Note pubs.acs.org/Macromolecules
Poly(2-imino-4-oxazolidinone)s via the Condensation of Diamidocarbenes with Bis(isocyanate)s Young-Gi Lee,† Songsu Kang,‡ Jonathan P. Moerdyk,§ and Christopher W. Bielawski*,∥,⊥ †
Department of Electronic Materials, Samsung SDI Company Ltd., Suwon 443-803, Republic of Korea Department of Chemistry, University of Texas at Austin, 1 University Station, A5300, Austin, Texas 78712, United States § Seton Hill University, One Seton Hill Drive, Greensburg, Pennsylvania 15601, United States ∥ Center for Multidimensional Carbon Materials (CMCM), Institute for Basic Science (IBS), Ulsan 689-798, Republic of Korea ⊥ Department of Chemistry and Department of Energy Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan 689-798, Republic of Korea ‡
S Supporting Information *
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INTRODUCTION Polycyclic polymers continue to attract interest as high performance materials on account of their high degrees of chemical and thermal stabilities.1,2 In particular, spiro- and other ladder-like polymers comprised of bicyclic repeat units wherein the rings connect through a common atom have received increasing attention due to their restricted rotational freedom and utility in forming thermally robust materials.3,4 In some cases (e.g., the poly(spirobifluorene)s), the polycycles have been shown to adopt twisted helical structures that effectively prevented aggregation and enhanced their optical properties.5−12 Unfortunately, the synthesis of spiro-polycycles is challenging due to the need for an efficient polymerization reaction that forms cyclic linkages between the repeating units.12 Toward this end, we recently demonstrated that readily available N,N′diamidocarbenes (DACs)13−15 rapidly and selectively afford 2imino-4-oxazolidinones16 upon exposure to 2 equiv of an isocyanate. The reactivity was remarkable as most NHCs catalyze the cyclotrimerization of isocyanates to isocyanurates rather than undergoing cycloaddition chemistry.17 We envisioned condensing DACs with bifunctional isocyanates to access new classes of polycycles with functionally rich heterocyclic cores. Herein we report the synthesis and study of novel polyspiro(iminooxazolidinone)s via the formal [2 + 2 + 1] cycloaddition of a stable DAC with various diisocyanates. The chemistry presented herein overcomes many of the challenges associated with the synthesis of stable spirocenters in polymeric materials and provides a unique approach to rapidly access a new structural class of polyheterocycles.
additional 1 h at room temperature. The product of this reaction (2) was subsequently isolated in high yield (98%) as a white solid by recrystallization from benzene/pentane. Characterization using 1H and 13C NMR spectroscopy, high-resolution mass spectrometry, and elemental analysis supported the formation of a 2:1 isocyanate/DAC adduct. The structure of this product was unambiguously elucidated using single crystal X-ray diffraction (XRD) analysis (Figure 1, left), which confirmed the formation of the oxazolidinone heterocycle 2, rather than the isomeric hydantoin 3.16 We postulated that the formation of 2 originated from the generation of a carbimidate intermediate, 1′. To test this hypothesis, one equivalent of phenylisocyanate was combined with 1 in C6H6, which resulted in the nearly instantaneous formation of a purple solution. Recrystallization from benzene/ pentane afforded pure, crystalline 1′ (86%) as evidenced by NMR spectroscopy (CDCl3), mass spectrometry and an XRD analysis (Figure 1, right).18 In support of the intermediacy of 1′ in the formation of the aforementioned cycloadduct, adding an additional equivalent of phenylisocyanate to 1′ cleanly afforded 2. Thus, the cycloaddition to afford 2 likely involved a two-step process: (i) carbimidate formation via nucleophilic attack of DAC 1 on the phenylisocyanate followed by (ii) a [3 + 2] cycloaddition of the carbimidate 1′ with a second equivalent of phenylisocyanate to form the 2-imino-4-oxazolidinone 2. Building on the above-mentioned results, focus shifted toward the preparation of the corresponding polymeric heterocycles. Table 1 summarizes a series of experiments aimed at optimizing the reaction of the DAC with MDI (4) to form high molecular weight polymer. At 20 °C, the polymerization of 1 and 4 ([1]0 = [4]0 = 0.2 M) in benzene afforded the poly(2-imino-4oxazolidinone) 8 which was found to exhibit a moderate number-average molecular weight (Mn = 9100 Da, entry 1) and a relatively broad polydispersity index (Đ = 1.54) consistent with a stepwise polymerization reaction. In general, increasing the reaction temperature and/or concentration afforded higher molecular weight polymers and increased yields, although the use of initial monomer concentrations that exceeded 0.4 M
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RESULTS AND DISCUSSION Previously, we demonstrated that an oxazolidinone forms upon treatment of DAC 1 with two equivalents of 3-nitrophenylisocyanate. In order to use this chemistry to form high molecular weight polymers, we anticipated that less polarized and more solubilizing diisocyanates, such as methylene diphenyl diisocyanate (MDI), would be advantageous. Thus, initial experiments focused on probing the reactivity and product selectivity of 1 with phenylisocyanate, a nonpolymerizable model. As summarized in Scheme 1, adding two equivalents of phenylisocyanate to a solution of 1 in benzene resulted in the initial formation of a purple solution that became colorless after stirring for an © 2015 American Chemical Society
Received: September 2, 2015 Revised: November 7, 2015 Published: December 4, 2015 9081
DOI: 10.1021/acs.macromol.5b01930 Macromolecules 2015, 48, 9081−9084
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Scheme 1. Treatment of 1 with 2 equiv of Phenylisocyanate Resulted in the Spiro 2-Imino-4-oxazolidinone 2, Rather Than the Hydantoin 3
Figure 1. (Left) ORTEP diagram of 2 with thermal ellipsoids drawn at 50% probability and H atoms omitted for clarity. Selected distances (Å) and angles (deg): C1−C25, 1.536(3); C25−03, 1.198(2); C25−N3, 1.373(2); N3−C26, 1.389(2); C26−O4, 1.370(2); O4−C1, 1.477(2); C1−C25−N3, 106.13(16); C25−N3−C26, 112.37(15); N3−C26−O4, 108.44(15); C26−O4−C1, 110.47(14); O4−C1−C25, 102.32(14). (Right) ORTEP diagram of 1′ with thermal ellipsoids drawn at 50% probability and H atoms omitted for clarity. Selected distances (Å) and angles (deg): C1−C25, 1.526(5); C25−O3, 1.252(4); C25−N3, 1.286(4); C1−C25−O3, 111.4(3); C1−C25−N3, 112.0(3); N3−C25−O3, 136.6(3).
Table 1. Effects of Concentration and Temperature on the Polymerization of DAC 1 and Diisocyanate 4.a
entry
[4]0 (M)
temp (°C)
yield (%)b
Mnc
Đc
1 2 3 4 5 6 7
0.20 0.20 0.10 0.20 0.25 0.40 0.50
20 40 60 60 60 60 60
68 78 72 89 98 84 71
9100 15400 10000 15500 41000 8600 4000
1.54 2.01 1.60 1.81 2.09 1.70 1.56
a
All polymerizations were performed in benzene for 24 h. bIsolated yield. cDetermined by gel permeation chromatography (GPC) using polystyrene standards (eluent: THF).
Analysis of the 1H NMR spectrum recorded for 8 revealed the presence of a signal attributed to a methylene linker at 3.87−3.96 ppm as well as the expected upfield shift of the aromatic protons to 7.12 and 6.69 ppm (cf., the analogous signals displayed by 2 were recorded at 7.36 and 6.73 ppm in CDCl3) (Figure 2). Additionally, the FT-IR spectra recorded for 2 and 8 revealed
afforded polymers with significantly lower molecular weights, presumably due to the limited solubility of the monomer. Under optimized conditions (entry 5: 60 °C, [1]0 = [4]0 = 0.25 M), a relatively high molecular weight polymer (Mn = 41000 Da) was produced and isolated in 98% yield. 9082
DOI: 10.1021/acs.macromol.5b01930 Macromolecules 2015, 48, 9081−9084
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Figure 2. 1H NMR spectra of 2 (top) and poly(2-imino-4-oxazolidinone) 8 (bottom).
signals at 1689 and 1690 cm−1, respectively, which were consistent with a CN vibration mode; similarly, broad signals were measured at 1709 and 1700 cm−1 and attributed to the C O vibration mode of the amide groups of 8. Collectively, these data along with 13C NMR spectral data (see ESI) supported the formation of a poly(iminooxazolidinone). A variety of poly(2-imino-4-oxazolidinone)s (i.e., 9 − 11) were obtained by exchanging MDI for various aryl or alkyl diisocyanates, as determined by GPC as well as 1H and 13C NMR spectroscopy (see Scheme 2 and Table 2). The formation of 9 from toluene-2,4-diisocyanate (5) and 1 proceeded to a relatively high molecular weight (Mn = 11000 Da, entry 2); however, 10 exhibited a relatively low molecular weight (Mn = 3400 Da, entry 3), likely due to reduced solubility as precipitation was observed over the course of the polymerization. Similarly, combining hexamethylene-1,6-diisocyanate 7 and 1 afforded a poly(2imino-4-oxazolidinone) 11 albeit with a modest molecular weight (Mn = 5200 g/mol) and yield (53%). The thermal properties of 8−11 were studied via differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) (Table 2). Using DSC, the glass transition temperature (Tg) ranged from 46 to 91 °C, depending on the structure of the diisocyanate monomers. The decomposition temperatures (Td5) measured for the aforementioned polymers were also dependent on structure and varied from 201 to 265 °C.19,20
Scheme 2. Synthesis of Various Poly(2-imino-4oxazolidinone)s
cycloaddition of an N,N′-diamidocarbene with various diisocyanates. The resultant poly(2-imino-4-oxazolidinone)s were obtained in good yields with moderate to high molecular weights and successfully cast into transparent films. Small molecule model studies with phenylisocyanate indicated that the polymerization proceeded via a step-growth polymerization wherein the formation of the iminooxazolidinone core proceeded through a carbimidate intermediate. Beyond establishing a new approach for the synthesis of thermally stable polyspiroheterocycles and carbene-derived polymers, the method is straightforward and
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CONCLUSION In sum, a new method for the synthesis of thermally robust polymeric heterocycles was achieved via the formal [2 + 2 + 1] 9083
DOI: 10.1021/acs.macromol.5b01930 Macromolecules 2015, 48, 9081−9084
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(13) Hudnall, T. W.; Bielawski, C. W. J. Am. Chem. Soc. 2009, 131, 16039−16041. (14) Moerdyk, J. P.; Bielawski, C. W. Nat. Chem. 2012, 4, 275−280. (15) César, V.; Lugan, N.; Lavigne, G. Eur. J. Inorg. Chem. 2010, 2010, 361−365. (16) Lee, Y.-G.; Moerdyk, J. P.; Bielawski, C. W. J. Phys. Org. Chem. 2012, 25, 1027−1032. (17) Duong, H. A.; Cross, M. J.; Louie, J. Org. Lett. 2004, 6, 4679− 4681. (18) Treating 1′ with excess sulfur (10 equiv) in C7H8 at 23 °C for 30 min afforded the thiourea derivative of DAC 1 in quantitative yield. The result indicated that the initial coupling reaction of the DAC and phenylisocyanate may be reversible. (19) Transparent, freestanding films were obtained through the evaporation of concentrated solutions of the aforementioned polymers onto Teflon-coated surfaces. Unfortunately, these films were too brittle for dynamic mechanical analysis. (20) For examples of related poly(1,2,4-oxadiazolidin-5-one)s, see: (a) Dickmeis, M.; Cinar, H.; Ritter, H. Angew. Chem., Int. Ed. 2012, 51, 3957−3959. (b) Dickmeis, M.; Cinar, H.; Ritter, H. Macromolecules 2012, 45, 3285−3288.
Table 2. Molecular Weights and Thermal Properties of Various Poly(2-imino-4-oxazolidinones)a entry
polymer
Mn (kDa)b
Đb
yield (%)c
Tg (°C)d
Td (°C)e
1 2 3 4
8 9 10 11
41000 11000 3400 5200
2.09 1.54 1.48 1.68
98 69 82 53
91 87 50 46
218 211 201 265
a
All polymerizations were performed in benzene ([monomer]0 = 0.25 M) at 60 °C for 24 h. bDetermined by GPC and reported as a polystyrene equivalent (eluent: THF). cIsolated yield. dObtained at a heating rate of 10 °C·min−1 under N2 (flow rate = 100 mL·min−1). e Obtained at a heating rate of 10 °C·min−1 under N2 (flow rate = 100 mL·min−1).
may be generally applied to access a variety of other macromolecules through the use of other cycloaddition partners (e.g., alkynes, isothiocyanates, etc.)
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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/acs.macromol.5b01930. Detailed experimental procedures and additional characterization details (PDF) Crystallographic file (CIF)
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AUTHOR INFORMATION
Corresponding Author
*(C.W.B.) E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We acknowledge the Institute for Basic Science (IBS-R019-D1), BK21 Plus Program as funded by the Ministry of Education and the National Research Foundation of Korea, the National Science Foundation (NSF-ECCS-1120823) and the U.S. Army Research Office (W911NF-09-1-0446) for their support.
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REFERENCES
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DOI: 10.1021/acs.macromol.5b01930 Macromolecules 2015, 48, 9081−9084
