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Tandem Reaction of Cationic Copolymerization and Concertedly Induced Hetero-Diels−Alder Reaction Preparing SequenceRegulated Polymers Suzuka Matsumoto, Arihiro Kanazawa, Shokyoku Kanaoka, and Sadahito Aoshima* Department of Macromolecular Science, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan S Supporting Information *

a reactive site for the functional transformation. In the present study, we are interested in an electron deficiency-activated reaction because we aim to explore the tandem reaction in the presence of a carbocation. A possible carbocation-activated reaction is the Diels−Alder (DA) reaction of a conjugated diene and a dienophile, or a cycloaddition reaction, which is a well-known click reaction. The reaction occurs between substrates with substantially different electron densities, such as a conjugated diene with an electron-donating substituent and a dienophile with an electron-withdrawing substituent and vice versa (inverseelectron-demand DA reaction).19 In addition, the accelerator or catalyst, such as a Lewis acid or carbocation, is necessary to activate the electron-deficient substrate by withdrawing electrons in some cases.20 For example, Grieco et al. reported an intramolecular DA reaction that proceeds through the activation by the carbocation adjacent to the dienophile moiety.21 Thus, a propagating carbocation in the cationic polymerization may activate a DA reaction. In this study, we focus on furfural, which is an aldehydesubstituted furan from inedible plants. In cationic polymerization, furfural can generate the propagating carbocation adjacent to the furan ring through the addition of its aldehyde moiety. Hence, the furan ring can be activated for a DA reaction with a dienophile such as a CC or OC bond. The primary aim of this study is to examine the feasibility of a new tandem reaction composed of cationic copolymerization and DA reactions of VEs and furfural. As a result, a highly efficient hetero-DA reaction was specifically induced at the side furan ring adjacent to the growing carbocation with the aldehyde moiety of another furfural molecule during their controlled cationic copolymerization. Furthermore, the resulting copolymers exhibited unprecedented 2:(1 + 1)-type alternating structures, which were corroborated by 1H NMR and MALDI-TOF-MS analyses of the products in the copolymerization and a model reaction (Scheme 1). The cationic copolymerization of 2-acetoxyethyl VE (AcOVE) and furfural was examined using a suitable initiating system for the controlled alternating copolymerization of various VEs and conjugated aldehydes.15 Both monomers were consumed at similar rates in the reaction in which GaCl3 was used as a catalyst in conjunction with EtSO3H initiator in the presence of 1,4-dioxane as an added base in toluene at −78

ABSTRACT: A unique tandem reaction of sequencecontrolled cationic copolymerization and site-specific hetero-Diels−Alder (DA) reaction is demonstrated. In the controlled cationic copolymerization of furfural and 2acetoxyethyl vinyl ether (AcOVE), only the furan ring adjacent to the propagating carbocation underwent the hetero-DA reaction with the aldehyde moiety of another furfural molecule. A further and equally important feature of the copolymerization is that the obtained copolymers had unprecedented 2:(1 + 1)-type alternating structures of repeating sequences of two VE and one furfural units in the main chain and one furfural unit in the side chain. The specific DA reaction is attributed to the delocalization of the positive charge to the side furan ring.

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equence control in a polymer chain is generally achieved using sequence-controlled copolymerization1−6 or siteselective transformation of side groups after a polymerization reaction.7 In order to realize more precise control of a sequence, a rather complicated system and/or a designed monomer is required. Thus, a simple reaction system of general compounds is important for polymer sequence control. An easier method is a tandem side-group transformation with living polymerization. One of the fundamental keys to successful tandem reactions is that the catalysts or additives for one reaction do not interrupt the other reactions. This type of reaction has been applied to polymer synthesis when the propagating species of a polymerization reaction remains intact in the presence of catalysts or active species for other reactions. Reported tandem reactions with polymerization include polymerization reactions via two different mechanisms8−11 and living polymerization combined with an organic reaction.12−14 In most cases, two or more reactions independently occur without interfering with the active species or sites of the other reactions. However, the involvement of active species in a tandem reaction would generate structures that have never been obtained via conventional polymerization and organic reactions. In cationic polymerization, high sequence control has been achieved through copolymerization of vinyl ethers (VEs) and conjugated aldehydes15−17 although the cationic homopolymerization of aldehyde compounds is difficult because of their low ceiling temperature.18 An optimum set of an initiating system and reaction conditions even produces alternating copolymers. In such copolymers, either a VE unit or an aldehyde unit can be © 2017 American Chemical Society

Received: April 8, 2017 Published: June 1, 2017 7713

DOI: 10.1021/jacs.7b03526 J. Am. Chem. Soc. 2017, 139, 7713−7716

Communication

Journal of the American Chemical Society Scheme 1. Tandem Cationic Copolymerization and HeteroDiels−Alder Reaction Activated by the Adjacent Carbocation: Generation of 2:(1 + 1)-Type Alternating Copolymer

Figure 1. 1H NMR spectra of (A) poly(AcOVE-co-furfural) purified by reprecipitation [Mn (GPC) = 11.5 × 103, Mw/Mn (GPC) = 1.33, aldehyde content: 48%; Table 1, entry 1; Integral ratio: (a + a′):h:f:g = 4.2:2.0:2.0:1.0], (B) 2,5-dihydrofuran, and (C) furan in CDCl3 at 30 °C.

°C. Moreover, control of the polymerization was indicated by the linear increase in Mn and by the relatively narrow molecular weight distributions of the product polymers (Table 1 and Figure S1). The structures of the obtained polymers were derived from the copolymerization and the structures resulting from the reaction of the pendant furan ring. The 1H NMR spectrum showed resonances associated with acetals in the main chain (e.g., peaks b and e, Figure 1A); these acetals were derived from the copolymerization. In addition, the cationic homopolymerization of furfural never proceeded under similar reaction conditions.15 Other side reactions such as chain transfer reactions to the furan ring in the side chain negligibly proceeded, which was confirmed by 1H NMR and GPC analyses of the products (Figures 1A and S1C). Furthermore, the spectrum of poly(AcOVE-co-furfural) showed peaks attributable to the side group resulting from the DA reaction of furfural (Figure 1A). For example, peak h, which is similar in chemical shift to the olefinic protons of 2,5-dihydrofuran (Figure 1B), was indicative of the structure that resulted from the DA reaction on the furan ring of furfural. Moreover, peaks assigned to the intact furan rings (peaks g and f) were observed; their integral ratios showed that the dihydrofuran-like olefin units (peak h) were equivalent in molar ratio to the furan rings. This unexpected structural feature suggests that half of the furfural-derived furan rings were consumed by the DA reaction during the copolymerization. These results and the assignments of the other peaks indicate that the DA reaction on the furan ring proceeded exclusively with the aldehyde group of furfural and in almost quantitative yield. 13C NMR analysis also supported the occurrence of the hetero-DA reaction (Figure

S2). The structural analysis also demonstrates that no DA reaction of the furan ring occurred with the CC bond of AcOVE,22 other furan rings,23 nor the unsaturated bonds resulting from DA reactions. The uniqueness of this tandem reaction is the specific occurrence of the hetero-DA reaction at a side furan ring adjacent to the propagating carbocation, which is generated in the cationic copolymerization of furfural with a VE. This mechanism is likely supported by model reactions of furfural and furan. The following three patterns are possible in the DA reaction between the furan ring and the aldehyde group of furfural in the examined tandem reaction: (i) a random reaction of a furfural monomer with the furan ring, which is incorporated into the polymer side chain [Scheme 2, reaction Scheme 2. Possible Mechanisms of the Hetero-DA Reaction of Furfural

Table 1. Cationic Copolymerization of AcOVE with Furaldehyde Derivativesa Conv.c entry 1 2e 3 4 5 6

VE

Lewis acid

time

VE

aldehyde

Mn × 10−3b

Mw/Mnb

aldehyde contentd

Diels−Alder reactiond

Furfural

AcOVE

3-Furaldehyde

CEVE AcOVE

GaCl3 GaCl3 BF3OEt2 EtAlCl2 GaCl3 GaCl3

4h 2h 24 h 24 h 3h 20 h

50% 62% 72% 7% 57% 45%

37% 42% 92% 13% 50% 34%

8.1 5.9 1.9 − 5.7 4.5

1.51 1.56 1.68 − 1.75 1.79

48% 40% 50% − 48% 46%

98% 100% 96% − 98% 0%

aldehyde

a

[Aldehyde]0 = 0.60 M, [VE]0 = 0.60 M, [EtSO3H]0 = 4.0 mM, [Lewis acid]0 = 4.0 mM (for GaCl3) or 20 mM (for BF3OEt2 and EtAlCl2), [1,4dioxane] = 1.0 M in toluene at −78 °C. bBy GPC (polystyrene calibration). cDetermined by 1H NMR of the products before reprecipitation. d Determined by 1H NMR of the products after purified by reprecipitation in methanol. eIn dichloromethane. 7714

DOI: 10.1021/jacs.7b03526 J. Am. Chem. Soc. 2017, 139, 7713−7716

Communication

Journal of the American Chemical Society 1]; (ii) the reaction of two furfural monomers to yield a dimer, which subsequently copolymerizes with VE [Scheme 2, reaction 2)]; and (iii) the reaction of a furfural monomer with the furan ring at the side chain of the propagating chain end [Scheme 2, reaction 3]. Mechanisms (i) and (ii) were ruled out because the model reactions between furfural and furan, which is a model compound of the furan ring incorporated into the side chain of a copolymer, or between two furfural molecules did not induce any cycloaddition. By contrast, a cycloaddition product with no aldehyde was obtained in their reaction in the presence of an IBVE-HCl adduct, which is a model compound for the propagating species, with GaCl3. The compound obtained using LiBH4 as a quencher was composed of one IBVE and two furfural molecules, as supported by 1H NMR (Figure S3) and high-resolution ESI-MS analyses (Figure 2). Thus, the DA

Figure 3. (A) MALDI-TOF-MS spectrum and (B) the copolymer composition curves of poly(AcOVE-co-furfural) obtained with the EtSO3H/GaCl3 initiating system [(A) Mn (GPC) = 3.7 × 103, Mw/Mn (GPC) = 1.30, aldehyde content: 48%; (B) polymerization conditions: [AcOVE]0 + [furfural]0 = 1.2 M, [EtSO3H]0 = 4.0 mM, [GaCl3]0 = 4.0 mM, [1,4-dioxane] = 1.0 M in toluene at −78 °C].

in the main chain were approximately one-third (green circles in Figure 3B). The small effects of the monomer feed ratio suggest high selectivity in generating the 2:(1 + 1)-regulated alternating sequences. Moreover, the acid hydrolysis of poly(AcOVE-co-furfural) proceeded through the scission of acetal linkages in the main chain. The structure of the acid hydrolysis product reflects the 2:(1 + 1)-alternating sequences of the original copolymer, as confirmed by the NMR and ESI-MS analyses (Figures S4 and S5). The unique 2:(1 + 1)-alternating sequence may be derived from the steric hindrance of the side chain, which results from the DA reaction. The bulkiness of the furfural-attached side chain at the penultimate unit of the AcOVE-derived propagating chain end prevents the addition of a furfural monomer to the carbocation, which causes the sequential addition of two VE monomers. Incorporation of furfural into the main chain without accompanying DA reaction (0−4%) likely produces the conventional 1:1-type alternating sequences. The efficiency of a tandem hetero-DA and copolymerization reaction was not affected by polymerization conditions (Table 1 and Figure S6). The tandem reaction of AcOVE and furfural efficiently proceeded in dichloromethane (entry 2) similarly to the previously demonstrated reaction in toluene (entry 1). The DA reactions have been reported to proceed with similar efficiency in any solvent.24 The use of BF3OEt2 instead of GaCl3 did not affect the efficiency of the DA reaction and sequence control, although a copolymer with a smaller molecular weight was obtained (entry 3). However, with EtAlCl2, cyclic trimerization exclusively occurred from one VE and two furfural molecules (entry 4; Figures S6C and S7), similar to the results of previous reports involving other conjugated aldehydes.15 The reaction using 2-chloroethyl VE (CEVE) instead of AcOVE also yielded products of the DA and 2:(1 + 1)-regulated alternating copolymerization reactions (entry 5 in Table 1; Figures S8 and S9). A VE with higher reactivity (isobutyl VE) was also efficient for the tandem copolymerization (Figure S10). Other dienophiles such as benzaldehyde was incorporated into the side chains in competition with furfural (Figure S11). 3-Furaldehyde (3FA), which is an isomer of furfural, was demonstrated to be ineffective for the tandem hetero-DA and cationic copolymerization reactions. The copolymerization of AcOVE and 3FA smoothly proceeded when the EtSO3H/ GaCl3 initiating system was used in the presence of 1,4-dioxane in toluene at −78 °C, to give copolymers selectively (entry 6 in

Figure 2. ESI-MS spectrum of the product obtained by model reaction (reaction conditions: [furfural]0 = 0.60 M, [IBVE-HCl]0 = 40 mM, [GaCl3]0 = 4.0 mM, [1,4-dioxane] = 1.0 M in toluene at −78 °C).

reaction between the furan ring and the aldehyde group of another furfural molecule occurred when the furan-derived carbocation was generated via the addition to the IBVE-HClderived carbocation. This type of DA reaction is classified as an inverse-electron-demand DA reaction, which is the reaction between an electron-deficient diene and an electron-abundant dienophile.19 In the present system, the DA reaction most likely proceeded because of the electron-withdrawing effect of the carbocationic center adjacent to the furan ring; the carbocationic center was generated via the reaction of the aldehyde moiety with the VE-derived carbocation. The copolymer obtained from the tandem reaction was demonstrated to have unprecedented 2:(1 + 1)-regulated alternating sequences of two VE and one furfural units in the main chain and one furfural unit in the side chain. The molar ratios based on the integral ratios of AcOVE and furfural units were equal in the 1H NMR spectrum (Figure 1A), which indicates that the AcOVE-to-furfural ratio is 2 to 1 in the main chain. Moreover, the MALDI-TOF-MS spectrum of the copolymer (Figure 3A) showed a main series of peaks at m/z intervals of 452, which corresponds to the total molecular weight of two AcOVE and two furfural units. These results indicate that the copolymer had 2:(1 + 1)-regulated alternating sequences. In addition, no series of peaks was assigned to sequences (322) with one unit of VE and two units of furfural, which suggests that the 1:(1 + 1)-alternating sequences of VE and furfural were not generated in the main chain, unlike previous copolymerization reactions of VEs and conjugated aldehyde.15−17 The copolymer composition curves, which were obtained from the copolymerization results with different feed ratios of the monomers, show that the furfural contents were approximately 50% in copolymers at all feed ratios (red inverted triangles in Figure 3B), whereas the furfural contents 7715

DOI: 10.1021/jacs.7b03526 J. Am. Chem. Soc. 2017, 139, 7713−7716

Communication

Journal of the American Chemical Society Table 1; Figure S12). However, the 1H NMR spectrum of the product exhibited no peaks for the structures derived from DA reactions (Figure S13). Moreover, the copolymer had 1:1instead of 2:(1 + 1)-alternating sequences of AcOVE and 3FA in the main chain. The inactivity of the furan ring to the DA reaction is likely due to the resonance structure of cationic species at the propagating chain end. Propagating carbocations derived from the addition reaction of the aldehyde moiety of furaldehydes are delocalized in the furan ring; however, furfural and 3FA have different degrees of delocalization. The furfuralderived carbocation is delocalized through all carbon atoms of the furan ring, whereas the resonance structure of the 3FAderived carbocation includes two of the four carbon atoms on the furan ring. In addition to the smaller degree of delocalization of the latter carbocation compared to that of the former, the steric hindrance around the diene structure may also be responsible for the inertness. The 2:(1 + 1)- and 1:1type alternating copolymers had different glass transition temperatures (Table S1). In conclusion, the tandem cationic copolymerization and hetero-DA reaction proceeded when furfural and VEs were used as monomers. The concerted DA reaction was demonstrated to proceed selectively between the furan ring adjacent to the carbocation, which was generated at the propagating chain end, and the aldehyde moiety of furfural. Moreover, the repeated sequences in the main chain were composed of two units of VE and one unit of furfural, which indicates that the product was a 2:(1 + 1)-regulated alternating copolymer. The copolymerization using 3FA proceeded without an accompanying DA reaction and produced 1:1alternating copolymers. The greater degree of delocalization of the furfural-derived carbocation was indispensable for the tandem reaction.

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(3) Zhang, J.; Matta, M. E.; Hillmyer, M. A. ACS Macro Lett. 2012, 1, 1383−1387. (4) Atallah, P.; Wagener, K. B.; Schulz, M. D. Macromolecules 2013, 46, 4735−4741. (5) Pfeifer, S.; Lutz, J.-F. J. Am. Chem. Soc. 2007, 129, 9542−9543. (6) Satoh, K.; Ozawa, S.; Mizutani, M.; Nagai, K.; Kamigaito, M. Nat. Commun. 2010, 1, 1−6. (7) Tasdelen, M. A. Polym. Chem. 2011, 2, 2133−2145. (8) Mecerreyes, D.; Moineau, G.; Dubois, P.; Jérôme, R.; Hedrick, J. L.; Hawker, C. J.; Malmström, E. E.; Trollsas, M. Angew. Chem., Int. Ed. 1998, 37, 1274−1276. (9) Bielawski, C. W.; Louie, J.; Grubbs, R. H. J. Am. Chem. Soc. 2000, 122, 12872−12873. (10) Mahanthappa, M. K.; Bates, F. S.; Hillmyer, M. A. Macromolecules 2005, 38, 7890−7894. (11) Zhou, Y.; Chen, Z.; Wei, C.; Luo, Z. Macromol. Chem. Phys. 2015, 216, 329−333. (12) Nakatani, K.; Terashima, T.; Sawamoto, M. J. Am. Chem. Soc. 2009, 131, 13600−13601. (13) Lundberg, P.; Hawker, C. J.; Hult, A.; Malkoch, M. Macromol. Rapid Commun. 2008, 29, 998−1015. (14) Mallakpour, S. E.; Hajipour, A.; Mahdavian, A. Polym. Int. 1999, 48, 109−116. (15) Ishido, Y.; Aburaki, R.; Kanaoka, S.; Aoshima, S. Macromolecules 2010, 43, 3141−3144. (16) Aoshima, S.; Oda, Y.; Matsumoto, S.; Shinke, Y.; Kanazawa, A.; Kanaoka, S. ACS Macro Lett. 2014, 3, 80−85. (17) Kawamura, M.; Kanazawa, A.; Kanaoka, S.; Aoshima, S. Polym. Chem. 2015, 6, 4102−4108. (18) Aso, C.; Tagami, S.; Kunitake, T. J. Polym. Sci., Part A-1: Polym. Chem. 1969, 7, 497−511. (19) Jiang, X.; Wang, R. Chem. Rev. 2013, 113, 5515−5546. (20) Yates, P.; Eaton, P. J. Am. Chem. Soc. 1960, 82, 4436−4437. (21) Grieco, P. A.; Dai, Y. J. Am. Chem. Soc. 1998, 120, 5128−5129. (22) If a VE monomer acts as a dienophile, the content of the consumed furfural should be less than 33% in a copolymer because furfural is not homopolymerizable. The actual content of approximately 50% indicates that a VE did not undergo DA reactions. (23) No aldehyde moiety was observed in the 1H NMR spectrum, which suggests that the CC bond of the furfural ring did not function as a dienophile. (24) Reichardt, C.; Welton, T. Solvents and Solvent Effect in Organic Chemistry; Wiley: New York, NY, 2011.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b03526. Experimental section, NMR spectra, polymerization results, and DSC analysis (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Suzuka Matsumoto: 0000-0002-6483-0877 Arihiro Kanazawa: 0000-0002-8245-6014 Sadahito Aoshima: 0000-0002-7353-9272 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by JSPS KAKENHI Grant Nos. JP14J01127 and No. 26288063. We thank the group of Prof. T. Inoue (Osaka University) for DSC measurements.

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REFERENCES

(1) Lutz, J.-F.; Ouchi, M.; Liu, D. R.; Sawamoto, M. Science 2013, 341, 1238149. (2) Satoh, K.; Matsuda, M.; Nagai, K.; Kamigaito, M. J. Am. Chem. Soc. 2010, 132, 10003−10005. 7716

DOI: 10.1021/jacs.7b03526 J. Am. Chem. Soc. 2017, 139, 7713−7716