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Polymer Valence Isomerism: Poly(Dewar‑o‑xylylene)s Rong Zhu and Timothy M. Swager* Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States S Supporting Information *
ABSTRACT: Poly(o-xylylene) (POX) has long been a challenging synthetic target despite its simple structure and potentially useful physical properties. In this report, we demonstrate a valence isomer strategy that leads to the formation of high molecular weight POX via an intermediate polymer of a unique structure, namely poly(Dewar-o-xylylene) (PDOX). We show that the free radical polymerization of highly strained Dewar-o-xylylene (DOX) monomer afforded PDOX, a material with a high density of Dewar benzene units in the backbone through ring-retaining propagation. The thermal- and photoinduced isomerizations of PDOX to produce POX were investigated. This chemistry yields POXs that are difficult to obtain using traditional methods. Moreover, it also provides a potential entry into new reconfigurable materials featuring highly efficient postpolymerization main chain structural transformations.
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INTRODUCTION Polymers containing arylene structures in the main chain constitute a class of materials of great academic interest as well as industrial importance. Conjugated polyarylenes and poly(arylenevinylene)s are widely employed in optoelectronic devices,1 while nonconjugated main-chain arylene-polymers are especially relevant as engineering plastics and coatings.2 A prominent example of the latter is poly(p-xylylene)s, which display both low gas permeability and low dielectric constants. These polymers are derived from free radical polymerization of in situ generated p-xylylenes (Scheme 1a).3 Highly efficient polymerization of halogen-substituted p-xylylenes can also be found in the classical Gilch synthesis of poly(phenylenevinylene)s.4 In contrast to the well-developed polymerization of pxylylene, the pursuit of poly(o-xylylene) (POX, 1), derived from its isomer, o-xylylene, has proven challenging (Scheme
1b) as a result of facile formal [4 + 2] and [4 + 4] dimerization pathways that consume the highly reactive o-xylylene.5 Direct polymerization of the parent o-xylylene via thermolysis of benzocyclobutenes only affords short oligomers.6 Extensive efforts have been made in substituting benzocyclobutene with electron-donating and -withdrawing groups to lower the temperature required for the thermal retroelectrocyclization. Such functionalization has led to success in the preparation of alkoxy- or siloxy-substituted POXs of molecular weights ranging from 5 kDa to 11 kDa.7 Gilch conditions have also recently been applied to the synthesis of poly(ophenylenevinylene)s.8 In the 1960s, Errede discovered the dimerization of o-xylylene at low temperature to produce an unstable spiro-dimer. Upon warming this molecule autopolymerizes to afford POX, which is the first report on radical ringopening polymerization.9 However, the polymerization of electronically unmodified o-xylylenes remains largely unknown. Indeed, o-xylylenes dimerize so rapidly that they are frequently employed as highly efficient thermal cross-linking agents.10 The elusive efficient synthesis of POX has prevented the exploration of its physical properties and potential applications. Herein, we address this challenge using a valence isomer strategy, which leads not only high molecular weight POX, but also a new polymer with a nonaromatic isomeric precursor structure. We targeted the highly strained Dewar-o-xylylene (DOX, 2, Scheme 2a) as our monomer. Analogous to the well-known Dewar benzene (bicyclo[2.2.0]hexa-2,5-diene)/benzene isomerism, 2 is a Dewar-type valence isomer of o-xylylene.
Scheme 1. Challenging Synthesis of Poly(o-xylylene)
Received: January 29, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/jacs.8b01106 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
Article
Journal of the American Chemical Society Scheme 2. A Valence Isomer Strategy to Poly(o-xylylene)s and Poly(Dewar-o-xylylene)s (6)
proceeds through a highly unsymmetrical transition state where one of the bridgehead carbons has a developing radical character.17 At the UB3LYP/6-31G(d, p) level the difference in the reaction barriers for Pathways A and B (ΔΔ⧧GAB) was estimated to be ca. 0.4 kcal/mol at 298 K, suggesting comparable rates under the standard conditions. On the basis of these results, we hypothesized that the polymer structure could be tuned by simply varying the polymerization temperature and concentration, taking the advantage of the large difference in the entropy cost. We began our investigations by synthesizing tetraalkyl DOX 2b-d through a three-step sequence (Scheme 3). The Koster
Despite the added strain energy, 2 is kinetically stable, which has been previously established.11 The addition of tetraalkyl substitution to the parent DOX (2a) is expected to be responsible for further increasing its thermal stability (2b−2d). Free radical 1,4-polymerization of diene 2 can produce POX through ring-opening propagation via the Dewar-benzyl radical 3 (Pathway A). This process is expected to be thermodynamically driven by the strain release and aromaticity gained in intermediate benzyl radical 4. Polymerization of 2 also provides an opportunity en route to an even more interesting target polymer structure, namely poly(Dewar-o-xylylene) (PDOX, 6), via ring-retaining propagation (Pathway B). PDOX is a new polymer composed of Dewar benzene units stitched together by only ethylene linkages, thereby building up high energy density in the polymer backbone.12 In fact, the isomerization of PDOX to POX (1) is estimated to be exothermic by ∼0.27 kcal/g in the case of R = Et, which is over a quarter of the energy density of the well-known explosive TNT (∼1 kcal/g).13 On the other hand, the high reaction barrier as a result of the forbidden symmetry of the PDOX to POX isomerization provides kinetic stability. It is noteworthy that the transformation from PDOX to POX will be accompanied by a dramatic change in the backbone conformation and main chain rigidification. The resulting sharp contrast in the physical properties could be useful for the development of new reconfigurable materials. Electron transfer14 and energy transfer15 are also known to trigger the aromatization of Dewar benzenes. The chain reaction nature and electronic structure changes of these reactions has led to interest in developing Dewar benzenecontaining organic materials for imaging and optical data storage.16 In this context, the photoinduced isomerization process of PDOX could be interesting to explore.
Scheme 3. Monomer Synthesisa
Conditions: (a) AlCl3, CH2Cl2, −15 °C; then DMSO −15 °C. Yields: 12b, 92%; 12c, 49%; 12d, 45%. (b) LiAlH4, THF, 0 °C; TsCl, py. 0 °C to r.t. (c) KOtBu, DMSO, r.t. Yields over two steps: 2b, 40%; 2c, 38%; 2d, 42%.
a
procedure was carried out using dimethyl fumarate to intercept the in situ formed tetraalkylcyclobutadienes, furnishing bicyclo[2.2.0]hexene derivatives 12b−d.18 Following reduction, tosylation, and final elimination, 2b-d were obtained at gram scale. These compounds were not observed to undergo detectable decomposition in the neat form at 0 °C under an inert atmosphere for >4 weeks, and they were stable in degassed hexane solutions indefinitely at ambient temperature. With the monomers in hand, we set out to test our hypothesis by subjecting DOX 2b to a variety of different radical polymerization conditions (Table 1). In all cases, 1,4addition was observed exclusively. AIBN-initiated free-radical polymerization was carried out neat at 80 °C (entry 1). Nearly full conversion of 2b was observed within 20 h, and a white
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RESULTS AND DISCUSSIONS To assess the competing radical reaction Pathways A and B, we performed preliminary computational analysis on a model system in which the isomerization of Dewar benzyl radical 7 and its addition to diene 9 were studied (Scheme 2b). The conversion of 7 into the corresponding benzyl radical 8 likely B
DOI: 10.1021/jacs.8b01106 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
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Journal of the American Chemical Society Table 1. Evaluation of Reaction Conditions for the Free Radical Homopolymerization of 2
entry
monomer
initiator
T (°C)
t (h)
conv. (%)a
yield (%)b
Mn (kg/mol)c
Mw/Mnc
xa
1 2d 3 4 5 6 7 8 9 10
2b 2b 2b 2b 2b 2b 2b 2b 2c 2d
AIBN (1 wt %) AIBN (1 wt %) AIBN (1 wt %) V-65 (2 wt %) none BAPO (1 wt %)/UV BAPO (10 wt %)/UV BAPO (4 wt%)/UV V-65 (2 wt %) V-65 (2 wt %)
80 80 55 45 80 25 25 − 15 35 45
20 40 64 24 40 20 20 16 136 24
94 >95 52 58 87 29 68 n.a. n.a. n.a.
52 49 30 45 30 25 64 41 47 46
15.4 5.1 42.3 10.7 12.7 5.7 3.4 4.1 80.7 41.8
2.3 1.4 2.0 1.9 3.9 1.5 1.4 1.4 2.1 1.6
0.30 0.15 0.56 0.70 0.16 0.78 0.82 0.95 0.76 0.74
a
Determined by 1H NMR spectroscopy. bYield of purified polymer. cDetermined by size-exclusion chromatography (SEC) in tetrahydrofuran calibrated with polystyrene standard. dIn toluene (ca. 0.7 M). V-65 = 2,2′-azobis(2,4-dimethylvaleronitrile). BAPO = phenylbis(2,4,6trimethylbenzoyl)phosphine oxide. UV wavelength: 365 nm.
solid Poly-2b with a molecular weight of over 15 kDa was isolated after precipitation in methanol. This polymer was readily soluble in CH2Cl2, CHCl3, and tetrahydrofuran. 1H NMR spectroscopic analysis provided an estimation that 30% of the repeating units remained as Dewar benzenes with the balance as ring-opened units. The presence of both structures is consistent with our computational analysis. The polymerization also produces a significant amount of dimer (∼10% by 1H NMR analysis), indicating that thermal ring opening of 2b occurs to afford the corresponding o-xylylene at this temperature. As expected, the percentage of Dewar benzene repeating units decreases with lower monomer concentration (x = 0.15, entry 2) and increases when the polymerization is conducted at lower temperature (x = 0.56 and 0.70, entries 3 and 4). In addition, a higher molecular weight (>42 kDa) was achieved (entry 3). In the absence of an initiator, autopolymerization occurred at 80 °C (entry 5), affording a polymer with broad molecular weight distribution (Mw/Mn ∼ 4). o-Xylylene formation is likely the basis of the initiation.19 We next attempted to maximize the Dewar benzene content of our polymers by running the polymerizations at lower temperatures. Photoinitiated polymerizations were performed at ambient and reduced temperatures (entries 6−8). Under these conditions, o-xylylenes are not produced and no dimers were detectable in the crude reaction mixtures. Thus, any aromatic units in the product are not derived from o-xylylene addition, but the ring opening of the radical in propagation steps of 2b. Low conversion and low molecular weights (ca. 4− 5 kDa) result as a consequence of the slower propagation and increased viscosity. Increasing initiator loading improved the conversion at the expense of a slightly lower molecular weight because of the higher radical concentration (entry 7). Nonetheless, the Dewar benzene content in the product increased steadily as the system was cooled, reaching over 95% in the case of the reaction performed at −15 °C (entry 8). In addition, the n-propyl and n-butyl analogues Poly-2c and Poly2d were synthesized from corresponding monomers respectively (entries 9 and 10), with molecular weights of up to 81 kDa.
The structures of PDOX polymers Poly-2 were confirmed by H and 13C NMR spectroscopic analysis. The spectra of Poly2b containing 95% Dewar benzene units are shown as an example (Figure 1). The main-chain methylene groups (e)
1
Figure 1. 1H and Poly-2b′ (b/d). C
13
C NMR spectra overlay of Poly-2b (a/c) and DOI: 10.1021/jacs.8b01106 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
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Journal of the American Chemical Society
activation enthalpy of ∼34 kcal/mol, which is in good agreement with the literature values for hexamethyl Dewar benzene isomerization (∼36 kcal/mol).13 These polymers are indeed energy-rich as demonstrated by the reaction enthalpies obtained (>0.1 kcal/g). Assuming the isomerization heat of the Dewar benzene unit in Poly-2 is similar to that of hexamethyl Dewar benzene (−56 kcal/mol), the thermochemistry-based Dewar benzene contents (x′) for Poly-2b−d were calculated respectively, and were found to be consistent with those assigned by 1H NMR analysis. Poly-2 possesses relatively low glass transition temperatures (Poly-2b, 45 °C; Poly-2c, 33 °C; Poly-2d, 1 °C) despite the bicyclic structures present in the main chain (Table 2).20 This is attributable to the flexible alkyl side chains and unsaturated backbone. The Dewar benzene bridgehead substituents are pointing away from the main chain methylene groups, attenuating the steric hindrance and likely contribute to lowering Tg values. This was contrasted by an elevation (>60 °C) in T g observed for Poly-2′. The planar bulky hexasubstituted benzene structure presumably adds a high barrier for segmental motion, affording a much more rigid main chain. A significant loss in solubility was also noticed. Poly-2′ possesses good thermal stability, as demonstrated in Table 2 with decomposition temperatures (Td) > 350 °C measured by thermogravimetric analysis (TGA) (Figures S5−S7). The thermal conversion of PDOX to POX is quantitative as determined by NMR analysis. The spectra of thermally produced Poly-2b′ without any purification was compared to those of Poly-2b in Figure 1. Only two proton signals corresponding to the methylene (2.0−3.0 ppm) and methyl groups (0.5−1.2 ppm) are present (Figure 1b). The alkene and bridgehead carbon signals completely disappear, and aromatic
display a broad signal between 2.3 and 3.0 ppm and the side chains give sharper peaks (a−d) (Figure 1a). The bridgehead carbon signal (f) is diagnostic of a Dewar benzene structure (Figure 1c). Both the 1H and 13C spectra indicate the presence of a minimal amount of aromatic structures. Thermal characterizations of PDOXs were then carried out to test our hypothesis about their thermal isomerization (Table 2). Differential scanning calorimetry (DSC) curves of Poly-2 all Table 2. Thermal Properties of Poly-2a
Poly2b Poly2c Poly2d
xb
Tg (°C)
ΔHiso (kcal/g)
x′c
0.70
45
− 0.178
0.69
0.76
33
− 0.154
0.75
0.74
1
− 0.112
0.66
Poly2b′ Poly2c′ Poly2d′
Tg (°C)
Td, 5% (°C)d
117
414
94
390
61
354
a
Glass transition temperatures (Tg) and isomerization enthalpies (ΔHiso) were determined by DSC. bEstimated by 1H NMR spectroscopy. cEstimated by isomerization enthalpy. dDetermined by TGA.
revealed a single large exothermic peak with the onset at 153 °C and maximum at 182 °C, corresponding to the isomerization to fully aromatized POX polymers Poly-2′ (Figures S5− S7). Eyring plots confirmed the first-order kinetics with an
Figure 2. Photoredox-induced isomerization of Poly-2c. (a) Reaction conditions: (1) thermal reaction; (2) photoisomerization in solution; (3) photoisomerization in film. (b) Overlay of the 1H NMR spectra of Poly-2c and the crude materials obtained from reactions (1)−(3). (c) SEC traces (in tetrahydrofuran) of Poly-2c and Poly-2c′ obtained from reactions (1) and (2). D
DOI: 10.1021/jacs.8b01106 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
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Journal of the American Chemical Society Scheme 4. Synthesis of a Random Copolymer
retaining and ring-opening propagations were shown to be tunable to produce Dewar benzene or benzene structures. We have studied the isomerization reactions of these polymers triggered by either thermal or photoredox processes. These reactions quantitatively produced high molecular weight POXs, which has long been a challenging synthetic target because of the limitations of the traditional o-xylylene polymerization chemistry. Moreover, the isomerization involves main chain aromatization, which leads to a sharp contrast in the physical properties during the transition. Such features are promising for developing new amorphous reconfigurable photoresponsive materials.
carbon signals emerge around 138 ppm (Figure 1d). The broadening of both the proton and carbon signals in Poly-2b′ when compared to the starting polymer, corroborate with the increase chain rigidity suggested by the increased Tg. ATRFTIR provided additional evidence for the structures of Poly2b and Poly-2b′ (Figure S10). The isomerization was indicated by the disappearance of the very weak alkene stretching band at 1670 cm−1 and blue shifts of the methylene stretching and scissoring bands. Next, photoredox-induced isomerization of Poly-2c was studied (Figure 2). Acridinium photo-oxidant 13 (5 wt %; excited state E1/2red* = 2.06 V vs SCE) was used for the blue light-mediated generation of hexaalkyl Dewar benzene radical cations (∼1.6 V vs SCE), thereby initiating the chain isomerization process.21 We monitored this reaction by 1H NMR in CDCl3 and observed full conversion within only 10 s at r.t. (Figure 2a, conditions (2)). The 1H NMR spectrum and SEC trace of the photoproduced Poly-2c′ were essentially identical to those of the material obtained via thermal process (Figure 2b,c, plots 1 and 2). The same reaction proceeded in solid state as well (conditions (3)). Exposing a thin-film of Poly-2c containing 13 to blue light triggers the isomerization, as determined by 1H NMR analysis (Figure 2b, plot 3). We noticed that the SEC traces of Poly-2c′, while still in the highmolecular-weight regime, shifted slightly compared to Poly-2c (Figure 2c) indicative of changes in the hydrodynamic volume. Similar observations were made for all Poly-2 synthesized. No major degradation under the isomerization conditions was detected by either NMR or TGA analysis. Copolymerization of DOX would allow convenient physical property tuning as well as chemical modifications of the original PDOX. Introducing Dewar benzene structures into functional polymers could also bring interesting thermal/photoresponsive behaviors. To this end, random copolymerization of 2c with pbromostyrene (14) was demonstrated (Scheme 4). In a typical experiment, a monomer feed of equal weights of 2c and 14 (molar ratio, 0.40:0.60) produced 15 in good yield. Copolymer 15 was estimated to contain 63% styrene unit by 1H NMR analysis. Thermochemical data indicated a Dewar benzene content of 34% (Figure S8). The ratio between ring-retaining and ring-opening structures was higher (∼10:1) than that was typically observed in the homopolymerization reactions performed at similar temperatures (Table 1). This could be attributable to the faster propagation of 14.
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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/jacs.8b01106. Experimental procedures, characterization and spectra data; DSC and TGA curves and SEC traces of the polymers; photos of the experimental setup of photoinduced isomerization; DFT calculation details (PDF)
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AUTHOR INFORMATION
Corresponding Author
*
[email protected] ORCID
Timothy M. Swager: 0000-0002-3577-0510 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors would like to acknowledge the financial support of the Army Research Office through the MIT Institute for Soldier Nanotechnology. The authors would like to thank Drs. M. B. Herbert (M.I.T.) and N. A. Romero (M.I.T.) for insightful discussions.
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REFERENCES
(1) (a) Pang, Yi. Poly(phenylnevinylenes). In Design and Synthesis of Conjugated Polymers; Leclerc, M., Morin, J. F., Eds.; Wiley-VCH Veriag GmbH & Co. KGaA: Weinheim, Germany, 2010; p 147. (b) Grimsdale, A. C.; Chan, K. L.; Martin, R. E.; Jokisz, P. G.; Holmes, A. B. Chem. Rev. 2009, 109, 897−1091. (2) Handbook of Thermoplastics, 2nd ed.; Olabisi, O., Adewale, K., Eds.; CRC Press: Boca Raton, FL, USA, 2016. (3) (a) Gorham, W. F. A. J. Polym. Sci., Part A-1: Polym. Chem. 1966, 4, 3027−3039. (b) Itoh, T. Prog. Polym. Sci. 2001, 26, 1019−1059. (4) Gilch, H. G.; Wheelwright, W. L. J. Polym. Sci., Part A-1: Polym. Chem. 1966, 4, 1337−1349.
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CONCLUSION We have synthesized a new class of high molecular weight (up to 81 kDa) PDOX polymers. Their unique structures feature a high density of Dewar benzene units in the polymer main chain. The synthesis was achieved by the free radical polymerization of highly strained DOXs, for which the ringE
DOI: 10.1021/jacs.8b01106 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
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Journal of the American Chemical Society (5) (a) Segura, J. L.; Martín, N. Chem. Rev. 1999, 99, 3199−3246. (b) Sisido, K.; Kusano, N.; Noyori, R.; Nozaki, Y.; Simosaka, M.; Nozaki, H. J. Polym. Sci., Part A: Gen. Pap. 1963, 1, 2101−2107. (6) Jensen, F. R.; Coleman, W. E.; Berlin, A. J. Tetrahedron Lett. 1962, 3, 15−18. (7) (a) Chino, K.; Takata, T.; Endo, T. Macromol. Rapid Commun. 1996, 17, 339−345. (b) Chino, K.; Takata, T.; Endo, T. Macromolecules 1997, 30, 6715−6720. (c) Chino, K.; Takata, T.; Endo, T. J. Polym. Sci., Part A: Polym. Chem. 1999, 37, 1555−1563. (d) Chino, K.; Endo, T. J. Polym. Sci., Part A: Polym. Chem. 2000, 38, 3434−3439. (8) Rittscher, V.; Kuch, S.; Rehahn, M. Macromol. Rapid Commun. 2016, 37, 814−819. (9) (a) Errede, L. A. J. Polym. Sci. 1961, 49, 253−265. (b) Errede, L. A. J. Am. Chem. Soc. 1961, 83, 949−954. (c) Iwatsuki, S. Polymerization of quinodimethane compounds. In Polymerization Reactions; Springer: Berlin, Heidelberg, 1984; pp 93−120. (10) Harth, E.; Horn, B. V.; Lee, V. Y.; Germack, D. S.; Gonzales, C. P.; Miller, R. D.; Hawker, C. J. J. Am. Chem. Soc. 2002, 124, 8653− 8660. (11) (a) Bauld, N. L.; Farr, F. J. Am. Chem. Soc. 1970, 92, 6695− 6696. (b) Freiermuth, B.; Wirz, J. Chimia 1985, 39, 22. (12) Swager, T. M.; Dougherty, D. A.; Grubbs, R. H. J. Am. Chem. Soc. 1988, 110, 2973−2974. (13) (a) Adam, W.; Chang, J. C. Int. J. Chem. Kinet. 1969, 1, 487− 492. (b) Weinheimer, R. Properties of selected high explosives. Proceedings of the 18th International Pyrotechnics Seminar, Grand Junction, CO., July 16−20, 2000. pp 13−17. (14) (a) Evans, T. R.; Wake, R. W.; Sifain, M. M. Tetrahedron Lett. 1973, 14, 701−704. (b) Jones, G.; Becker, W. G. J. Am. Chem. Soc. 1983, 105, 1276−1283. (15) (a) Turro, N. J.; Ramamurthy, V.; Katz, T. J. Nouveau J. Chim. 1977, 1, 363. (b) Merkel, P. B.; Roh, Y.; Dinnocenzo, J. P.; Robello, D. R.; Farid, S. J. Phys. Chem. A 2007, 111, 1188−1199. (c) Ferrar, L.; Mis, M.; Dinnocenzo, J. P.; Farid, S.; Merkel, P. B.; Robello, D. R. J. Org. Chem. 2008, 73, 5683−5692. (16) (a) Gillmore, J. G.; Neiser, J. D.; McManus, K. A.; Roh, Y.; Dombrowski, G. W.; Brown, T. G.; Dinnocenzo, J. P.; Farid, S.; Robello, D. R. Macromolecules 2005, 38, 7684−7694. (b) Mis, M. R.; Robello, D. R. Chem. Mater. 2012, 24, 1950−1953. (c) Khan, A.; Stucky, G. D.; Hawker, C. J. Adv. Mater. 2008, 20, 3937−3941. (d) Marsella, M. J.; Meyer, M. M.; Tham, F. S. Org. Lett. 2001, 3, 3847−3849. (e) Marsella, M. J.; Estassi, S.; Wang, L. S.; Yoon, K. Synlett 2004, 1, 192−194. (17) See Supporting Information for calculation details. (18) Koster, J. B.; Timmermans, G. J.; van Bekkum, H. Synthesis 1971, 139−140. (19) The high polydispersity could be attributable to the coupling of diradical species derived from the o-xylylene. (20) Luo, K.; Rzayev, J. Macromolecules 2009, 42, 9268−9274. (21) Fukuzumi, S.; Kotani, H.; Ohkubo, K.; Ogo, S.; Tkachenko, N. V.; Lemmetyinen, H. J. Am. Chem. Soc. 2004, 126, 1600−1601.
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DOI: 10.1021/jacs.8b01106 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
