (39, 40) as a means of polymer degradation (41), synthesis (42–45), or (network) re-arrangement (46, 47). Recent interest in the use of ultrasound-induced mechano-chemistry for a controlled polymerization via atom transfer radical polymerization (ATRP) (48, 49) led us to investigate the chemical effects of ultrasound for controlled polymerization. Importantly, these two effects (mechanical and chemical) can both be accessed via ultrasonic irradiation of a liquid by changing the frequency of the applied sound waves. The chemical effects of ultrasound usually stem from cavitation within a solvent, followed by the violent collapse of cavitation bubbles resulting in the localized formation of various free radical species. In water, these species are typically hydrogen (•H) and hydroxyl (•OH) radicals (50). We have recently demonstrated that this pathway toward hydroxyl radical generation can be utilized as an initiation system for aqueous RAFT polymerization (Figure 7a) (51). The apparatus used to study Sono-RAFT polymerization consisted of an ultrasonic plate and transducer capable of supplying high frequency ultrasound (414 kHz) to a jacketed water bath. The aqueous reaction mixture consisting solely of monomer and RAFT agent was firstly degassed with argon, then submerged into the ultrasonic bath, and the power switched on to start the reaction. The polymerization of water-soluble (meth)acrylates and acrylamides proceeded smoothly for a range of different targeted degrees of polymerization, giving monomodal GPC peaks with low dispersities and molecular weights in agreement with their targeted values (Figure 8a-b). Control experiments without added RAFT agent gave a high molecular weight polymer with uncontrolled characteristics, as expected for a free radical polymerization. Interestingly, temporal control was achievable in this system by switching the ultrasound on/off (Figure 8c). This effect is thought to be similar to that observed in photo-initiated RAFT polymerization, whereby a constant supply of radicals is required to sustain the RAFT process. This has intriguing implications for the operating mechanism, as it would imply a larger degree of irreversible termination events than are commonly seen in the resultant polymer structure. 316
In the case of the sonochemical ATRP reaction described by Esser-Kahn et al. (48), and further developed by Matyjaszewski and co-workers (49, 52), a mechanically-active agent is used so that under high shear conditions generated by low frequency ultrasound (typically < 50 Hz) this “piezocatalytic” agent can enact the reduction of Cu(II) to Cu(I), which in turn activates the ATRP process via its well-known mechanism (53). Aside from the lower frequencies used for this process potentially limiting the obtainable polymer chain lengths due to shear-induced polymer degradation reactions (54), the polymerization kinetics were found to be highly dependent on the crystal structure and particle size of the added piezocatalyst (49). Conversely, in the Sono-RAFT approach, the ability to generate hydroxyl radicals without the addition of exogenous catalysts, substrates, or reagents makes it highly promising for the “clean” synthesis of controlled polymers. However, sonochemical events can be strongly affected by physical parameters such as reagent vapour pressures, reaction mixture viscosity, and even dissolved gas type (55). For this reason, a generalizable approach to Sono-RAFT remains a significant challenge, particularly as many of these sonochemical parameters can change dramatically throughout the course of a polymerization reaction.
Figure 7. a) Sonolysis of water results in the generation of hydroxyl radicals which can then initiate a RAFT polymerization (note: in the absence of monomer these will rapidly combine to produce hydrogen peroxide); b) Water-soluble monomers and chain transfer agents investigated for Sono-RAFT polymerization. 317
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Figure 8. a) GPC chromatograms of polymer growth in Sono-RAFT; b) Molecular weights increase with monomer conversion, with low dispersities maintained throughout; c) Temporal control over polymer growth demonstrated by switching the ultrasound on and off at regular intervals. Adapted with permission from ref. (51). Copyright 2017 John Wiley and Sons.
Conclusions In this chapter, we have discussed the use of hydroxyl radicals as the initiating species for RAFT polymerization, and described several “initiation systems” for generating hydroxyl radicals in situ. We believe that the promising early work on hydroxyl radical-initiated RAFT is worthy of consideration from the experimentalist when planning conditions for future polymerization studies. Moreover, the prevalence of hydroxyl radicals (and reactive oxygen species in general) in biology may raise interesting new avenues for bio-inspired systems, or for direct utilization of bio-produced radicals for the initiation of RAFT polymerizations.
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