Renewable Benzofuran Polymerization Initiated by Lewis Acid Al

Oct 26, 2017 - When a strong Lewis acid Al(C6F5)3 was used, PBFs with high molecular weights (Mn = 17.9 × 104) and narrow molecular weight distributi...
0 downloads 7 Views 3MB Size
Download PDF
Article Cite This: Macromolecules XXXX, XXX, XXX-XXX

pubs.acs.org/Macromolecules

Renewable Benzofuran Polymerization Initiated by Lewis Acid Al(C6F5)3 and Mechanism Fei Lin,†,‡ Meiyan Wang,§ and Dongmei Cui*,† †

State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China ‡ University of Chinese Academy of Sciences, Changchun Branch, Changchun 130022, China § Institute of Theoretical Chemistry, Jilin University, Changchun 130022, China S Supporting Information *

ABSTRACT: The polymerization of a nature resourced aromatic compound benzofuran (BF) encounters the problems of low activity and molecular weight. Herein we report BF polymerization initiated by Lewis acids, which showed significant dependence on the type of Lewis acids, reaction solvents, and temperature. When a strong Lewis acid Al(C6F5)3 was used, PBFs with high molecular weights (Mn = 17.9 × 104) and narrow molecular weight distributions (Mw/Mn = 1.56) were achieved for the first time. In addition, these PBFs are amorphous polymer possessing high glass-transition temperature (Tg = 210 °C), good thermal stability (Td,5% loss = 344 °C) as well as high transparency (transmittance >89%). The active species was isolated and characterized through NMR and X-ray diffraction analyses; meanwhile, the polymer chain ends were identified by MALDI−TOF MS and NMR studies, which allowed us to elucidate the mechanism for benzofuran polymerization.

■

INTRODUCTION Hydrocarbon polymers with cyclic repeating units arranged in a controlled manner along the polymer chains have unique properties, such as low dielectric constants, high thermal stability, and high transparency. Such polymers are synthesized through cyclopolymerization of nonconjugated dienes, such as 1,5-hexadiene,1 metathesis polymerization of bicyclic olefins, such as norbornene and dicyclopentadiene,2 and addition polymerization of cyclic alkenes, such as 3-methylencyclopentene,3 indene,4 and α-pinene5 (Chart 1), some of which have been commercialized by Mitsui and Ticona under the trade names of APEL and Topas, respectively.6,7 However, owing to lack of polar groups, these polymers have poor surface and adhesive properties and low affinity with dyes as well as noncompatibility with other polar polymers, which limit their

application areas. In addition, most of these polymers are synthesized by using complicated and expensive organometallic catalysts and monomers derived from petroleum resources. Polybenzofuran (PBF) containing cyclic rings in the main chains, meanwhile bearing heteroatom oxygen, can meet the requirements of the functional materials.8−10 Moreover, the monomer benzofuran (BF) is very abundant, which can be extracted from coal tar of the petroleum side product, or obtained by decarboxylative intramolecular C−O coupling of a biorenewable coumarin11 rich in many plants such as tonka bean, vanilla grass, and sweet woodruff etc. However, BF is an aromatic compound bearing a stable fused ring, which arouses predicament of polymerization and usually gives PBFs with low molecular weight. As early as 1961, Natta and his colleagues investigated BF polymerization by using EtAlCl2 as the catalyst precursor at extremely low temperatures (−80 to −100 °C) to obtain optically active PBF,12 but the molecular weight was not determined and the true catalytic species was not clear because of the harsh experiment conditions then. Over the following semicentury, some catalytic systems, such as SnC1 4 · CC13COOH13 and C6H5C(CH3)2Cl/SnCl4/

Chart 1. Structures of Cyclic Olefins

Received: September 6, 2017 Revised: October 19, 2017

© XXXX American Chemical Society

A

DOI: 10.1021/acs.macromol.7b01928 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules Table 1. Polymerization of BF Catalyzed by Lewis Acids

entrya 1 2 3 4 5 6 7 8 9 10 11

cat.

[M]/[Cat.]

T (°C)

solvent

t (h)

convn (%)

activityb

Mnc (×104)

Mw/Mnc

Al Bu3 Al(OiPr)3 AlCl3 B(C6F5)3 Al(C6F5)3·0.5TOL Al(C6F5)3·0.5TOL Al(C6F5)3·0.5TOL Al(C6F5)3·0.5TOL Al(C6F5)3·0.5TOL Al(C6F5)3·0.5TOL Al(C6F5)3·0.5TOL

500 500 500 500 500 500 500 500 1000 1000 1000

20 20 20 20 20 20 20 20 20 20 0

toluene toluene toluene toluene toluene hexane C6H5Cl C6H5Cl C6H5Cl C6H5Cl C6H5Cl

24 24 24 24 8 48 8 32 8 96 32

0 0 33.8 0 56.2 4.0 67.5 80.7 58.4 66.1 58.1

0 0 0.83 0 4.14 0.05 4.98 − 8.61 − 2.14

− − 5.2 − 22.5 16.7 5.4 8.9 10.1 11.8 17.9

− − 7.70 − 3.02 2.30 1.77 2.32 2.32 2.42 1.56

i

a

Polymerization conditions: entry 1 for example, BF (2.36g, 20 mmol), [BF]/[AliBu3] = 500:1 (mol/mol), toluene (6 mL). bGiven in kg of polymer/(molAl·h). cDetermined by GPC in 1,2,4-trichlorobenzene at 150 °C against a polystyrene standard.

Figure 1. 1H NMR and 13C NMR spectra of PBF (Table 1, entry 9).

low temperature (Tp = −70 °C) with low activity and initiation efficiency (I* = 7−9%).15 Therefore, to develop new catalytic systems to obtain stable active species and achieve a more controlled BF polymerization has been a problem to be solved in polymer science. Lewis acid catalysts in modern organic synthesis have grown by leaps and bounds in the past few decades,16−18 which also nurture the synthesis of new polymers.19 The strong Lewis

ClCH 2COOCH3 ,14 were employed, but still gave low molecular weight (Mn under 30000) polymers with moderate molecular weight distribution (Mw/Mn around 2.0) at extremely low temperatures. Okuda group first employed a chiral (OSSO) bis(phenolato)aluminum precursor in combination with B(C6F5)3 to obtain high molecular weight PBFs (Mn = 90100−149500) with moderate polydispersities (Mw/Mn = 2.45−2.87), but the polymerization was also performed under B

DOI: 10.1021/acs.macromol.7b01928 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules acids B(C6F5)3 and Al(C6F5)3 have attracted an increasing attention because of being able to catalyze polymerizations of a broad range monomers, such as alkyleneoxides,20,21 vinyl ether,21 ε-caprolactone,22 and styrene,23 and even the concurrent cationic vinyl-addition and ring-opening copolymerizations of vinyl ethers and isobutylene oxide.24 Furthermore, the high Lewis acidity and the steric hindrance of B(C6F5)3 and Al(C6F5)3 allow them to combine with strong Lewis bases (LBs) to form the exiting frustrated Lewis pairs (FLPs), a new generation catalysts for controlled polymerization of conjugated polar alkenes.25,26 Herein, we report the highly active and efficient polymerization of BF using a strong Lewis acid Al(C6F5)3 under mild conditions. The resultant PBFs have high molecular weights and narrow molecular weight distributions, which show good thermal stability and excellent transmittance, indicating a promising optical material. The possible mechanism is proposed based on NMR and X-ray diffraction analyses of the active species and the chain end analysis of the low molecular weight polymer.

Scheme 1. Mechanism of IPVE and Styrene Polymerization Catalyzed by Lewis Acids

■

RESULTS AND DISCUSSION First, some rare-earth metal catalysts developed by our group, (Flu−CH2−Py)Ln(CH2SiMe3)2(THF)x (Flu = fluorenyl; Py =

Scheme 2. Reaction between Al(C6F5)3 and BF

Figure 2. Transmittance of polybenzofuran film.

pyridyl) and (BDI)Ln(CH2SiMe3)2(THF)x (BDI = bis(2,6dimethyl anilido)ketamine) (Ln = Sc, x = 0; Ln = Y or Lu, x = 1), which are highly active and specifically selective for polymerizations of styrene27 and methoxystyrenes,28 were employed to catalyze the coordination polymerization of BF, but were unsuccessful. By merit of the “frustrated Lewis pairs (FLPs)” systems of high activity for the polymerizations of polar monomers, FLPs generated by mixing a Lewis acid, Al(C6F5)3·0.5TOL or B(C6F5)3, and a Lewis base, Ph3P or Nheterocyclic carbenes, was used to catalyze BF polymerization, however, afforded little polymer even after prolonging the polymerization time to 24 h. Then, we turned to using aluminum based Lewis acids such as AliBu3, Al(OiPr)3, and AlCl3, of which AliBu3 and Al(OiPr)3 were completely inert and AlCl3 exhibited a low activity at room temperature in toluene (ε = 2.38) to give a PBF with extremely wide molecular weight distribution (convn = 33.8%, Mn = 5.2 × 104, Mw/Mn = 7.7) (Table 1, entries 1−3). By employing B(C6F5)3, a known cationic catalyst for the polymerization of vinyl ethers,24 no polymer was isolated at

Figure 3. X-ray structures of Al(C6F5)3·BF.

all (Table 1, entry 4). When we switched to a much stronger Lewis acid Al(C6F5)3·0.5TOL, excitingly, the polymerization under a BF-to-Al ratio of 500:1 reached a conversion of 56.2% in 8 h to afford a high molecular weight PBF (Mn = 22.5 × 104) with a moderate polydispersity (Mw/Mn = 3.02) (Table 1, entry 5). The polymerization was significantly influenced by the polarity of solvent. When the polymerization was performed in the nonpolar hexane (ε = 0.06), only a 4.0% conversion was obtained, probably because solubility of Al(C6F5)3·0.5TOL and PBF in hexane was very low, leading to precipitation and aggregation of the active species (Table 1, entry 6). With polar chlorobenzene (ε = 2.7) instead, under the same conditions, the monomer conversion reached to 67.5% in 8 h, which enhanced substantially to 80.7% by prolonging polymerization C

DOI: 10.1021/acs.macromol.7b01928 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

Figure 4. 19F NMR spectra of Al(C6F5)3·0.5TOL and Al(C6F5)3·BF.

Figure 5. 1H NMR spectra of BF and Al(C6F5)3·BF.

Strikingly, a high molecular weight PBF (Mn = 17.9 × 104) with narrow polydispersity (Mw/Mn = 1.56) was obtained (Table 1, entry 11) albeit at a longer polymerization time,30 suggesting that the chain transfer reaction typical for benzofuran polymerization was suppressed at this temperature. The 1H NMR spectrum of this PBF as shown in Figure 1 gives two groups of broad peaks at 4.05 and 2.76 ppm assigned to the methine protons H2 and H1 on the five-membered Oheterocyclic ring, while the three broad resonances in the aromatic region are attributed to phenyl protons. The 13C NMR spectrum shows four distinct peaks at δ 109.57, 120.68, 126.29, and 129.35 ppm arising from aromatic carbons C3, C4, C5 and C6 of the phenyl ring, respectively; while the resonances at 124.51 and 159.33 are associated with the two rings’ junction carbons C7 and C8. Note that the two methine carbons C1 and C2 at the five-membered O-heterocyclic ring give multiple resonances, suggesting that this PBF may exist in a variety of stereoisomers of cis and trans configurations.31,32 DSC analysis reveals that the resultant amorphous PBF possesses a high Tg of 210 °C (Figure S3),33 which is much higher than those of widely applied transparent polymers such as polystyrene (Tg = 100 °C), poly(methyl methacrylate) (Tg = 105 °C) and the polycarbonate (Tg = 150 °C). Meanwhile this PBF displays excellent thermal stability of less than 5% weight loss at 344 °C (Figure S4). More delighted to us, the PBF film under the UV−vis measurement, shows higher than 89% transmittance for visible lights from 400 to 800 nm owing to the amorphous structure (Figure 2), suggesting a new optical material having a wide process window. Polymerizations initiated by strong Lewis acids usually involve the cationic procedure but the structures of the active

Scheme 3. Possible Mechanism for BF Polymerization Catalyzed by Al(C6F5)3

time to 32 h to give a high molecular weight PBF (Mn = 8.9 × 104). Further increasing BF-to-Al ratio to 1000:1 brought about a doubled activity, although the molecular weight of the resultant PBF increased slightly not correspondingly, which might be attributed to the chain-transfer side reaction to monomer at high concentrations (Table 1, entries 7−9). On the basis of these results, we reasoned that the catalyst with discrete, single metal site should hold the potential for an even higher conversion.29 Furthermore, the polymerization was conducted at a low temperature (Tp = 0 °C) and a dilute Al(C6F5)3 concentration (0.1 mol %) to reduce side reactions. D

DOI: 10.1021/acs.macromol.7b01928 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

Figure 6. MALDI−TOF mass spectrum of low molecular weight PBF with DCTB as matrix.

Figure 7. 13C NMR spectrum of low molecular weight polybenzofuran produced by Al(C6F5)3 (Mn = 3900, Mw/Mn = 2.56 measured by GPC) (CDCl3, 125 M Hz).

To understand the mechanism of BF polymerization catalyzed by Al(C6F5)3, the stoichiometric reaction between Al(C6F5)3·0.5TOL and BF was carried out and a benzofuranalane adduct Al(C6F5)3·BF was obtained as the major product (yield: 95%) (Scheme 2). Al(C6F5)3·BF was used to initiate the polymerization of 1000 equiv. BF in C6H5Cl at room temperature to reach a 59.2% yield. The isolated PBF had Mn = 11.7 × 104 and Mw/Mn = 2.20, comparable to that obtained by using the in situ generated catalytic system under the same conditions. These results suggested that Al(C6F5)3·BF was the true active species in the above polymerization procedure, and the possibility of BF polymerization initiated by the activated toluene was ruled out. Fortunately, the Al(C6F5)3·BF adduct was isolated as fine crystals, so we defined its molecular structure by X-ray diffraction analysis. As shown in Figure 3, BF coordinates to

species are different. Aoshima reported concurrent vinyladdition and ring-opening copolymerization of isopropyl vinyl ethers (IPVE) with isobutylene oxide using B(C6F5)3. The active species was H+ arising from the reaction of adventitious water and B(C6F5) (Scheme 1, (1)).24 Chen reported styrene polymerization with Al(C6F5)3·0.5TOL,23 in which Al(C6F5)3 activated toluene to form the canonical forms A and B serving as a protogen active species (Scheme 1, (2), path a),34 or Al(C6F5)3 activated styrene to form the zwitterionic active species C (Scheme 1, (2), path b). For our system, both the solvent and monomer were dried rigorously and the BF polymerization was performed by excluding oxygen and moisture, in addition, B(C6F5)3 was inert toward BF polymerization, therefore cationic H+ could not be the active species for BF polymerization. E

DOI: 10.1021/acs.macromol.7b01928 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

104) and narrow molecular weight distribution (Mw/Mn = 1.56) was obtained unprecedentedly. This polymer presents the characteristics of high glass transition temperature (Tg = 210 °C), distinguished thermal decomposition temperature (Td, 5% loss = 344 °C) and excellent transparency (transmittance >89%). The possible mechanism of this polymerization was confirmed by the isolation of the true active species and chain end analysis of the low molecular weight polymer. BF was activated by Al(C6F5)3 through polarization of the CC bond in furan ring to form the carbocation active species. The cationic polymerization was performed by continuously electrophilic addition of the carbocation active species to benzofuran. The high Lewis acidity and oxophilicity of Al(C6F5)3 contributed the stability of the carbocation in the polymerization, which made the polymerization more controllable with less side reactions. Further investigation of the stereoselective BF polymerization is in progress.

aluminum through oxygen atom, which combining with three C6F5 groups forms a distorted tetrahedron geometry around the central aluminum.35 The 19F NMR spectrum analysis of Al(C6F5)3·BF reveals that the resonances of F atoms at ortho, meta, and para positions of one C6F5 unit shift downfield to δ −122.9, −150.5, and −160.4 ppm, respectively, as compared to the corresponding those at δ −122.5, −149.3, and −159.8 ppm in Al(C6F5)3·0.5TOL (Figure 4), which are contrary to the upfield shift of F atom resonances in the adduct of Al(C6F5)3 with MMA,36 NHCs or P ligands.37 Protons HA and HB in furan ring of Al(C6F5)3·BF give resonances at δ 6.06 and 6.93 in the 1H NMR spectrum, shifting upfield as compared to δ 6.33 and 7.12 of the corresponding protons in BF monomer (Figure 5). Moreover, the LUMOs of the adduct Al(C6F5)3·BF showed that the activated benzofuran in the complex could accept the electron from the monomer and form new bond (Figure S7). All these analyses are consistent with the crystal structure of Al(C6F5)3·BF, where BF coordinates to the highly Lewis acidic and oxophilic Al(C6F5)3 via oxygen atom, leading to the adjacent carbon of the CC bond more positive. Indeed, the NBO charge of this carbon connected to oxygen in furan ring of the adduct Al(C6F5)3·BF is δ = +0.143, higher than that in neutral BF monomer (δ = +0.136) (Figure S8). On the basis of these studies, we proposed BF polymerization following the mechanism as illustrated in Scheme 3. First, BF is activated by Al(C6F5)3 through forming the adduct Al(C6F5)3·BF, as a result, the carbon atom of the CC bond connecting to the oxygen of the furan ring becomes the more positive carbocation. Second, the carbocation attacks the electron enriched carbon atom of the CC bond of furan ring of the incoming BF monomer (electrophilic addition), completes a propagation step. The high Lewis acidic and oxophilic of Al(C6F5)3 can disperse electron from the newly generated carbanion and suppresses the combination of the carbocation and carbanion, which may inhibit chain termination and avoid some chain transfer reaction such as β-H elimination. Therefore, high molecular weight PBFs were obtained. The mechanism elucidated above for BF polymerization catalyzed by Al(C6F5)3, was further confirmed by analyzing the polymer chain-ends via matrix assisted laser desorption/ ionization time-of-flight mass spectroscopy (MALDI−TOF MS). A low-molecular-weight PBF was obtained by performing the polymerization under a low monomer-to-initiator ratio ([BF]:[Al] = 5:1) and terminated with HCl-acidified methanol. As shown in Figure 6, the difference between two neighboring molecular ion peaks equals to the molecular weight of a repeat unit (m/z = 118.13). Accordingly, a formula [H(BF)nCl] was established, where the end groups H and Cl were arising from the terminator H+Cl−. This formular [H(BF)nCl] was further confirmed by 13C NMR spectrum analysis (Figure 7).38 The major signals are attributed to the main chain carbons (Ca−Ch), while the minor resonances at δ 29.68 and 103.14 are arising from the chain end carbons C1 and C10, respectively. Overall, these results are quite consistent with cationic polymerization mechanism composed of chain initiation, propagation and termination steps. This can also explain why “FLPs” could not catalyze BF polymerization. Because, NHCs or P ligand in a FLPs is a stronger Lewis base than BF, which can react with the active carbocation species, leaving no chance for BF being incorporated into the polymer chain. In conclusion, the renewable and aromatic benzofuran polymerization initiated by a strong Lewis acid Al(C6F5)3 was reported and PBF with high molecular weight (Mn = 17.9 ×

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b01928. Experimental section, selected bond distances and angles of the complex Al(C6F5)3·BF, GPC traces, 16 possible combinations of dyads in polybenzofuran, DSC thermogram of the polybenzofuran, TGA curves for polybenzofuran, WAXD profile of the polybenzofuran, log plot of SAXS integrated intensity profile of the obtain polybenzofuran, HOMOs and LUMOs of BF and Al(C6F5)3·BF, NBO charge values in BF and Al(C6F5)3·BF, and 1H NMR spectrum of low molecular weight polybenzofuran (PDF) Crystallographic data (CIF)

■

AUTHOR INFORMATION

Corresponding Author

*(D.C.) E-mail: [email protected]. Fax: (+86) 431 85262774. Telephone: +86 431 85262773. ORCID

Dongmei Cui: 0000-0001-8372-5987 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by the NSF China for Projects Nos. 21634007, 21374112, and 21574125 and by the MST for “973” Project No. 2015CB654702.

■

REFERENCES

(1) Resconi, L.; Waymouth, R. M. Diastereoselectivity in the Homogeneous Cyclopolymerization of 1,5-Hexadiene. J. Am. Chem. Soc. 1990, 112, 4953−4954. (2) Hou, X. H.; Nomura, K. Ring-Opening Metathesis Polymerization of Cyclic Olefins by (Arylimido)vanadium(V)-Alkylidenes: Highly Active, Thermally Robust Cis Specific Polymerization. J. Am. Chem. Soc. 2016, 138, 11840−11849. (3) Kobayashi, S.; Lu, C.; Hoye, T. R.; Hillmyer, M. A. Controlled Polymerization of a Cyclic Diene Prepared from the Ring-Closing Metathesis of a Naturally Occurring Monoterpene. J. Am. Chem. Soc. 2009, 131, 7960−7961. (4) Thomas, L.; Polton, A.; Tardi, M.; Sigwalt, P. Living Cationic Polymerization of Indene 0.1. Polymerization Initiated with Cumyl

F

DOI: 10.1021/acs.macromol.7b01928 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules Methyl-Ether Titanium Tetrachloride and Cumyl Methyl-Ether NButoxytrichlorotitanium Initiating Systems. Macromolecules 1992, 25, 5886−5892. (5) Miyaji, H.; Satoh, K.; Kamigaito, M. Bio-Based Polyketones by Selective Ring-Opening Radical Polymerization of alpha-PineneDerived Pinocarvone. Angew. Chem., Int. Ed. 2016, 55, 1372−1376. (6) Mitsui Chemicals boosts cycloolefins. Chem. Eng. News 2007, June 11, 17. (7) Topas launches under new owner. Chem. Eng. News 2006, February 6, 15. (8) Boffa, L. S.; Novak, B. M. Copolymerization of polar monomers with olefins using transition-metal complexes. Chem. Rev. 2000, 100, 1479−1493. (9) Nakamura, A.; Ito, S.; Nozaki, K. Coordination-Insertion Copolymerization of Fundamental Polar Monomers. Chem. Rev. 2009, 109, 5215−5244. (10) Stempfle, F.; Ortmann, P.; Mecking, S. Long-Chain Aliphatic Polymers To Bridge the Gap between Semicrystalline Polyolefins and Traditional Polycondensates. Chem. Rev. 2016, 116, 4597−4641. (11) Pu, W. C.; Mu, G. M.; Zhang, G. L.; Wang, C. Copper-catalyzed decarboxylative intramolecular C-O coupling: synthesis of 2arylbenzofuran from 3-arylcoumarin. RSC Adv. 2014, 4, 903−906. (12) Natta, G.; Farina, M.; Peraldo, M.; Bressan, G. Asymmetric Synthesis of Optically Active Di-Isotactic Polymers from Cyclic Monomers. Makromol. Chem. 1961, 43, 68−75. (13) Mizote, A.; Tanaka, T.; Higashimura, T.; Okamura, S. Cationic Polymerization of Cyclic Olefins. J. Polym. Sci., Part A-1: Polym. Chem. 1966, 4, 869−879. (14) Yonezumi, M.; Kanaoka, S.; Aoshima, S. Living cationic polymerization of dihydrofuran and its derivatives. J. Polym. Sci., Part A: Polym. Chem. 2008, 46, 4495−4504. (15) Lian, B.; Ma, H. Y.; Spaniol, T. P.; Okuda, J. Neutral and cationic aluminium complexes containing a chiral (OSSO)-type bis(phenolato) ligand: synthesis, structures and polymerization activity. Dalton Trans. 2009, 9033−9042. (16) Yamamoto, H. Lewis acids in organic synthesis; Wiley-VCH: Weinheim, Germany, 2000; p 1. (17) Jakobsson, K.; Chu, T.; Nikonov, G. I. Hydrosilylation of Olefins Catalyzed by Well-Defined Cationic Aluminum Complexes: Lewis Acid versus Insertion Mechanisms. ACS Catal. 2016, 6, 7350− 7356. (18) Stahl, T.; Klare, H. F. T.; Oestreich, M. Main-Group Lewis Acids for C-F Bond Activation. ACS Catal. 2013, 3, 1578−1587. (19) Aoshima, S.; Kanaoka, S. A Renaissance in Living Cationic Polymerization. Chem. Rev. 2009, 109, 5245−5287. (20) Asenjo-Sanz, I.; Veloso, A.; Miranda, J. I.; Alegría, A.; Pomposo, J. A.; Barroso-Bujans, F. Zwitterionic Ring-Opening Copolymerization of Tetrahydrofuran and Glycidyl Phenyl Ether with B(C6F5)3. Macromolecules 2015, 48, 1664−1672. (21) Chakraborty, D.; Rodriguez, A.; Chen, E. Y. X. Catalytic ringopening polymerization of propylene oxide by organoborane and aluminum Lewis acids. Macromolecules 2003, 36, 5470−5481. (22) Xu, J.; Song, J.; Pispas, S.; Zhang, G. Metal-free controlled ringopening polymerization of ε-caprolactone in bulk using tris(pentafluorophenyl)borane as a catalyst. Polym. Chem. 2014, 5, 4726−4733. (23) Chakraborty, D.; Chen, E. Y. X. Neutral olefin polymerization activators as highly active catalysts for ROP of heterocyclic monomers and for polymerization of styrene. Macromolecules 2002, 35, 13−15. (24) Kanazawa, A.; Kanaoka, S.; Aoshima, S. Concurrent Cationic Vinyl-Addition and Ring-Opening Copolymerization Using B(C6F5) (3) as a Catalyst: Copolymerization of Vinyl Ethers and Isobutylene Oxide via Crossover Propagation Reactions. J. Am. Chem. Soc. 2013, 135, 9330−9333. (25) Zhang, Y. T.; Miyake, G. M.; John, M. G.; Falivene, L.; Caporaso, L.; Cavallo, L.; Chen, E. Y. X. Lewis pair polymerization by classical and frustrated Lewis pairs: acid, base and monomer scope and polymerization mechanism. Dalton Trans. 2012, 41, 9119−9134.

(26) Knaus, M. G. M.; Giuman, M. M.; Pothig, A.; Rieger, B. End of Frustration: Catalytic Precision Polymerization with Highly Interacting Lewis Pairs. J. Am. Chem. Soc. 2016, 138, 7776−7781. (27) Pan, Y. P.; Rong, W. F.; Jian, Z. B.; Cui, D. M. Ligands Dominate Highly Syndioselective Polymerization of Styrene by Using Constrained-geometry-configuration Rare-earth Metal Precursors. Macromolecules 2012, 45, 1248−1253. (28) Liu, D. T.; Yao, C. G.; Wang, R.; Wang, M. Y.; Wang, Z. C.; Wu, C. J.; Lin, F.; Li, S. H.; Wan, X. H.; Cui, D. M. Highly Isoselective Coordination Polymerization of ortho-Methoxystyrene with betaDiketiminato Rare-Earth-Metal Precursors. Angew. Chem., Int. Ed. 2015, 54, 5205−5209. (29) Hong, M.; Chen, E. Y. X. Completely recyclable biopolymers with linear and cyclic topologies via ring-opening polymerization of gamma-butyrolactone. Nat. Chem. 2016, 8, 42−49. (30) The representative GPC curves are shown in Figure S1. (31) The sixteen possible combinations of two adjacent stereocenters (dyads) in polybenzofuran were shown in Figure S2. (32) Hahn, S. F.; Hillmyer, M. A. High glass transition temperature polyolefins obtained by the catalytic hydrogenation of polyindene. Macromolecules 2003, 36, 71−76. (33) WAXD and SAXS profiles of the obtained polybenzofuran are shown in Figure S5 and S6, respectively. (34) Belgardt, T.; Storre, J.; Roesky, H. W.; Noltemeyer, M.; Schmidt, H. G. Tris(Pentafluorophenyl)Alane - a Novel Aluminum Organyl. Inorg. Chem. 1995, 34, 3821−3822. (35) Selected bond distances and angles of Al(C6F5)3·BF were shown in Table S1. (36) Rodriguez-Delgado, A.; Chen, E. Y. X. Single-site anionic polymerization. Monomeric ester enolaluminate propagator synthesis, molecular structure, and polymerization mechanism. J. Am. Chem. Soc. 2005, 127, 961−974. (37) Zhang, Y. T.; Miyake, G. M.; Chen, E. Y. X. Alane-Based Classical and Frustrated Lewis Pairs in Polymer Synthesis: Rapid Polymerization of MMA and Naturally Renewable Methylene Butyrolactones into High-Molecular-Weight Polymers. Angew. Chem., Int. Ed. 2010, 49, 10158−10162. (38) 1H NMR spectrum of low molecular weight polybenzofuran produced by Al(C6F5)3 was shown in Figure S9.

G

DOI: 10.1021/acs.macromol.7b01928 Macromolecules XXXX, XXX, XXX−XXX