Rapid Visible Light-Mediated Controlled Aqueous Polymerizationwith In Situ MonitoringJia Niu,†,‡,§ Zachariah A. Page,‡ Neil D. Dolinski,‡ Athina Anastasaki,†,‡ Andy T. Hsueh,∥

H. Tom Soh,⊥,# and Craig J. Hawker*,†,‡,∥

†California NanoSystems Institute, ‡Materials Research Laboratory, and ∥Department of Chemistry and Biochemistry, University ofCalifornia, Santa Barbara, California 93106, United States§Department of Chemistry, Boston College, Chestnut Hill, Massachusetts 02467, United States⊥Department of Radiology, School of Medicine, and #Department of Electrical Engineering, Stanford University, Stanford, California94305, United States

*S Supporting Information

ABSTRACT: We report a simple procedure for rapid, visible light-mediated, controlled radical polymerization in aqueous solutions.Based on the photoelectron transfer reversible addition−fragmenta-tion chain transfer (PET−RAFT) polymerization, fast chainpropagation at room temperature in water was achieved in thepresence of reductant and without prior deoxygenation. Asystematic study correlating irradiation intensity and polymerizationkinetics, enabled by in situ nuclear magnetic resonance spectrosco-py, provided optimized reaction conditions. The versatility of thisprocedure was demonstrated through a rapid triblock copolymersynthesis, and incorporation of water-labile activated esters for directconjugation of hydrophilic small molecules and proteins. In addition, this technique boasts excellent temporal control andprovides a wide range of macromolecular materials with controlled molecular weights and narrow molecular weight distributions.

The development of controlled radical polymerization(CRP) techniques has enabled the synthesis of well-

defined functional polymers with predefined molecular weightand low dispersity (Đ).1,2 In particular, CRP techniques thatoperate in water have attracted significant attention, since theygrant access to hydrophilic monomers and serve as a moreenvironmentally friendly and biocompatible alternative toorganic solvents.3−7 Nevertheless, traditional aqueous CRPtechniques often rely on either metastable Cu(I) catalysts thatreadily undergo disproportionation in water or operation atelevated temperatures, resulting in diminished control and aloss in chain end-fidelity during the polymerization.3,4,8

Recently, there has been interest in the development ofvisible-light mediated CRP techniques that offer spatiotemporalcontrol and room temperature operation.9−19 For example,Hawker, Miyake, Yagci, and others have developed visible light-mediated atom transfer radical polymerization (photoATRP)strategies,9,20−22 while Cai and Boyer expanded the platform tophotoinitiated and photoelectron transfer reversible addition−fragmentation chain transfer (PET-RAFT). These advancesprovide polymers with low Đ and excellent chain-endfidelity.23−28 Concurrently with this work, PET-RAFTconditions were developed that allow for room temperatureaqueous CRPs, though reaction rates are slow, resulting insignificant irradiation times.26 Faster reaction rates are highlydesired as they would reduce exposure time, making themethodology compatible with hydrolytically stable monomers

and in situ bioconjugation.7,29 Alternatively, the photoinitiatedRAFT system developed by Cai and co-workers showedimpressive reaction rates, but required deoxygenation prior topolymerization.27,30 In this report, we address this challengethrough the design of simple, room temperature-based aqueousCRP procedures that reach high conversion within minutes.Optimal reaction conditions that balance propagation rate andpolymerization control are systematically identified using arecently developed in situ NMR monitoring technique.31 Giventhat high conversion and chain-end fidelity is achievable withthis platform, the rapid fabrication of triblock copolymers, andincorporation of hydrolytically unstable activated ester buildingblocks for in situ bioconjugation of small molecules andproteins are explored.I n i t i a l s t u d i e s e x am in ed t r i s ( 2 , 2 - b i p y r i d y l ) -

dichlororuthenium(II) (Ru(bpy)3Cl2) as a water-soluble photo-catalyst for PET-RAFT, which, upon photoexcitation, under-goes electron/energy transfer to induce polymerization (FigureS3a).23,26 However, in this mechanism the presence of traceoxygen and the generation of singlet oxygen greatly retardschain propagation (Figure S3b).32 It was hypothesized that theaddition of a reductant would mitigate this issue by preferential

Received: August 8, 2017Accepted: September 18, 2017Published: September 22, 2017

Letter

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© 2017 American Chemical Society 1109 DOI: 10.1021/acsmacrolett.7b00587ACS Macro Lett. 2017, 6, 1109−1113

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reaction with singlet oxygen, while simultaneously serving as anelectron source to improve catalyst turnover (Figure S3c).32−36

To this end, the polymerization of N,N′-dimethylacrylamide(DMA) as the monomer, 2-(butylthiocarbonothioyl) propionicacid (BTPA) as the chain transfer agent (CTA), and sodiumascorbate (NaAsc) as the reductant was studied under ambientconditions in water (Figure 1a). The initial experiments werescreened inside an NMR spectrometer using a fiber-coupled470 nm light emitting diode (LED; Figure 1b), revealing fastand controlled polymerization kinetics. Specifically, targeting adegree of polymerization (DP) of 400 at a light intensity of 26mW/cm2 provided high monomer conversion (∼85%) in lessthan 40 min. We attributed the short induction period of ∼3min to oxygen consumption;32,37,38 accordingly, deoxygenationof the reaction mixture prior to polymerization eliminated thisinitial inhibition (Figure S4). The evolution of the experimentalnumber-average molecular weight (Mn), as measured by sizeexclusion chromatography (SEC), with respect to monomerconversion is linear during the course of the reaction, indicatinggood control over polymerization (Figure S5). Additionally,replacing NaAsc with sodium acetate (NaOAc) led to noobservable polymerization over a period of 4 h (Figure S6),which attests to the importance of a reducing agent to quenchsinglet oxygen. The role of oxygen in polymerization inhibitionwas further probed via a method established by Boyer and co-workers, where anthracene-9,10-dipropionic acid disodium salt(ADPA) is used as a singlet oxygen sensitizer.32,39,40 Using thistechnique, we observed a significant reduction of singlet oxygenevolution in the presence of NaAsc being observed (Figure S7).While the presence of NaAsc proved critical to achieve rapid

and controlled aqueous PET-RAFT polymerizations withoutthe need for deoxygenation, a deeper understanding of theeffects of light intensity was particularly desirable for furtherdevelopment of this system.A digitally controlled LED coupled with in situ NMR

monitoring offers the capability to simply and systematicallystudy the effect of light intensity on these PET-RAFT systems.Tuning the light intensity (470 nm) from 8 to 139 mW/cm2

revealed a direct linear relationship with kp, while maintaininglog−linear kinetics for all intensities measured as well as narrowmolecular weight distributions (≲1.4) at high monomerconversion (≥85%), consistent with controlled polymerizationprocesses (Figure 1c,d). In addition, for faster polymerizationspeeds at higher light intensities, induction times are shortened(Figure S8). However, it is noteworthy that a slightcompromise between light intensity and Đ exists (i.e., higherintensity leads to larger Đ), as seen in Figure 1d and in the SECtraces of the resulting polymers (Figure S9), which may beattributed to an increase in side reactions at high kp

app, such asphotolysis of the trithiocarbonate chain-end (Figure S10 andS11) and radical termination by chain−chain coupling. Thissystematic study revealed a desirable balance between kp

app andĐ at a light intensity of 26 mW/cm2, and as such, this intensitywas used in all subsequent studies, unless otherwise indicated.Temporal control was then investigated using in situ NMR

monitoring with automated “on”/“off” cycling of the LED(Figure 1d). Rapid chain propagation during illumination issignificantly decreased when the light is turned “off” andincreases quickly with the next “on” cycle with minimalinhibition and similar rates of polymerization. This “on”/“off”

Figure 1. PET-RAFT monitored using in situ NMR spectroscopy. (a) Reaction scheme. (b) Illustration of the in situ NMR setup. (c) Reactionkinetics at different light intensities, represented by the plot of ln(M0/MT) vs reaction time (T), whereM0 andMT are the concentration of monomerat time zero and T, respectively. (d) Apparent propagation rate constants (kp

app) and dispersity of the resulting polymers with respect to lightintensity. (e) Temporal control during automated “ON” and “OFF” cycling of the light source. Light intensity: 26 mW/cm2.

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cycling was repeated four times, exemplifying the excellenttemporal control that is achievable under these conditions.To examine the versatility of this technique, the targeted

molecular weights for DMA polymerizations were varied andthe monomer scope was expanded. Reactions with DP rangingfrom 25 to 400 were conducted at a constant monomerconcentration and CTA/catalyst/reductant ratio, along withlight intensity (26 mW/cm2, Table 1, entries 1−5). Addition-ally, at low DPs (i.e., high CTA concentrations) 20 vol %DMSO was added as cosolvent to solubilize BTPA.Significantly, for a wide range of targeted molecular weights,monomer conversions of greater than 80% could be achievedwithin ∼15−30 min. This reaction rate is markedly higher thanpreviously reported PET-RAFT systems, which take 5−10×longer to reach the same monomer conversion.26,37 Addition-ally, polymerizations of N,N′-diethylacrylamide (DEA) and N-acryoylmorpholine (NAM) with a targeted DP = 40 weresuccessfully conducted without deoxygenation, achieving >95%monomer conversion within 15 min. Both polymers exhibitedlow Đ and molecular weights that were consistent withtheoretical values (Table 1, entries 6 and 7).One of the hallmarks of a CRP process is the ability to

generate well-defined architectures, such as multiblockcopolymers. To achieve such structures, high polymer chain-end fidelity is essential, which was confirmed with electrosprayionization mass spectrometry (ESI-MS) for a poly(NAM)sample prepared using the presented approach (Figure S12).To take advantage of the rapid PET-RAFT system and theresulting excellent chain-end fidelity, one-pot block copoly-merizations were conducted, which mitigates the need forintermediate purification as it relies on quantitative monomerconversion for each consecutive block. First, NAM waspolymerized to nearly quantitative conversion (97%) after 15min of irradiation (26 mW/cm2), followed by DEA (97%, 15min) and DMA (94%, 20 min). The shift to higher molecularweight after each block addition was tracked using SEC,revealing a low Đ (<1.3) that was maintained throughout(Figure 2). The small high molecular weight peak observed forpoly(NAM) in Figure 2 is attributed to chain−chain coupling athigh conversion, indicated by its continued presence inpoly(NAM-b-DEA) and poly(NAM-b-DEA-b-DMA). Notably,the triblock copolymer was synthesized in less than 1 h, withoutthe need for deoxygenation. These results clearly demonstratethe efficiency of the fast PET-RAFT process, and highlight itsutility to prepare well-defined water-soluble triblock copoly-mers.Given the significant implications of polymer bioconjugates

in biotechnology and medicine,41,42 it was desirable toinvestigate whether common bioconjugation approaches were

compatible with the presented technique. For example,activated esters, such as N-hydroxysuccinimidyl (NHS) andpentafluorophenyl (PFP) esters, have been extensively used forbioconjugation of amine-containing compounds. Althoughthese activated ester monomers have been incorporated intocopolymers,43−47 their poor water solubility, and propensity tohydrolyze over time limits their utility in aqueous systems. Toovercome these deficiencies, a water-soluble monomer, N-acryloxysulfosuccimide (NASS), was synthesized and itsincorporation in hydrophilic copolymers was examined usingthe present aqueous PET-RAFT procedure. It is noteworthythat the rapid nature of the polymerization enabled theincorporation of NASS with minimal degradation, therebyallowing for subsequent in situ conjugation (Figure 3a). Tofurther quantify the rate of hydrolysis, NASS was placed in ananalogous polymerization environment (100 mM NaAsc(aq),pH = 8) and monitored with 1H NMR spectroscopy, revealinga half-life of ∼60 min (Figure S13). Therefore, a 15 mincopolymerization should afford ≥85% of the activated estergroups intact. To demonstrate this ability, the polymerization

Table 1. Summary of Polymerization Conditions and Results

entry[M]/[BTPA]/ [catalyst]/

[reductant] monomer solventcatalyst concn

(mM)time(min) αa (%) Mn,theo

bMn,exp

c

(SEC) Đ

1 400:1:1 × 10−3:2 DMA water 6.25 × 10−3 45 90 39900 43200 1.232 200:1:1 × 10−3:2 DMA water 1.25 × 10−2 30 90 18100 22100 1.243 100:1:1 × 10−3:2 DMA water 2.5 × 10−2 30 80 8150 7010 1.284 40:1:1 × 10−3:2 DMA water + 20 v% DMSO 6.25 × 10−2 20 95 4000 6000 1.215 25:1:1 × 10−3:2 DMA water + 20 v% DMSO 1.0 × 10−1 20 90 2460 1500 1.306 40:1:1 × 10−3:2 DEA water + 20 v% DMSO 6.25 × 10−2 15 96 5100 7020 1.267 40:1:1 × 10−3:2 NAM water + 20 v% DMSO 6.25 × 10−2 15 95 5600 6100 1.22

aMonomer conversion (α) determined by 1H NMR spectroscopy. bTheoretical molecular weight calculated based on monomer conversion.cMolecular weight and Đ (Mw/Mn) determined using SEC (DMF with 0.1% LiBr as eluent, relative to PS standards). Light intensity: 26 mW/cm2.

Figure 2. SEC analysis of the poly(NAM-b-DEA-b-DMA) one pottriblock copolymer synthesis. Samples were withdrawn from thereaction after each block and SEC was performed with DMF as theeluent and molecular weights are relative to polystyrene standards.

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was performed with a 9:1 monomer mixture of NAM/NASSand an irradiation intensity of 80 mW/cm2, affording monomerconversions of 95% and 87% for target DPs of 40 and 400,respectively. NAM was chosen as the comonomer due to itssimilar reactivity ratio with NHS-containing acrylate mono-mers.44,48 Both polymers demonstrated low Đ (≤1.3; FigureS14), suggesting good control. Monitoring the reaction kineticswith in situ NMR spectroscopy indicates that NASSincorporates slightly faster than NAM (Figure 3b), where 1HNMR resonances at 2.58, 2.80, and 3.92 ppm correspond toNASS, indicating successful incorporation (∼12 mol %; Figure3c, top spectrum). To further confirm the existence of intactactive esters and demonstrate the feasibility of in situconjugation, amine-containing small molecules and proteinswere added to the crude postpolymerization mixture. Both L-phenylalanine (Phe) and 5-[(2-aminoethyl)amino]-naphthalene-1-sulfonic acid sodium salt (EDANS) were reactedin molar excess with poly(NAM-co-NASS) over the course of 4h at room temperature. 1H NMR spectroscopy revealed 3% and8% incorporation of Phe and EDANS, respectively (Figure 3c,middle and bottom spectra, and Figure S15). The graftingdensity for the EDANS−polymer conjugate was also quantifiedusing fluorescence spectroscopy, finding 11.7 mol %, which wasin reasonable agreement with NMR and with the original feedratio (Figure S16). The difference in grafting efficiency isattributed to the reduced steric hindrance and highernucleophilicity of the amino group of EDANS relative toPhe. Additionally, the in situ grafting-to conjugation of proteinswas tested using bovine serum albumin (BSA). Significantly,this was shown to proceed with high efficiency using sodiumdodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), affording 88% yield of conjugation (Figure S17).Overall, the in situ postpolymerization conjugation of smallmolecules and proteins highlights the utility of rapid PET-

RAFT polymerizations for the preparation of functionalmaterials and bioconjugates.In summary, a visible light-mediated, rapid, and controlled

radical polymerization strategy in aqueous solution at roomtemperature is reported. In situ NMR spectroscopy was used tosystematically correlate light intensity to propagation rate,dispersity, and inhibition time, while simultaneously demon-strating temporal control during “on”/“off” cycling. The utilityof this rapid polymerization technique is highlighted by thefacile one-pot fabrication of triblock copolymers in less than 1h. Furthermore, the synthesis of a water-soluble activated estermonomer and subsequent in situ incorporation into copoly-mers and conjugation with amine-containing compounds inwater was achieved with high levels of functionalization andonly minimal hydrolysis. Given the versatility and fast kineticsof the presented approach, a wide range of hydrophilic andfunctional polymers are now available to nonexperts withexciting potential in biorelated applications.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acsmacro-lett.7b00587.

Additional experimental details (PDF).

■ AUTHOR INFORMATIONCorresponding Author*E-mail: hawker@mrl.ucsb.edu.ORCIDJia Niu: 0000-0002-5622-6362Zachariah A. Page: 0000-0002-1013-5422Craig J. Hawker: 0000-0001-9951-851X

Figure 3. In situ postpolymerization conjugation. (a) Reaction scheme. The light intensity was increased to 80 mW/cm2 to increase the reaction rate.(b) Reaction kinetics of the copolymerization of NAM/NASS = 9:1, target DP = 400. (c) 1H NMR characterization of the small moleculeconjugation. From top to bottom: the purified polymer with NASS incorporated (top); the polymer conjugated with Phe (middle); and the polymerconjugated with EDANS (bottom). The insets show the magnified characteristic peaks of the conjugates.

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NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSWe thank the financial support of DARPA (N66001-14-2-4055), MRSEC program of the National Science Foundation(DMR 1121053), and the Institute for CollaborativeBiotechnologies through Grant W911NF-09-0001 from theU.S. Army Research Office for financial support. The content ofthe information does not necessarily reflect the position or thepolicy of the Government, and no official endorsement shouldbe inferred. A.A. is grateful to the European Union (EU) for aMarie Curie Postdoctoral Fellowship and the CaliforniaNanoSystems Institute for an Elings Prize Fellowship inExperimental Science. We thank Dr. Sivaprakash Shanmugamfor his suggestions regarding ADPA and Dr. Hongjun Zhou fortechnical help with the NMR experiments.

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