Polyselenoureas via Multicomponent Polymerizations Using

Letter Cite This: ACS Macro Lett. 2018, 7, 898−903

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Polyselenoureas via Multicomponent Polymerizations Using Elemental Selenium as Monomer Bryan T. Tuten,*,† Fabian R. Bloesser,† David L. Marshall,§ Lukas Michalek,† Christian W. Schmitt,† Stephen J. Blanksby,§ and Christopher Barner-Kowollik*,†,‡ School of Chemistry, Physics and Mechanical Engineering and §Central Analytical Research Facility, Institute for Future Environments, Queensland University of Technology (QUT), 2 George Street, Brisbane, Queensland 4000, Australia ‡ Macromolecular Architectures, Institut für Technische Chemie und Polymerchemie, Karlsruhe Institute of Technology (KIT), Engesserstr. 18, 76131 Karlsruhe, Germany Downloaded via UNIV OF SOUTH DAKOTA on July 12, 2018 at 01:28:05 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

†

S Supporting Information *

ABSTRACT: Multicomponent polymerizations (MCPs) have emerged as a powerful tool in the synthesis of advanced, sequence-regulated polymers based on their mild reaction conditions, ease of use, and high atom economy. Herein, we exploit MCP methodology to introduce elemental selenium into a polymer chain, accessing a unique polymer class,i.e., polyselenoureas. These polyselenoureas can be synthesized from a broad range of commercially available starting materials, in a simple ambient temperature one-step procedure. The incorporation of selenium directly into the polymer backbone provides a unique handle for polymer characterization based on the distinctive isotope profiles exposed by high-resolution mass spectrometry, along with diagnostic signals observed in infrared and X-ray photoelectron spectroscopies. In addition, diffusion ordered spectroscopy provides access to hydrodynamic diameter information on the generated unique polymer class. the Biginelli reaction,8 the Hantzsch reaction,9 and alkyne/ sulfonyl azide systems.10 Interesting specialized MCRs, which utilize elemental sulfur, have been pioneered by the Nguyen team. These systems have utilized various components such as benzyl amines/aryl alkynes,11 benzyl amines/alkyl amines,12 and isocyanides/ amines.13 The reactions allow access to a large library of compounds from commercially available or readily synthesized starting materials. Drawing on the benzyl amine/aryl alkyne system and, very recently, the isocyanide/amine system, Tang and co-workers translated this methodology into poly(thioamide) and poly(thiourea), enabling the synthesis of luminescent polymeric materials capable of removing Hg2+ ions from water with greater than 99.99% efficiency.14,15 In addition to the previously noted features of MCR-based polymerizations, a critical advantage in the two systems developed by the Tang team is the utilization of one of the largest byproducts in the petroleum refining industry,

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ulticomponent reactions (MCRs) have emerged as powerful tools in advanced sequence-regulated stepgrowth polymerization techniques. These simple, high-yielding reactions offer access to a massive library of bespoke macromolecular architectures simply by adjusting the monomer functionality. Isocyanide-based multicomponent polymerization reactions (MCPs) are the most widely known processes, particularly the Passerini and the Ugi reaction.1 The efficacy of Passerini and Ugi reactions is based on the impressively large array of accessible polymers with varying linking motifs arising from just a few monomers, as demonstrated by the Meier team as well as Luxenhofer and co-workers.2,3 The ability to finely tailor the linking motifs in MCPs has led to the development of advanced functionality in these macromolecular architectures such as photodegradable single-chain polymer nanoparticles (SCNPs),4 SCNPs capable of being analyzed by high resolution mass spectrometry,5 polymers with highly precise sequence definition,6 and simple access to acyclic diene metathesis (ADMET) monomers.7 Equally elegant work outside the scope of isocyanide-based MCPs has also been accomplished via various MCRs, such as © XXXX American Chemical Society

Received: June 5, 2018 Accepted: July 9, 2018

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DOI: 10.1021/acsmacrolett.8b00428 ACS Macro Lett. 2018, 7, 898−903

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ACS Macro Letters

Scheme 1. Diisocyanide and Diamine Monomers Used in the Selenium-Mediated MCP To Form a Library of Polymers, P1− P6a

a

The inset demonstrates the proposed mechanism for the MCP.

elemental sulfur.16 Inspired by the idea of exploiting elements as building blocks, we looked to another chalcogen, that is, selenium. Selenium is a common byproduct in many metal refining industries, most commonly in the electrolytic refining of copper metal and as a decoloring agent used in glass manufacturing.17 The unique properties of selenium can be attributed to an atomic radius larger than that of sulfur, while at the same time being less electronegative. The combination of both these properties leads to lower bond energies ((C−Se) ∼ 244 kJ mol−1 and (C−S) ∼ 272 kJ mol−1) while simultaneously allowing for easier oxidation at lower valence states than that of sulfur analogs.18 To date, there are only limited examples of selenium containing polymers, which predominately exist as mono- or diselenide containing macromolecules or dendrimers.19 However, evident even in the limited number of examples, these selenium containing polymers show remarkable promise as enzyme mimics,20 redox responsive materials,21 electrically conducting polymers,22,23 red-light-induced drug release systems,24 degradable micelles,18,25−28 and self-healing thermosets.29 There have also been reports of selenium-based reversible addition chain transfer (RAFT) agents, where the Se-RAFT end group can be readily aminolyzed, affording dynamically reactive selenol end groups.30−32 The critical limitation toward developing selenium containing polymers has been the relative synthetic difficulty in incorporating selenium into polymers beyond mono- and diselenide moieties. Herein, we introduce an entirely new class of polymer, utilizing elemental selenium as a polymeric feedstock. Its synthesis proceeds via the multicomponent reaction of elemental selenium, isocyanides, and amines to form selenoureas with 100% atom economy. This process is analogous to isoselenocyanate chemistry, whereby isocyanides are reacted with elemental black selenium powder to form isoselenocyanates.33,34 We demonstrate that this simple methodology proceeds rapidly under atmospheric conditions at ambient temperature in the presence of various amine monomers. Isoselenocyanates are generated in situ upon the reaction of the elemental selenium with the isocyanides, subsequently reacting with the amine moieties forming the selenourea linking motifs. Furthermore, we demonstrate that the unique spectroscopic fingerprint of selenium makes it possible to derive accurate structural data through high resolution mass spectrometric techniques, infrared spectroscopy (IR), and X-ray photoelectron spectroscopy (XPS). These techniques, in conjunction with NMR analysis by diffusion ordered spectroscopy (DOSY), provide a detailed image of the

structure and size when traditional techniques, such as size exclusion chromatography (SEC), are not possible. Starting with commercially available amines and simple alkyl isocyanides, we demonstrate that a library of polyselenoureas can be generated. To demonstrate the general efficacy of our approach, we selected branched and asymmetric monomers to facilitate solubility (refer to Scheme 1) and carried out our reactions in scintillation vials at ambient temperature for 18 h. All monomer ratios for diisocyanides/amines/selenium were 1:1:2.05, respectively. We found that the polymerizations at ambient temperature in dichloromethane (DCM) with no base catalyst were often successful, however, 10 mol % triethylamine (TEA) provided the most consistently reproducible results. MCPs were performed at high concentration (2 M) in order to minimize cyclization of the growing oligomers. After vacuum drying, the resulting polymers are readily resoluble in dimethylformamide (DMF), dimethylacetamide (DMAc), and dimethyl sulfoxide (DMSO). Classical means of molecular weight characterization via SEC proved difficult, caused by a specific property of polyselenoureas. When dissolved in 0.8 wt % LiBr DMAc solvent (for SEC analysis) a fine, black precipitate appeared over the course of 10 min, indicating that elemental selenium was precipitating from the polymer backbone and THF as an alternative SEC solvent could not remedy the situation due to nonsolubility.35 To overcome this analytical challenge, we turned to electrospray ionization mass spectrometry (ESI-MS), 2D DOSY NMR, IR, and XPS to derive accurate size and structural information on our polyselenourea-based materials. Despite lacking the ability to generate relative molecular weight data via SEC, we were able to measure P1-P6 via 2D DOSY NMR (Figures S10−S15) in order to determine a diffusion coefficient that, through the Stokes−Einstein equation, can be translated to a hydrodynamic diameter (DH, Table S1). The log of the hydrodynamic diameters have been shown to correlate linearly with the log of molecular weight.36 Ranging from approximately 4 to 16 nm, we demonstrate that our polyselenoureas are well above oligomeric molecular weight and within the polymeric size regime. When compared to a calibration curve using monodisperse poly(methyl methacrylate) (PMMA) standards, the polyselenoureas are estimated to be between 6.5 and 99 kDa. It is important to note that just as in single detector size exclusion chromatography (SEC), the number-average molecular weight is dependent on the selected polymer standard. Subsequently, we turned to high-resolution mass spectrometry, a powerful tool for absolute characterization of polymer 899

DOI: 10.1021/acsmacrolett.8b00428 ACS Macro Lett. 2018, 7, 898−903

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ACS Macro Letters

Figure 1. Analysis of polymer P2. (A) A region of the positive ion ESI-MS spectrum showing 3 polymer repeat units. (B) Theoretical isotopic pattern of one repeat unit of P2 compared to the experimentally observed isotopic pattern (resolving power was 50000 at this mass-to-charge ratio). (C) High resolution XPS of the Se 3d region with peak deconvolution demonstrating three Se 3d species within the polymer. (D) IR spectrum of P2 demonstrating the diagnostic N−H, C−H, CSe, and C−O bands.

repeat units and end groups.37,38 The sequence-regulated nature of the MCP (two selenium atoms for every repeat unit) and the broad distribution of naturally occurring selenium isotopes provides a unique molecular ion fingerprint (Figure S16) that can be read by high resolution ESI-MS in order to precisely identify the polymeric repeat units. It is important to note that the molecular weight observed in ESI-MS is strongly biased toward low molecular weight oligomers as the polyselenoureas are poorly soluble in solvents suitable for ESI-MS (see Supporting Information for details). As a representative example, the overview spectrum of P2 is depicted in Figure 1A. In the case of P2, the predominant end group entails one isocyanide and one amine (refer to the structure above Figure 1). The agreement between the theoretical mass of the repeat unit (516.1118 Da) and the experimentally observed spacing of the singly charged ions is excellent, with a mass error below 4 ppm. Furthermore, in Figure 1B, the theoretical isotopic pattern simulation (at a mass resolving power of 50000) of a P2 oligomer (n = 2) provides an excellent match with the experimental spectrum. Further evidence for the repeat unit structure is provided by

tandem mass spectrometry (MS/MS). Mass selection and collision-induced dissociation (CID) of the [M + H]+ and [M + 2H] 2+ ions from each oligomer yields diagnostic isoselenocyanate and amine product ions arising from cleavage of the selenourea C−N bonds. This is illustrated in Figure 2, depicting the CID spectrum of the [M + 2H]2+ precursor ion of P4 at m/z 883.3. Given the basicity of the terminal amines of the P4 structure, it is most probable that the two charges are localized on the α and ω amine end groups. This is consistent with the formation of predominantly singly charged product ions (some with higher m/z values than that of the precursor ions) upon CID. All product ions present in the spectrum could be assigned based on single-point heterolytic cleavages within the chain and provide strong evidence for the structure proposed in Figure 2. Possible dissociation mechanisms, consistent with literature precedent for amide cleavages, are provided in the Supporting Information (Scheme S1) and are found to be reproducible across the library of polyselenoureas (refer to Figures S23−S27). The repeat units and end groups of polymers P1−P6 were also characterized via ESI-MS and MS/MS (refer to Figures S17−S22). High resolution ESI-MS 900

DOI: 10.1021/acsmacrolett.8b00428 ACS Macro Lett. 2018, 7, 898−903

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ACS Macro Letters

proton signals were observed as well as the methyl, ethyl, or polyethylene glycol-like protons, depending on the monomer structure. In summary, we introduce the synthesis of polyselenoureas, resting on a simple MCP utilizing black elemental selenium, commercially available amines, and simple isocyanides. The synthesis is carried out at ambient temperature and under atmospheric conditions. Due to the unique characteristics of selenium, accurate size and structural information was obtained, even when classical techniques such as SEC were not applicable. Critically, high resolution ESI-MS and ESI-MS/ MS were utilized to precisely identify the unique molecular ion fingerprint of the selenium within the polymer backbone. XPS was exploited to arrive at accurate atomic compositions of spin coated polyselenourea thin films, while selenourea linking motifs could also be observed via FTIR. For accurate size information, 2D DOSY NMR demonstrated that DH values in the polymeric size regime were observed. We submit that our simple MCP based on elemental selenium for selenium incorporation directly into each repeat unit of a polymer chain is the to-date most powerful approach to selenium containing macromolecules, opening a synthetic access route to a new materials class with potential applications in biomaterials and sensing as well as materials for advanced imaging applications.

Figure 2. ESI-MS/MS of the [M + 2H]2+ ion of P4. The chemical structure of the precursor ion is shown indicating the likely location of the charges. Vertical lines designate the C−N bonds cleaved upon CID giving rise to product ions indicated by the m/z values in red.

provides evidence for the formation of cyclic oligomers in modest abundance, as well as carbodiimide linking motifs due to the oxidative loss of selenium in aged samples.39 ESI-MS thus confirmed the observations regarding the shelf life of the polymers. After being left on the benchtop and exposed to air over the course of about 3 weeks, the polymers gradually turn red, indicating the precipitation of elemental selenium. One drawback to ESI-MS characterization of step-growth polymers is that it is difficult to distinguish isomeric repeat units. Therefore, further characterization is required to elucidate the structures of P4, P1, and P6, from their structural isomers (P2, P3, and P5, respectively). Here, classical means of macromolecular characterization, such as 1H NMR, IR spectroscopy, and XPS are critical in fully elucidating the internal chemical structure of these polymers (refer to Figure 1C,D). Qualitative structural analysis via FTIR clearly demonstrates the appearance of both the N−H as well as the C = Se bands (∼3200 and 1550 cm−1, respectively) for polymers P1−P6. Next, the unique X-ray cross section of the selenium nucleus was exploited in atomic compositional analysis of P1−P6 via wide angle as well as high resolution XPS. Polymers were spin-coated and subjected to a wide angle scan, where not only the C 1s, N 1s, and, when applicable, the O 1s signals can be identified, but critically the 3s, 3p, and 3d signals of the selenium. Based on XPS, the atomic composition of each polymer was determined (Figures S34−S39), in agreement with the expected values. A high resolution scan of the Se 3d region clearly identifies the C−Se binding energy (53 eV). Peak deconvolution identified three species, namely the selenone and selenol tautomers, as well as a small peak corresponding to oxidation of the selenium to selenic acid. There is still some debate in the literature as to the exact nature of the tautomerization of selenourea moieties, however, the general consensus is that due to the large π-delocalization in the N−CSe system, as well as the substantial rotational barrier between the N−C bond, an equilibrium between the selenone and the selenol/zwitterionic moiety exists approximately 1:1.40−42 The entire library of Se-containing polymers P1−P6, were additionally analyzed via 1H NMR (Figures S3− S8), where the expected broad pronounced urea-like N−H

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.8b00428. Monomer synthesis, polymerization procedures, and additional experimental data (PDF).

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AUTHOR INFORMATION

Corresponding Authors

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

Bryan T. Tuten: 0000-0002-5419-7561 Stephen J. Blanksby: 0000-0002-8560-756X Christopher Barner-Kowollik: 0000-0002-6745-0570 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS C.B.-K. acknowledges continued key funding from the Queensland University of Technology (QUT) as well the Australian Research Council (ARC) in the form of a Laureate Fellowship. Additional support by the Karlsruhe Institute of Technology (KIT) is gratefully acknowledged. This work was enabled by use of the Central Analytical Research Facility hosted by the Institute for Future Environments at QUT and cofunded by the Science and Engineering Faculty.

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REFERENCES

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