Photoinduced Oxygen Reduction for Dark Polymerization

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Photoinduced Oxygen Reduction for Dark Polymerization Sivaprakash Shanmugam,† Jiangtao Xu,†,‡ and Cyrille Boyer*,†,‡ †

Centre for Advanced Macromolecular Design (CAMD), School of Chemical Engineering, and ‡Australian Centre for NanoMedicine, School of Chemical Engineering, UNSW Australia, Sydney, NSW 2052, Australia S Supporting Information *

ABSTRACT: Photopolymerization systems for controlled/ living radical polymerization (CLRP) have often been dependent on continuous irradiation to sustain radical production. Although this approach offers an opportunity to impose spatial and temporal control, it remains an energy inefficient process. As energy storage for CLRP remains an unexplored area in polymer chemistry, it may provide an opportunity for designing energy efficient polymerization. In this contribution, we propose a novel energy storage system where in situ production of hydrogen peroxide from molecular oxygen was achieved after a brief period of visible light irradiation in the presence of photo-organocatalyst and ascorbic acid. Upon ceasing irradiation, the slow generation of hydroxyl radicals from hydrogen peroxide in the presence of ascorbic acid allows for continuous radical generation in the dark. The highlight of this system stems from the fact that irradiation as brief as 5 min allows storage of enough energy as hydrogen peroxide to perform continuous polymerization to reach high monomer conversions in the dark. In addition, these aqueous polymerizations do not require nitrogen purging as oxygen is required for the production of hydrogen peroxide which becomes the radical source that initiates the polymerization. Interestingly, the amount of oxygen present in the reaction mixture affects the rate of polymerization. The system was found to be robust and versatile as it is able to accommodate different monomer families (acrylate, acrylamide, and methacrylate) and RAFT agents (dithiobenzoates and trithiocarbonates). Finally, this approach can help to solve one of the major limitations of photopolymerization pertaining to light penetration.

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INTRODUCTION Photosynthesis in green plants is made possible by two important cycles: light harvesting cycle and the Calvin−Benson cycle (dark cycle). In the light dependent cycle, absorption of photons by the light-harvesting systems helps to drive electron transport through the thylakoid membrane of the chloroplasts. In this process, splitting of water in photosystem II (PSII) generates electrons that reduce nicotinamide adenine dinucleotide phosphate (NADP+) to NADPH through photosystem I (PSI).1 In addition, a proton gradient is created across the thylakoid membrane due to electron transport by the cytochrome b6 f complex intermediating PSII and PSI which results in the generation of adenosine triphosphate (ATP). Both NADPH and ATP provide chemical energy to the lightindependent Calvin−Benson cycle where atmospheric carbon dioxide (CO2) is transformed into a three-carbon sugar called glyceraldehyde-3-phosphate (G3P), a precursor for the synthesis of complex sugars such as glucose.1,2 Recent advances in controlled/living radical polymerization (CLRP), especially made by Hawker,3−12 Fors,13,14 Matyjaszewski,15−20 Haddleton and Anastasaki,11,21−26 Kamigaito,27,28 Johnson,29−32 Lalevée,33−38 Miyake,39−42 Yagci,20,43−46 Qiao,47−50 Cai,51−53 others,54−56 and ourselves,57−60 have seen a surge in the techniques for light-mediated polymerization which allows for precision polymer synthesis. Unlike photosynthesis where energy stored in ATP and NADPH activates sugar synthesis in the dark reaction, photomediated CLRP is often dependent © XXXX American Chemical Society

on continuous irradiation to sustain radical production. An important avenue that remains unexplored in photomediated CLRP is the ability to continuously generate radicals over a period of time in the dark after a brief period of light irradiation which can lead to new opportunities for applications where light penetration becomes an issue. The current photomediated CLRP approach is focused on spatiotemporal control where the activation/deactivation of the polymerization is accomplished by switching the light ON/ OFF.5 Although this approach is highly desirable in patterning nanoscale objects,9,12,61,62 it becomes an inefficient approach for applications in areas concerning dental curing,63,64 coatings,65 photolithography,66 and adhesives,67 where limited light absorption, scattering, and reflection lead to sluggish polymerization as light penetration becomes an issue.68−70 In an effort to overcome this limitation, Stansbury and co-workers71 introduced an elegant approach through the use of methylene blue and a tertiary amine where energy is stored as leuco methylene blue upon brief irradiation period. Upon leaving the system in darkness, latent production of radicals by activation of initiator through electron transfer from leuco methylene blue resulted in free radical polymerization with high monomer conversions. Received: January 25, 2017 Revised: February 17, 2017

A

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Figure 1. Investigating the effects of concentration of Rose Bengal (RB) on RAFT photopolymerization by monitoring online Fourier transform near-infrared (FTNIR) measurements of DMA polymerization at room temperature under yellow light irradiation (λmax = 560 nm, intensity = 8 mW/cm2) with BTPA as the chain transfer agent ([DMA]:[BTPA]:[Asc acid] = 200:1:1, 4.9 M monomer concentration) without nitrogen sparging. (A) Plot of conversion against time for polymerization of DMA with 20, 50, and 100 ppm of RB with initial irradiation period of 45 min (green region) followed by commencement of polymerization in complete darkness (gray region). (B) Plot of ln([M]0/[M]t) against time for polymerization of DMA with 20, 50, and 100 ppm of RB. (C) GPC profile of final polymer synthesized with 20 ppm RB.

carried out to characterize the intermediates formed during periods of initiation and propagation.

Peroxide−amine formulation is also well-known for generating thick polymeric materials; however, this approach lacks temporal control over the initiation step as polymerization is initiated upon mixing.72 In addition, the use of amine has often been a health concern as it has been shown to cause visual disturbances, such as mydriasis, cycloplegia, corneal edema, and blurry vision, and lead to unwanted systemic health effects.73 Consequently, amine-free peroxide initiating systems have been developed using enzymes. For instance, the use of enzymes, including glucose oxidase (GOx) and horseradish peroxidase (HRP), for removal of oxygen by generating hydrogen peroxide has been recently explored for CLRP.74−77 The generated hydrogen peroxide has also been successfully used to initiate polymerization in the presence of ascorbic acid.77 Although the enzyme-mediated approach is highly efficient in oxygen scrubbing, this approach lacks temporal control over the initiation of the reaction as the polymerization starts upon mixing the different compounds together. In this study, we showcase the development of a novel approach for RAFT photopolymerization that allows for radical initiation in the dark after a brief period of irradiation analogous to photosynthesis. In this process, visible light irradiation in the presence of xanthene dyes generates singlet oxygen that is reduced by ascorbic acid to form in situ hydrogen peroxide. Unlike aforementioned peroxide systems, we are able to retain initial temporal control as polymerization is only initiated in the presence of visible light. Therefore, prior mixing of the formulation does not result in polymerization until brief irradiation period is introduced, which allows storage for long period in dark conditions. Furthermore, we are able to avoid the need for adding external hydrogen peroxide which is a strong oxidant and highly corrosive that requires extreme care during handling. Moreover, with irradiation periods as short as 5 min, we are able to sustain polymerization for several hours in the dark to achieve high monomer conversions (α > 70%). Unlike other radical polymerization systems where continuous light irradiation is needed to sustain radical generation, this approach is highly energy efficient as it is carried out under aqueous conditions without the need for nitrogen sparging and shows compatibility to different monomer families and RAFT agents. In contrast to conventional radical polymerization, the rate of polymerization increases with the amount of oxygen present in the system; i.e., a high concentration of oxygen results in fast polymerization rate. Finally, detailed mechanistic studies on this novel form of peroxide energy storage are

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RESULTS AND DISCUSSION In our preliminary studies, we decided to use Rose Bengal in the presence of ascorbic acid (Asc acid) to carry out RAFT photopolymerization of N,N-dimethylacrylamide (DMA) in water without nitrogen sparging. Rose Bengal has an absorption maximum around 560 nm (yellow), which is ideal to prevent unwanted RAFT or monomer activation. Upon irradiation for 45 min under yellow light, less than 5% monomer conversion was detected via Fourier transform near-infrared (FTNIR) measurements. In addition, the initial red color before irradiation was completely bleached after 45 min irradiation period with the final reaction mixture turning yellow. Because of the low monomer conversion despite complete reduction of Rose Bengal, it was assumed that either the photocatalyst degraded or the reduced species is unable to activate the polymerization. Surprisingly, upon removing the reaction vessel from the light source and leaving the reaction cuvette overnight (around 12 h) in darkness, formation of a viscous mixture was observed with the solution remaining yellow. FTNIR measurement revealed that the monomer conversion reached around 70% (Mn = 15 900 and Mw/Mn = 1.05). As the behavior of the Rose Bengal/ascorbic acid system was quite unprecedented and deviated from a typical PET-RAFT system57 and other conventional photopolymerization, where polymerization was only initiated in the presence of light and completely suppressed in the absence of light, further characterization of this system was carried out by manipulating the photocatalyst concentration, period of irradiation, and concentration of reducing agent. In order to understand the effects of concentration of RB on the latent initiated polymerization, polymerization of DMA was monitored via FTNIR measurements. The polymerization was carried out in a cuvette using BTPA as the RAFT agent with the molar ratio of [DMA]:[BTPA]:[Asc acid] determined to be 200:1:1. The concentration of RB was varied between 20, 50, and 100 ppm, which is the ratio of the molar concentration of photocatalyst to the molar concentration of the monomer, with the concentration of ascorbic acid and length of initial irradiation remained fixed for the three reactions. During the initial irradiation period of 45 min under yellow light (λmax = 560 nm, intensity = 8 mW/cm2), monomer conversions for the B

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Figure 2. The 300 MHz 1H NMR spectrum in deuterated methanol (MeOD) for characterization of RAFT end group and molecular weight (Mn,NMR = (I1.8−1.3 ppm/2)/(I5.2−5.3 ppm/1) × MWM + MWRAFT = 16 500 g/mol) of poly(N,N-dimethylacrylamide) synthesized through dark polymerization with GPC trace (inset) for polymerization in the presence of 50 ppm Rose Bengal.

rate constant for [RAFT]:[Asc acid] of 1:0.1 (kpapp = 3.21 × 10−3 min−1) (Figure 3) was found to be lower than the apparent propagation rate constant for [RAFT]:[Asc acid] of 1:1 (kpapp = 5.53 × 10−3 min−1) (Figure 1B). The lower concentration of ascorbic acid, however, did not affect the initial trend observed for higher concentration of ascorbic acid where minimum polymerization of DMA (∼5%) was observed during the 45 min irradiation period and the polymerization continuing and reaching high monomer conversion (∼80%) in the dark (Figure 3B). In addition, the final polymer product was also found to have close correlation between theoretical and experimental molecular weights (Figure S2B). As the decrease in the molar ratio of [RAFT]:[Asc acid] from 1:1 to 1:0.1 led to a slightly slower polymerization rate, we were curious as to whether decreasing the initial irradiation time would lead to a similar effect. In order to investigate the effects of irradiation time on the polymerization rate, the concentration of RB (20 ppm) and the concentration of ascorbic acid ([RAFT]:[Asc acid] = 1:0.1) were held constant while the initial irradiation periods were varied. For irradiation periods of 5 and 20 min, both reactions portrayed a similar trend to the 45 min irradiation period where less than 5% conversion was observed under irradiation with the polymerization continuing and reaching high conversions (∼80%) in the dark (Figure 3A,B and Figure S3). The 5 min irradiation (kpapp = 3.93 × 10−3 min−1), 20 min irradiation (kpapp = 3.51 × 10−3 min−1), and 45 min irradiation (kpapp = 3.21 × 10−3 min−1) led to apparent propagation rate constants (Figure 3C) that were close and almost similar. Interestingly, only the concentration of RB and concentration of ascorbic acid had significant effects on the polymerization rate of DMA while the period of irradiation, between 5 and 45 min, had almost no contribution in speeding up or slowing down the reaction.

three different concentrations of RB were monitored (Figure 1A) and determined to be less than 5%. At the end of the irradiation period, the cuvettes were left in the dark (inside the FTNIR instrument chamber), and measurements were continuously taken in automation for every 15 min. Upon cessation of irradiation, the monomer conversions for all three concentrations of Rose Bengal continued to increase in the dark. The apparent propagation rate constants (kpapp) were determined for each reaction by plotting ln[M]0/[M]t against exposure time as shown in Figure 1B. The concentrations of propagating radical was found to be the highest in the 20 ppm RB (kpapp = 5.53 × 10−3 min−1) followed by 50 ppm RB (kpapp = 4.08 × 10−3 min−1) and the lowest in 100 ppm RB (kpapp = 2.87 × 10−3 min−1). In comparison to PET-RAFT systems where the apparent propagation rate constant increases with the concentration of the photocatalyst,78 the trend that is shown by the Rose Bengal/ascorbic acid system is quite different and unexpected. The final polymer product that was obtained through the dark polymerization showed good correlation between the experimental and theoretical molecular weights with excellent control over the molecular weight distributions for all three concentrations of photocatalyst (Figures 1C and 2 and Supporting Information, Figure S1). Furthermore, analysis with 1H NMR showed good retention of the RAFT end group (Figure 2). As 20 ppm of RB was shown to provide the fastest polymerization rate for DMA, further investigations for the RB/ ascorbic acid system were carried out at this catalyst concentration. Curious as to whether the changes in concentration of ascorbic acid would make an impact on the dark polymerization, we decided to lower the molar ratio of [RAFT]:[Asc acid] from 1:1 to 1:0.1 while maintaining the initial irradiation period of 45 min. The apparent propagation C

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presence of RB after 5 min irradiation, we were curious about the effects of further shortening the initial irradiation periods to 30 s and 1 min. Irradiation for 1 min led to continuous polymerization in the dark after 30 min inhibition period (Figure S5A). However, further shortening the irradiation time to 30 s led to a 90 min inhibition period in the dark before polymerization commenced (Figure S5D). In addition, the propagation rate constants for the 30 s irradiation (kpapp = 3.3 × 10−3 min−1) (Figure S5B) and 1 min irradiation (kpapp = 3.43 × 10−3 min−1) (Figure S5E) were quite close to the propagation rate constant for 5, 20, and 45 min irradiation (Table 1, entries 5−9). Polymerizations under 30 s and 1 min irradiation suffered from a long inhibition period due to inefficient removal of oxygen from the reaction mixtures. The initial radicals generated in the dark were probably used to consume the residual oxygen in the solution and therefore resulting in inhibition periods. The removal of oxygen and generation of initiators for the polymerization will be further discussed under Mechanistic Understanding of Dark Polymerization section of this paper. As we have previously shown that Eosin Y (EY) is able to activate and mediate PET-RAFT polymerization with TEA,79 we were interested to know whether substituting TEA with ascorbic acid would change the general trend of polymerization that was observed for PET-RAFT. As the ratio of [RAFT]:[Asc acid] of 1:0.1 was proven effective in the polymerization of DMA with RB, we decided to maintain this ratio for polymerization of DMA in the presence of EY. Surprisingly, the substitution of TEA to ascorbic acid led to a similar polymerization that was observed with the RB−ascorbic acid system. In the presence of 20 ppm of EY and under green light irradiation (λmax = 530 nm, intensity = 4 mW/cm2), the EY− ascorbic acid system in either 5 min irradiation (Figure 4A) or 45 min irradiation (Figure S6A) led to less than 10% monomer conversion. Upon cessation of irradiation and leaving the reaction mixtures in the dark, continuous polymerization was observed for both reactions with final monomer conversions reaching between 80 and 90%. The final polymer products synthesized in the presence of EY for both irradiation periods led to close consistency between theoretical and experimental molecular weights and narrow polydispersities (Figure 4C and

Figure 3. Investigating the effects of different initial irradiation periods of Rose Bengal (RB) on RAFT photopolymerization of DMA by FTNIR measurements at room temperature under yellow light irradiation (λmax = 560 nm, intensity = 8 mW/cm2) with BTPA as the chain transfer agent ([DMA]:[BTPA]:[RB]:[Asc acid] = 200:1:0.004:0.1, 20 ppm RB with respect to molar ratio of monomer and 4.9 M monomer concentration) without nitrogen sparging. (A) Plot of conversion against time for polymerization of DMA with initial irradiation period of 5 min (green region) followed by commencement of polymerization in complete darkness (gray region). (B) Plot of conversion against time for polymerization of DMA with initial irradiation period of 45 min (green region) followed by commencement of polymerization in complete darkness (gray region). (C) Plot of ln([M]0/[M]t) against time for polymerization of DMA in complete darkness after irradiation periods of 5, 20, and 45 min. (D) GPC profile of final polymer product synthesized in darkness after 5 min irradiation.

Consequently, an energy efficient polymerization is made possible where irradiation of the reaction mixture with as minimum as 5 min was sufficient in activating the polymerization and allowing for synthesis of polymer with high retention of RAFT end group (Figure 3D and Figure S4). As successfully polymerization of DMA was possible in the

Table 1. Summary of DMA Polymerization with EY and RB under Different Periods of Irradiationa no.

time of irradiation (min)

induction period in dark (min)b

1f 2f 3f 4f

0.5 1 5 45

150 30 0 0

5g 6g 7g 8g 9g

0.5 1 5 20 45

90 30 0 0 0

kpapp (min−1) (±0.2 × 10−3) Eosin Y 1.50 × 10−3 3.73 × 10−3 5.48 × 10−3 6.00 × 10−3 Rose Bengal 3.3 × 10−3 3.43 × 10−3 3.93 × 10−3 3.51 × 10−3 3.21 × 10−3

αc (%)

Mn,FTNIRd (g/mol)

Mn,GPCe(g/mol)

Mw/Mne

49 87 92 82

10000 17500 18500 16500

9800 14600 17400 16000

1.10 1.08 1.06 1.06

70 71 82 77 83

14100 14300 16500 15600 16600

12700 14300 15500 15600 14600

1.07 1.06 1.06 1.05 1.05

a

Polymerizations were performed under green light (530 nm, intensity = 4 mW/cm2) or yellow light (560 nm, intensity = 8 mW/cm2) for different irradiation periods before being placed in darkness. bInduction period in darkness before initiation of polymerization. cMonomer conversion was determined by using Fourier transform near-infrared (FTNIR) spectroscopy. dTheoretical molecular weight was calculated using the equation Mn,th = [M]0/[RAFT]0 × MWM × α + MWRAFT, where [M]0, [RAFT]0, MWM, α, and MWRAFT correspond to initial monomer concentration, initial RAFT concentration, molar mass of monomer, conversion determined by Fourier Transform near-infrared (FTNIR) spectroscopy, and molar mass of RAFT agent. eMolecular weight and polydispersity index (Mw/Mn) were determined by GPC analysis (DMAC as eluent) calibrated to poly(methyl methacrylate) standard. fEY was used as the photocatalyst. gRB was used as the photocatalyst. D

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Figure 4. Investigating the effects of different initial irradiation periods with Eosin Y for RAFT photopolymerization of DMA by FTNIR measurements at room temperature under green light irradiation (λmax = 530 nm, intensity = 4 mW/cm2) with BTPA as the chain transfer agent ([DMA]:[BTPA]:[EY]:[Asc acid] = 200:1:0.004:0.1, 20 ppm EY with respect to molar ratio of monomer and 4.9 M monomer concentration) without nitrogen sparging. (A) Plot of conversion against time for polymerization of DMA with initial irradiation period of 5 min (green region) followed by commencement of polymerization in complete darkness (gray region). (B) Plot of ln([M]0/[M]t) against time for polymerization of DMA in complete darkness after irradiation periods of 5 and 45 min. (C) GPC profile of final polymer product synthesized in darkness after 5 min irradiation.

Table 2. Polymerization of Different Monomers by RAFT Photopolymerization Using Rose Bengal (RB) and Eosin Y (EY) (4.9 M Monomer Concentration) with Various Thiocarbonylthio Compounds with Yellow (λmax = 560 nm, Intensity = 8 mW/cm2) and Green (λmax = 530 nm, Intensity = 4 mW/cm2) Lighta no. e

1 2e 3f 4f 5e,g 6e,g 7e 8e 9f,h 10f,h 11i 12i

exptl conda [M]:[RAFT]:[PC]:[Asc acid]

monomer

RAFT

PC

time in darkness (h)

αb (%)

Mn,thc (g/mol)

Mn,GPCd (g/mol)

Mw/Mnd

400:1:0.004:0.1 1000:1:0.004:0.1 400:1:0.004:0.1 1000:1:0.004:0.1 100:1:0.004:0.1 100:1:0.004:0.1 200:1:0.004:0.1 200:1:0.004:0.1 200:1:0.004:0.1 200:1:0.004:0.1 200:1:0.004:0.1 200:1:0.004:0.1

DMA DMA DMA DMA OEGA OEGA HEA HEA DEA DEA OEGMA OEGMA

BTPA BTPA BTPA BTPA BTPA BTPA BTPA BTPA BTPA BTPA CPADB CETCPA

EY EY RB RB EY RB EY RB EY RB RB RB

9 9 9 8 2 2 0.5 0.5 8.5 8 12 18

58 69 68 48 60 47 32 24 45 27 21j 46j

28800 68900 27100 47400 29000 22600 7900 5000 11700 7100 12900 28000

28600 74600 25300 45600 26000 20000 16900 11300 10600 6400 11800 20800

1.09 1.21 1.14 1.28 1.20 1.19 1.15 1.14 1.13 1.20 1.20 1.34

a Reactions were performed with no nitrogen sparging at room temperature in water. bMonomer conversion was determined by using 1H NMR spectroscopy. cTheoretical molecular weight was calculated using the equation Mn,th = [M]0/[RAFT]0 × MWM × α + MWRAFT, where [M]0, [RAFT]0, MWM, α, and MWRAFT correspond to initial monomer concentration, initial RAFT concentration, molar mass of monomer, conversion determined by 1H NMR, and molar mass of RAFT agent. dMolecular weight and polydispersity index (Mw/Mn) were determined by GPC analysis (DMAC as eluent) calibrated to poly(methyl methacrylate) standard. e10 min irradiation followed by darkness. f20 min irradiation followed by darkness. gMonomer concentration determined to be 1.56 M. hMonomer concentration determined to be 3.72 M. i45 min irradiation followed by darkness. jMonomer conversion determined with FTNIR measurement.

led to no significant changes to the apparent propagation rate constants. Sampling during the dark period for EY and RB was made difficult as even trace amounts of oxygen led to inhibition of polymerization by quenching the low concentration of radicals generated. In order to overcome this shortcoming, polymerization of DMA for each photocatalyst was carried out in three different cuvettes with each cuvette having identical composition of the reaction mixture. The monomer conversion was monitored with FTNIR, and each reaction stopped at different time points. For both EY (Figure S8A) and RB (Figure S9A), the plot of molecular weight against monomer conversion demonstrated a good correlation between experimental and theoretical molecular weights with decreasing molecular weight distributions with increasing monomer conversion. The evolution of molecular weight obtained from GPC profiles at different time points during the polymerization were symmetrical with monomodal distributions (Figures S8B and S9B). As RB and EY both showed the ability to carry out dark polymerization, we decided to test the versatility of this system

Figure S6B). In addition, the apparent propagation rate constants (Figure 4B) for 5 min irradiation (Table 1, entry 3) and 45 min irradiation (Table 1, entry 4) were quite close. Further shortening of the irradiation periods to 1 min or 30 s resulted in long inhibition periods before commencement of polymerization (Figure S7A,D), 30 and 150 min, respectively. Unlike RB, we observed a drop in the apparent propagation rate constants for polymerizations under 30 s (Table 1, entry 1) and 1 min irradiation (Table 1, entry 2) for EY (Figure S7B,E) in comparison to the 5 and 45 min irradiation. As RB has a higher singlet oxygen quantum yield as compared to EY (Table 4), the former is more likely to generate higher concentration of initiator species (H2O2) than the latter for a given period of irradiation. Therefore, oxygen quenching in the EY system which has a lower concentration of initiator radical led to a more prominent effect as compared to the RB system by depressing the polymerization rate of DMA. For both EY and RB, it is essential that minimum length of irradiation of 5 min is introduced in order to ensure complete removal of oxygen, and irradiation above this minimum threshold, i.e., 20 and 45 min, E

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Macromolecules Scheme 1. List of Monomers, RAFT Agents, and Organic Photocatalysts Investigated in This Studya

a

N,N-Dimethylacrylamide (DMA), N,N-diethylacrylamide (DEA), N-isopropylacrylamide (NIPAAM), 2-hydroxyethyl acrylate (HEA), oligo(ethylene glycol) methyl ether acrylate (OEGA480) and oligo(ethylene glycol) methyl ether methacrylate (OEGMA300), 4-cyanopentanoic acid dithiobenzoate (CPADB), 2-(n-butyltrithiocarbonate)propionic acid (BTPA), 4-((((2-carboxyethyl)thio)carbonothioyl)thio)-4-cyanopentanoic acid (CETCPA), Rose Bengal (RB), and Eosin Y (EY).

mixture was then left in the dark for 9 h before characterization with FTNIR and GPC (α = 99%, Mn,theo = 19 900 g/mol, Mn,GPC = 21 000 g/mol, and Mw/Mn = 1.06). The polymer was then purified by dialysis against methanol for 2 days to remove any residual monomer, EY and ascorbic acid. The macro-CTA was then characterized with NMR (Mn,NMR = 19 400 g/mol) (Figure S11). Chain extensions were carried out with the PDMA macro-CTA with monomers, such as NIPAM and DMA, in the presence of RB under 20 min yellow light irradiation followed by 3 h in darkness. For DMA, chain extension was carried out with a molar ratio of [monomer]: [macro-CTA]:[RB]:[Asc acid] of 500:1:0.004:1 using 8 ppm catalyst while chain extension with NIPAM was carried out with a molar ratio of [monomer]:[macro-CTA]:[RB]:[Asc acid] of 200:1:0.008:1 using 40 ppm catalyst. Both chain extensions (Figure 5) resulted in a well-defined diblock copolymers with a complete shift in molecular weight and minimum formation of dead chains (Mn,GPC = 39 000 g/mol, Mw/Mn = 1.10, and 50% DMA monomer conversion for PDMA-block-PDMA (Figure

to synthesize DMA with different molecular weights as well as polymerize different water-soluble monomers with various RAFT agents. Different monomers were polymerized with either 10 or 20 min irradiation followed by keeping the reaction in the dark. As shown in Table 1, synthesis of PDMA in the range between 20 000 and 60 000 g/mol (Table 2, entries 1−4) was made possible with either RB or EY with good correlation between experimental and theoretical molecular weights and acceptable molecular weight distributions. Expanding this approach to DEA was proven effective as we were able to synthesize well-defined polymers (Table 2, entries 9 and 10). However, we observed a much slower polymerization in comparison to DMA due to a lower monomer concentration. In terms of acrylates, both OEGA and HEA were successfully polymerized. Polymerization of OEGA (Table 2, entries 5 and 6) led to the synthesis of well-defined polymer with good correlation between theoretical and experimental molecular weights and acceptable polydispersities. On the other hand, polymerization of HEA (Table 2, entries 7 and 8) was kept at low monomer conversions in order to avoid cross-linking. Poor correlations between experimental and theoretical molecular weights were observed due to the differences in hydrodynamic volume of PHEA to the PMMA standards used as described in the literature.80,81 Polymerization of OEGMA (Table 2, entries 11 and 12) was also carried out with a dithiobenzoate (CPADB) (Table 2, entry 11, and Figure S10) and a trithiocarbonate (CETCPA) (Table 2, entry 12) with the former yielding a better control over molecular weight and molecular weight distributions compared to the latter. Scheme 1 demonstrates the compatibility of the RB−ascorbic acid and EY−ascorbic acid systems with various acrylamides, acrylates, and methacrylate with both trithiocarbonates and dithiobenzoate. In order to reinforce the livingness of this approach, chain extension experiments were carried out using both EY and RB. Synthesis of PDMA macromolecular chain transfer agent (macro-CTA) was carried out using EY and ascorbic acid with BTPA as the chain transfer agent with no nitrogen sparging under green light irradiation for 20 min. The reaction

Figure 5. Chain extension of PDMA macro-CTA synthesized in the presence of EY photocatalyst and chain extended with DMA and NIPAM in the presence of RB photocatalyst at room temperature in water with no nitrogen sparging. (A) GPC profiles of PDMA macroCTA before (black trace) and after (red trace) chain extension with DMA leading to the synthesis of PDMA-block-PDMA. (B) GPC profiles of PDMA macro-CTA before (black trace) and after (green trace) chain extension with NIPAM leading to the synthesis of PDMAblock-PNIPAM. F

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Macromolecules 5A) and Mn,GPC = 33 100 g/mol, Mw/Mn = 1.07, and 73% NIPAAM monomer conversion for PDMA-block-PNIPAM (Figure 5B)). Mechanistic Understanding of Dark Polymerization. In the presence of ascorbic acid, the behavior of xanthene dyes, primarily Eosin Y (EY) and Rose Bengal (RB), were quite unprecedented for aqueous RAFT photopolymerization. Control experiments (Tables S1 and S2) performed in the darkness for 24 h suggest that activation of dark polymerization is only possible in the presence of a brief initial irradiation period. Although Pemberton and Johnson,82,83 Yang and Oster,84 Smets, Delzenne, and Toppets,85 Tigulla and Vuruputuri,86 Takura and Takayama,87 and others88−90 had proposed a similar free radical polymerization system with ascorbic acid, these mechanisms disagreed upon two key issues: the role of the reduced dyes and the importance of oxygen. Role of Reduced Dyes. In our investigation, upon initial irradiation as brief as 5 min, polymerization systems with both dyes were able to continuously generate radicals in darkness that enabled the synthesis of well-defined high molecular weight polymers. In addition, further increase to the irradiation times (20 and 45 min irradiation) did not increase the polymerization rates for both EY and RB. Therefore, it is safe to assume that the concentration of the reduced dye or the leuco dye adds no contribution to the generation of radicals during the course of polymerization. It is interesting to note that very little polymerization took place during the course of irradiation up to 45 min, but ceasing the irradiation anytime during this period, with a minimum irradiation period of 5 min, and placing the reaction mixture in the dark led to immediate activation of polymerization. During the 45 min irradiation period, a clear change in the color of the reaction mixture was observed for both RB (Figure 6, top) and EY (Figure S12, top). Examining the UV−vis absorption of the reaction mixture showed a reduction in the RB (Figure 6, bottom) and EY (Figure S12, bottom) absorption upon continuous irradiation. The change in color and reduction of absorption of both RB and EY upon

continuous irradiation in the presence of ascorbic acid suggested that the excited state photocatalysts were undergoing reduction upon interacting with ascorbic acid in the triplet state. In addition, the reduction cycle had a significant effect on the polymerization as it introduced an induction period. Removing irradiation anytime during the inhibition period stimulated polymerization as the ascorbic acid stopped reducing the photocatalysts and started initiating the polymerization. Similarly, when the photocatalysts were completely reduced after continuous irradiation for more than 45 min as seen in Figure 7A,B, further irradiation was not necessary as there was

Figure 7. Comparative FTNIR study to determine the effects of continuous irradiation and 45 min irradiation on the polymerization of DMA with EY and RB with a fixed concentration of photocatalysts and ascorbic acid at room temperature under yellow and green light irradiation (λmax = 560 nm, intensity = 8 mW/cm2 and λmax = 530 nm, intensity = 4 mW/cm2) with BTPA as the chain transfer agent ([DMA]:[BTPA]:[RB or EY]:[Asc acid] = 200:1:0.004:1, 20 ppm RB or EY with respect to molar ratio of monomer and 4.9 M monomer concentration) without nitrogen sparging. (A) Plot of ln([M]0/[M]t) against time for polymerization of DMA with EY and RB upon continuous irradiation. (B) Plot of ln([M]0/[M]t) against time for polymerization of DMA with EY and RB upon irradiation for 45 min followed by polymerization in complete darkness.

no absorbing species to reduce. In this case, ascorbic acid was used primarily for initiation of polymerization. In order to test the validity of this theory, FTNIR study was carried out to compare the polymerization rates of completely bleached photocatalysts that were continuously irradiated (Figure 7A and Figure S13A,B) to the polymerization rates of completely bleached photocatalysts where the irradiation was stopped upon reaching bleaching point (Figure 7B and Figure S13C). Polymerization in the continuously irradiated samples for both EY and RB started after an inhibition period of 45 min and continued during the period of irradiation. Similarly, polymerizations with 45 min irradiation to enable bleaching had a similar lag phase with the polymerization commencing after leaving the mixture in the dark. The apparent propagation rate constants for polymerization of DMA under continuous irradiation (RB: kpapp = 6.35 × 10−3 min−1; EY: kpapp = 6.06 × 10−3 min−1) were found to be quite close to the apparent propagation rate constants for polymerization where irradiation was stopped after bleaching (RB: kpapp = 6.23 × 10−3 min−1; EY: kpapp = 5.02 × 10−3 min−1). Again, the concentration of propagating radicals that was generated was not dependent on the irradiation time, but a longer irradiation led to a longer inhibition period (up to 45 min). After the bleaching of the photocatalysts, the polymerizations proceeded under darkness or under continuous irradiation. Nevertheless, the initial concentration of photocatalyst before irradiation affected the polymerization rates (Figure 1A,B). With an increase in concentration of RB, a slower propagation was observed for

Figure 6. Mapping visual changes of Rose Bengal (RB) (top) and changes in UV−vis absorption profile (bottom) in a typical DMA polymerization mixture ([DMA]:[BTPA]:[RB]:[Asc acid] = 200:1:0.004:0.1) upon irradiation under yellow light (λmax = 560 nm, intensity = 8 mW/cm2) for a period of time. G

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Macromolecules the same period of irradiation. It is highly likely that as the concentration of photocatalyst increases, more ascorbic acid is used up to reduce the photocatalysts. Therefore, this left a lower concentration of ascorbic acid available for initiation in reactions with 100 ppm RB as compared to reactions with 20 ppm RB. The role of ascorbic acid in initiation will be described in the following discussions. Importance of Oxygen. Oxygen plays a crucial role in the latent initiated dark polymerization with RB and EY as it is reduced into a radical initiator (H2O2) during light irradiation. Nevertheless, introduction of oxygen after the irradiation period (i.e., during the dark period) was proven to be detrimental as it completely inhibited polymerization. In addition, the polymerization mixture was not stirred or agitated during FTNIR measurements or during the course of polymerization to avoid the diffusion of oxygen from the headspace of the cuvette into the reaction mixture, which would result in the inhibition of polymerization. Furthermore, the free headspace in most experiments was maintained at around 5− 10% of the total volume of the cuvette in order to minimize the diffusion of oxygen into the reaction mixture after irradiation. Nevertheless, the importance of oxygen in initiating polymerization was revealed when 50 min nitrogen sparging of DMA polymerization mixtures with either EY (Figure S14A) or RB (Figure S14B) followed by 20 min irradiation led to less than 10% monomer conversion even after 10 h in darkness. To understand the effects of oxygen in dark polymerization system, polymerization of DMA with varying volumes of injected air was carried out using EY as a model system. In these polymerizations, three reactions mixtures were sparged with nitrogen for a period of 50 min to ensure complete removal of oxygen from the reaction mixture. Each reaction mixture was then injected with 3, 5, or 10 mL of air (see Supporting Information, Polymerization of DMA with Addition of Different Volumes of Air). The amount of oxygen present in these three polymerization mixtures was estimated by a separate UV−vis experiment using quenching of dimethylanthracene (9,10-dimethylantharacene, DMeA), which is an oxygen probe. In this experiment, oxygen is converted into singlet oxygen by the photocatalysts, which then reacts with DMeA. By following the decrement in the absorption of DMeA, we were able to estimate the amount of oxygen. As expected, the amount of dissolved oxygen increases with the amount of air injected. A similar fourth reaction mixture was prepared as a positive control as no deoxygenation was carried out. The four reaction mixtures were irradiated under green light for 20 min before being placed in the dark with continuous FTNIR measurements to monitor monomer conversion (Figure 8). The monomer conversions increased with increasing volume of injected air before irradiation, with the highest monomer conversion observed for the nondeoxygenated sample (Table 3 and Figure 8A,C,E,G). In addition, the apparent propagation rate constants were also found to increase with the volume of injected air (Table 3 and Figure S15). Interestingly, the rate of polymerization appears proportional to the amount of oxygen present in the system. Assuming that one ascorbic acid molecule reacts with one molecule of O2 to generate one molecule of H2O2,91 the amount of hydrogen peroxide generated should be close to the initial concentration of oxygen dissolved in the reaction mixture where 3, 5, and 10 mL of added air should generate 0.24, 0.34, and 0.41 μmol of H2O2, respectively. In the reaction mixture without deoxygenation, a theoretical estimate of H2O2 of 1.06

Figure 8. Investigating the effects of addition of different volumes of air in reaction mixtures sparged with nitrogen on RAFT photopolymerization of DMA in the presence Eosin Y through FTNIR measurements at room temperature under 20 min green light irradiation (λmax = 530 nm, intensity = 4 mW/cm2) with BTPA as the chain transfer agent ([DMA]:[BTPA]:[EY]:[Asc acid] = 200:1:0.004:0.1, 20 ppm EY with respect to molar ratio of monomer and 4.9 M monomer concentration). (A) Plot of conversion against time for polymerization of DMA injected with 3 mL of air before irradiation. (B) GPC profile of final polymer product synthesized in darkness after 20 min irradiation with injection of 3 mL of air before irradiation. (C) Plot of conversion against time for polymerization of DMA injected with 5 mL of air before irradiation. (D) GPC profile of final polymer product synthesized in darkness after 20 min irradiation with injection of 5 mL of air before irradiation. (E) Plot of conversion against time for polymerization of DMA injected with 10 mL of air before irradiation. (F) GPC profile of final polymer product synthesized in darkness after 20 min irradiation with injection of 10 mL of air before irradiation. (G) Plot of conversion against time for polymerization of DMA with no nitrogen sparging or addition of air. (H) GPC profile of final polymer product synthesized in darkness after 20 min irradiation with no nitrogen sparging and injection of air before irradiation. H

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Macromolecules Table 3. Summary of DMA Polymerization with Different Volumes of Injected Aira no. b

1 2 3 4

volume of injected air (mL) b

0 3 5 10

oxygen concentration (mmol)c 1.06 0.24 0.34 0.41

× × × ×

−3 g

10 10−3 10−3 10−3

kpapp (min−1) 4.21 0.89 1.14 1.76

× × × ×

−3

10 10−3 10−3 10−3

αd (%)

Mn,FTNIRe (g/mol)

Mn,GPCf (g/mol)

Mw/Mnf

99 24 38 51

19900 5000 7800 10400

20600 7400 10100 13400

1.09 1.11 1.09 1.08

a

Reactions were sparged under nitrogen for 50 min before injection of different volumes of air followed by green light irradiation (530 nm, intensity = 4 mW/cm2) for 20 min before being placed in darkness. bReaction was not sparged under nitrogen and no addition of air was made. cDetermined by the consumption of 9,10-dimethylanthracene (DMeA) as described in the Supporting Information, Polymerization of DMA with Addition of Different Volumes of Air. dMonomer conversion was determined by using Fourier transform near-infrared (FTNIR) spectroscopy. eTheoretical molecular weight was calculated using the equation Mn,th = [M]0/[RAFT]0 × MWM × α + MWRAFT, where [M]0, [RAFT]0, MWM, α, and MWRAFT correspond to initial monomer concentration, initial RAFT concentration, molar mass of monomer, conversion determined by Fourier transform near-infrared (FTNIR) spectroscopy, and molar mass of RAFT agent. fMolecular weight and polydispersity index (Mw/Mn) were determined by GPC analysis (DMAC as eluent) calibrated to poly(methyl methacrylate) standard. gDue to difficulties in measuring the amount of oxygen based on quenching of DMeA concentration, a theoretical estimation was made as described in Supporting Information, Semiquantitative Calculation of Oxygen Amount in the Reaction Vessel.

μmol was made (see Supporting Information, Semiquantitative Calculation of Oxygen Amount in the Reaction Vessel) due to difficulties of solubilizing high concentrations of DMeA. This trend suggests that the concentration of propagating radical is proportional to the concentration of H2O2 generated in situ. In contrast to conventional free radical polymerization, a faster polymerization rate was observed with the highest concentration of dissolved oxygen (Table 3) as it was assumed to be completely converted to H2O2 in the presence of ascorbic acid upon irradiation. The reaction mixtures with different volume of air were also found to yield final polymers with excellent correlation between theoretical and experimental molecular weights and narrow polydispersities (Table 3 and Figure 8B,D,F,H). As we have demonstrated the important role of oxygen, we decided to investigate if the reaction could be reactivated by subsequent addition of small amount of air, followed by a short exposure to visible light. We repeated the polymerization in the presence of 3 mL of air, which resulted in a similar low monomer conversion (∼22%). Upon further addition of 3 mL of air followed by 20 min green light irradiation, the polymerization restarted and resulted in a higher final monomer conversion (∼41%) (Figure 9A). Interestingly, the rates of polymerization for the initial polymerization and after addition of air are similar. Both first and second addition of air resulted in final polymer products that portrayed excellent control (Figure 9B). Singlet Oxygen Generation and Trapping. As it has been clearly shown that the initial volume of air before irradiation dictates the concentration of propagating radical, the role of oxygen in creating reactive initiating species was further studied. Both EY and RB are known for their ability to generate singlet oxygen from triplet oxygen through energy transfer in their excited state.92 The generation of singlet oxygen was verified by singlet oxygen trapping using DMeA in the presence of Eosin Y (EY) upon green light irradiation.93−96 The trapping of singlet oxygen by DMeA resulted in the decrease in the UV−vis absorption due to the formation of endoperoxide.97−99 In the absence of ascorbic acid, a complete disappearance of the DMeA absorption peak (Figure S16) was observed after 5 min irradiation under conditions similar to a typical polymerization setup. In the presence of ascorbic acid, addition of DMeA to a typical polymerization, in the presence of RB and DMA with molar ratio of [DMeA]:[Asc acid] of 0.2:1, did not result in complete quenching of DMeA (Figure S17A−C) with the absorption of DMeA stabilizing after 5 min.

Figure 9. Investigating reinitiation of dark polymerization upon further addition of air for RAFT photopolymerization of DMA in the presence Eosin Y through FTNIR measurements at room temperature under 20 min green light irradiation (λmax = 530 nm, intensity = 4 mW/cm2) with BTPA as the chain transfer agent ([DMA]:[BTPA]:[EY]:[Asc acid] = 200:1:0.004:0.1, 20 ppm EY with respect to molar ratio of monomer and 4.9 M monomer concentration). (A) Plot of conversion against time for polymerization of DMA injected with 3 mL of air before irradiation followed by further addition of 3 mL air and irradiation after reaching a plateau in monomer conversion. (B) GPC profiles of final polymer products synthesized in darkness after 20 min irradiation in the first and second addition of 3 mL of air. Note: 3 mL of air was readded after 380 min, and the solution was then irradiated for 20 min and then placed in the dark. The monomer conversion was monitored by FTNIR measurement.

The absence of complete quenching of DMeA could be due to competitive pathways for singlet oxygen quenching by both ascorbic acid and DMeA with most of the oxygen in the system consumed after 5 min. However, when the irradiation period was increased beyond 10 min, a decrease in the absorption of DMeA was observed. This can be attributed to the increase in monomer conversion (Figure S17D) which may have resulted in the consumption of DMeA by the propagating radicals. In addition, the presence of DMeA did not affect the reductive quenching of RB upon continuous irradiation. Similar observations were also seen with EY where reduction of DMeA was observed due to singlet oxygen quenching with the activated propagating radicals also contributing to this process (Figure S18). In order to avoid DMeA quenching by propagating radicals, DMA was replaced with N,N-dimethylacetamide (DMAc), which has a chemically similar structure. The reaction conditions were similar to a typical DMA polymerization without addition of BTPA as the chain transfer agent. As we have observed ascorbic acid competing with DMeA to quench singlet oxygen, we decided to test the effects of different I

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Figure 10. UV−vis measurements of singlet oxygen (1O2) quenching by 9,10-dimethylanthracene (DMeA) in the presence of Eosin Y (EY) upon green light irradiation (λmax = 530 nm, intensity = 4 mW/cm2) at 1, 5, 10, 20, and 30 min. (A) Complete absorption profile of EY and DMeA at different time points of green light irradiation with the molar ratio of [EY]:[DMeA]:[Asc acid] = 1:0.8:2.6. (B) Narrowed absorption profile of DMeA at different time points of green light irradiation from (A). (C) Narrowed absorption profile of EY at different time points of green light irradiation from (A). (D) Complete absorption profile of EY and DMeA at different time points of green light irradiation with the molar ratio of [EY]:[DMeA]:[Asc acid] = 1:0.8:26. (E) Narrowed absorption profile of DMeA at different time points of green light irradiation from (D). (F) Narrowed absorption profile of EY at different time points of green light irradiation from (D).

concentrations of ascorbic acid on singlet oxygen quenching as well as the ability to photochemically reduce EY. In our typical DMA polymerization setup, the molar ratio of [EY]:[Asc acid]: [DMeA] was [1]:[25]:[0]. Therefore, we decided to test two different concentrations of ascorbic acid, with the lower extreme having [EY]:[Asc acid]:[DMeA] of [1]:[2.6]:[0.8] and the higher extreme having [EY]:[Asc acid]:[DMeA] of [1]: [26]:[0.8]. The absorption of DMeA was completely quenched by singlet oxygen reaction for the lower extreme (Figure 10A,B). On the other hand, the absorption of DMeA initially decreased but stabilized for the higher extreme after 5 min irradiation (Figure 10D,E). The higher concentration of ascorbic acid was able to outcompete the quenching of singlet oxygen by DMeA leading to lower concentration of DMeA consumption. The reduction of EY upon continuous irradiation was quite inhibited (Figure 10C) for the lower extreme as the ascorbic acid was most likely used up for singlet oxygen quenching leading to less events of reduction. For the higher extreme of ascorbic acid, reduction of EY proceeded in an uninterrupted manner (Figure 10F) upon continuous irradiation. A useful insight that is obtained by comparing the results from the two extremes is that the event of singlet oxygen quenching happens before reductive quenching of the photocatalyst. The generated singlet oxygen is an energetic molecule that can rapidly react with an electron-rich molecule, such as ascorbic acid. The antioxidant properties of ascorbic acid results in the reduction of singlet oxygen to form hydrogen peroxide (H2O2) and dehydroascorbate (DHA).100 The formation of hydrogen peroxide in the ascorbic acid−EY/RB system was characterized by proton NMR (10.60−10.65 ppm), which was in agreement with the signal observed for a standard stock solution of H2O2 (0.000165% w/v) (Figure 11 and Figures

Figure 11. The 600 MHz 1H NMR spectrum in deuterated DMSO for characterization of hydrogen peroxide from commercial source (top) with hydrogen peroxide generated from interaction between singlet oxygen and ascorbic acid (bottom) upon irradiation.

S19−S21).101−103 In contrast to its role as an antioxidant, ascorbic acid has also been shown to have pro-oxidant properties under certain conditions.104 Previous studies have shown that ascorbic acid can be oxidized by hydrogen peroxide leading to the formation of hydroxyl radical that initiates free radical polymerization.105 The presence of hydroxyl radicals in the ascorbic acid−EY/RB system was verified by radical trapping with terephthalic acid to form 2-hydroxyterephthalate (Figure 12, top).98,106 Terephthalic acid, which does not emit any fluorescence, was used as a probe for hydroxyl radical generation as the 2-hydroxy terephthalate formed emits a strong fluorescence at 430 nm. As terephthalic acid is insoluble at acidic pH in water, sodium ascorbate is used to increase the pH of the solution to near neutral (∼ pH 7.2) and therefore J

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Figure 12. Trapping of hydroxyl radical generated during oxidation of sodium ascorbate by hydrogen peroxide (H2O2) using terephthalic acid to generate 2-hydroxy terephthalate (top). (A) Glass vials with sodium ascorbate and EY photocatalyst with one vial placed in the dark for 6 h while the other was irradiated for 6 h. (B) Normalized fluorescence measurements of the reaction mixture before irradiation (0 h) and after leaving in darkness for 6 h. (C) Normalized fluorescence measurements of the reaction mixture after irradiation for 6 h.

increase the solubility of terephthalic acid. The use of sodium ascorbate as a substitute for ascorbic acid provided a similar trend to that observed for ascorbic acid−EY/RB systems (Figures S22−S25). In the absence of irradiation and in the presence of sodium ascorbate and terephtalic acid, only fluorescence for RB (Figure S26A) and EY (Figure 12A,B) were detected. Upon irradiation for 6 h to maximize the formation of singlet oxygen and hydroxyl radical (see Supporting Information, Procedure for Hydroxyl Radical Detection), the fluorescence spectra of EY (Figure 12A,C) and RB solutions (Figure S26B) revealed the presence of a peak at 430 nm which is attributed to 2-hydroxy terephthalate. An Overview of the Mechanism. By combining the different parts of the mechanistic studies, the mechanism of latent radical initiated dark polymerization is proposed to take place as shown in Scheme 2. Upon irradiation of RB or EY, the photocatalyst enters the excited state and is able to transfer its energy to a nearby oxygen molecule. This leads to the generation of reactive singlet oxygen (1O2) from unreactive triplet oxygen (O2). The singlet oxygen is then able to oxidize ascorbic acid to generate hydrogen peroxide (H2O2) and dehydroascorbate (DHA). Based on the UV−vis studies carried out, reduction of oxygen to hydrogen peroxide happens in the period between 1 and 5 min. Therefore, this creates a minimum irradiation threshold of 5 min for complete removal of oxygen to enable efficient polymerization as shown in the previous sections. Although the photocatalyst is also reduced upon continuous irradiation in the presence of ascorbic acid, the leuco-dye does not aid the dark polymerization. The generated H2O2 then reacts with ascorbic acid to form hydroxyl radical which subsequently initiates monomers that enter the RAFT polymerization cycle. Direct initiation of monomers by hydroxyl radical was proven using control experiments. High monomer conversions (70−100%) were observed for the RB (Figure S27) and EY (Figure S28) reaction mixtures in the absence of RAFT agent (i.e., BTPA as the chain transfer agent). An estimation of the amount of H2O2 generated theoretically based on a typical experimental condition (see Supporting Information, Semiquantitative Calculation of Oxygen Amount

Scheme 2. Proposed Mechanism for Dark Polymerization after Brief Period of Light Irradiationa

a

The dashed arrow is used for reduction of photocatalyst (PC) as it less important in activating the polymerization and is proposed to happen only after all the oxygen is reduced to H2O2.

in the Reaction Vessel) revealed a rough estimate of molar ratio of [DMA]:[BTPA]:[RB/EY]:[Asc acid]:[H2O2] ratio to be 200:1:0.004:0.1:0.05. Although studies on stability of dithiobenzoates showed that dithioester could be converted to thioester functionalities by peroxy induced radical oxidation, these studies often required heating of reaction mixtures to temperature ranges between 60 and 100 °C.107,108 In addition, trithiocarbonates have been demonstrated to be quite stable due to electron-donating abilities of lone pair substituent, alkylthio functionality, which forms resonance structures that raise the energy of radical addition to thiocarbonylthio moiety.108,109 In order to further K

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which suggests that reduction of MB was more likely favored over singlet oxygen generation. On the other hand, FL has a reduction potential that is relatively close to EY, but its singlet oxygen quantum yield is much lower than all the other photoorganocatalysts tested. This low singlet oxygen quantum yield resulted in poor activation of polymerization (Figure S34) as less hydrogen peroxide was generated. In short, the outcomes from FL and MB further reinforce our proposed mechanism and establish selection criteria for future development of photocatalysts for this system.

demonstrate the stability of BTPA toward peroxy induced radical oxidation, changes in the absorption profile in the visible region, between 400 and 500 nm which pertained to spinforbidden n to π* transition, were monitored for reaction mixtures consisting BTPA, ascorbic acid, and photocatalysts (RB and EY) before and after irradiation in the absence of monomer (Figure S29). These reactions were performed in water with DMA substituted with DMAc to increase the solubility of BTPA under conditions similar to typical polymerization formulation. In the presence of EY or RB, reaction mixtures were irradiated for 20 min before placing them in the dark. Both reactions mixtures showed reduction of photocatalysts upon irradiation, but no changes were observed in the spin-forbidden n to π* transition of trithiocarbonates (400−500 nm) under irradiated and dark conditions. The final reaction mixtures were then analyzed with 1H NMR (Figures S30 and S31) which revealed that no significant degradation had taken place as BTPA integrity was maintained. In order to negate any contributions of DMAc in the stability analysis of BTPA, water-soluble PDMA macro-CTA with trithiocarbonate functionality was used. Stability experiments performed in the absence of DMAc with both EY and RB (Figure S32) by monitoring the CS bond at 305 nm, which resulted in similar conclusions where no significant degradation of thiocarbonylthio group was observed. Therefore, high RAFT end-group fidelity was maintained due to the inherent stability of trithiocarbonates and the fact that all polymerizations were conducted at room temperature (∼25 °C). To validate the mechanism, we decided to carry out further testing with other photo-organocatalysts such as fluorescein (FL) and methylene blue (MB). We discovered that these dyes resulted were in inefficient for polymerizations of DMA, i.e., low monomer conversions. These outcomes were quite consistent with our proposed mechanism. To obtain an efficient hydrogen peroxide generation, and therefore efficient polymerization, the photocatalyst needs to fulfill two important criteria: high singlet oxygen quantum yield and low reduction potential. Although MB has a similar singlet oxygen quantum yield to EY (Table 4), it has a higher reduction potential than EY or RB. Upon irradiation in the presence of ascorbic acid, MB underwent photobleaching in less than 5 min (Figure S33),

■

CONCLUSION In this investigation, we explored a novel form of polymerization that introduces energy storage in RAFT photopolymerization. By using molecular oxygen as a precursor, hydrogen peroxide can be generated under visible light and stored as a radical source for polymerization in complete darkness. As irradiation is only introduced for generation of hydrogen peroxide, only a brief period of irradiation is needed. This approach is an energy efficient and green process carried out under aqueous condition with the ability to regulate temporal control over initiation of different monomer functionalities and RAFT agents. This system could solve a significant limitation of photomediated polymerization that pertains to light penetration.

■

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b00192. Experimental section, UV−vis spectra, NMR spectra, GPC traces, Figures S1−S33, oxygen concentration calculation (PDF)

■

Rose Bengal (RB) Eosin Y (EY) fluorescein (FL) methylene blue (MB)

singlet oxygen quantum yield (ΦΔ)

reduction potential (V) (PC/PC•−)

time in dark (h)

α (%)

0.75 0.57 0.03 ∼0.50

−0.78 −1.14 −1.22 +0.01

6 6 6 6

>70 >70 10 0

AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (C.B.). ORCID

Jiangtao Xu: 0000-0002-9020-7018 Cyrille Boyer: 0000-0002-4564-4702

Table 4. Summary of Singlet Oxygen Quantum Yield and Reduction Potential of Different Photoorganocatalysts and Their Ability to Initiate Polymerization of DMA79,92,110−112,a photoorganocatalyst (PC)

ASSOCIATED CONTENT

S Supporting Information *

Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS C.B. acknowledges Australian Research Council (ARC) for his Future Fellowship (FT12010096). The authors acknowledge Dr. Doug Lawes of the NMR Facility within the Mark Wainwright Analytical Centre at the University of New South Wales for NMR support and Dr. James Hook for his contribution of terephthalic acid.

■

a

Reactions were performed with no nitrogen sparging at room temperature in water, and monomer conversions were determined using FTNIR for [DMA]:[BTPA]:[Asc acid] = 200:1:1 for polymerization of DMA with RB (λmax = 560 nm, intensity = 8 mW/cm2), EY (λmax = 530 nm, intensity = 4 mW/cm2), FL (λmax = 505 nm, intensity = 4 mW/cm2), and MB (λmax = 650 nm, intensity = 9 mW/cm2). All reactions were irradiated for 45 min before being placed in the dark except methylene blue where irradiation was only carried out for 5 min.

REFERENCES

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DOI: 10.1021/acs.macromol.7b00192 Macromolecules XXXX, XXX, XXX−XXX