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with a hydrogen atom abstraction step that cre-ates a stereogenic carbon center (41). Using ele-ments of thework demonstrated herein, itmay bepossible to create previously inaccessible stereo-chemical permutations—a process ideal from thevantage point of diverse analog preparation.
REFERENCES AND NOTES
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Synthesis (Wiley, 2011).6. M. Razzak, J. K. De Brabander, Nat. Chem. Biol. 7, 865–875 (2011).7. R. A. Yoder, J. N. Johnston, Chem. Rev. 105, 4730–4756 (2005).8. S. Kim, D. S. Shin, T. Lee, K. B. Oh, J. Nat. Prod. 67, 448–450
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ACKNOWLEDGMENTS
Financial support for this work was provided by the Universityof California, Berkeley (UC Berkeley); the UC Berkeley HellmanFellows Fund; the Alfred P. Sloan Foundation; and the NIH NationalInstitute of General Medical Sciences (grant R01GM116952).
Z.G.B. acknowledges UC Berkeley and the NSF for a BerkeleyFellowship and an NSF Graduate Research Fellowship (awardDGE-1106400), respectively. We thank N. Shin for technicalassistance, A. DiPasquale for x-ray crystallographic analysis andsupport from an NIH Shared Instrument grant (S10-RR027172),and N. Burns and F. Seidl (Stanford University) for various TADDOLdiols used in early stages of this work. Metrical parameters for thestructures of compounds 15 and SI-6 are available free of chargefrom the Cambridge Crystallographic Data Centre under accessionnumbers CCDC-1474968 and CCDC-1474969, respectively.
SUPPLEMENTARY MATERIALS
www.sciencemag.org/content/352/6289/1078/suppl/DC1Materials and MethodsFigs. S1 to S5Tables S1 to S12NMR SpectraReferences (42–49)
11 March 2016; accepted 26 April 201610.1126/science.aaf6742
POLYMER CHEMISTRY
Organocatalyzed atom transferradical polymerization driven byvisible lightJordan C. Theriot,1 Chern-Hooi Lim,1,2 Haishen Yang,1 Matthew D. Ryan,1
Charles B. Musgrave,1,2,3 Garret M. Miyake1,3*
Atom transfer radical polymerization (ATRP) has become one of the most implementedmethods for polymer synthesis, owing to impressive control over polymer composition andassociated properties. However, contamination of the polymer by the metal catalyst remains amajor limitation. Organic ATRP photoredox catalysts have been sought to address this difficultchallenge but have not achieved the precision performance of metal catalysts. Here, weintroduce diaryl dihydrophenazines, identified through computationally directed discovery, as aclass of strongly reducing photoredox catalysts.These catalysts achieve high initiatorefficiencies through activation by visible light to synthesize polymers with tunable molecularweights and low dispersities.
Over the past two decades, atom transferradical polymerization (ATRP) (1–4) has ma-tured into one of the most powerful meth-odologies for precision polymer synthesis (5).Strict control over the equilibrium between
a dormant alkyl halide and an active propagatingradical dictates a low concentration of radicalsand minimizes bimolecular termination to achievecontrolled polymer chain growth (6). ATRP hashistorically relied on transition-metal catalysts tomediate this equilibrium and polymerizemonomerswith diverse functionality into macromolecules withcontrolled molecular weight (MW), low MW dis-persity (Ð), defined chemical composition, andcomplex architecture (7).The caveat of traditional ATRP has been that
the transition-metal catalysts present purificationchallenges for the polymer products and impedetheir use in biomedical and electronic applications(8). Despite substantial strides in lowering catalystloading (9, 10) and facilitating purification (11),organocatalyzed methods remain highly desirablefor circumventing the need for metal removal, re-ducing toxicity concerns, and avoiding interference
with electronic systems. Organocatalyzed variantsof ATRP by use of alkyl iodide initiators have beenestablished, although they are not a broadly ap-plicable replacement for metal-catalyzed ATRP(12–14).Our interest in this field originated in 2013
with the discovery that perylene could serve asan organic visible-light photoredox catalyst (PC)to mediate an ATRP mechanism with alkyl bro-mide initiators, albeit with less control over thepolymerization than has become the benchmarkfor traditional metal-catalyzed ATRP (15–17). Ourongoing work has striven to establish organo-catalyzed ATRP (O-ATRP) for the synthesis ofpolymers with the precision of traditional ATRP,using visible-light PCs to realize energy-efficientpolymerization methods that eliminate a majorlimitation of ATRP. Although photoredox catal-ysis has been established for decades, visible-lightphotoredox catalysis has drawn increasing atten-tion by presenting the opportunity to harness solarenergy to mediate chemical transformations undermild conditions (18, 19). Phenyl phenothiazine de-rivatives have since also proven effective as PCsfor the ATRP of methacrylates (20) and acrylo-nitrile (21) but require irradiation by ultravioletlight and leave much room for improvement forgenerating polymers with higher molecular weightsand lower dispersities coupled with increased ini-tiator efficiency.Our proposed mechanism of photoredox O-
ATRP posits reversible electron transfer (ET) from
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1Department of Chemistry and Biochemistry, University ofColorado Boulder, Boulder, CO 80309, USA. 2Department ofChemical and Biological Engineering, University of Colorado,Boulder, CO 80309, USA. 3Materials Science and EngineeringProgram, University of Colorado Boulder, Boulder, CO 80309,USA.*Corresponding author. Email: [email protected]
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the photoexcited PC in order to reversibly activatean alkyl bromide initiator (Fig. 1C). In addi-tion to the requirement that the excited tripletstate 3PC* possess sufficiently strong reducingpower to activate the initiator, a delicate inter-play must be balanced between the stability ofthe radical cation 2PC•+ and its capacity to ox-idize the propagating radical so as to efficientlydeactivate the propagating polymer and yielda controlled radical polymerization.Computationally directed discovery (22, 23)
inspired us to focus on 5,10-diphenyl-5,10-dihydrophenazines as a potential class of PCs forO-ATRP (Fig. 1B). The phenazine core is sharedby several biologically relevant molecules thatserve as redox-active antibiotics (24, 25), whereassynthetic derivatives have drawn interest in organicphotovoltaics (26–28) and organic ferromagnets(29, 30). We hypothesized that an appropriateunion between the excited-state reduction potential(E0*) and the stability of the radical cation PC•+
resulting from ET to the initiator would be re-quired for the production of polymers with con-trolled MW and low Ð. As such, we investigatedelectron-donating (OMe, 1), neutral (H, 2), andelectron-withdrawing (CF3, 3, and CN, 4) moietieson the N-phenyl substituents.Density functional theory (DFT) was used to
calculate the reduction potentials of the tripletexcited-state PCs, initiator, and propagating rad-icals (Fig. 1B) (31). We found that 2 possessesa triplet excited-state reduction potential ofE0(PC•+/3PC*) = –2.34 V versus saturated calomelelectrode (SCE). Functionalization of the phenylsubstituents with an electron-donating group OMe(1) strengthened the E0* to –2.36 V, whereasintroduction of CF3 or CN electron-withdrawinggroups (EWGs) weakened the E0* to –2.24 and–2.06 V for 3 and 4, respectively, all of which iscorroborated by the measured values within ex-perimental error (table S1). The triplet excited statesof these PCs are all strongly reducing with respectto 1e– transfer to the ethyl a-bromophenylacetate(EBP) initiator; we calculated that E0(EBP/EBP•–) = –0.74 V versus SCE for an adiabatic ET,which is consistent with our cyclic voltammetryresults, which show that the onset of EBP reduc-tion occurs at ~–0.8 V versus SCE (fig. S28). Thesephenazine derivatives are significantly more re-ducing than are commonly used metal PCs (18),including polypyridyl iridium complexes (E0* asnegative as –1.73 V versus SCE) that have beenused in photomediated ATRP (32, 33). IridiumPCs are expensive, challenging to remove fromthe product, and have only been demonstratedto produce polymers with Ð as low as 1.19.The remarkable reducing power of these
dihydrophenazine-based PCs arises from a dis-tinct combination of their high triplet-state en-ergies (~2.2 to 2.4 eV) and the formation ofrelatively stable radical cations [E0(PC•+/PC) =~–0.1 to 0.2 V] upon their oxidation. These rad-ical cations are also sufficiently oxidizing todeactivate the propagating chains. We computedE0s for propagating radicals with n monomerrepeat unit (or units) bound to ethyl phenylacetate(EPA) of E0[(EPA – MMAn)/(EPA – MMAn)
•–] =
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Table 1. Results for the organocatalyzed ATRP of MMA catalyzed by 3 using white LEDs or sunlight.Asterisk indicates use of sunlight. Experimental details are provided in the supplementary materials.
Run [MMA]:[EBP]:[3] Time
(hours)
Conversion
(%)
Mw
(kDa)
Ð
(Mw/Mn)
I
[Mn(theo)/Mn(exp)]
1 [1000]:[10]:[1] 8 98.4 17.9 1.17 65.9. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .
2* [1000]:[10]:[1] 7 33.8 7.54 1.10 52.9. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .
3 [1000]:[20]:[1] 8 78.9 7.12 1.18 69.5. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .
4 [1000]:[15]:[1] 8 67.8 8.74 1.18 64.3. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .
5 [1000]:[5]:[1] 8 86.9 37.3 1.26 59.6. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .
6 [1000]:[2]:[1] 8 95.2 85.5 1.54 86.3. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .
7 [5000]:[10]:[1] 8 74.7 77.4 1.32 64.2. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .
8 [2500]:[10]:[1] 8 96.3 61.3 1.31 52.0. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .
9 [750]:[10]:[1] 6.5 53.2 7.75 1.30 71.1. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .
10 [500]:[10]:[1] 6.5 64.0 4.83 1.12 79.9. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .
Fig. 1. PC development for O-ATRP. (A) Polymerization of MMA to well-defined polymers by using photo-redoxO-ATRPdriven by sunlight. (B) Structures of the diphenyl dihydrophenazine PCs 1 to4 used in this study.(C) A proposedmechanism for ATRPmediated by a PC via photoexcitation to 1PC*, intersystem crossing (ISC)to the triplet state 3PC*,ETto formthe radical cationdoublet 2PC•+, andbackETto regeneratePCand reversiblyterminate polymerization.
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–0.74, –0.86, and –0.71 V for n = 0, 1, and 2,respectively. These E0s are sufficiently nega-tive with respect to oxidization by the radicalcations to drive rapid radical deactivation andregeneration of the PC to complete the photo-catalytic cycle.An initial series of target PCs (1 to 4) were
synthesized in two steps from commercial re-agents in good yields. Under otherwise identicalconditions, all of the PCs were tested in the poly-merization of methyl methacrylate (MMA) by usingEBP as the initiator and white light-emitting
diodes (LEDs) for irradiation in dimethylacetamide(Table 1, run 1, and table S2, runs S1 to S3). Allfour PCs proved effective in polymerization after8 hours of irradiation, with the PCs bearing EWGsexhibiting the best catalytic performance. PC3 proved superior in producing polymers witha combination of not only the lowest dispersity(Ð = 1.17) but also the highest initiator efficiency(I* = 65.9%) (I* is the ratio between the theoreticaland experimentally measured number averagemolecular weight) (Table 1, run 1). Using methyla-bromoisobutyrate as the initiator was also ef-
ficient but did not achieve the same level of con-trol of the polymerization achieved with EBP (tableS2, run S5). Additionally, polymerization couldbe driven by sunlight to produce poly(methylmethacrylate) (PMMA) with a low dispersity ofÐ = 1.10 (run 2).Time-point aliquots were taken during poly-
merization to monitor the MW and Ð progressionas a function of monomer conversion (Fig. 2,A and B). The control provided by 3 was evi-denced by the linear increase in polymer MW andlow Ð throughout the course of polymerization.
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Fig. 2. Polymerization results using PC 3. (A) Plot of molecular weight as a function of monomer conversion and (B) plot of dispersity as a function ofmonomer conversion for the polymerization of MMAmediated by 3. (C) Chain-extension from a PMMAmacro-initiator (black) to produce block copolymers withMMA (green), BMA (blue), and BA (red). (D) GPC traces of each polymer depicted in (C) (color coded).
Fig. 3. Calculated tripletstate (3PC*) frontier orbitalsand excited-state reductionpotentials E0* of diphenyldihydrophenazine PCs 1 to4. (Top) The higher-lyingSOMO. (Bottom) The low-lyingSOMO. Phenyl functionalizationwith electron-withdrawinggroups (CF3 and CN) localizesthe high-lying SOMO onthe phenyl.
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However, the y intercept of the number-averagemolecular weight (Mn) versus conversion plotwas 3.46 kDa, suggesting an uncontrolled chain-growth period with addition of ~32 MMAequivalents during the onset of polymerizationbefore precise control was attained, whereas anideal polymerization would have a y interceptequal to the mass of the initiator (MW of EBP =243 Da).We next examined the effect of adjusting the
initiator ratio relative to monomer and PC (runs3 to 6). The weight-average molecular weight (Mw)of the resulting PMMA could be modulated from7.12 to 85.5 kDa. High EBP ratios resulted incontrolled polymerizations and low dispersities(Ð = 1.26 to 1.17), and despite the moderate lossof precise control over the polymerization atlow EBP ratios (Ð = 1.54), high-MW polymer wasproduced with high initiator efficiency (Mw =85.5 kDa, I* = 86.3%). Alternatively, adjusting
the monomer ratio relative to EBP and PC reg-ulated polymer MW while also maintaining lowÐ (runs 7 to 10).One of the greatest strengths of traditional
ATRP is its capacity to synthesize advanced poly-meric architectures, including block copolymers.The reversible-deactivation mechanism enforcedin ATRP repeatedly reinstalls the Br chain-endgroup onto the polymer, and thus, isolated poly-mers can be used to reinitiate polymerization. Acombination of nuclear magnetic resonance spec-troscopy and matrix-assisted laser desorption ion-ization mass spectroscopy were used to confirmthe expected EBP-derived polymer chain-end groupsfor a polymer produced through the proposedphotoredox O-ATRPmechanism (figs. S12 and S13).Additionally, to further support the posited O-ATRP mechanism, a series of block polymeriza-tions were performed to probe the Br chain-endgroup fidelity.
First, after initial polymerization of MMA pro-ceeded for 12 hours, additional MMA was addedto the reaction mixture. Gel permeation chro-matography (GPC) analysis revealed that theMW of the resulting polymer quantitatively in-creased (fig. S22). Second, after polymerizationof MMA was allowed to proceed for 8 hours,the reaction mixture was placed in the dark for8 hours, and subsequently, additional MMA,benzyl methacrylate (BMA), or butyl acrylate(BA) was added. No polymerization took placeduring the dark period, whereas the subsequentaddition of monomer and further illuminationresulted in continued and controlled polymer chaingrowth (figs. S23, S24, and S26). Third, an isolatedpolymer was reintroduced to a solution of mono-mer and catalyst and exposed to light in order toascertain whether it would serve as a macro-initiator for the synthesis of block polymers. Thischain-extension proved successful with MMA,BMA, and BA (Fig. 2, C and D). The chain-extension polymerization from an isolated poly-mer produced from this polymerization methodfirmly supports the conclusion that this meth-odology proceeds through the O-ATRP mecha-nism, whereas all of these experiments revealedbaseline resolved peaks in the GPC traces, dem-onstrating high chain-end group fidelity.DFT calculations were performed in order to
gain insight into the differences in the perform-ances of the PCs, all of which possess similarE0(PC•+/3PC*)s and E0(PC•+/PC)s that are suffi-ciently reducing and oxidizing, respectively, to drivethe photocatalytic cycle of Fig. 1C. As such, we re-asoned that the superior performances of 4 and,in particular, 3must be qualitatively different fromthose of 1 and 2 and result from a more complexeffect.Inspection of the triplet state (3PC*) frontier
orbitals reveals qualitative differences in these PCs(Fig. 3). The low-lying singly occupied molecularorbitals (SOMOs) of all the PCs are similar, withthe electron localized over the phenazine p system.For PCs 1 (OMe) and 2 (H), the high-lying SOMO
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Fig. 4. Computationally directed discovery of PCs 5 and 6. (A) Structures of 5 and 6 and thecalculated E0*. (B) Triplet-state frontier orbitals of 5 and 6 showing the (top) higher-lying SOMO and(bottom) low-lying SOMO.
Fig. 5. Results for the polymerization of MMA using PC 6. (A) Plot of Mn and Đ versus monomer conversion for the polymerization of MMA undercontinuous irradiation. (B) Plot of monomer conversion versus time and (C) plot of Mn and Đ (solid symbols indicate after irradiation, and open symbolsindicate after dark period) versus monomer conversion, using 6 as the PC during pulsed light irradiation with white LEDs. Experimental details areprovided in the supplementary materials.
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is also localized on the phenazine rings; incontrast, for 3 (CF3) and 4 (CN), the high-lyingSOMO, occupied by the reducing e–, resides onthe phenyl ring (or rings). We contend that theCF3 and CN EWGs of 3 and 4 stabilize their p*orbitals localized on the phenyl rings relative tothe phenazine-localized p* orbital that is the high-lying SOMO of 1 and 2. This reorders the energiesof the p* orbitals so that a p* orbital localized onthe phenyls becomes the high-lying SOMO of 3and 4, although the low-lying SOMO localized onthe phenazine moiety remains singly occupied.Thus, 3 and 4 differ qualitatively from 1 and 2 inthat their two triplet electrons reside on eitherthe phenazine or the phenyl substituent and arethus spatially separated.Furthermore, a comparison of 3 to 4 elucidates
another important distinction. For 3, the high-lying SOMO is localized on one of the phenyl rings,whereas in 4, the reducing e– is delocalized overboth phenyl rings. Surprisingly, calculations re-vealed one of the C–F bonds of the CF3 func-tionalized phenyl that bears the high-lying SOMOof 3 is lengthened from 1.35 to 1.40 Å, indicatingpartial localization of electron density on the C–Fantibond. This symmetry-breaking effect in thetriplet state of 3 creates a more localized, higherelectron density of the reducing electron of 3relative to 4 while also maintaining the spatialseparation between the two SOMO electrons thatpreserves the reducing potential of the triplet.With the above observations in mind, we at-
tempted to discover even more efficient PCs tomediate O-ATRP using computational chemistryto design diaryl dihydrophenazines that possesssufficiently strong E0*s and spatially separatedexcited-state SOMOs, with the higher-energy SOMOlocalized over only one of the aromatic substituentsoff the dihydrophenazine core. Using these prin-ciples, we designed and synthesized 2-napthyl (5)and 1-napthyl (6) derivatives—with strong E0*s of–2.20 and –2.12 V, respectively—and SOMOs withthe targeted desirable geometric features (Fig. 4).Using EBP as the initiator, both PCs proved suc-cessful in the polymerization of MMA (table S2,runs S8 and S9). Although 5 produced PMMAwith an impressively low Đ of 1.03 (Mw = 9.35 kDa,I* = 46.1%)—rivaling metal ATRP catalysts—6 pro-duced PMMA with a slightly higher I* (47.5%),faster polymerization rates, and a similarly low
Đ of 1.08 (Mw = 12.3 kDa). The plot ofMn versusmonomer conversion exhibits a y intercept of850 Da, demonstrating the attainment of controlover polymerization after the addition of only~6 MMA units, which is correspondingly muchmore efficient control in the O-ATRP mediatedby 6 than is achieved with 3 (Fig. 5A). Thus, weinvestigated 6 in more detail as the PC in thepolymerization of MMA.A survey of initiators commonly used in tradi-
tional metal-catalyzed ATRP in conjunction with6 (Table 2, run 11, and table S2, runs 9 to 12)revealed that methyl 2-bromopropionate (MBP)provided the best overall results for the polym-erization of MMA (Mw = 10.6 kDa; Đ = 1.28; I* =88.1%). Furthermore, temporal control was real-ized by using a pulsed-irradiation sequence (Fig.5, B and C). Polymerization was only observedduring irradiation and paused during dark periods,and the MW steadily increased with continued ir-radiation while producing a polymer with a low Đof 1.17. Efficient control over the polymerizationby 6 is highlighted by the consistently high I*achieved over broad reaction conditions to pro-duce polymers with tunable MWs through varyinginitiator (runs 11 to 14) or monomer (runs 15 to17) ratios.We envision that this O-ATRP catalyst plat-
form will expand the application scope for po-lymers beyond those synthesized by metal-catalyzedATRP, and their impressively strong reducingpower presents great promise for their applica-tion toward other challenging chemical trans-formations. We also anticipate that the governingprinciples that afford these organic photocatalystswith their desirable properties will be exploitedthrough computational design to discover addi-tional photochemical platforms with capacitiesfor a variety of applications.
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ACKNOWLEDGMENTS
G.M.M. is grateful for financial support from the University of ColoradoBoulder and Advanced Research Projects Agency–Energy. This workwas supported in part by NSF grant CHE-1214131 (C.B.M. and C.-H.L.).J.C.T. is thankful for a NSF Graduate Research Fellowship Programfellowship. M.D.R. acknowledges support from a Graduate Assistance inAreas of National Need fellowship. We also gratefully acknowledge useof Extreme Science and Engineering Development Environmentsupercomputing resources (NSF ACI-1053575) and the Janussupercomputer, which is supported by NSF (CNS-0821794) and theUniversity of Colorado Boulder. We thank L. Hansman, A. Lockwood,S. Fatur, and N. Damrauer for technical assistance and enlighteningdiscussions. We have filed a provisional patent application onthe work described here.
SUPPLEMENTARY MATERIALS
www.sciencemag.org/content/352/6289/1082/suppl/DC1Materials and MethodsFigs. S1 to S28Tables S1 to S3Coordinates of Calculated Molecular StructuresReferences (35–39)
3 February 2016; accepted 22 March 2016Published online 31 March 201610.1126/science.aaf3935
1086 27 MAY 2016 • VOL 352 ISSUE 6289 sciencemag.org SCIENCE
Table 2. Results for the organocatalyzed ATRP of MMA catalyzed by 6 using white LEDs.Experimental details are provided in the supplementary materials.
Run [MMA]:[MBP]:[6] Time
(hours)
Conversion
(%)
Mw
(kDa)
Ð
(Mw/Mn)
I
[Mn(theo)/Mn(exp)]
11 [1000]:[10]:[1] 8 71.7 10.6 1.28 88.1.. .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. .
12 [1000]:[20]:[1] 8 73.1 5.24 1.29 94.5.. .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. .
13 [1000]:[15]:[1] 8 70.8 7.52 1.36 88.5.. .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. .
14 [5000]:[10]:[1] 8 69.5 46.9 1.32 98.7.. .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. .
15 [2500]:[10]:[1] 8 64.5 21.9 1.34 99.3.. .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. .
16 [750]:[10]:[1] 8 69.0 6.93 1.23 94.7.. .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. .
17 [500]:[10]:[1] 8 76.4 5.74 1.39 95.7.. .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. .
RESEARCH | REPORTSCorrected 27 May 2016; see full text.
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Organocatalyzed atom transfer radical polymerization driven by visible lightJordan C. Theriot, Chern-Hooi Lim, Haishen Yang, Matthew D. Ryan, Charles B. Musgrave and Garret M. Miyake
originally published online March 31, 2016DOI: 10.1126/science.aaf3935 (6289), 1082-1086.352Science
, this issue p. 1082; see also p. 1053SciencePerspective by Shanmugam and Boyer).precise control in atom transfer radical polymerization, alleviating concerns about residual metal contamination (see the
used theory to guide the design of a metal-free light-activated catalyst that offerset al.from the plastic product. Theriot steady, orderly steps. However, order comes at a price, and often it's the need for metal catalysts that are hard to removeOne of modern chemistry's great accomplishments has been the development of methods to assemble polymers in
Polymerization can be a rather dangerous free for all, with molecules joining randomly in chains at a chaotic pace.Precise control from a metal-free catalyst
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REFERENCES
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