Metal-Free Photoinduced Decarboxylative Radical Polymerization

Mar 23, 2017 - (1) A large effort in the past several decades has been devoted to the study of introducing complex molecules or unstable functional gr...
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Metal-Free Photoinduced Decarboxylative Radical Polymerization Using Carboxylic Acids as Benign Radical Initiators: Introduction of Complex Molecules into Polymer Chain Ends Mugen Yamawaki, Akari Ukai, Yuki Kamiya, Shinji Sugihara,* Miku Sakai, and Yasuharu Yoshimi* Department of Applied Chemistry and Biotechnology, Graduate School of Engineering, University of Fukui, 3-9-1 Bunkyo, Fukui 910-8507, Japan S Supporting Information *

ABSTRACT: Metal-free photoinduced decarboxylative radical polymerization of aliphatic carboxylic acids with a variety of monomers was found to proceed smoothly to give the corresponding polymers under mild conditions. Complex carboxylic acids such as those of sugars, steroids, and peptides can function as benign radical initiators via decarboxylation and can be incorporated at the polymer chain ends. This synthetic methodology represents a facile introduction of molecules and functionalities to polymers by using commercially available carboxylic acids.

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nation reactions and interconversion must be utilized.8 Recently, remarkable advances in ATRP have demonstrated metal-free polymerization mediated by light and catalyzed by an organic-based photoredox catalyst.9,10 Thus, the challenge of facile metal-free end-functionalization is urgent. Recently, we have reported a decarboxylative radical reaction of carboxylic acids that is promoted by a radical cation of phenanthrene (Phen), formed by single electron transfer (SET) from the excited state of Phen (Phen*) to 1,4-dicyanobenzene (DCB) as an electron acceptor (Scheme 1).11 The alkyl

ontrolling the molecular structures of polymeric materials is a key issue facing modern polymer synthesis for providing more complex structures needed in the rapidly growing fields of nanotechnology and nanobiotechnology. However, simple and facile access is required for the synthetic methodology. In this context, photoinduced radical polymerization continues to receive significant attention owing to its high value in many fields of industry such as paints, printing inks, dental materials, lithography, and photoresins.1 A large effort in the past several decades has been devoted to the study of introducing complex molecules or unstable functional groups at a polymer chain end (α-end from initiator) in the common radical polymerization; however, this requires complicated preparation of peroxides and azo- and disulfide-compounds as radical initiators.2,3 A few other methods for directly introducing functionality into polymer chain ends using RBr and transition metal catalysts such as Cu and Ru (atom transfer radical polymerization; ATRP) have been developed, but they require harsh conditions.4,5 The current challenges are to avoid metal contamination and to conjugate various compounds to the polymer chain ends and use the resulting end-functional polymers for biomedical purposes.4b Thus, development of metal-free and facile radical polymerization methods for introducing functionality at the polymer chain end under mild conditions using benign radical initiators is strongly desired. The representative metal-free method is RAFT (reversible addition−fragmentation chain transfer) polymerization. The incorporation of functionality into the initiating species results in end-functionalized polymers.6,7 However, complex RAFT agents must be synthesized for the end-functionalization.6b Furthermore, for introduction of complex functionality into the ω-end of the polymer, postpolymerization including termi© XXXX American Chemical Society

Scheme 1. Mechanistic Pathway for Photoinduced Decarboxylative Radical Addition of Carboxylic Acids to Alkenes

radicals, produced in this process by SET from carboxylates to the Phen radical cation followed by decarboxylation of the intermediate carboxy radicals, react with a variety of reagents such as electron-deficient alkenes, oxime ethers, thiols, and DCB, to produce addition,11c−e,g−i reduction,11a,f and substitution11b products in high yields. For example, alkyl radicals Received: March 13, 2017 Accepted: March 21, 2017

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DOI: 10.1021/acsmacrolett.7b00193 ACS Macro Lett. 2017, 6, 381−385

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ACS Macro Letters formed by irradiation of a mixture of carboxylic acids containing catalytic amounts of Phen and DCB11i undergo efficient addition to electron-deficient alkenes such as acrylonitrile. The results of this early effort demonstrated that this photochemical process serves as an efficient method for generating alkyl radicals from aliphatic carboxylic acids under mild conditions. This finding encouraged us to apply the photoinduced decarboxylation to metal-free radical polymerization using commercially available complex carboxylic acids as benign radical initiators. As far as we are aware, there have been reported some photoinduced decarboxylative initiation of polymerizations such as photodecarboxylation of ketoprofen to initiate anionic polymerization,12 photodecarboxylative elimination of base for the polymerization of epoxides,13 and thiol-Michael polymerization.14 These are also significant polymerization tools using benign initiators. In this study, the environmentally friendly method can provide facile access to the introduction of complex molecules or unstable functional groups to the polymer chain ends, that is, complex endfunctional polymers. Initially, we investigated photoinduced radical polymerization using the representative carboxylic acid of sugar 1 as summarized in Table 1. Photoreaction of aqueous acetonitrile

Figure 1. 1H NMR spectrum in CDCl3 of poly(methyl acrylate) 3 with sugar end (entry 3 in Table 1).

Table 1. Photoinduced Decarboxylative Radical Polymerization of 1 with 2

entry

Phen (mM)

DCB (mM)

conversiona (%)

Mn

Mw/Mn

1 2 3 4 5 6b

5 10 20 50 100 20

5 10 20 50 100 20

80 78 74 74 67 54

24500 25100 25200 18700 7300 5600

2.62 3.19 2.73 2.83 4.41 2.48

a Measured by 1H NMR analysis. bUsing biphenyl and DCN in the place of Phen and DCB.

solutions (CH3CN/H2O = 9/1 w/w) containing Phen (5 mM), DCB (5 mM), 1 (10 mM), and methyl acrylate 2 (1 M) under argon for 3 h at room temperature induced by a 100 W high-pressure mercury lamp in Pyrex vessels (15 mm × 180 mm, λ > 280 nm) yielded polymer 3 in high conversion (entry 1). In the absence of Phen or DCB or both Phen and DCB or 1, polymer 3 was not obtained by irradiation. The 1H NMR spectrum of the obtained polymer 3 after reprecipitation shows the chemical shifts of the corresponding sugar moiety at 4−5 ppm (Figure 1). In addition, the MALDI-TOF-MS spectrum reveals the incorporation of the sugar moiety in the polymer at the α-end at full end-functionality (Figure 2). For n = 22 in the structure, one of the experimental signals (2145.8 g/mol) fits the calculated molecular weight with Na+, that is, Mn (calcd) = 229.25 + 86.04 × 22 + 1.01 + 22.99 = 2146.13 g/mol. The spectrum also shows a single series of peaks for the repeating unit of methyl acrylate (MA). The distance between individual peaks (e.g., between 2059.8 and 2145.8 or 2145.8 and 2231.8) for 86.0 mass units also corresponds to the molar mass (86.04 g/mol) of the MA repeating unit.

Figure 2. Typical MALDI-TOF-MS spectrum of poly(methyl acrylate) 3 with sugar end (entry 5 in Table 1).

Intriguingly, increasing the concentration of Phen and DCB to more than 20 mM decreased the number-average molecular weight (Mn) and conversion (entries 2−5). The decrease of conversion may be due to inner filter effects of Phen. Additionally, the Mn and conversion were reduced using biphenyl and 1,4-dicyanonaphthalene (DCN) in the place of Phen and DCB (entry 6). Thus, the proposed mechanism for photoinduced decarboxylative radical polymerization is shown in Scheme 2. The alkyl radical generated from 1 via photoinduced decarboxylation by using Phen and DCB acts as a radical initiator that is incorporated in the polymer at the polymer chain end (Scheme 2, initiation and propagation). Both increasing the concentrations of Phen and DCB and replacing DCB with DCN improved the efficiency of SET from radical anions of electron acceptors to the adduct radicals to form the anion, which led to increased efficiency of chain 382

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ACS Macro Letters Scheme 2. Proposed Mechanism for Photoinduced Decarboxylative Radical Polymerization

Table 2. Photoinduced Decarboxylative Radical Polymerization of 4−9 with 2a,b

termination in the polymerization (Scheme 2, termination). Thus, Mn and conversion in the photoinduced polymerization were found to be influenced both by concentrations of Phen and DCB and types of electron acceptors. However, as the future work, light absorption efficiencies, redox potentials, and reaction rates of the competing processes should be further considered. In an effort aimed at elucidating the substrate scope of the new process, photoinduced polymerization of simple aliphatic carboxylic acids 4 and 5, the acid of steroid 6, N-Boc-Lphenylalanine 7 (Boc = tert-butoxycarbonyl), N-Boc dipeptide (N-BocValValOH) 8, and N-Boc tripeptide having glutamate 9 (N-BocValGluValOCH3) with 2 was carried out under the same conditions (Table 2). Although Mn is somewhat dependent on the type of carboxylic acid, a variety of aliphatic carboxylic acids can serve as a radical initiator in photoinduced polymerization, indicating that a variety of molecules and complex functional groups can be introduced at the terminal chain end under mild conditions. In particular, peptides having carboxylic acids in a side chain such as 9 are easily modified by this method to peptides bearing a polymer. This may lead to protein modification for bioconjugation.15 Moreover, the photoinduced polymerization of 1 proceeded smoothly with other monomers such as ethyl acrylate, t-butyl acrylate, Nisopropylacrylamide, and styrene, although a long irradiation time (24 h) was required in the case of styrene (Table 3). Thus, this method is applicable to the preparation of polymers having functionalities at the chain end by using a variety of aliphatic carboxylic acids and monomers. Finally, sequential photoinduced decarboxylative radical polymerization of monomethyl succinate 10 with two different monomers was demonstrated (Scheme 3). In the first step, photoinduced polymerization of 10 having both a carboxylic acid and methyl ester with t-butyl acrylate led to the formation of polymer 11 (Mn = 27300). After evaporation to eliminate tbutyl acrylate, the resulting polymer 11 having an ester moiety was hydrolyzed by NaOH (100 mM) in aqueous acetonitrile at 50 °C for 3 h to form the corresponding carboxylate ion as reported previously,16 followed by the second photoinduced polymerization with N-isopropylacrylamide to obtain polymer 12 with a higher molecular weight (Mn = 41300) having different segments (GPC and 1H NMR are shown in Figure S11 of the Supporting Information). Similarly, the sequential photoinduced radical polymerization of 11 with t-butyl acrylate and styrene took place to obtain the corresponding polymer 13. Investigations to control these molecular weight distributions by a precision polymerization are currently in progress. In conclusion, we found that the metal-free photoinduced decarboxylative polymerization of carboxylic acids can be

a

Photoinduced polymerization using carboxylic acids 4−9 was conducted in the presence of Phen (10 mM), DCB (10 mM), and 2 (1 M) using a 100 W high-pressure mercury lamp under an argon atmosphere for 3 h. bAll 1H NMR data are shown in Figures S1−S6 of the Supporting Information.

Table 3. Photoinduced Decarboxylative Radical Polymerization of 1 With a Variety of Monomersa

entry 1 2 3 4d

monomer ethyl acrylate, X = CO2C2H5 t-butyl acrylate, X = CO2t-C4H9 N-isopropyl acrylamide, X = CONHi-C3H7 styrene, X = Ph

conversionb (%)

Mn

Mw/Mn

77 88 100c

30200 13300 81000

2.29 3.69 2.38

16

26700

2.20

a

All 1H NMR data are shown in Figures S7−S10 of the Supporting Information. bMeasured by 1H NMR analysis. cIsolated conversion. d Irradiation time is 24 h.

utilized for direct introduction of molecules and functional groups at the polymer chain ends. A variety of complex carboxylic acids can serve as radical initiators to provide 383

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ACS Macro Letters

Defined Environmentally Friendly Polymeric Materials. Chem. Rev. 2007, 107, 2270−2299. (5) Kato, M.; Kamigaito, M.; Sawamoto, M.; Higashimura, T. Polymerization of Methyl Methacrylate with the Carbon Tetrachloride/Dichlorotris-(triphenylphosphine)ruthenium(II)/Methylaluminum Bis(2,6-di-tert-butylphenoxide) Initiating System: Possibility of Living Radical Polymerization. Macromolecules 1995, 28, 1721−1723. (6) (a) Chiefari, J.; Chong, Y. K.; Ercole, F.; Krstina, J.; Jeffrey, J.; Le, T. P.; Mayadunne, R. T.; Meijs, G. F.; Moad, C. L.; Moad, G.; Rizzardo, E. S.; Thang, H. Living Free-Radical Polymerization by Reversible Addition−Fragmentation Chain Transfer: The RAFT Process. Macromolecules 1998, 31, 5559−5562. (b) Keddie, D. J.; Moad, G.; Rizzardo, E.; Thang, S. H. RAFT Agent Design and Synthesis. Macromolecules 2012, 45, 5321−5342. (7) Sugihara, S.; Kawamoto, Y.; Maeda, Y. Direct Radical Polymerization of Vinyl Ethers: Reversible Addition−Fragmentation Chain Transfer Polymerization of Hydroxy-Functional Vinyl Ethers. Macromolecules 2016, 49, 1563−1574. (8) Willcock, H.; O’Reilly, R. K. End Group Removal and Modification of RAFT Polymers. Polym. Chem. 2010, 1, 149−157. (9) Treat, N. J.; Sprafke, H.; Kramer, J. W.; Clark, P. G.; Barton, B. E.; de Alaniz, J. R.; Fors, B. P.; Hawker, C. J. Metal-Free Atom Transfer Radical Polymerization. J. Am. Chem. Soc. 2014, 136, 16096− 16101. (10) Theriot, J. C.; Lim, C.-H.; Yang, H.; Ryan, M. D.; Musgrave, C. B.; Miyake, G. M. Organocatalyzed Atom Transfer Radical Polymerization Driven by Visible Light. Science 2016, 352, 1082−1086. (11) (a) Yoshimi, Y.; Itou, T.; Hatanaka, M. Decarboxylative Reduction of Free Aliphatic Carboxylic Acid by Photogenerated Cation Radical. Chem. Commun. 2007, 5244−5246. (b) Itou, T.; Yoshimi, Y.; Morita, T.; Tokunaga, Y.; Hatanaka, M. Decarboxylative Reduction of Free Aliphatic Carboxylic Acid by Photogenerated Cation Radical. Tetrahedron 2009, 65, 263−269. (c) Yoshimi, Y.; Masuda, M.; Mizunashi, T.; Nishikawa, K.; Maeda, K.; Koshida, N.; Itou, T.; Morita, T.; Hatanaka, M. Inter- and Intramolecular Addition Reactions of Electron-Deficient Alkenes with Alkyl Radicals, Generated by SET-Photochemical Decarboxylation of Carboxylic Acids, Serve as a Mild and Efficient Method for the Preparation of γAmino Acids and Macrocyclic Lactones. Org. Lett. 2009, 11, 4652− 4655. (d) Yoshimi, Y.; Hayashi, S.; Nishikawa, K.; Haga, Y.; Maeda, K.; Morita, T.; Itou, T.; Okada, Y.; Hatanaka, M. Influence of Solvent, Electron Acceptors, and Arenes on Photochemical Decarboxylation of Free Carboxylic Acid via Single Electron Transfer (SET). Molecules 2010, 15, 2623−2630. (e) Yoshimi, Y.; Kobayashi, K.; Kamakura, H.; Nishikawa, K.; Haga, Y.; Maeda, K.; Morita, T.; Itou, T.; Okada, Y.; Hatanaka, M. Addition of Alkyl Radicals, Generated from Carboxylic Acids via Photochemical Decarboxylation, to Glyoxylic Oxime Ether: A Mild and Efficient Route to α-Substituted α-Aminoesters. Tetrahedron Lett. 2010, 51, 2332−2334. (f) Itou, T.; Yoshimi, Y.; Nishikawa, K.; Morita, T.; Okada, Y.; Ichinose, N.; Hatanaka, M. A Mild Deuterium Exchange Reaction of Free Carboxylic Acids by Photochemical Decarboxylation. Chem. Commun. 2010, 46, 6177− 6179. (g) Nishikawa, K.; Yoshimi, Y.; Maeda, K.; Morita, T.; Takahashi, I.; Itou, T.; Inagaki, S.; Hatanaka, M. Radical Photocyclization Route for Macrocyclic Lactone Ring Expansion and Conversion to Macrocyclic Lactams and Ketones. J. Org. Chem. 2013, 78, 582−589. (h) Nishikawa, K.; Ando, T.; Maeda, K.; Morita, T.; Yoshimi, Y. Photoinduced Electron Transfer Promoted Radical Ring Expansion and Cyclization Reactions of α-(ω-Carboxyalkyl) βKeto Esters. Org. Lett. 2013, 15, 636−638. (i) Yoshimi, Y.; Washida, S.; Okita, Y.; Nishikawa, K.; Maeda, K.; Hayashi, S.; Morita, T. Radical Addition to Acrylonitrile via Catalytic Photochemical Decarboxylation of Aliphatic Carboxylic Acids. Tetrahedron Lett. 2013, 54, 4324−4326. (12) Wang, Y. H.; Wan, P. Ketoprofen as a Photoinitiator for Anionic Polymerization. Photochem. Photobiol. Sci. 2015, 14, 1120−1126. (13) Arimitsu, K.; Kushima, R.; Endo, R. Novel Photobase Generators and Their Application to Photopolymers. J. Photopolym. Sci. Technol. 2009, 22, 663−666.

Scheme 3. Sequential Photoinduced Decarboxylative Radical Polymerization of Monomethyl Succinate 10 with Two Different Monomers

polymers from several monomers. The Mn of the resulting polymers was dependent on both the concentration of Phen and DCB and the type of electron acceptors. Photoindued decarboxylative radical reactions of carboxylic acids have been reported by using Ir,17 TiO2,18 and Fukuzumi catalysts19 and further developments are expected in the field of polymer and industrial chemistry. As a metal-free and facile system, we anticipate that many researchers will utilize our photoinduced decarboxylative polymerization using Phen, DCB, and electron acceptors.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.7b00193. Detailed experimental section and 1H NMR and GPC data (PDF).

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Shinji Sugihara: 0000-0002-7091-3994 Notes

The authors declare no competing financial interest.

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REFERENCES

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