Aqueous Metal-Free Atom Transfer Radical Polymerization

Publication Date (Web): March 15, 2018 ... Good controllability of the polymerization regulated by light on and off confirmed the high degree of tempo...
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Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

Aqueous Metal-Free Atom Transfer Radical Polymerization: Experiments and Model-Based Approach for Mechanistic Understanding Chao Bian, Yin-Ning Zhou, Jun-Kang Guo, and Zheng-Hong Luo* Department of Chemical Engineering, School of Chemistry and Chemical Engineering, State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai 200240, P. R. China S Supporting Information *

ABSTRACT: Metal-free atom transfer radical polymerization (ATRP) was successfully achieved in aqueous media for the first time. Polymerization of poly(ethylene oxide) methyl ether acrylate (PEGA480) was well controlled (Đ < 1.40) under visible light irradiation using tetrabromofluorescein (Eosin Y) as catalyst and pentamethyldiethylenetriamine (PMDETA) as electron donor. A validated kinetic model was developed to investigate the process of photoredox catalytic cycle via reductive quenching pathway. Experimental and simulation results showed that electron donor not only had an important influence on the ATRP activation, but also participated in the ATRP deactivation. Furthermore, the effects of water content, catalyst concentration, and degree of polymerization on the polymerization were studied thoroughly by a series of experiments. Good controllability of the polymerization regulated by light on and off confirmed the high degree of temporal control. The livingness of the chains was proved by a successful chain extension experiment. Both experimental and simulation techniques were used to study aqueous metal-free ATRP, which provided a promising method to synthesize polymers in the absence of metal and organic solvent.

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INTRODUCTION Since the discovery of controlled radical polymerization (CRP) techniques such as nitroxide-meditated radical polymerization (NMP),1 reversible addition−fragmentation chain transfer (RAFT) polymerization,2 and atom transfer radical polymerization (ATRP),3 polymerization engineering has experienced great development. These CRP techniques have proven to be a powerful tool to synthesize polymers with tailor-made architecture. ATRP is one of the most extensively employed CRP techniques due to the wide availability of various catalysts and initiators.4 Particularly, photoATRP receives great interests in recent years.5,6 Metal catalysts such as copper (Cu),7−11 iridium (Ir),12−14 iron (Fe),15−19 and ruthenium (Ru)20,21 are used to meditate the equilibrium between active species and dormant species in photoATRP systems. However, there exist two challenges in ATRP. One is that the use of metal catalysts results in an inevitable contamination to the final products by metal residues, which impedes the application of this technology to synthesize biomedical and electronic materials.22,23 Though low-ppm catalyst ATRP methods24−26 and metal catalyst removal procedures are developed to avoid the metal residues,27−29 there are still some limitations such as the limited practicability and the complicated postprocessing. Another one is that potentially hazardous and volatile organic solvent is commonly used in ATRP systems.30−32 In order to overcome these challenges, ATRP with metal-free catalyst needs to develop and environ© XXXX American Chemical Society

ment friendly water is considered to be a good choice to replace the organic solvent. Fortunately, metal-free ATRP utilizing organic catalyst under light irradiation has been discovered, which entirely circumvent the problem of metal contamination to the products.33−37 However, the current research about metal-free ATRP is still in the initial stage and most of researches about metal-free ATRP are performed in organic solvent. Therefore, aqueous metal-free ATRP is highly desired. In a robust metal-free ATRP, the main task is to establish photoredox catalytic cycle through single electron transfer, which requires a catalyst having long lifetime in the excited state, appropriate redox potential and good reversibility in the redox process.38−40 Currently, photoredox catalytic cycle can operate via oxidative quenching pathway or reductive quenching pathway, as shown in Scheme 1. When the excited state of photoredox catalysts possess sufficient negative reduction potential and sufficient long lifetime to directly reduce the initiator, the polymerization system can proceed through an oxidative quenching pathway.41 For example, a series of aromatic compounds (e.g., perylene, diaryl dihydrophenazines and phenoxazines) were investigated by Miyake and co-workers, and are able to mediate the polymerization under light irradiation via oxidative quenching pathway.42−47 OtherReceived: February 13, 2018 Revised: March 7, 2018

A

DOI: 10.1021/acs.macromol.8b00348 Macromolecules XXXX, XXX, XXX−XXX

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Scheme 1. Photoredox Catalytic Cycles via (A) Oxidative Quenching Pathway and (B) Reductive Quenching Pathway in Photoinduced Metal-Free ATRPa

Key: Cat, ground state catalyst; Cat*, excited state catalyst; Cat•+, oxidized radical cation; Cat•−, reductive radical anion; D, electron donor; D•+, electron donor radical cation; PnX, dormant chains, Pn•/Pm•, propagating radical chains, X−, halide anion; M, monomer; Pn+m/PnH/Pm=, dead chains.

a

Scheme 2. Metal-Free ATRP of PEGA480 in Water under Visible Light

wise, if the excited state of photoredox catalysts cannot directly reduce the initiator, a sacrificial electron donor is required for the catalyst to operate the polymerization via reductive quenching pathway. Yagci group and Zhu group explored dye/amine systems for metal-free ATRP, which operated in the polymerization via reductive quenching pathway.48,49 Compared to oxidative quenching pathway, reductive quenching pathway is easier to conduct in metal-free ATRP due to lower requirement of the reducing power in the excited state of photoredox catalysts. In addition to the experiment method, the approach of modeling has a distinctive ability in polymerization processes. Kinetic model validated by experimental data is considered to be reliable to provide some underlying reaction information, which is not able to acquire from the experimental method easily.50−58 For instance, Matyjaszewski et al. thoroughly investigated metal-free ATRP through experiment and simulation methods, which provided a clear understanding on the process of oxidative quenching pathway.59 However, there are few studies for a deep insight into the process of reductive quenching pathway. The polymerization mechanism and the process of reductive quenching pathway are still unclear in some respects. In this work, metal-free ATRP in water was first reported for the polymerization of poly(ethylene glycol) methyl ether acrylate using Eosin Y as photoredox catalyst and PMDETA as electron donor under visible light, as shown in Scheme 2. Kinetic modeling validated by experimental data was employed to give a deep insight into the polymerization mechanism and the process of reductive quenching pathway. Furthermore, the influence of water content on the polymerization was studied and the effects of catalyst concentration and degree of polymerization were analyzed by polymerization kinetics experiments. The ability of temporal control for this technology was evaluated by the experiment turning light on and off. A chain extension experiment was used to examine the chain end functionality. It is confident that this study provides a promising ATRP technology to synthesize polymers and improves the

understanding about mechanism of reductive quenching pathway via combination of experiment and simulation methods.

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EXPERIMENTAL SECTION

Materials. Poly(ethylene oxide) methyl ether acrylate (PEGA480, Sigma-Aldrich, average molecular weight 480 g/mol) was passed through a column of basic alumina (Sinopharm Chemical Reagent Co.) to eliminate the inhibitors. Tetrabromofluorescein (Eosin Y, J&K Scientific Ltd., analytical reagent), pentamethyldiethylenetriamine (PMDETA, Energy Chemical, 99%), 2-hydroxyethyl 2-bromoisobutyrate (HEBiB, J&K Scientific Ltd., 95%), water (H2O, Fisher Scientific, HPLC grade), tetrahydrofuran (THF, Adamas, analytical reagent), nhexane (Adamas, analytical reagent), and anhydrous sodium sulfate (Adamas, analytical reagent) were used as received. Instrumentation. Monomer conversions were recorded on Bruker AV400 MHz via 1H NMR spectra in D2O. Analysis of number-average molecular weight (Mn), weight-average molecular weight (Mw) and molecular weight distributions (Đ = Mw/Mn) of the polymers were performed by gel permeation chromatography (GPC, DAWNEOS, Wyatt Technology Corp.) equipped with PLgel column (MIXED-B, 5 μm) and a multiangle laser light scattering instrument, using tetrahydrofuran (THF) as the eluent at a flow rate of 1.0 mL/min at 30 °C. Ultraviolet−visible spectroscopy was detected on a PerkinElmer Lambda 35 spectrometer. A long-arc xenon lamp coupled with visible light filter (ranging from 400 to 800 nm) was used for the photochemical reactions. The intensity of visible light source was measured to 7.0 ± 0.5 mW/cm2, and the reaction temperature was about 36 ± 1 °C. The equations based on the method of moments were solved numerically in simulation software (MATLAB). General Procedure for Aqueous Metal-Free ATRP of PEGA480. PEGA480 (2.2 mL, 5.0 mmol), HEBiB (5.28 mg, 25.0 μmol), Eosin Y (16.2 mg, 25.0 μmol), PMDETA (43.3 mg, 250 μmol), and H2O (8.8 mL) were successively transferred into a Schlenk flask. The Schlenk flask was sealed with rubber plug and deoxygenated via bubbling with nitrogen for 30 min before being placed into the photochemical reactor. After a predetermined time, samples were obtained by using a syringe from the Schlenk flask for GPC and 1H NMR measurement. Several drops of the sample were dissolved in D2O for 1H NMR measurement. The rest was dissolved in THF and dried by anhydrous sodium sulfate. Subsequently, the polymers were precipitated into n-hexane and dried in a vacuum oven. The final B

DOI: 10.1021/acs.macromol.8b00348 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules product was obtained for GPC analysis. Aqueous metal-free ATRP systems under different conditions were performed in the same procedure. Chain Extension. The macroinitiator P(PEGA480)Br was obtained via aqueous metal-free ATRP in the optimized conditions of water content {[PEGA480]:[HEBiB]:[EosinY]:[PMDETA] = 200:2:1:10, [PEGA480] = 0.45 M in H2O/PEGA480 (4:1 v/v, VH2O = 8.8 mL)}. The polymerization was stopped after 2 h, with conversion around 18.0%. The macroinitiator was purified for GPC analysis. The experiment of chain extension followed this methodology: PEGA480 (2.2 mL, 5.0 mmol), macroinitiator (154.0 mg, 10 μmol), Eosin Y (16.2 mg, 25.0 μmol), PMDETA (43.3 mg, 250 μmol) were disovled in H2O (8.8 mL). The mixture was added into a Schlenk flask and bubbled with nitrogen for 30 min. Then, the Schlenk flask was irradiated in the photochemical reactor for 4 h. The final polymer (9.1% conversion) was purified prior to GPC measurement.

1). These species showed almost no absorbance, suggesting negligible photochemical activity. The transitions between different electronic states of Eosin Y during the light irradiation and the redox potentials of the ground and excited states of Eosin Y were illustrated in Figure 2. As shown in Figure 2A, the ground state catalyst (EY) was turned into singlet excited state (1EY*) via the absorption of visible light photon. 1EY* could experience an intersystem crossing (ISC) to become triplet excited state (3EY*). Both 1 EY* and 3EY* presented reactive redox potentials (Figure 2B, calculated by eqs 1−4 in the Supporting Information), whereas 3 EY* usually participated in the photoredox reaction due to its longer lifetime (τ1EY* = 2.1 ns, τ3EY* = 24 μs).61−63 In ATRP system, though the reduction potential of 3EY* (EEY•+/3EY* = −1.15 V) was more negative than the initiator (∼-0.80 V vs SCE for the alkyl bromides41), the 3EY* still cannot directly reduce the initiator. This was because the lifetime of 3EY* was not long enough to persist to undergo the reaction with ATRP initiator under such small electric potential difference. On the other hand, the radical anion of Eosin Y (EY•−) with EEY/EY•− = −1.08 V was a relatively stable state, which owned enough long lifetime for reducing the initiator. In this case, 3EY* could be first quenched by addition of electron donor to generate EY•−, and then the EY•− initiated the initiator. Therefore, the Eosin Y catalyzed polymerization system could be conducted via reductive quenching pathway in the presence of electron donor.48 As an indirect evidence of current system experiencing reductive quenching pathway, two comparative trials in the absence and presence of electron donor were presented in Table 1. No polymerization occurred in the absence of electron donor, while it could conduct the polymerization in a controlled manner in the presence of electron donor. This phenomenon suggested that electron donor was a prerequisite for the polymerization via reductive quenching pathway. Kinetic Model of Aqueous Metal-Free ATRP via the Reductive Quenching Pathway. To better investigate the significance of electron donor on current system, experimental and modeling approaches were undertaken to give more information on reactions. Under the mechanism of metal-free ATRP via reductive quenching pathway as depicted in Scheme 1B, the cycle began with photoexcitation of Cat into Cat*. Cat* was reduced to Cat•− by accepting D, at the same time D•+ was generated in this process. Then, the Cat•− activated the initiator to form primary radicals (P0•), X− and Cat. P0• could add to monomer and initiate the polymerization. The deactivation of Pn•, as previously proposed, involved with a back electron transfer from the X− to the D•+. D•+ oxidized X− to halide

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RESULTS AND DISCUSSION Characterization and Property of the Catalyst. As the photoexcitation of catalyst is the first step in photochemical process, the catalyst should have a strong absorption in the wavelength region of light source. The ultraviolet−visible spectroscopy of Eosin Y in water was shown in Figure 1. Eosin

Figure 1. UV−vis spectra of a diluted solution of each component in the reaction system with [PEGA480]:[HEBiB]:[EosinY]: [PMDETA] = 200:1:1:10 (PEGA480 (10.4 mM), HEBiB (5.2 × 10−5 M), Eosin Y (5.2 × 10−5 M), and PMDETA (5.2 × 10−4 M) in water).

Y displayed a strong absorption in the visible light region and the maximum absorption wavelength (λmax) was about 515 nm. The molar absorption coefficient (ε) at maximum absorption wavelength was 55385 M−1 cm−1 which calculated according to Beer−Lambert law.60 Furthermore, the monomer (PEGA480), initiator (HEBiB) and electron donor (PMDETA) in water were also recorded by ultraviolet−visible spectroscopy (Figure

Figure 2. (A) Electronic states of Eosin Y and the transitions under irradition and (B) redox potentials (V vs SCE) of the ground and excited states of Eosin Y in MeOH. C

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Table 1. Results of Aqueous Metal-Free ATRP of PEGA480 Using Eosin Y as Catalyst in the Absence and Presence of Electron Donor PMDETA entrya

conditions ([PEGA480]:[HEBiB]:[EosinY]:[PMDETA])

time (h)

conversion (%)c

Mnd

Đd

1 2b

200:1:1:0 200:1:1:10

6 6

0 39.2

− 76300

− 1.20

a

Reaction conditions: [PEGA480] = 0.45 M in H2O/PEGA480 (4:1 v/v, VH2O = 8.8 mL) under visible light irradiation. b[EosinY]:[PMDETA] = 1:10 was used here due to the moderate polymerization rate. The detailed study about the influence of the concentration of PMDETA on the polymerization will be given in the following section. cConversions determined by 1H NMR. dDetermined by GPC in THF.

Table 2. Elementary Reactions of Aqueous Metal-Free ATRP PEGA480 via Reductive Quenching Pathway elementary reaction

rate coefficienta

equation khv

photoexcitation

8.8

Cat ⎯→ ⎯ Cat*

reduction by electron donor

7.3 × 10

this work

−

5.2 × 10

this work

1.1 × 10

this work

2.3 × 10

this work

8.8 × 10

this work

3

1.5 × 10

64b

2.4 × 104

64b

k t0

3.7 × 108

64b

k t1

6.8 × 107

64b

k tc

1.4 × 107

64b

k td

0

64b

1.4 × 108

65

•−

•− ka0

•

P0X + cat •

PnX + cat

6 10

⎯→ ⎯ P0X + D •

6

−

→ Pn + X + Cat •+ kd

−

Pn + X + D propagation

+D

•+ kd0

•− ka

k in

•

9

⎯→ ⎯ P0 + X + Cat

−

P0 + X + D

•

P0 + M →

12

→ PnX + D

P1•

kp

Pn• + M → Pn + 1•

P0• + P0• ⎯→ ⎯ P0P0

termination

P0• + Pn• → P0Pn Pn• + Pm• → Pn + m

Pn• + Pm• ⎯→ ⎯ PnH + Pm2 − kde

catalyst decay

Cat* ⎯→ ⎯ Cat

The units for all rate coefficients were M s , except that of kd, kd0 were expressed in M s and khv, kde were expressed in s−1. bThe rate coefficients at 36 °C were calculated based on ref64 via Arrhenius equation k = Ae−Ea/RT, and the value of Ea was 17.0 kJ mol−1.66 Other unknown rate coefficients were assessed by adjusting to the experimental data. The standardized residual analysis of rate coefficients estimation (Figure S1) was shown in the Supporting Information. The adjusting curves (dash lines) in Figure 3 agreed well with experimental data, suggesting the reliability for illustration purposes (qualitative description). a

−1

this work

•+

k re

Cat* + D → Cat

ATRP equilibrium

ref

−1

−2

−1

Figure 3. Comparison of kinetic experimental data (points) and simulation results (dash lines) for aqueous metal-free ATRP of PEGA480. (A) semilogarithmic kinetic plots; (B) evolution of Mn and Đ (Mw/Mn) versus conversion, under conditions [PEGA480]:[HEBiB]:[EosinY]:[PMDETA] = 200:1:1:5/200:1:1:10/200:1:1:20 and [PEGA480] = 0.45 M in H2O/PEGA480 (4:1 v/v, VH2O = 8.8 mL) with visible light irradiation at 36 °C.

radical that deactivate the Pn•.48,49 Here, the deactivation reaction between Pn•, X− and D•+ was considered as one elementary reaction for simplicity because the halide radical was an intermediate product and did not participate other reactions. Finally, the three forms of catalysts (Cat, Cat* and Cat•−) established a reversible cycle via reductive quenching pathway to mediate the polymerization. Table 2 listed the elementary reactions and relevant rate coefficients for aqueous metal-free ATRP of PEGA480 via

reductive quenching pathway using Eosin Y as catalyst and PMDETA as electron donor. The main reactions were consisting of photoexcitation of ground state catalyst, chain initiation, chain propagation, ATRP activation/deactivation equilibrium, chain termination, and decay of the excited state catalyst. In addition, the key process of reductive quenching pathway is the reduction of excited state catalyst by accepting electron from electron donor, which was also included in this system. The chain transfer reactions for D

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Figure 4. the comparison of the evolution of reactant concentrations (A) [D], (B) [Cat•−], (C) [D•+], (D) [Pn•] with conversion in aqueous metalfree ATRP of PEGA480, under conditions [PEGA480]:[HEBiB]:[EosinY]:[PMDETA] = 200:1:1:5/200:1:1:10/200:1:1:20 and [PEGA480] = 0.45 M in H2O/PEGA480 (4:1 v/v, VH2O = 8.8 mL) with visible light irradiation at 36 °C.

Figure 5. Evolution of PnX concentration, X− concentration, and P0X concentration with conversion in aqueous metal-free ATRP of PEGA480, under the following conditions: (A) [PEGA480]:[HEBiB]:[EosinY]:[PMDETA] = 200:1:1:5; (B) [PEGA480]:[HEBiB]:[EosinY]: [PMDETA] = 200:1:1:10; (C) [PEGA480]:[HEBiB]:[EosinY]:[PMDETA] = 200:1:1:20. [PEGA480] = 0.45 M in H2O/PEGA480 (4:1 v/v, VH2O = 8.8 mL) with visible light irradiation at 36 °C.

concentration, Cat•− concentration, D•+ concentration and Pn• concentration with conversion were contrasted between the reaction conditions with different amounts of electron donor, which extracted from simulation strategy. The increased amounts of electron donor promoted the reduction of excited state catalyst which generated more Cat•− and D•+. More activator Cat•− produced a higher concentration of propagating radicals Pn• in the system that resulted the polymerization rate faster. Hence, the electron donor had an important influence on the ATRP activation. Though the electron donor was continuously consumed during the process of the reduction of excited state catalyst, the concentration of D remained constant in the polymerization systems. This was attributed to the back electron transfer from X− to D•+ in the ATRP deactivation process which generated the initial D. Therefore, the electron donor not only had an important influence on the ATRP activation, but also participated in the ATRP deactivation. Furthermore, taking a look into the evolution of Mn with conversion (Figure 3B), one could find that the numberaverage molecular weights were about one time over theoretical ones. This means that only about 50% initiation efficiency was obtained in current ATRP system. Figure 5 showed the

PEGA480 polymerization were ignored due to their limited contribution.60 It should be noted that the chain length dependent termination have an important effect on the kinetics of ATRP at high conversion.67 Here, chain length dependent termination was not taken into account due to the low monomer conversion. The readers are referred to the Supporting Information for detailed model equations. It should be noted that no reaction occurred when the reactant concentration of electron donor was set as 0 in our model, which is consistent with the result of experiment (Table 1). Figure 3 showed the simulation results of metal-free ATRP of PEGA480 with different amounts of electron donor, which matched well with the experimental data. Linear semilogarithmic kinetic behaviors, linearly increasing Mn with conversation and low values of dispersity (Đ < 1.40) indicated that the aqueous metal-free ATRP of PEGA480 proceeded in a controlled manner and the photoredox catalytic cycle for the catalyst via reductive quenching pathway was well-run after the addition of electron donor. The polymerization rate increased as the amounts of electron donor increased (Figure 3A). This was because the increased amounts of electron donor indirectly promoted the ATRP activation. As depicted in Figure 4, the evolution of D E

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Macromolecules evolutions of P0X concentration, PnX concentration and X− concentration with conversion under different conditions, P0X concentration was almost zero at the early stage, indicating nearly all the initiator was reduced by the Cat•−. It was worth noting that the sum of PnX concentration (1.212 × 10−3 M in Figure 5A, 1.090 × 10−3 M in Figure 5B, and 1.006 × 10−3 M in Figure 5C) and X− concentration (1.061 × 10−3 M in Figure 5A, 1.183 × 10−3 M in Figure 5B, and 1.267 × 10−3 M in Figure 5C) was equal to the initial initiator concentration (2.273 × 10−3 M), respectively. This phenomenon suggests that not all the reductive initiator participated in the polymerization to become PnX. And the reductive initiator that did not participate in the polymerization was transformed into X− and P0P0 due to the combination termination of P0• at the initial stage, which resulted in the decreased initiator efficiency. The initiator efficiency was calculated to be 0.53, 0.48, and 0.44 for [EosinY]: [PMDETA] = 1:5, 1:10, and 1:20, respectively. Effect of Water Content, Catalyst Concentration, and Degree of Polymerization on Aqueous Metal-Free ATRP of PEGA480. The influence of water content on aqueous metalfree ATRP of PEGA480 was shown in Table 3. As the volume

upon enhancing water content. When the volume ratio of H2O and PEGA480 increased to 4:1, it was applicable for this technology to conduct in a controlled manner. The polymer behavior of aqueous metal-free ATRP with different catalyst concentration {[PEGA480]:[EosinY] = 200:0.5/200:1/200:2}was investigated in Figure 6. Linear semilogarithmic kinetic plots versus time {ln [M0]/[M] vs time} in Figure 6A demonstrated that the concentration of propagating radicals remained almost constant in the polymerization systems. It can be deduced that the equilibrium between the active species and dormant species was well maintained and a reversible photoredox cycle of the catalyst was well established. Furthermore, when the amounts of catalyst increased, the polymerization rate showed an obvious decrease. This could be contributed to the phenomenon in specific chromophores. As reported in literatures, the formation of excited state dimer of Eosin Y was promoted in high catalyst concentration,48,69 which was not beneficial to the redox reaction. Figure 6B presented the evolution of Mn and Đ (Mw/ Mn) versus conversation. The molecular weights increased as the monomer consumed. The values of Đ were lower than 1.40, suggesting a good controllability of aqueous metal-free ATRP system. Furthermore, it was noteworthy that the Mn deviated from the theoretical values, which might attribute to the low initiation efficiency as discussed above. Aqueous metal-free ATRP of PEGA480 with varying degrees of polymerization (DP = 100, 200, 300) was studied in Figure 7. The monomer, catalyst and electron donor concentrations were kept constant in the experiments, while the amounts of initiator were varied corresponding to DP. The lower amounts of initiator used, the higher molecular weights obtained. Furthermore, the values of dispersity were still low (Đ < 1.40), indicating the polymerizations with different DPs were conducted in a controlled manner. The well-defined polymers with different molecular weights could be synthesized by manipulation of the initiator loading. Temporal Control of Aqueous Metal-Free ATRP by Switching Light on and off. Temporal control is one of key features for photopolymerizations. To confirm the ability of temporal control for aqueous metal-free ATRP, the polymerization of PEGA480 was determined by simply turning the light on and off. As shown in Figure 8, the polymerization behavior and the photoredox catalytic cycle process showed a rapid response to light switching regulation. The monomer conversion (Figure 8A) and molecular weights of products (Figure 8B) remained almost unchanged during turning the

Table 3. Effect of Water Content on Aqueous Metal-Free ATRP of PEGA480 entrya

VH2O/ Vmonomer

time (h)

conversion (%)b

Mn,th (g/mol)c

Mn,GPC (g/mol)d

Đd

1 2 3 4

1:1 2:1 4:1 5:1

12 12 12 12

79.9 72.6 57.5 39.4

76900 69900 55400 38000

242900 185600 96900 69090

2.11 1.83 1.26 1.26

a

Reaction conditions: [PEGA480]:[HEBiB]:[EosinY]:[PMDETA] = 200:1:1:10; VPEGA480 = 2.2 mL under visible light irradiation at 36 °C. b Conversions determined by 1H NMR. cMn,th = MHEBiB + DP × conversion × Mmonomer. dDetermined by GPC in THF.

ratio of H2O and PEGA480 increased from 1:1 to 5:1, the monomer conversion decreased from 79.9% to 39.4% and the value of dispersity also showed a decrease from 2.11 to 1.26. The controllability of the polymerization became better accompanied by enhancing water content, even though the polymerization rate had an obvious decrease. In aqueous ATRP, as previously reported, water content had a positive relationship with deactivation rate coefficient (kdeact).68 Wellcontrolled ATRP was successfully carried out in water-rich solutions because of the high value of kdeact. Therefore, we speculated that the kdeact in this system also had an increase

Figure 6. Effect of Eosin Y concentrations on aqueous metal-free ATRP of PEGA480: (A) semilogarithmic kinetic plots versus time (B) evolution of Mn and Đ (Mw/Mn) versus conversion, under conditions [PEGA480]:[HEBiB]:[EosinY]:[PMDETA] = 200:1:0.5:5/200:1:1:10/200:1:2:20 and [PEGA480] = 0.45 M in H2O/PEGA480 (4:1 v/v, VH2O = 8.8 mL) with visible light irradiation at 36 °C. F

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Figure 7. Effect of degree of polymerization on aqueous metal-free ATRP of PEGA480: (A) semilogarithmic kinetic plots versus time (B) evolution of Mn and Đ (Mw/Mn) versus conversion, under conditions [PEGA480]:[HEBiB]:[EosinY]:[PMDETA] = 200:2:1:10/200:1:1:10/200:0.67:1:10 and [PEGA480] = 0.45 M in H2O/PEGA480 (4:1 v/v, VH2O = 8.8 mL) with visible light irradiation at 36 °C.

Figure 8. Temporal control of aqueous metal-free ATRP by switching light on and off: (A) semilogarithmic kinetic plots versus time (B) evolution of Mn and Đ (Mw/Mn) versus conversion, under conditions [PEGA480]:[HEBiB]:[EosinY]:[PMDETA] = 200:1:1:10 and [PEGA480] = 0.45 M in H2O/PEGA480 (4:1 v/v, VH2O = 8.8 mL) with visible light irradiation at 36 °C.

light off, indicating the polymerization stopped in the dark period. This was because the generation of excited state catalyst (Cat*) by photoexcitation of ground state catalyst (Cat) was stopped after removal of light, and then the activator radical anion (Cat•−) was no longer produced which resulted in stopping the ATRP activation. The residual radical species (very low concentration) were rapidly deactivated to stable and dormant species. When re-exposure the mixture to light, the polymerization restarted and the photoredox catalytic cycle was rebuild. On/off light switching cycle was repeated twice. Nearly a linear increase for the polymer chains between monomer consumption and light irradiation time and low values of dispersity confirmed that the polymerization conducted in a well-controlled manner which was not influenced by intermittent of light irradiation. Chain Extension of Poly(PEGA480)Br. Retention of chain end functionality is an important characteristic for a successful ATRP technology, which allows to synthesize block polymers. An experiment of chain extension was performed to confirm the livingness of the chains. Macroinitiator P(PEGA480)Br (Mn = 15400 g/mol, Đ = 1.35) was prepared. After purification, the chain extension of P(PEGA480)Br was carried out. The reaction was successfully initiated, and yielded a well-controlled chainextended polymer (Mn = 27500 g/mol, Đ = 1.31). The GPC trace (Figure 9) showed a clear shift to higher molecular weight region, indicating the retention of chain end functionality in the products of aqueous metal-free ATRP.

Figure 9. GPC traces of the macroinitiator P(PEGA480)Br under conditions [PEGA480]:[HEBiB]:[EosinY]:[PMDETA] = 200:2:1:10 and [PEGA480] = 0.45 M in H2O/PEGA480 (4:1 v/v, VH2O = 8.8 mL) and the corresponding chain-extended polymer P(PEGA480) under conditions [PEGA480]:[P(PEGA480)Br]:[EosinY]:[PMDETA] = 200:0.4:1:10 and [PEGA480] = 0.45 M in H2O/PEGA480 (4:1 v/v, VH2O = 8.8 mL).

information on the photoredox catalytic cycle via reductive quenching pathway was provided by experimental and kinetic modeling techniques. The results revealed that electron donor not only had an important influence on the ATRP activation, but also participated in the ATRP deactivation. More electron donor resulted in faster polymerization rate. And not all of initiator participated in the polymerization resulted in decreased initiator efficiency. The initiator efficiency was calculated to be 0.53, 0.48, and 0.44 for [EosinY]:[PMDETA] = 1:5, 1:10, and 1:20, respectively. Furthermore, aqueous metal-free ATRP is conducted in a controlled manner (Đ < 1.40) with different catalyst concentration. Polymers with predictable molecular weight could be prepared by manipulation of the initiator loading. Study about the effect of water

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CONCLUSION In conclusion, metal-free ATRP was successfully performed in aqueous media using Eosin Y as catalyst and PMDETA as electron donor under visible light irradiation. Reaction G

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content to the polymerization showed that it was well suited for this technology when the volume ratio of H2O and PEGA480 increased to 4:1. A high degree of temporal control for this technology was verified by the experiment of turning the light on and off. A successful chain extension experiment further confirmed the “living” nature of this technology. This work provided a detailed study about aqueous metal-free ATRP by a combination of experiment and simulation methods, which promoted the development of ATRP technology.

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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.8b00348. Redox potential calculations, detailed kinetic model, and the standardized residual analysis of rate coefficients estimation. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*(Z.-H.L.) E-mail: [email protected]. Telephone: +86-2154745602. Fax: +86-21-54745602. ORCID

Yin-Ning Zhou: 0000-0003-3509-3983 Zheng-Hong Luo: 0000-0001-9011-6020 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the National Natural Science Foundation of China (Nos. 21625603, 21606148) for supporting this work and acknowledge the Center for Advanced Electronic Materials and Devices of Shanghai Jiao Tong University.

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REFERENCES

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