Letter pubs.acs.org/macroletters
Polybutadiene Functionalization via an Efficient Avenue Christina M. Geiselhart,†,‡ Janin T. Offenloch,†,‡ Hatice Mutlu,*,†,‡ and Christopher Barner-Kowollik*,†,‡,§ †
Preparative Macromolecular Chemistry, Institut für Technische Chemie und Polymerchemie, Karlsruhe Institute of Technology (KIT), Engesserstraße 18, 76128 Karlsruhe, Germany ‡ Soft Matter Synthesis Laboratory, Institut für Biologische Grenzflächen, Karlsruhe Institute of Technology (KIT), Hermann-von-Helmholtz-Platz 1, 76344 Karlsruhe, Germany § School of Chemistry, Physics and Mechanical Engineering, Queensland University of Technology (QUT), 2 George Street, QLD 4000, Brisbane, Australia S Supporting Information *
ABSTRACT: We introduce a facile and quantitative postpolymerization functionalization methodology for 1,4-polybutadienes, allowing us to decorate their pendent alkene functionalities with bromine and alkoxyether motifs carrying an array of functional groups ranging from tetrazoles to pyrenes. Specifically, the approach makes use of a mild, metal-free, electrophilic cascade reaction employing N-bromosuccinimide (NBS), a cyclic ether (i.e., THF), and a functional carboxylic acid. Detailed NMR, SEC, and ATR-IR studies confirm the successful modification.
E
nent reaction,10 which has not yet been applied to macromolecular transformations, as a highly efficient avenue for nonpolar polymer postpolymerization functionalization (Scheme 1a). Indeed, a careful literature survey reveals that various multicomponent reactions (MCRs) have recently received attention in polymer chemistry for polymer synthesis11 or postpolymerization modification.12 However, examples aimed at exploiting the reactivity profiles of halogen electrophiles with other components in a one-pot cascade reaction involving both nucleophilic addition and electrophilic coupling are scarce. As a unique example, an electrophilic MCR13 utilizing N-bromosuccinimide (NBS) as the brominating species was employed in a number of electrophilic cascade reactions using various nucleophilic partners (Scheme 1b). The most plausible mechanism of the MCR proceeds as follows: The activated olefin A is reactive toward nucleophilic attack by cyclic ethers affording the intermediate B14 shown in Scheme 1b. The resulting oxonium intermediate B can be further attacked by a nucleophilic partner to afford the desired alkoxyetherification product. For a more in-depth description of the reaction and of its applications, refer to the literature.13 Thus, in the current letter we introduce the first utilization of an electrophilic multicomponent reaction in a postpolymerization modification approach of unsaturated polyolefins (Scheme
ver since Staudinger1 coined the term polymer analogous reaction and hailed such reactions as an attractive approach for the generation of functional macromolecular materials, the scope of postpolymerization modifications has been critically expanded by introducing an array of ligation protocols,2 some adhering to the stringent click criteria.3 Despite the fact that many of these ligations lead to quantitative conversions during the postpolymerization modification step, increasing the functional density along the lateral chain via the installation of multiple functional groups per monomeric unit remains challenging. To the best of our knowledge, only a few examples have been reported so far, andnotablymost of these modifications are performed on polar polymers.4 Despite these significant efforts, the postpolymerization modification of nonpolar polymers (i.e., unsaturated polyolefins such as polybutadiene) is still challenging5 due to the limited number of reactive functional groups present within these polymers. In particular, addition reactions play a critical role in the chemistry of unsaturated polyolefins based on the high reactivity of the CC bond in the polymer. These addition reactions6 include epoxidations,7 thiol−ene ligations,8 or halohydrin formation.9 In any case, the aforementioned postpolymerization methods do not allow for an efficient installation of multiple functional groups per monomeric unit on the unsaturated polyolefin backbone. It is therefore critical to introduce a new postpolymerization modification for unsaturated polyolefins. Herein, we introduce a catalyst- and metal-free Nbromosuccinimide (NBS) initiated electrophilic multicompo© XXXX American Chemical Society
Received: September 5, 2016 Accepted: September 20, 2016
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DOI: 10.1021/acsmacrolett.6b00679 ACS Macro Lett. 2016, 5, 1146−1151
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ACS Macro Letters Scheme 1. Schematic Representation of (a) the NBSInitiated Electrophilic Multicomponent Reaction, (b) the Proposed Mechanism of the MCR, and (c) the MCR Modification of Unsaturated Polyolefins (i.e., 1,4Polybutadiene) as Introduced in the Current Report
Scheme 2. Carboxylic Acid Derivatives Which Have Been Successfully Employed in the Electrophilic MCR for Poly(butadiene) Modification
1c). Initially, we examined the MCR cascade by adopting a slightly modified procedure of the already reported reaction conditions for low molecular olefins.13b A commercially available polybutadiene (PBD, Mn,SEC of 9200 g mol−1,15 Đ of 1.06; 23% vinyl and 77% trans-1,4 + cis-1,4 olefin CC bonds, Figure S1 in the Supporting Information)16 was mixed with 1.0 equiv of a carboxylic acid derivative17 (2,3,4,5,6pentafluorobenzoic acid, 1) to afford polymer P1 in Scheme 2. PFB is used as an abbreviation to indicate the attached nucleophile in dry tetrahydrofuran (THF) under dilute conditions (0.16 M). Once the mixture was cooled to 0 °C, 6.0 equiv of NBS was added. Subsequently, the reaction proceeded at ambient temperature for 22 h. Generally, the nucleophile partner (i.e., the functional carboxylic acid derivative) should be rather acidic.13 Accordingly, 1 (in Scheme 2) was initially assessed since it has a pKa of 1.48.18 In addition, a modification with a fluorine moiety will alter the physical properties (for example the refractive index) of the PBD.19 With the idea to alter and transform the properties of PBDs into new/specialty materials with tailor-made properties and custom-designed performance for target applications, less acidic partners (in comparison to acid 1 in Scheme 2) were additionally examined. Functional elements such as tetrazoles (Scheme 2, 4-(2-(4-methoxyphenyl)-2H-tetrazol-5-yl)benzoic acid, 2, pKa = 3.76)20 and maleimides ((4-(2-(1,3-dioxohexahydro-1H-4,7-epoxyisoindol-2(3H)-yl) ethoxy)-4-oxo-butanoic acid, 3, pKa = 3.8−4.0) are recognized as synthetically important units due to their high (photo)reactivity toward various reactions including the nitrile imine-mediated tetrazoleene cycloaddition (NITEC),21 which is a versatile tool not only for SCNP formation22 but also for, e.g., surface modifications,23 polymer ligation,24 protein functionalization,25 and peptide modification.26 Thus, in an application example, we show the synthesis of single-chain nanoparticles (SCNPs) via photoinduced NITEC by employing the tetrazole and maleimidefunctionalized PBD. Moreover, introducing chromophores such as 1-pyrenecarboxylic acid (4, pK a = 4.0) or anthraquinone-2-carboxylic acid (5, pKa = 4.3) furnishes photosensitizing characteristics27 to the postfunctionalized
PBD. Finally, modifying PBD with 3,6,9-trioxadecanoic acid (6, pKa = 4.0), a mimic of 2-(2-methoxyethoxy)ethyl 1147
DOI: 10.1021/acsmacrolett.6b00679 ACS Macro Lett. 2016, 5, 1146−1151
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ACS Macro Letters methacrylate, as a new family of monomers for the synthesis of thermoresponsive polymers, may pave the way toward waterdispersible polybutadiene derivatives. In an illustrative postmodification example, the olefin functions of the commercially available PBD were decorated with a bromine and a pentafluorophenyl motif via an alkoxyether bond to yield polymer P1 (Scheme 2). The SEC trace of the isolated polymer P1 after precipitation in ice cold MeOH indicates a clear and clean shift toward higher molecular weights (Mn,SEC of 19 000 g mol−1, Figure 1a). Moreover, the SEC traces obtained with UV (λ = 254 nm) and RI detection are virtually identical, indicating that the modification occurred uniformly over the entire molecular weight regime (Figure S2 in the Supporting Information). Furthermore, the essentially unaltered polydispersity index (Đ of 1.06 and 1.09, respectively, for PBD and P1) serves as an indicator for the absence of side reactions such as cross-linking or degradation during the postpolymerization functionalization (compare Figure 1a). In order to confirm that the reaction proceeds via the multicomponent reaction, 1H and 13C NMR measurements of the generated polymer P1 were carried out (Figures 1b and S3). To confirm the chemical structure of polymer P1,28 2D NMR analysis, i.e., COSY, has been also conducted (Figure S4 in the Supporting Information). As depicted in Figure 1b, the characteristic magnetic resonances associated with the olefin CC bonds (5.75−4.75 ppm, respectively, assigned as 6, 6′, 9, and 10) of the parent polymer were virtually absent, which confirms that quantitative conversion of the olefin bonds was achieved. Further, new resonances between 4.55 and 3.15 ppm appeared, associated with the postfunctionalized backbone of the polymer. More specifically, the new resonance signals associated with the methine and methylene signals of the functionalized 1,4-cis/trans and 1,2-vinyl butadiene units (b and c as shown in Figure 1b) underpin the successful functionalization, which is also observed in the 13C NMR spectrum of P1 (compare Figures S1b and S3 in the Supporting Information). The resonance signals of the protons e and g (compare Figure 1b) indicate the successful incorporation of the ether (i.e., THF) as a linker between the polymer backbone and the pendent carboxylic acid (PFB). Virtually identical results were obtained by 19F NMR (Figure S5 in the Supporting Information), evidencing the successful functionalization: three characteristic resonances corresponding to the fluorine atoms in the ortho, para, and meta positions of the pentafluorophenyl motif are observed. The anticipated structure of the postfunctionalized polymer is additionally supported by ATR-IR analysis (Figure 1c). The characteristic vibrations of unmodified PBD, which exhibits strong bands in the 730−665, 960−980, and 905−915 cm−1 regions (corresponding to the bending vibrations (C−H) out-of-plane of the isomers: cis/trans (1,4 addition) and vinyl (1,2 addition), respectively) are absent in the polymer P1.29 However, an intense peak appears at 1735 cm −1 , corresponding to the carbonyl CO stretching vibration, indicating the formation of the ester group. Furthermore, the two bands typical for the C−O in the ester bond were detected at 1100 and 1225 cm−1, respectively. In summary, these results for the functionalization reactions underpin the operational principle of our efficient method to densely and efficiently code two different functionalities into the polymer backbone. One other halogen source, i.e., N-chlorosuccinimide (NCS), was tested, yet only slight formation of the halohydrine derivative of PBD and no detectable MCR reaction were
Figure 1. Comparative SEC traces (in THF at 30 °C) and 1H NMR (500 MHz, CDCl3, ambient temperature, * solvent impurities) and ATR-IR spectra of PBD and the postpolymerization functionalized polymer (P1), respectively, (a), (b), and (c). To simplify the complex structure of the product P1 (shown in a), only the product of the postpolymerization modification of the 1,4-cis pattern of PBD (as in b) is depicted.
observed (P2 in Figures S6 and S7 in the Supporting Information, Entry 2 in Table 1). As aforementioned, the scope of the MCR postpolymerization functionalization was probed, and the product diversity is laid out in Scheme 2. Next, carboxylic acids 2 and 3 were reacted with PBD to synthesize polymers P3 and P4, respectively. After a reaction time of 22 h, quantitative conversion was indicated by 1H NMR analysis since the characteristic magnetic resonances associated with the olefin region of the parent polymer were absent (compare Figures S8 1148
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multimodal polymer peaks were observed during SEC analysis (Figure S12 in the Supporting Information shows the representative SEC trace for the reaction performed with acid 6). However, control over the postpolymerization modification reaction can be achieved by shortening the reaction time. Thus, the MCRs for acid 4 and 6, respectively, were carried out for 5 and 8 h. The SEC traces of the resulting polymers P5 and P7, depicted in Figures S13 and S14, respectively, showed clear and clean shifts in retention time as compared to the starting polymer PBD. In addition, low molar-mass dispersity (Đ < 1.25) was maintained throughout the functionalization process. However, the 1H NMR analysis indicated that 4 reacts less efficiently than 6. Additional experiments were conducted to determine if our new protocol can be utilized to generate modified polybutadienes that contain more than one type of alkoxyether derivative (P8.1, in Table 1; for more details see section A.5 in the Supporting Information). We observed that the reaction of polybutadiene with 2 and 3 in a 1:1 ratio produces polybutadiene containing only tetrazole acid derived alkoxyether moieties (Figure S15). On the other hand, the use of a slight excess of 3 in the presence of 2 (i.e., 1:1.9 ratio of 2:3) leads to the exclusive formation of polybutadiene containing an approximately 1:1 ratio of the corresponding photoreactive acids (P8.2 in Table 1; for more details see section A.6 in the Supporting Information). Indeed, the functional linear precursor P8.2 can be employed in the synthesis of welldefined fluorescent single-chain nanoparticles (SCNPs) via the photoinduced nitrile imine intramolecular cross-ligation (for more details see section A.7 in the Supporting Information). Thus, the intramolecular cross-linking of the linear precursors P8.2 was carried out in highly diluted solutions (cprecursor = 23 mg·L−1 in dichloromethane) at an excitation wavelength of 315 nm to afford single-chain nanoparticles SCNP1. The 1H NMR, SEC, and DLS characterization data of the linear, functional precursor P8.2 as well as of the SCNP1 are shown in Figures S16, 17, and 18, respectively. Finally, in a representative experiment, a further postpolymerization modification of the pendent bromine group of precursor P6 was carried out using a mild azidation in order to obtain a modified polymer containing azide pendent groups, constituting a potent platform for further orthogonal postmodification reactions (for details refer to section A.8 in the Supporting Information). Notably, this reaction rapidly generates the desired azide-modified polymer in good yields without any need for further optimization. The 1 H NMR and SEC characterization data of the azide-modified polymer P6M are shown in Figures S19 and S20, respectively.
Table 1. Overview of the Conducted Postpolymerization Modificationsa,b polymer
RCOOH
conv. of CC Hc[%]
yieldd [%]
P1 P2e P3 P4f P5g P6 P7h P8.1i P8.2j
1 1 2 3 4 5 6 2:3 2:3
>99 ∼10 >99 >99 ∼70 >99 ∼80 >99 >99
43.4 78.5 26.9 32.6 99.5 50.0
a
The polymer structures are shown in Scheme 2. bThe reaction conditions are as follows: PBD (1.0 equiv), NBS (6.0 equiv), and oxygen nucleophile RCOOH (1.0 equiv of carboxylic acid with respect to the double bonds in the polymer backbone) in THF (0.16 M) at 0− 25 °C for 22 h under inert gas atmosphere. cDetermined by 1H NMR (500 MHz) in CDCl3 at ambient temperature. dIsolated yields are based on modified polybutadiene. eNCS is employed as a halogen source. f1H NMR analysis indicated side reactions on the furan moiety of the carboxylic acid derivative. gReaction was performed for 5 h. h Reaction was performed for 8 h. iReaction of PBD with 2 and 3 in 1:1 ratio. jReaction of PBD with 2 and 3 in a 1:1.9 ratio.
and S9 in the Supporting Information). Moreover, the SEC traces of the postfunctionalized polymers P3 and P4 showed clear shifts in retention time toward higher molecular weights as compared to starting polymer PBD (Figures S10 and S11, respectively, in the Supporting Information). Surprisingly, the polydispersity index for P3 remained low (Đ of 1.09), while the Đ for P4 has a slightly elevated value (1.24) along a high molecular weight shoulder (Figure S11), suggesting that some minimal interpolymer chain coupling or a side reaction has occurred on the double bond of the furan moiety of the carboxylic acid derivative 3 during the MCR (which was also detected during 1H NMR analysis, compare Figure S9). Nevertheless, the successful postpolymerization modification with the less acidic carboxylic acid derivate 2 triggered us to assess carboxylic acid derivative 5, which possess even less acidic character than acid 1 (compare Scheme 2 and P6 in Table 1). The full incorporation of 5 was clearly confirmed by 1 H NMR and SEC analysis (Figure 2). In addition, 4 and 6, which have relatively similar acidic pKa values, were separately tested in the multicomponent postmodification reaction. Interestingly, long reaction times (22 h) resulted in an uncontrolled postpolymerization functionalization, although the conversion of the reaction was quantitative. Moreover,
Figure 2. Comparative SEC traces (in THF at 30 °C) of PBD and postpolymerization-functionalized polymer (P6) and the 1H NMR spectrum (500 MHz, CDCl3, ambient temperature) of P6, respectively, (a) and (b). 1149
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(6) McGrath, M. P.; Sall, E. D.; Tremont, S. J. Chem. Rev. 1995, 95, 381−398. (7) Saffer, A.; Johnson, B. L. Ind. Eng. Chem. 1948, 40, 538. (8) (a) Justynska, J.; Schlaad, H. Macromol. Rapid Commun. 2004, 25, 1478−1481. (b) Justynska, J.; Hordyjewicz, Z.; Schlaad, H. Polymer 2005, 46, 12057−12064. (c) ten Brummelhuis, N.; Diehl, C.; Schlaad, H. Macromolecules 2008, 41, 9946−9947. (9) Johnson, A.; Nudenberg, W. US Pat. 3,733,313, 1973. (10) Tan, C. K.; Yeung, Y.-Y. Chem. Commun. 2013, 49, 7985−7996. (11) (a) Kreye, O.; Toth, T.; Meier, M. A. R. J. Am. Chem. Soc. 2011, 133, 1790−1792. (b) Kakuchi, R.; Theato, P. ACS Macro Lett. 2013, 2, 419−422. (c) Hartweg, M.; Becer, C. R. Green Chem. 2016, 18, 3272− 3277. (12) (a) Kakuchi, R. Angew. Chem., Int. Ed. 2014, 53, 46−48. (b) Jiang, X.; Feng, C.; Lu, G.; Huang, X. Sci. China: Chem. 2015, 58, 1695−1709. (13) (a) Zhou, L.; Tan, C. K.; Zhou, J.; Yeung, Y.-Y. J. Am. Chem. Soc. 2010, 132, 10245−10247. For other examples of electrophilic NBS initiated cascades, see: (b) Chen, J.; Chng, S.; Zhou, L.; Yeung, Y.-Y. Org. Lett. 2011, 13, 6456−6459. (c) Zhou, L.; Zhou, J.; Tan, C. K.; Chen, J.; Yeung, Y.-Y. Org. Lett. 2011, 13, 2448−2451. (d) Zhou, L.; Chen, J.; Zhou, J.; Yeung, Y.-Y. Org. Lett. 2011, 13, 5804−5807. (e) Zhou, J.; Zhou, L.; Yeung, Y.-Y. Org. Lett. 2012, 14, 5250−5253. (f) Tay, D. W.; Tsoi, I. T.; Er, J. C.; Leung, G. Y. C.; Yeung, Y.-Y. Org. Lett. 2013, 15, 1310−1313. (g) Ke, Z.; Yeung, Y.-Y. Org. Lett. 2013, 15, 1906−1909. (h) Zhou, J.; Yeung, Y.-Y. J. Org. Chem. 2014, 79, 4644−4649. (i) Zhou, J.; Yeung, Y.-Y. Org. Lett. 2014, 16, 2134−2137. (j) Zhou, J.; Yeung, Y.-Y. Org. Biomol. Chem. 2014, 12, 7482−7485. (14) (a) Serguchev, Y. A.; Ponomarenko, M. V.; Lourie, L. F.; Chernega, A. N. J. Fluorine Chem. 2003, 123, 207−215. (b) Braddock, D. C.; Millan, D. S.; Perez-Fuertes, Y.; Pouwer, R. H.; Sheppard, R. N.; Solanki, S.; White, A. J. P. J. Org. Chem. 2009, 74, 1835−1841. (15) Determined by SEC in THF at 30 °C vs linear PS standards. (16) The content of the vinyl and trans-1,4 + cis-1,4 olefin CC bonds was measured using 1H NMR spectroscopy. In particular, the 1,4-cis amount was difficult to quantify accurately in the 1H NMR spectrum since the cis signal is superimposed with the residual vinyl resonances. The respective 1H NMR spectrum is provided in the Supporting Information (Figure S1a). (17) 1.0 equiv of carboxylic acid with respect to the double bonds in the polymer backbone (18) Prakash, G. K. S.; Hu, J. e-EROS Encyclopedia of Reagents for Organic Synthesis 2006, DOI: 10.1002/047084289X.rn00682. (19) Reisinger, J. J.; Hillmyer, M. A. Prog. Polym. Sci. 2002, 27, 971− 1005. (20) McMulkin, C. J.; Massi, M.; Jones, F. CrystEngComm 2015, 17, 2675−2681. (21) Clovis, J. S.; Eckell, A.; Huisgen, R.; Sustmann, R. Chem. Ber. 1967, 100, 60−70. (22) Willenbacher, J.; Wuest, K. N. R.; Mueller, J. O.; Kaupp, M.; Wagenknecht, H.-A.; Barner-Kowollik, C. ACS Macro Lett. 2014, 3, 574−579. (23) Blasco, E.; Piñol, M.; Oriol, L.; Schmidt, B. V. K. J.; Welle, A.; Trouillet, V.; Bruns, M.; Barner-Kowollik, C. Adv. Funct. Mater. 2013, 23, 4011−4019. (24) de Hoog, H.-P. M.; Nallani, M.; Liedberg, B. Polym. Chem. 2012, 3, 302−306. (25) Wang, J.; Zhang, W.; Song, W.; Wang, Y.; Yu, Z.; Li, J.; Wu, M.; Wang, L.; Zang, J.; Lin, Q. J. Am. Chem. Soc. 2010, 132, 14812−14818. (26) Madden, M. M.; Rivera Vera, C. I.; Song, W.; Lin, Q. Chem. Commun. 2009, 5588−5590. (27) (a) Crivello, J. V.; Jiang, F. Chem. Mater. 2002, 14, 4858−4866. (b) Diaz, A. N. J. Photochem. Photobiol., A 1990, 53, 141−167. (28) The full NMR characterization of the postfunctionalized polymers is challenging since the MCR resulted in various regioand stereoselective additions. (29) The strong band attributed to the stretching vibration (C−H) of the hydrogen of the olefin double bond appearing near 3000 cm−1 was absent. In addition, close to 1436 cm−1 there is a strong absorption
In summary, the postpolymerization functionalization of polybutadienes, introducing polar alkoxyether units, is possible under mild conditions in the presence of NBS. Successful postfunctionalization with quantitative conversion was achieved with acidic carboxylic acid 1 and the considerably less acidic carboxylic acid derivatives 2 and 5, while the functionalization with 3 resulted in uncontrolled postpolymerization modification presumably due to the side reaction occurring on the double bond of the furan moiety. On the other hand, MCRs with 4 and 6 required shorter reaction times to obtain the desired functionalized polymers. Finally, we have shown that our protocol can be employed for the preparation of polybutadienes containing two different alkoxyether groups, to afford linear precursor polymers which can undergo a photoinduced nitrile imine intramolecular cross-ligation. Further investigation of this intriguing approach for polymer modification and its potential for modifying macromolecules for specialty materials applications, such as biomaterials, and polymer degradation are currently underway in our laboratories.
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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.6b00679. Experimental section and additional characterization data (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]; christopher.
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS C.B.-K. acknowledges continued support from the Karlsruhe Institute of Technology (KIT) in the context of the Helmholtz BioInterfaces in Technology and Medicine (BIFTM) and Science and Technology of Nanosystems (STN) programs as well as from the Queensland University of Technology (QUT). Additional support by the GRK 2039 (project A1) funded by the German Research Council (DFG) is gratefully acknowledged. The authors are thankful to Kuraray Europe GmbH for the kind donation of polybutadiene.
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REFERENCES
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ACS Macro Letters band corresponding to the rocking vibration of C−H in combination with a scissoring vibration of the terminal methylene (−CH2) from the vinylic portion.
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