Heterogeneous Photocatalysis as a Means for Improving Recyclability

Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

Heterogeneous Photocatalysis as a Means for Improving Recyclability of Organocatalyst in “Living” Radical Polymerization Sivaprakash Shanmugam,† Sihao Xu,†,‡ Nik Nik M. Adnan,†,‡ 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: A novel heterogeneous catalytic system consisting of Eosin Y conjugated to silica nanoparticle (EY-SNP) is explored in this work to promote visible light photoinduced electron/energy transfer−reversible addition−fragmentation chain transfer (PET-RAFT) polymerization. In contrast to unconjugated Eosin Y (EY) that has been extensively studied and employed in free radical and controlled/“living” radical polymerization, EY-SNP was found to be tolerant to photodegradation during the polymerization. In addition, ultralow concentration of EY-SNP (less than 10 ppm) was found to efficiently catalyze polymerization of hydrophobic and hydrophilic monomers to reach high monomer conversions with narrow molecular weight distributions in a range of different solvents, including water, dimethyl sulfoxide, and N-methyl-2-pyrrolidone. Finally, the EY-SNP was also recovered via centrifugation and was reused to perform multiple cycles of polymerization.

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INTRODUCTION In recent years, the incorporation of photocatalysts in controlled/living radical polymerization (CLRP) has allowed polymerization in mild conditions under visible lights.1−6 In these systems, photopolymerizations mediated by reversible addition−fragmentation chain transfer (RAFT), atom transfer radical polymerization (ATRP), and nitroxide-mediated polymerization (NMP) rely on the use of photocatalysts that are either metal-based7−31 or metal-free.32−42 However, the presence of photocatalysts can result in further polymer degradation and side reactions if the catalysts are not eliminated after polymerization, therefore limiting the potential applications of these polymers in fields including advanced materials and biomaterials.32,43 Therefore, development of biocompatible and green methods that reduce concentration of catalysts in the synthesized materials and/or removal of catalysts from reaction solutions has been the central theme in polymer chemistry and chemical industry.44−48 Several alternatives have been developed to achieve this objective. For instance in photoRAFT polymerization, catalyst-free systems that rely on photoreduction of initiators by amines49 and direct initiation of chain transfer agents36,37,50−55 have been developed. The polymerization rates of these systems are relatively slow (i.e., 24 h for acrylate to achieve a high monomer conversion (∼70− 80%) when polymerizations were performed using blue light at λ = 460 nm) or limited to a specific monomer family. Although Cai and co-workers developed the use catalyst-free visible-lightmediated RAFT polymerization in both aqueous56−58 and organic media59 with monomer versatility, these studies often depend on the use of high molar amounts of photoinitiators © XXXX American Chemical Society

that cannot be recycled and also the use of high-energy violet light (λ = 420 nm). To eliminate residual photocatalysts, polymer purification via ion exchange columns,45,60 catalyst extraction through polymer precipitation,61,62 and the use of catalyst supports have been proposed.43,63−66 For instance, Bergbreiter and Liang explored the use of polyisobutylene polymer support for phenothiazine as means of extracting the photocatalyst from the polymer solution via biphasic separation.67 In another example, Chen, Zhang, and co-workers proposed the recyclable-catalyst-aided, opened-to-air, and sunlight-photolyzed RAFT (ROS-RAFT) polymerization technique.20 This approach was successfully implemented for the synthesis of poly(methyl methacrylate) with a good control of molecular weight with relatively narrow molecular weight distribution (Mw/Mn ∼ 1.4−1.5). More recently, Johnson and co-workers designed poly(N-isopropylacrylamide) hydrogel conjugated with 10-phenylphenothiazine to mediate polymerization by both light and temperature and afford the removal of the gel at the end of the polymerization.68 The use of photocatalysts is a promising route for RAFT photopolymerization as it enables oxygen tolerance,13,20,69,70 the use of low-energy wavelength (including red light, far-red, and near-infrared),71−73 fast polymerization rates (i.e., from a few minutes to a couple of hours to reach full monomer conversion for acrylates),74,75 and also selective chemical activation.76,77 However, there have been limited studies on Received: October 16, 2017 Revised: December 16, 2017

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DOI: 10.1021/acs.macromol.7b02215 Macromolecules XXXX, XXX, XXX−XXX

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Scheme 1. Stepwise Synthesis of Mesoporous Silica Nanoparticle Conjugated to Eosin Y (EY-SNP) and Transmission Electron Microscopy (TEM) Micrograph of Amino-Functionalized Silica Nanoparticle Conjugated with Eosin Y To Yield EY-SNP

Figure 1. (A) UV−vis absorptions of EY-SNP and unconjugated EY in water and their respective photos of the solutions. (B) Photos of polymerization setup for DMA in water with 3.0 ppm of EY-SNP before (1) and after (2) polymerization and photo 3 exhibiting removal of the EYSNP by centrifugation of reaction mixture 2.

ppm) over multiple polymerization cycles. In addition, this work utilizes part per millions (ppm) amount of recyclable photocatalyst that efficiently promotes polymerization under low-energy green light irradiation (λmax = 515 nm, 2.8−3.6 mW/cm2). This heterogeneous catalyst was found to catalyze the polymerization of a range of hydrophobic and hydrophilic monomers in different solvents (water, dimethyl sulfoxide, and N-methyl-2-pyrrolidone) and was easily removed by centrifugation at the end of the polymerization.

methods to remove these photocatalysts after polymerization, except for the examples highlighted above. Furthermore, in an effort to reduce metallic contamination in synthesized polymers, organodyes have been developed and implemented in light-mediated ATRP,32,33,78−80 RAFT,40,74,75,81−83 and iodine transfer polymerization.84 In recent works by Hawker,74 Sumerlin,75 Cheng,42 and our group,53 Eosin Y (EY) was implemented as a photocatalyst not only to produce welldefined polymers from a myriad range of monomers but also to mediate grafting of polymer from surfaces of proteins and living cells. Despite the remarkable potential of EY in catalyzing visible light RAFT polymerization, it presents two main disadvantages. First, EY has been known to undergo oxidative photobleaching upon direct electron transfer to initiator which reduces its efficiency and results in its degradation.85 Second, EY, when used at high concentrations, is often trapped within the polymer matrix and therefore contaminates the final polymer product. In this work, we showcase solutions to both problems by conjugating EY to silica nanoparticle (EY-SNP) to provide stability from photodegradation as well as to enhance catalyst removal. In addition, solving both these issues not only allowed minimization of catalyst contamination but also enabled the recycling of EY conjugated to silica particle (EY-SNP). This work strictly adheres to the principles of “green chemistry”86,87 in terms of atom efficiency as we demonstrate the ability to recycle ultralow EY-SNP catalyst loadings (in the range 3.0−7.5

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RESULTS AND DISCUSSION Silica nanoparticles coated with Eosin Y (EY-SNP) were synthesized as shown in Scheme 1. Initial synthesis of aminefunctionalized silica nanoparticles (SNP-NH2) following a previous procedure88 was necessary to enable covalent attachment of Eosin Y onto the nanoparticles. In order to remove unconjugated EY, i.e., physically adsorb EY on the silica nanoparticles, several cycles of centrifugation and redispersion of nanoparticles in fresh ethanol were carried out. The morphology and size of silica nanoparticles were investigated via TEM. TEM of bare silica nanoparticles (SNP), aminofunctionalized silica nanoparticles (SNP-NH2), and EY-SNPs revealed average nanoparticle diameter of 100 nm (Supporting Information, Figures S1−S3). Covalent coupling of Eosin Y to SNP-NH2 was then verified via FTIR spectroscopy by the appearance of a vibrational band at 1385 cm−1 corresponding to the C−N stretching mode of amide while the vibrational bands B


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Table 1. Control Experiments for Polymerization of DMA with RAFT Photopolymerization Using EY-SNP and BTPA under Green LEDs (λmax = 515 nm, Intensity = 3.6 mW/cm2) as a Light Source in Watera no. 1c 2c 3c 4 5 6

exptl conda [M]:[RAFT]:[EY-SNP] 200:1:0 200:0:6.0 200:1:6.0 200:0:6.0 200:0:6.0 200:1:0

× × × ×

10−4 10−4 10−4 10−4

monomer

RAFT

amount of catalyst (ppm)

time (h)

αb (%)

DMA DMA DMA DMA DMA DMA

BTPA

0 3.0 3.0 3.0 3.0 0

12 36 12 0.25d 12e 12

0 0 0 8 96 0

BTPA

BTPA

a

Reactions were performed in the absence of oxygen at room temperature with monomer concentration of 4.81 M. bMonomer conversion was determined by using 1H NMR spectroscopy. cReaction was kept in darkness for the designated period of time. dViscous polymer solution was obtained. eGel formation.

Scheme 2. Proposed Mechanism for RAFT Photopolymerization Catalyzed by EY-SNP under Green Light Irradiation

at 1660 and 1600 cm−1 correspond to CO stretching and N−H bending modes, respectively (Figure S4B).89 XPS analysis confirmed the presence of EY on silica nanoparticle by detecting the presence of bromine at 67.9 and 70.1 eV (Figure S5).88 XPS analysis of EY-SNP also revealed that the amount of nitrogen atom (2.9%) present was in slight excess to the amount of Br atom (0.6%). Although the excess of primary amine groups on the silica nanoparticle may lead to aminolysis and degradation of trithiocarbonate chain-ends, the amount of EY-SNP used to catalyze the polymerization reactions is typically around 0.06% relative to trithiocarbonates (RAFT:EYSNP = 1:6.0 × 10−4). As the amount of excess primary amines is about 4-fold the amount of Eosin Y conjugated to the silica particle, it is likely that a maximum degradation of 0.24% of trithiocarbonate is estimated in a typical polymerization reaction. Therefore, the amount of degradation of trithiocarbonate due to primary amines is quite negligible. Thermal gravimetric analysis (TGA) on EY-SNPs (Figure S6 and Table S1) revealed EY grafting density of 0.94 EY molecules/nm2. Visible absorption of the resultant EY-SNP was then analyzed and compared with the (free) unconjugated EY in water as shown in Figure 1A. In comparison to unconjugated EY, EY-SNPs displayed a 15 nm red-shift in absorption with maximum absorption at 530 nm. This shift was attributed to the conjugation of EY onto the silica surface. Indeed, studies on silica nanoparticles coated with organic dyes revealed stabilization of the chromophore in the porous channels.90,91 In addition, previous report by Gianotti and co-workers on silica nanoparticle functionalized with Rose Bengal revealed a

similar bathochromic shift compared to its monomeric form in water.89 The authors ascribed the red-shift as confinement effect and change in chemical environment of Rose Bengal within the silica nanoparticle, which may also be the contributing factors for the bathochromic shift observed for EY-SNP. Upon successful characterizations of EY-SNP, preliminary investigations on the efficiency of EY-SNP to catalyze PET-RAFT polymerization were carried out using N,N-dimethylacrylamide (DMA) as monomer in water (Figure 1B (1)). Under green light irradiation (λmax = 515 nm, 2.8 mW/cm2), polymerization of DMA with 2-(n-butyltrithiocarbonate)propionic acid (BTPA) in the presence of EY-SNP photocatalyst (3.0 ppm relative to molar ratio of monomer) led to a viscous mixture within 10 h (Figure 1B (2)) (monomer conversion of 68%, Mn,GPC = 14 300, Mn,theo = 13 700, and Mw/ Mn = 1.08). As the EY-SNP is a heterogeneous catalyst dispersed in the reaction mixture, it can be easily removed at the end of the polymerization. Consequently, centrifugation of the polymer mixture twice in Eppendorf tubes, after dilution in methanol, led to a separation between the pink EY-SNP photocatalyst and the yellow polymer solution (Figure 1B (3)). Upon successful polymerization, we decided to perform a series of control experiments to confirm the role of EY-SNPs in the activation of the polymerization. The control studies were carried out with DMA as the model monomer in water. First, the absence of irradiation led to no observable polymerization (Table 1, entries 1−3). As there have been previous reports on the activation of RAFT agents under visible light through photolysis,50 polymerization in the absence of EY-SNP was C


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Table 2. Polymerization of Different Monomers by PET-RAFT Polymerization Using EY-SNP as Photocatalyst and BTPA under Green LEDs (λmax = 515 nm, 3.6 mW/cm2) as a Light Sourcea no e

1 2e 3e 4e 5 6 7 8 9 10 11 12

exptl conda [M]:[RAFT]:[EY-SNP] 200:1:7.5 200:1:1.5 200:1:3.0 200:1:6.0 100:1:6.0 100:1:6.0 100:1:6.0 100:1:6.0 100:1:6.0 100:1:6.0 100:1:6.0 100:1:6.0

× × × × × × × × × × × ×

−5

10 10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−4

monomer

solvent

amount of catalyst (ppm)

time (h)

αb (%)

Mn,thc(g/mol)

Mn,GPCc(g/mol)

Mw/Mn

DMA DMA DMA DMA DEGA BzA HEA THFA IA t-BA n-BA EHA

H2O H2O H2O H2O DMSO DMSO DMSO DMSO NMP NMP NMP NMP

0.375 0.75 1.5 3.0 6.0 6.0 6.0 6.0 6.0 6.0 6.0 6.0

4 4 4 4 4.5 4.5 12 12 12 12 12 12

37 50 55 66 76 53 42 59 47 82 81 75

7500 10200 11100 13200 14540 8900 5200f 9400 10000 10800 10600 14100

7700 10100 11500 13400 15330 8100 11700 7300 8100 10400 10400 13500

1.19 1.16 1.11 1.10 1.17 1.13 1.20 1.31 1.20 1.13 1.15 1.15

a

Reactions were performed in the absence of oxygen at room temperature with monomer concentrations between 2.4 to 3.2 M (formulations for each reaction are provided in the Supporting Information). bMonomer conversion was determined by using 1H NMR spectroscopy. cTheoretical molecular weight was calculated using the following 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 calibrated to poly(methyl methacrylate) and poly(styrene) standards. eIrradiated under green LEDs (λmax = 515 nm, 2.8 mW/cm2). fDeviation of the GPC molecular weight from the theoretical molecular weight is attributed to the differences in hydrodynamic volume. Monomers: di(ethylene glycol) ethyl ether acrylate (DEGA), benzyl acrylate (BzA), 2-hyroxyethyl acrylate (HEA), tetrahydrofurfuryl acrylate (THFA), isobornyl acrylate (IA), tert-butyl acrylate (tBA), n-butyl acrylate (n-BA), and 2-ethylhexyl acrylate (EHA).

Figure 2. Kinetic analysis of polymerization of DMA with different concentrations of EY-SNP as heterogeneous catalyst in water in the absence of oxygen at room temperature under green light irradiation (λmax = 515 nm, intensity = 2.8 mW/cm2) with BTPA as the chain transfer agent ([DMA]: [BTPA] = 200:1, 4.81 M monomer concentration). (A) Plot of ln([M]0/[M]t) against exposure time for polymerization with 3.0 and 7.5 ppm EYSNP. (B) Temporal control over polymerization in the presence of 3 ppm catalyst. (C) Mn versus monomer conversion for polymerization with 3.0 ppm catalyst. (D) Molecular weight distributions for polymerization with 3.0 ppm catalyst.

formation. Although these findings suggest a monomeractivated route for initiation of polymerization, this may result in the consumption of EY-SNP as it acts as a photoinitiator rather than a photocatalyst. However, this study, in the following discussions, will demonstrate that EY-SNP can be recovered and reused for further catalysis which reinforces the

carried out (Table 1, entry 6) under green light irradiation with no polymerization reported. In the absence of BTPA, direct monomer activation was observed under irradiation with a viscous polymer mixture forming in 15 min (Table 1, entry 4). Upon continuous irradiation for 12 h (Table 1, entry 5), almost complete monomer consumption was observed with gel D


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Figure 3. 400 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 = 14 000 g/mol) of poly(N,N-dimethylacrylamide) synthesized with 3.0 ppm EY-SNP.

Figure 4. Kinetic analysis of polymerization of DMA with 7.5 ppm EY-SNP and 7.5 ppm unbound EY as photocatalysts in water in the absence of oxygen at room temperature under green light irradiation (λmax = 515 nm, intensity = 2.8 mW/cm2) with BTPA as the chain transfer agent ([DMA]: [BTPA] = 200:1, 4.81 M monomer concentration). (A) Plot of ln([M]0/[M]t) against exposure time for polymerization with 7.5 ppm EY-SNP and EY. (B) Mn against conversion for polymerization with 7.5 ppm EY-SNP and EY. (C) GPC profiles for polymerization with 7.5 ppm EY-SNP. (D) Molecular weight distributions versus irradiation times for polymerization with 7.5 ppm EY.

unconjugated EY to an initiator.85 It is possible a similar mechanism is observed for interaction of unbound EY to BTPA. On the other hand, as conjugation of EY to silica nanoparticle potentially affects the photophysical properties of EY,90 a different pathway of polymerization that prevented photobleaching might have taken place.89 Furthermore, in our previous investigation on EY as a photocatalyst in PET (photoinduced electron/energy transfer polymerization)-RAFT polymerization, we discovered that the efficiency of EY in promoting polymerization depended on not only its low reduction potential in its excited state but also its low

role of EY-SNP as a photocatalyst rather than a photoinitiator in the presence of BTPA. Initiation of BTPA by the excited state EY-SNP is proposed to generate radicals that will get into the RAFT cycle (Scheme 2). As opposed to the recyclable nature of EY-SNP, unbound EY was proven in this study to behave quite differently as it is possibly consumed during the reaction, through either photodegradation or attack by a propagating radical, both of which result in photobleaching. In addition, a previous study by Padon and Scranton showed that bleaching of the unbound EY can also be attributed to photooxidative degradation upon electron transfer from the E


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Figure 5. Kinetic analysis of polymerization of DMA in dimethyl sulfoxide (DMSO), N-methyl-2-pyrrolidone (NMP), and water. (A) Plot of ln([M]0/[M]t) versus exposure time with EY-SNP as heterogeneous catalyst in the absence of oxygen at room temperature under green light irradiation (λmax = 515 nm, 3.6 mW/cm2) with BTPA as the chain transfer agent ([DMA]:[BTPA]:[EY-SNP] = 200:1:3.0 × 10−4, 4.81 M monomer concentration, 3.0 ppm catalyst). (B) Mn against conversion for polymerization in different solvents. (C) Mw/Mn against conversion for polymerization in different solvents. (D) Photographs of separation of EY-SNP from polymer solutions synthesized in different solvents through centrifugation.

3.0 ppm (kpapp = 2.41 × 10−3 min−1) (Figure 2A). Both catalyst loadings also afforded temporal control as the polymerizations were initiated upon irradiation and suppressed when the light was turned off (Figure 2B and Figure S7). Sampling during the course of the polymerizations revealed a linear increase in theoretical and experimental molecular weights with a gradual decrease in polydispersities with increasing monomer conversions (Figures 2C and 4B). This behavior is consistent with a controlled/living radical process and an efficient RAFT polymerization.94 In addition, GPC analysis showed that monomodal distributions were achieved throughout the polymerizations for both 3.0 and 7.5 ppm (Figures 2D and 4C) catalyst loadings. Furthermore, 1H NMR verified the presence of RAFT end group for 3.0 ppm (Figure 3) and 7.5 ppm catalyst concentrations (Figure S8) by the presence of signals at 0.95, 1.15, 3.45, and 5.25 ppm. Furthermore, Mn,NMR, Mn,theo, and Mn,GPC were in good agreement. As we were intrigued by the efficiency of EY-SNP in mediating PET-RAFT polymerization, we decided to compare its efficiency against EY. Again, a plot of ln[M]0/[M]t against time revealed a pseudo-first-order kinetics for 7.5 ppm (kpapp = 2.06 × 10−3 min−1) (Figure 4A) and 3.0 ppm (kpapp = 1.63 × 10−3 min−1) (Figure S9) with unconjugated EY with apparent propagation rate constants that were slightly lower but close to that of EY-SNP (kpapp = 3.04 × 10−3 min−1 for 7.5 ppm (Figures 2A and 4A) and kpapp = 2.41 × 10−3 min−1 for 3 ppm (Figure 2A). Nevertheless, complete bleaching of EY dye was observed for the unconjugated 3.0 ppm EY (Figure S10) at the end of the polymerization while retention of the pink silica (Figure 1B (3)) was observed for EY-SNP. A brief discussion on the photobleaching effects of unbound EY and EY-SNP is provided in the previous section of this paper.

fluorescence quantum yield and high intersystem crossing rate which ensures a longer lived triplet excited state as compared to other xanthene dyes.92,93 As conjugation of xanthene dye to silica nanoparticle has been shown to enhance the radiative rate and reduce nonradiative rate of the dye due to reduced mobility in the porous channel, it is highly likely that EY-SNP is subjected to the same fate.90 A possible reduction of nonradiative rate of EY-SNP could affect intersystem crossing, and therefore, hinder electron transfer to a RAFT agent. The discrepancy in behavior between the EY-SNP and unbound EY warrants different pathways of RAFT activation by these catalysts, which could be either through electron or energy transfer. We are currently attempting to characterize these mechanistic pathways. Upon establishing the role of EY-SNPs in the photoactivation of RAFT agent, different concentrations of EY-SNPs (Table 2, entries 1−4), in the range between 0.375 to 3.0 ppm, were investigated to optimize the catalyst loading. Polymerization of DMA for 4 h in water under green light (λmax = 515 nm, 2.8 mW/cm2) irradiation led to the successful synthesis of polymers with well-defined molecular weights and molecular weight distributions. As expected, higher catalyst loadings resulted in higher monomer conversions. To further reinforce this hypothesis, kinetic analyses of polymerization of DMA with two different catalyst concentrations, 3.0 and 7.5 ppm, were performed under green light irradiation in water. These polymerizations were carried out in glass vials using BTPA as the RAFT agent with the molar ratio of [DMA]:[BTPA]:[EYSNP] of 200:1:6.0 × 10−4 (3.0 ppm)/1.5 × 10−3 (7.5 ppm). A plot of ln[M0]/[M]t against time revealed a pseudo-first-order kinetics with catalyst loading of 7.5 ppm (kpapp = 3.04 × 10−3 min−1) having a higher apparent propagation rate constant than F


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Figure 6. UV−vis absorptions (A) and photos (B) of EY-SNP in DMSO, NMP, and water.

faster polymerization. Despite the differences in kinetic rates, the three solvents led to overall well-defined polymers with a linear increase in theoretical and experimental molecular weights and narrow molecular weight distributions (Figure 5B,C and Figure S12). Furthermore, 1H NMR confirmed the retention of RAFT end group (Figures S13−S15). In these three solvents, EY-SNP was easily separated from the polymers using centrifugation. After centrifugation, the polymer solution was analyzed via UV−vis spectroscopy to determine the amount of EY-SNP that might be trapped in the polymer matrix.98 Based on the calibration curve generated with EY (Figure S16B), about 0.53, 0.27, and 0.2 ppm of the EY-SNP were found in water, DMSO, and NMP, respectively (Figure S16A). The remaining Eosin Y molecules were mostly likely bound to the surface of the silica particle as the unbound EY generated during the course of the reaction were most likely bleached. Photobleaching of unbound EY was discussed in the previous section. In order to determine the versatility of EY-SNP for PETRAFT, polymerization of various hydrophilic acrylate monomers (including (di(ethylene glycol) ethyl ether acrylate (DEGA) and 2-hyroxyethyl acrylate (HEA)) and hydrophobic acrylates (including benzyl acrylate (BzA), tetrahydrofurfuryl acrylate (THFA), isobornyl acrylate (IA), tert-butyl acrylate (tBA), n-butyl acrylate (n-BA), and 2-ethylhexyl acrylate (EHA)) were attempted in different solvents including water, DMSO, and NMP (Table 2, entries 5−12). These polymerizations resulted in well-defined polymers with good control over molecular weight and molecular weight distributions (Mw/Mn < 1.4). To demonstrate that this catalyst approach could be employed for a broad range of molecular weights, we decided to prepare higher molecular weight PDMA homopolymers using [DMA]:[RAFT] ratio equal to 1000:1 and 500:1. To improve the control, we also decreased the monomer concentration to 3.2 M instead of 4.8 M. High monomer conversions were achieved above 95% in 2 h with a good control of the molecular weight and a relatively low dispersity (around 1.3, see Figure S17). In addition, as our previous study has shown the ability of Eosin Y to confer oxygen tolerance,92 we decided to perform a similar oxygen tolerant polymerization with DMA in DMSO using 7.5 ppm EY-SNP under green light irradiation (λmax = 515 nm, intensity = 2.8 mW/cm2). We were able to achieve 93% monomer conversion in 18 h, which led to a high molecular weight polymer (Mn,theo = 18 700 g/mol, Mn,GPC = 19 700 g/mol, and Mw/Mn = 1.14) with monomodal distribution (Figure S18). To further assert the reusable nature of EY-SNP, recycling of EY-SNP in polymerization reactions was implemented. To achieve this, polymerization of n-BA was repeated with 6 ppm

As in our previous reports on PET-RAFT, we observed that the nature of solvent played an important role in the polymerization rates.95 Therefore, we decided to investigate polymerization of DMA catalyzed by EY-SNP in the presence of DMSO, NMP, and water. All three solvents afforded polymerization in the presence of 3.0 ppm EY-SNP; however, significant differences in the apparent propagation rate constants were observed in all three solvents. A plot of ln([M]0/[M]t) against time (Figure 5A) revealed a pseudofirst-order kinetics for the different solvents with the concentration of propagating radical being much higher in DMSO (kpapp = 1.33 × 10−2 min−1), followed by water (kpapp = 3.51 × 10−3 min−1) and finally by NMP (kpapp = 2.76 × 10−3 min−1). In comparison to the initial studies on the effects of catalyst concentration on polymerization of DMA in water (Figure 2A), which was carried out under green light irradiation (λmax = 515 nm, intensity = 2.8 mW/cm2), a higher light intensity (λmax = 515 nm, intensity = 3.6 mW/cm2) was used to study solvent effects on polymerization (Figure 5A), and therefore this led to a faster polymerization. To confirm that this change in polymerization rate was not due to a change in the EY-SNPs physical properties, TEM analysis of the EY-SNP after polymerization of DMA was performed. No change in particle size and morphology were observed (Figure S11). Furthermore, TEM did not exhibit the presence of polymers coated onto the EY-SNPs after polymerization, which confirmed that there were no radical initiations from the EYSNPs. As solvent can affect the physical properties of photocatalysts, we decided to record the absorption spectra of EY-SNPs in these solvents. No significant change was observed in the UV−vis absorption of EY-SNPs (Figure 6A). Finally, the difference in polymerization rates could be attributed to solvent effects on the propagation rate coefficient. Indeed, previous reports revealed that higher dielectric constant of solvents afforded better stabilization to the propagating radicals, resulting in higher propagation rate coefficient.95,96 The dielectric constants of DMSO, water, and NMP are 46.6, 79.7, and 32.2, respectively.97 The lower apparent propagation rate constant in NMP compared with DMSO and water can be attributed to its lower dielectric constant. Despite water having the highest dielectric constant, polymerization of DMA was found to be much slower in water compared to DMSO. This discrepancy can be attributed to the poor stability of EY-SNP in water. Indeed, we observed precipitation of the EY-SNP in water, which reduces the surface area of the catalyst and therefore its activity. In contrast, the dispersions in DMSO and NMP were comparatively stable for several days (Figure 6B). The higher particle stabilization in DMSO over water may have improved the reactivity and interaction with BTPA leading to G


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poly(n-BA) with monomodal GPC traces (Figure 7B,C and Figure S19) with high retention of RAFT end group (Figures S21−S25). Although photobleaching of EY can be considered as a form of catalyst removal, this approach is inferior compared to the ability to remove and recycle the catalyst that is provided by EY-SNP. For the same concentration of photocatalysts used, EY-SNP ensures maximum atom efficiency through its recyclable nature as compared to EY. Furthermore, to confirm the versatility of our systems, we decided to investigate the polymerization of another monomer, DMA, using 7.5 ppm catalyst (Table S2) to determine the ability to recycle the catalyst in water. EY-SNPs could be reused up to three cycles; however slight decrease in efficiencies was observed after each cycle in water. The decreased catalytic activity by EY-SNPs could probably be due to the loss of photocatalyst through stabilization in the polymer matrix in each cycle as water was shown to be a poor solvent for EY-SNP.98 In order to reinforce the livingness of the polymerization catalyzed by EY-SNP, chain extensions were also performed. Initial syntheses of PDMA and PMA macro-RAFT agents were carried out with EY-SNP under green light irradiation in water and DMSO, respectively. The macroRAFT agents were successfully synthesized with good control over molecular weights and molecular weight distributions: (Mn,theo = 5200 g/ mol, Mn,GPC = 7000 g/mol, and Mw/Mn = 1.19 after 5 h of irradiation for PDMA and Mn,theo = 14 000 g/mol, Mn,GPC = 13 700 g/mol, and Mw/Mn = 1.08 for PMA after 14 h of irradiation; Figure S26). Chain extensions with EY-SNPs in DMSO were then carried out with purified PMA macro-RAFT to generate PMA-block-PDMA (Figure 8A) and purified PDMA macro-RAFT to generate PDMA-block-PNIPAM (Figure 8B). A complete shift in molecular weights with minimum formation of dead chains was observed for the macroRAFT agents, therefore resulting in the synthesis of well-defined diblock copolymers. To further reinforce the presence of minimum dead chains during the course of the polymerization, chain extensions of PDMA macro-RAFT with DMA at a higher feed ratio (DMA:macro-RAFT = 400:1, Figure 8C) led to successful synthesis of PDMA-block-PDMA with over 80 ± 10% of “living” chains calculated by deconvolution of molecular weight distribution (Figure S27).

Table 3. Recycling EY-SNP for Polymerization of n-BA Using BTPA under Green LEDs (λmax = 515 nm, Intensity = 2.8 mW/cm2) as a Light Source in NMPa no. of cycles

time (h)

αb (%)

Mn,thc (g/mol)

Mn,GPCc (g/mol)

Mn,NMRe (g/mol)

Mw/Mn

1 2 3 4 5

12 12 12 12 14

75 72 73 70 75

9800 9500 9600 9300 9900

9500 9500 9200 10200 11200

10400 9500 9700 9900 10200

1.16 1.16 1.14 1.13 1.14

a

Reactions were performed in the absence of oxygen at room temperature with monomer concentrations at 2.91 M with [n-BA]: [BTPA]:[EY-SNP] of 100:1:6.0 × 10−4. bMonomer conversion was determined by using 1H NMR spectroscopy. cTheoretical molecular weight was calculated using the following 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 calibrated to poly(methyl methacrylate) standards. eMolecular weight was determined using the following equation Mn,NMR = (I3.3 ppm/2)/ (I5.1 ppm) × MWM + MWRAFT, where I3.3 ppm and I5.1 ppm correspond to integration of proton 2 and 3, respectively (see Figures S21−S25).

EY-SNP as the photocatalyst in NMP (Table 3, Figure 7, and Figure S19). A concurrent experiment with EY was also performed as a comparison. As shown in Figure 7A, initial EYSNP and EY reaction mixtures before polymerization were dark orange and pale orange, respectively. After 12 h, a viscous polymerization mixture was observed for both EY (79% monomer conversion, Mn,theo = 10 100 g/mol, Mn,GPC = 7500 g/mol, and Mw/Mn = 1.14) and EY-SNP (Table 3, entry 1). However, only the EY-SNP reaction mixture retained its color while the reaction mixture with EY turned yellow (UV−vis analysis (Figure S20) revealed the absence of photocatalyst). The viscous reaction mixture with EY-SNP was then diluted in THF and centrifuged to separate EY-SNP from the synthesized polymer. The catalyst was then dried in a vacuum oven to remove any residual THF before being reused in a second polymerization cycle. This procedure was repeated up to five times. Each cycle of polymerization with the recycled catalyst afforded high monomer conversions (α ≥ 70%) (Table 3, entries 1−5). Moreover, each cycle led to the synthesis of

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CONCLUSIONS In this work, we developed a heterogeneous catalyst system that is able to work under ultralow concentrations under green light irradiation and can be further recycled multiple times. This was

Figure 7. Polymerization of n-BA with 6 ppm EY-SNP recycled up to five times. (A) Photographs of EY and EY-SNP solutions before and after polymerization of n-BA. (B) Photographs of polymer separated from EY-SNP at the end of the first cycle with the inset reflecting the molecular weight distribution of the polymer product. (C) Photographs of polymer separated from EY-SNP at the end of the fifth cycle with the inset reflecting the molecular weight distribution of the polymer product. H


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Macromolecules

Figure 8. Chain extensions of PMA and PDMA macro chain transfer agents (macro-RAFT) with DMA and NIPAM with EY-SNP under green light irradiation. (A) Molecular weight distributions of PMA macro-RAFT agent and PMA-block-PDMA diblock copolymers after 6 h irradiation. (B) PDMA macro-RAFT agent and PDMA-block-PNIPAM diblock copolymers after 50 min irradiation. (C) PDMA macro-RAFT agent and PDMAblock-PDMA diblock copolymers after 60 min irradiation. (3) Shanmugam, S.; Xu, J.; Boyer, C. Photocontrolled Living Polymerization Systems with Reversible Deactivations through Electron and Energy Transfer. Macromol. Rapid Commun. 2017, 38 (13), 1700143. (4) Corrigan, N.; Shanmugam, S.; Xu, J.; Boyer, C. Photocatalysis in organic and polymer synthesis. Chem. Soc. Rev. 2016, 45 (22), 6165− 6212. (5) Zivic, N.; Bouzrati-Zerelli, M.; Kermagoret, A.; Dumur, F.; Fouassier, J.-P.; Gigmes, D.; Lalevée, J. Photocatalysts in Polymerization Reactions. ChemCatChem 2016, 8 (9), 1617−1631. (6) Yagci, Y.; Jockusch, S.; Turro, N. J. Photoinitiated Polymerization: Advances, Challenges, and Opportunities. Macromolecules 2010, 43 (15), 6245−6260. (7) Xu, J. T.; Shanmugam, S.; Boyer, C. Organic Electron DonorAcceptor Photoredox Catalysts: Enhanced Catalytic Efficiency toward Controlled Radical Polymerization. ACS Macro Lett. 2015, 4 (9), 926− 932. (8) Kutahya, C.; Aykac, F. S.; Yilmaz, G.; Yagci, Y. LED and visible light-induced metal free ATRP using reducible dyes in the presence of amines. Polym. Chem. 2016, 7 (39), 6094−6098. (9) Konkolewicz, D.; Schröder, K.; Buback, J.; Bernhard, S.; Matyjaszewski, K. Visible Light and Sunlight Photoinduced ATRP with ppm of Cu Catalyst. ACS Macro Lett. 2012, 1 (10), 1219−1223. (10) Pan, X.; Malhotra, N.; Simakova, A.; Wang, Z.; Konkolewicz, D.; Matyjaszewski, K. Photoinduced Atom Transfer Radical Polymerization with ppm-Level Cu Catalyst by Visible Light in Aqueous Media. J. Am. Chem. Soc. 2015, 137 (49), 15430−15433. (11) Anastasaki, A.; Nikolaou, V.; Zhang, Q.; Burns, J.; Samanta, S. R.; Waldron, C.; Haddleton, A. J.; McHale, R.; Fox, D.; Percec, V.; Wilson, P.; Haddleton, D. M. Copper(II)/Tertiary Amine Synergy in Photoinduced Living Radical Polymerization: Accelerated Synthesis of ω-Functional and α,ω-Heterofunctional Poly(acrylates). J. Am. Chem. Soc. 2014, 136 (3), 1141−1149. (12) Jones, G. R.; Whitfield, R.; Anastasaki, A.; Haddleton, D. M. Aqueous Copper(II) Photoinduced Polymerization of Acrylates: Low Copper Concentration and the Importance of Sodium Halide Salts. J. Am. Chem. Soc. 2016, 138 (23), 7346−7352. (13) Xu, J.; Jung, K.; Atme, A.; Shanmugam, S.; Boyer, C. A Robust and Versatile Photoinduced Living Polymerization of Conjugated and Unconjugated Monomers and Its Oxygen Tolerance. J. Am. Chem. Soc. 2014, 136 (14), 5508−5519. (14) Discekici, E. H.; Pester, C. W.; Treat, N. J.; Lawrence, J.; Mattson, K. M.; Narupai, B.; Toumayan, E. P.; Luo, Y.; McGrath, A. J.; Clark, P. G.; Read de Alaniz, J.; Hawker, C. J. Simple Benchtop Approach to Polymer Brush Nanostructures Using Visible-LightMediated Metal-Free Atom Transfer Radical Polymerization. ACS Macro Lett. 2016, 5 (2), 258−262.

achieved through conjugation of EY to silica nanoparticle, which not only allowed reduced contamination to the final polymer mixture but also ensured recycling of the catalyst. Polymerizations performed in this work relied on less than 10 ppm EY-SNP, therefore enabling maximum atom economy. This heterogeneous photocatalytic system generated a range of well-defined hydrophobic and hydrophilic polymers in biocompatible solvent such as water and organic solvents such as DMSO and NMP. We are currently performing investigations to understand mechanistic pathway of this heterogeneous photocatalytic system.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b02215. Experimental details, FTIR spectra, TEM images, GPC curves, and NMR spectra (Figures S1−S27 and Tables S1 and S2) (PDF)

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AUTHOR INFORMATION

Corresponding Author

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

Cyrille Boyer: 0000-0002-4564-4702 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS C.B. acknowledges Australian Research Council (ARC) for his Future Fellowship (FT12010096). The authors acknowledge the NMR Facility and Microscopy Facility within the Mark Wainwright Analytical Centre at the University of New South Wales for NMR support.

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REFERENCES

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