Controlled

Article pubs.acs.org/Macromolecules

Real-Time and in Situ Investigation of “Living”/Controlled Photopolymerization in the Presence of a Trithiocarbonate Hu Wang, Qianbiao Li, Jingwen Dai, Fanfan Du,* Haiting Zheng, and Ruke Bai* CAS Key Laboratory of Soft Matter Chemistry, Department of Polymer Science and Engineering, University of Science and Technology of China, Hefei 230026, People’s Republic of China S Supporting Information *

ABSTRACT: Polymerization of methyl acrylate under ultraviolet (UV) irradiation in the presence of S-1-dodecyl-S′-(α,α′dimethyl-α″-acetic acid) trithiocarbonate (DDMAT) was investigated by in situ 1H nuclear magnetic resonance spectroscopy. Effects of light intensity, wavelength, and concentration of DDMAT on the polymerization behaviors were studied in detail. The experimental results demonstrate that the “living” features of the photopolymerization are related to the concentration of DDMAT. “Living”/controlled radical polymerization was successfully achieved with a high concentration of DDMAT. However, with a low concentration of DDMAT, the polymerization proceeded in an uncontrolled manner and produced polymers with high molecular weights and broad polydispersities. Photochemical behavior of DDMAT was studied in detail, and the results showed that the photolysis of DDMAT was reversible at high concentration, whereas contrarily, DDMAT decomposed irreversibly at low concentration. A possible mechanism was proposed for the reversible photolysis of DDMAT at high concentration, which may involve both reversible termination and reversible addition−fragmentation chain transfer approaches.

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INTRODUCTION

Because of the lower dissociation energy of the C−S bond, various disulfide compounds can be readily used as photoinitiators of radical reactions, including not only photoiniferters20,21 such as dithiocarbamates but also RAFT/ MADIX (macromolecular design via the interchange of xanthates) agents such as xanthates,22−27 trithiocarbonates9 and dithioesters.10 In 2002, Rizzardo et al.10 reported on the RAFT polymerization of styrene (St) and methyl methacrylate (MMA) under UV irradiation using dithioesters as the source of primary radicals as well as CTAs at 42 °C. The polymerization was well controlled at low conversions (below 20%). However, it was significantly less controlled at relatively high conversions (over 30%). It has been demonstrated that common RAFT/MADIX agents are strongly sensitive to UV

Since its discovery in 1998, reversible addition−fragmentation chain transfer (RAFT) polymerization has become a highly versatile technique for preparation of the well-defined polymers with a variety of architectures.1−5 This “living” radical polymerization is carried out in the presence of a thiocarbonylthio compound as a chain transfer agent (CTA), which mediates the growing chain radicals via equilibrium of radical intermediates.1,6 In general, there are three types of initiation methods for RAFT polymerization: thermoinitiation,1−8 photoinitiation,9−16 and γ-radiation initiation.17−19 In comparison with the polymerizations initiated by the other initiation methods, the photoinitiated RAFT polymerization can be easily operated under mild conditions and shows potential applications in the polymerization of thermally unstable monomers and monomers containing thermodenaturalizable biomolecular moieties.9,11 © 2013 American Chemical Society

Received: January 30, 2013 Revised: March 15, 2013 Published: March 25, 2013 2576

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with subsequent radical addition of an olefin. Up to 78% monoinsertion product can be obtained by this reaction, suggesting that only a slight amount of the RAFT end groups were decomposed upon the irradiation. Referring to these reports, though the mechanism for the “living” character has not been well understood, it can be concluded that the suppression of photolysis of CTA under UV irradiation may be not essential toward a well-controlled RAFT polymerization as usually assumed.12 In situ nuclear magnetic resonance (NMR) spectroscopy provides a very powerful tool to elucidate mechanistic and kinetic features of radical polymerization processes.28−30 Herein, in situ proton NMR spectroscopy has been used first time to investigate the “living”/controlled polymerization initiated by UV irradiation at room temperature. UV light was introduced into the NMR tube by an optical fiber, and the effect of polymerization conditions such as concentration of DDMAT, irradiation wave range, and irradiation wave intensity on the “living” behavior of the photopolymerization was evaluated. The experimental results demonstrate that DDMAT is an effective photoinitiator under UV irradiation, and the controllability of the polymerization is mainly dependent on the concentration of DDMAT. The polymerization can be performed in a well-controlled manner when the concentration of DDMAT is relatively high even under high light intensity or full-wave UV irradiation. However, the polymerization becomes uncontrollable when concentration of DDMAT is very low. Further investigations on photochemical behavior of DDMAT reveal that the photolysis of DDMAT at high concentration is reversible, whereas DDMAT decomposes irreversibly at low concentration under UV irradiation. A possible mechanism was proposed for the reversible decomposition of DDMAT at high concentration under UV irradiation, and the present results highlight the possibility of a mixed mechanism operating in UVinitiated polymerization, which combines both the reversible termination and the RAFT approaches.

irradiation, and photolysis of the agents would generate two species including a reactive R• radical and a sulfur radical (Scheme 1).10,15 This fragmentation reaction is generally Scheme 1. Photolysis of CTA under UV Irradiation

reversible, but the intermediate sulfur radical is not completely stable, and it is possible to further fragment to a reactive Z• radical with the formation of CS2. The irreversible decomposition of the CTA moieties under UV irradiation in the duration of RAFT polymerization unavoidably leads to the permanently premature termination, thus undermining the “living” character at relatively high conversions or long irradiation time.12 In order to suppress the photolysis of CTAs under UV irradiation and facilitate the control of polymerization, Cai et al. chose long-wave (365−405 nm) UV as a irradiation source and achieved a “living”/controlled RAFT polymerization in 2005 in the presence of S-1-dodecyl-S′-(α,α′-dimethyl-α″-acetic acid) trithiocarbonate (DDMAT).11 The RAFT polymerization with DDMAT was carried out using acylphosphine oxide as a photoinitiator at ambient temperature. They further investigated the effect of light wavelenth on “living” behavior of the ambient temperature RAFT polymerization mediated by either trithiocarbonates or dithioesters and demonstrated that cutting off the CTA-sensitive UV irradiation can significantly improve the “living” behavior of the RAFT polymerizations.12−14 In the duration of the photopolymerization, the homolysis of C−S bond of CTAs took place at a very low level, and the polymerization showed good “living” character even at high conversions. In that particular case, the CTA only played a role of RAFT agent in the photopolymerization; thus, a suitable photoinitiator had to add into the reaction system to generate primary free radicals to initiate the polymerization. Alternatively, it is also possible to realize a well-controlled radical polymerization using CTA as both photoinitiator and RAFT agent under UV irradiation. In 2002, Pan’s group9 reported the radical polymerization of methyl acrylate (MA) and styrene mediated by dibenzyl trithiocarbonate (DBTC) under UV irradiation (without cutting off the short-wave UV light) at room temperature. The polymerization proceeded well without adding any photoinitiators, indicating the decomposition of DBTC under UV irradiation to generate initiating radicals. Interestingly, the decomposition of DBTC moieties under UV irradiation in the duration of photopolymerization did not undermine the “living” character of the polymerization. Recently, Klán et al.15 reported the utilizing of S-phenacyl xanthates both as photoinitiators and RAFT/MADIX agents in UV irradiation polymerization. The polymerization was wellcontrolled, giving rise to well-controlled poly(methyl methacrylate). In another related work, Barner-Kowollik et al.16 reported the photoinduced conjugation of polymers synthesized via RAFT polymerization with a number of low molecular weight (functional) olefins. The incorporation of the olefin at the polymer chain end was demonstrated to be feasible based on the fast β-cleavage of the photoexcited RAFT end group

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EXPERIMENTAL SECTION

Materials. All chemicals and reagents were purchased commercially unless otherwise stated. Methyl acrylate (MA) was purified by passing through a column of basic alumina to remove the inhibitor and stored at −19 °C before use. S-1-Dodecyl-S′-(α,α′-dimethyl-α″-acetic acid) trithiocarbonate (DDMAT) was synthesized according to ref 31. Photopolymerization of MA. Typically a solution of DDMAT (7.25 mM) and MA monomer (2.9 M) in d6-benzene (0.6 mL) was prepared gravimetrically. Photopolymerization was conducted in a 5 mm NMR tube at room temperature on a Bruker AVANCE II spectrometer opening at 400 MHz, capped with rubber septa, and deoxygenated by purging with argon gas for 10 min (Figure S1). The UV light was introduced into the NMR tube by a optical fiber (d = 3 mm). The head of the fiber was about 1 cm away from the surface of the solution. A high-pressure mercury vapor lamp with peak emissions at λ = 254, 302, 313, 365, 405, 436, 545, and 577 nm was used as a radiation source. Long-wave UV (λ > 320 nm) was obtained by an enclosed 320−480 nm filter. The intensity of UV irradiation was measured by a UV-A radiometer at 365 nm. The conversion of monomer was determined in situ by 1H NMR. Duplicates were similarly taken to account for variations the polymerization conditions. Mechanistic Studies. Two solutions of DDMAT in d6-benzene were prepared gravimetrically (0.45 and 13.5 mM). The photolysis of DDMAT was monitored in situ by 1H NMR at room temperature, capped with rubber septa, and deoxygenated by purging with argon gas for 10 min each. The 1H NMR spectrum of each sample was recorded after every 10 min. 2577

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Molecular Weight Analysis. The number-average molecular weights (Mn) and polydispersities (Mw/Mn) were measured by gel permeation chromatography (GPC) on a Waters 150 instrument equipped with 103, 104, and 105 Ǻ Waters Ultrastyragel columns and using THF (1.0 mL/min) as the eluent. Calibration was against polystyrene standards. Analysis of Decomposition of DDMAT. Decomposition of DDMAT was performed by irradiating the compound in a deoxygenated d6-benzene solution for 3 h. The resultant mixture was then air-dried for 2 h in order to remove the bulk of the d6benzene and then analyzed using a LTQ-Orbitrap XL liquid chromatograph/mass spectrometer.

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RESULTS AND DISCUSSION DDMAT as a Photoinitiator for UV-Initiated Radical Polymerization. DDMAT is frequently used as a CTA in RAFT polymerization in the presence of thermal initiators.31 On the other hand, it has also been used in the well-controlled RAFT polymerization of MA under long-wave UV irradiation (λ > 313 nm), as reported by Cai’s group.11,12 According to the papers, the long-wave UV irradiation could hardly homolytically cleave DDMAT to generate primary radicals, so the polymerization did not start in the time scale investigated (t ≤ 6 h). Thus, a photoinitiator was added to generate primary free radicals in the polymerization process. However, in general, it is considered that the decomposition of the weak C−S bond into an active carbon-centered radical and a stable sulfur-centered radical occurs under UV light between the wavelengths of 254 and 366 nm.12,32 Herein, we investigated the photolysis of DDMAT in detail by in situ 1H NMR spectroscopy under the polymerization conditions. We found that the decomposition of DDMAT was remarkably dependent on the intensity of UV light. Accordingly, it is possible to use DDMAT as a photoinitiator in the well-controlled polymerization even under long-wave UV irradiation. Then we performed the polymerization of MA in the presence of DDMAT without using additional photoinitiator under long-wave UV irradiation (λ > 320 nm) by increasing intensity of the UV light. When the deoxygenized solution of MA in d6-benzene (2.9 M) containing DDMAT (7.25 mM) was irradiated by UV light at a light intensity of 12 mW/cm2 (40 times that of Cai’s12), the polymerization took place and the monomer conversion reached about 20% in 150 min. However, without adding DDMAT, even under UV irradiation of 48 mW/cm2 intensity, no polymerization of MA was observed in d6-benzene in 180 min, indicating that no radical was generated from the monomer or the solvent. These preliminary results suggest that DDMAT can be used as a photoinitiator, and it is not necessary to add any additional photoinitiator in the polymerization of MA under UV irradiation. In order to understand the effect of DDMAT concentration on the polymerization, we further performed the UV-irradiated polymerization of MA with different concentrations of DDMAT. The polymerization process was monitored in situ by 1H NMR spectroscopy and the results are shown in Figure 1. Pseudo-first-order kinetics was confirmed in all the range of concentrations of DDMAT, and this result indicated that the steady state radical concentration was constant over the duration of the polymerization. Since DDMAT has already been proven to be very useful in the synthesis of well-defined polymers through RAFT polymerization,11,31 we inferred that DDMAT may play both roles of a photoinitiator and a CTA for the polymerization under UV irradiation. As shown in Figure 1, when the concentration of DDMAT was very low ([DDMAT]

Figure 1. Pseudo-first-order plot of DDMAT-mediated radical polymerization in d6-benzene under long-wave UV irradiation (λ > 320 nm). Polymerization conditions: [MA] = 2.9 M, I = 12 mW/cm2, T = 25 °C.

≤ 450 μM or [MA]/[DDMAT] ≥ 6400), an increase of the DDMAT concentration resulted in higher rates of polymerization. This can be attributed to the fact that more primary radicals were generated from decomposition of the DDMAT with the increase of its concentration. However, the primary free radical concentration cannot be increased indefinitely by further increasing the concentration of DDMAT. Actually, when the DDMAT concentrations were above 450 μM ([MA]/ [DDMAT] < 6400), no further increase of polymerization rate could be observed with the.higher DDMAT concentration. This means that only a fractional of trithiocarbonate molecules were decomposed at the high concentration in the initiation stage of polymerization because the photolysis rate was limited by the UV irradiation condition applied, so there was still a large amount of DDMAT remained in the reaction mixture. Then the retardation effect of DDMAT on the polymerization cannot be neglected because the presence of the undecomposed trithiocarbonate units can significantly decrease the concentration of the growing polymer chain radicals through RAFT equilibrium.31 As a result, the polymerization rates varied inversely with DDMAT concentration at the high concentration of DDMAT (Figure S2). In order to eliminate the effect of viscosity, all the polymerizations were stopped at low conversion ( 1.75), and the curve becomes asymmetric. Moreover, PMA with the molecular weight even up to 1 million Da is formed in the early state of polymerization. These results indicate that the polymerization was uncontrolled at the low concentration of DDMAT. The effect of DDMAT concentration on the polymerization controllability may be also ascribed to the change of the roles of DDMAT in the system. DDMAT at the high concentration not only plays a role of photoinitiator for initiation but also plays a role of CTA for controlling the polymerization. However, DDMAT at very low concentration mainly acts as a photoinitiator because most of DDMAT molecules should decompose into active radicals and irreversible decomposition 2578

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Figure 2. GPC traces for PMA prepared in the presence of different concentrations of DDMAT in d6-benzene under long-wave UV irradiation (λ > 320 nm). Polymerization conditions: [MA] = 2.9 M, [MA]/[DDMAT] = 25 600, 12 800, 6400, 3200, 1600, 800, 400, and 100 (a−h); I = 12 mW/cm2, T = 25 °C.

Figure 3. Dependence of Mn and Mw/Mn vs conversion for DDMATmediated radical polymerization of MA in d6-benzene under long-wave UV irradiation (λ > 320 nm). Polymerization conditions: [MA] = 2.9 M, [MA]/[DDMAT] = 400/1, I = 12 mW/cm2, T = 25 °C.

readily occurred in the initiation stage of polymerization (Scheme 1). “Living” Polymerization at High DDMAT Concentration. To evaluate the “living” character, kinetics of the UVirradiated polymerization of MA at a high concentration of DDMAT (7.25 mM, [MA]/[DDMAT] = 400) was studied typically. As shown in Table 1, the molecular weight Table 1. DDMAT-Mediated Polymerization of MA under Long-Wave UV Irradiation (λ > 320 nm)a entry

time (min)

convb (%)

Mn,thc (g/mol)

Mn,NMRd (g/mol)

Mn,GPCe (g/mol)

PDIe

1 2 3 4 5

95 150 210 275 360

10.2 20.3 30.0 40.1 50.0

3 900 7 300 10 700 14 200 17 600

4 400 7 700 11 200 14 800 18 400

5700 11 700 15 700 20 200 28 100

1.20 1.18 1.16 1.12 1.09

Figure 4. Results of DDMAT-mediated radical polymerization of MA in d6-benzene under long-wave UV irradiation (λ > 320 nm) with different light intensities. Polymerization conditions: [MA] = 2.9 M, [MA]/[DDMAT] = 400/1, T = 25 °C.

a

Polymerization conditions: [MA] = 2.9 M ([MA]/[DDMAT] = 400/1 in d6-benzene), I = 12 mW/cm2, T = 25 °C. bDetermined by 1 H NMR. cCalculated from the equation: Mn,th = conversion × 400 × 86 + 365. dCalculated by 1H NMR. eDetermined by GPC.

light intensity, as depicted by the slope of the monomer consumption curve. These results are reasonable because UV irradiation with the high light intensity can accelerate the decomposition of DDMAT. In order to eliminate the effect of viscosity, all the polymerizations were stopped at a monomer conversion of 20 mol %, and the polymers obtained were characterized by GPC (Table 2, entries 1−5). The molecular weight distribution of the polymer remained quite narrow (less than 1.20), and close to 100% of the growing chains have terminal units generated from the DDMAT even at a light intensity of 48 mW/cm2 (Figure S3). According to the literature,10 the RAFT polymerizations mediated by dithioesters such as 1-phenylethyl phenyldithioacetate became uncontrolled at the monomer conversion of 30% when UV irradiation was used as a source of initiation. However, in the present case, PMA with well controlled molecular weight and narrow molecular distribution (Mn = 41 000, Mw/Mn = 1.22) was successfully prepared at 82% monomer conversion in the UVirradiated polymerization of MA performed under stirring (Table 2, entry 6). We also carried out the polymerization under full-wave UV irradiation, and the results of the polymerization are summarized in Table 2 (entries 7−10). At the same time period, though the monomer conversion was higher than that under long-wave UV irradiation with the same light intensity, the polymerization still revealed “living” characters (see Figures S4 and S5): the number-average

distributions of the obtained polymers were quite narrow(50%). It can be envisaged that the structure of CTA should also affect the “living” behavior of the UV-initiated RAFT polymerization. According to the reports,9,12,15,16 we noticed that, compared with dithioesters, high concentrations of trithiocarbonates are more suitable for the “living”/controlled photopolymerization. It is important to study the photochemical behavior of DDMAT for exploring the mechanism of the DDMATmediated photopolymerization. Cai et al.12 have studied the UV−vis absorption behavior of DDMAT. Herein we reexamined the photochemistry of DDMAT using in situ NMR spectroscopy. As shown in Figure 6, when the low concentration solution of DDMAT (450 μM) in d6-benzene was deoxygenized and then irradiated by long-wave UV light, obvious changes in 1H NMR spectra was detected in 20 min. The disappearance of sharp signals located in the region of 1.60−1.65 ppm (peak b) suggests the homolytic cleavage of the weak bond (C−S) in DDMAT. Moreover, the resulting sulfur radical from the dissociation of the C−S bond is not stable enough in the present conditions and can undergo a fragmentation to produce CS2 and dodecyl radical; this can be confirmed by the signal decrease of protons of methylene

molecular weight increases linearly with the conversion of monomer, and a polymer was obtained with a high molecular weight and low polydispersity even at high conversions (conversion = 50%, Mn = 53 700, Mw/Mn = 1.19). In the previous report,9 only low conversions were achieved for the controlled radical polymerization of MA and styrene mediated by DBTC under UV irradiation at room temperature for a long time, e.g., 12−52% MA monomer conversions under UV irradiation for 15−50 h. However, as we demonstrated here, a more rapid photopolymerization can be realized at ambient temperature without the loss of control via optimizing irradiation conditions because the “living” behavior of the photopolymerization is independent of light intensity and wavelength. Chain Extension by the DDMAT-Mediated Polymerization. The above experimental results show that the “living” characters of the DDMAT-mediated polymerization are greatly dependent on the concentration of DDMAT. This issue has been further confirmed by the chain extension of MA using poly(methyl acrylate) as a macromolecular CTA. The PMA macro-CTA capped with a trithiocarbonate group was synthesized via RAFT polymerization at conversion of 40% with molecular weight of 6400 g/mol (measured by NMR spectroscopy). The GPC curves for chain-extension polymers prepared in various macro-CTA concentrations are shown in Figure 5. Similar to the observations in Figure 2, high concentration of macro-CTA is proved to be necessary for performing chainextension polymerization in a “living”/controlled manner. For example, when 5 mg/mL (780 μM) of macro-CTA was used, the peak for macro-photoinitiator completely shifted toward high molecular weight, indicating that the extension polymerization of MA is successful with macro-CTA. The extension polymerization is well-controlled, and no homopolymer is formed throughout the photopolymerization, as evidenced by the symmetric and narrow molecular weight distributions. In contrast, the chain-extension polymerization turns into an uncontrolled process when the concentration of macro-CTA is decreased from 5 to 2.5 mg/mL ([macro-CTA] ≤ 390 μM). At the low concentrations of macro-CTA, the GPC curves show asymmetrical or bimodal distribution, which can be attributed 2580

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shown in Scheme 2. The process involves both the reversible termination and the chain transfer reaction, and the key issue is Scheme 2. Proposed Mechanism for the Reversible Decomposition of High Concentration of DDMAT under UV Irradiation

Figure 6. 1H NMR spectra of deoxygenized solution of DDMAT in d6-benzene (450 μM) before and after full-wave UV radiation (I = 12 mW/cm2).

adjacent to trithiocarbonate group (3.10−3.20 ppm, peak a). The photochemical behavior of DDMAT at the low concentration is similar to that of dithioesters reported in previous papers (Scheme 1).10,15 When the high concentration solution of DDMAT (13.5 mM) was irradiated, the 1H NMR spectra in Figure 7a did not show any apparent changes after UV irradiation for 3 h. In fact, the homolytic cleavage of C−S bonds of DDMAT occurred in the present case. In order to confirm the decomposition, free radical scavenger, 2,2,6,6-tetramethyl-1-piperidine-1-oxyl (TEMPO), was added into the reaction system. Both 1H NMR spectra (Figure 7b) and liquid chromatography/mass spectrometry (Figure S6) demonstrated that the primary radicals generated from the homolytic cleavage of C−S bonds of DDMAT was captured by TEMPO. Since there was no new compound formed under the irradiation without the addition of TEMPO, we can reason that the primary radicals generated under UV irradiation did not undergo further fragmentation or bimolecular termination at the high DDMAT concentration, which was in favor of the “living”/controlled photopolymerization. Therefore, we can describe photochemical reaction of DDMAT at the high concentration under UV irradiation as

that the decomposition of DDMAT is reversible. Because of only a small fraction of DDMAT molecules are decomposed, a large amount of DDMAT molecules in the reaction mixture may stabilize the generated radicals by forming a stable radical intermediate, which leads to maintain the concentration of the reactive radicals in a very low level and effectively suppress the irreversible decomposition of DDMAT. A similar situation should exist in polymerization reaction with high concentration of DDMAT under UV irradiation wherein the reactive radical species are the growing polymer chain radicals. The polymerization results obtained in this research demonstrate that DDMAT is not only a photoinitiator but also a chain transfer agent. Thus, under UV irradiation at the high concentration of DDMAT, the “living”/controlled polymerization behavior may be dominated by both the reversible termination and RAFT processes.

Figure 7. 1H NMR results before and after full-wave UV irradiation for 3 h (I = 12 mW/cm2): (a) deoxygenized solution of DDMAT in d6-benzene (13.5 mM); (b) deoxygenized solution of DDMAT in d6-benzene (13.5 mM) with a stoichiometric amount of TEMPO. 2581

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CONCLUSION Without using additional photoinitiator, the photopolymerization of methyl acrylate has been studied under UV irradiation at room temperature in the presence of S-1-dodecyl-S′-(α,α′dimethyl-α″-acetic acid) trithiocarbonate (DDMAT) by in situ 1 H nuclear magnetic resonance spectroscopy. Effects of light intensity, wavelength, and concentration of DDMAT on the polymerization behaviors were investigated. It has been demonstrated that, in comparison with light intensity and wavelength, the concentration of DDMAT plays a more important role for the “living”/controlled photopolymerization. Well-controlled polymerization of MA has been successfully achieved with high concentration of DDMAT even at high conversions. The “living” character of the polymerization is confirmed by both the linear tendency of molecular weight evolution with conversion and a chain extension experiment. However, with low concentration of DDMAT, the polymerization proceeded in an uncontrolled manner and produced polymers with high molecular weights and broad polydispersities. The in-depth investigation on the photochemical reaction of DDMAT by liquid chromatograph/mass spectrometry analysis and in situ 1H NMR monitoring reveals that the photolysis of DDMAT is reversible at high concentration, whereas irreversible at low concentration. Based on all the results, a mechanism for the “living”/controlled photopolymerization is proposed to be a combination of the reversible termination and the RAFT processes. This work demonstrates that the trithiocarbonate can be used not only as a chain transfer agent but also as a photoinitiator toward a rapid and well-controlled ambient temperature photopolymerization.

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ASSOCIATED CONTENT

* Supporting Information S

Additional polymerization results and liquid chromatograph/ mass spectrometry. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (F.D.) or [email protected]. cn (R.B.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the financial support from the National Natural Science Foundation (NNSF) of China (No. 20974104 and No. 21074120) and Ministry of Science and Technology of China (No. 2007CB936401).

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REFERENCES

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