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Highly Isotactic and High-Molecular-Weight Poly(2-vinylpyridine) by Coordination Polymerization with Yttrium Bis(phenolate) Ether Catalysts Tie-Qi Xu, Guan-Wen Yang, and Xiao-Bing Lu ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b00881 • Publication Date (Web): 15 Jun 2016 Downloaded from http://pubs.acs.org on June 17, 2016

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Highly Isotactic and High-Molecular-Weight Poly(2-vinylpyridine) by Coordination Polymerization with Yttrium Bis(phenolate) Ether Catalysts Tie-Qi Xu,* Guan-Wen Yang, and Xiao-Bing Lu State Key Laboratory of Fine Chemicals, College of Chemistry, Dalian University of Technology, Dalian 116024, P. R. China

*Corresponding author. E-mail: [email protected]

Abstract The poly(2-vinylpyridine) combining highly isotactic (Pm = 0.97) and high-molecular-weight (~105 g/mol) was obtained for the first time using novel yttrium bis(phenolate) ether catalysts. These catalysts also showed high activity and tolerance to low loadings. Computational studies and kinetic experiments provided a fundamental understanding of the initiation, propagation, and stereocontrol mechanism governing the polymerization reactions mediated by yttrium catalysts. Keywords: 2-vinylpyridine; stereoselectivity; yttium catalyst; coordination polymerization; DFT

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Introduction Polymers containing heteroatoms are desirable materials for various applications. Pyridine-based synthetic polymers, a class of versatile N-containing polymers as self-assembling and ion exchange resin materials, have been attracting industrial and scientific interest because of their application in chemical synthesis.1 Controlling stereoselectivity of polymerization reaction is an effective approach to improve synthetic polymers. This process produces stereoregular polymers that often exhibit mechanical and physical properties superior to those of their stereorandom counterparts.2 Thus, the stereospecific polymerization catalysis of 2-vinylpyridine (2-VP) is of great interest academically and industrially. Several organic and metal-based catalyst systems show evident high activity in converting 2-VP into polymers with high molecular weight (Mn). However, the produced polymers are essentially atactic, amorphous materials.3 Only few catalysts actually exhibit moderate stereospecificity for the polymerization of 2-VP. Natta et al. reported that the anionic polymerization of 2-VP with magnesium amides produced isotactic poly(2-VP) with mm = 0.80.4 Rieger et al. developed 2-methoxyethylaminobis-(phenolate)-yttrium catalysts that afforded only iso-rich poly(2-VP) with mm = 0.61.5 Mashima et al. reported the generation of isotactic, low-Mn oligomeric 2-VP (Mn = ~3000 g/mol) while exploring end-functionalized polymerization of 2-VP through initial C-H bond activation of N-heteroaromatics.6 Thus, the development of effective polymerization catalysts that can produce highly isotactic poly(2-VP) with high-Mn still requires further studies. Coordination−addition polymerization is the most powerful technique to control the polymerization stereochemistry of polar vinyl monomer.7 In this context, the synthesis of highly isotactic

poly(methacrylate)s,8

poly(acrylamide)s,9

and

poly(β-methyl-α-methylene-γ-butyrolactone)s10 using cationic group 4 metallocene and related complexes has been achieved. Stereoselective polymerization of polar vinyl monomer ortho-methoxystyrene was facilitated using cationic β-diketiminate rare earth complexes11. On the other hand, neutral rare-earth metal complexes have been shown to be effective catalysts for 2-VP polymerization by the coordination−addition mechanisms with good catalytic activity and polymerization control12. Thus, our study was focused on developing new rare-earth metal


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catalysts to produce stereoregular poly(2-VP) via the coordination polymerization. Herein, we report the firs effective yttrium bis(phenolate) ether catalysts for highly isotactic 2-VP polymerization to produce high-Mn polymers.

Results and Discussion Synthesis and Structure of Yttrium Complexes 1−3 Complexes 1−3 spanning C2, C3, and C4 backbones were prepared by adding stoichiometric amounts of corresponding pro-ligands (partly used for Group IV Bisphenolate-Ether complexes13 in olefin polymerization) to a solution of the Y(CH2SiMe3)3(THF)2 in n-hexane (Scheme 1). The new yttrium complexes were characterized by 1H (13C) NMR spectroscopy and elemental analysis. The molecular structure of complex 1 was confirmed by single-crystal X-ray diffraction.14 The crystal structure, along with selected bond distances and angles, was depicted in Figure 1. One crystallographically independent Y atom in complex 1 adopted a six-coordinate, distorted octahedron geometry in which C(53), O(1), O(3), and O(4) were essentially coplanar with the metal atom Y15, forming the equatorial plane. The atoms of donors O(5) and O(2) atoms occupied the axial positions. The bis(phenolate) ether ligand adopted a β-cis configuration.

Scheme 1. Synthesis of yttrium bisphenolate ether complexes


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Figure 1. X-ray crystal structure of Y bis(ether phenolate) complex 1. Selected bond lengths (Å) and angles (°): Y−O(1) 2.135(3), Y−O(2) 2.411(3), Y−O(3) 2.582(4), Y−O(4) 2.133(3), Y−O(5) 2.304(3), Y−C(53) 1.820(5), O(5)−Y−O(2) 159.83(12), O(4)−Y−O(1) 155.65(13), C(53)−Y−O(3) 169.42(15), O(1)−Y−O(3) 85.51(12), O(3)−Y−O(4) 72.46(12), O(4)−Y−C(53) 96.99(16), C(53)−Y−O(1) 105.01(16) Stereoselective Polymerization of 2-VP Selected experimental results of 2-VP polymerization with complexes 1−3 as catalysts are summarized in Table 1. The catalysts exhibited moderate to high activity for 2-VP polymerization at ambient temperature in toluene, which was significantly modulated by the ligand structure (Table 1). Catalyst 1 showed the highest catalytic activity, with a turnover frequency of 473 h−1 (run 3, Table 1). This catalyst was also found to be efficient with a low initiator loading of 0.1% mol. Catalyst 1 achieved nearly complete conversion of 1000 equiv. of 2-VP in ~2 h (run 4, Table 1). Changing the solvent from nonpolar toluene (ε = 2.38) to relatively polar dichloromethane (CH2Cl2, ε = 8.93) decreased the polymerization activity (run 1 vs. 5, Table 1). The coordinating solvent (THF or pyridine) significantly decreased the catalytic activity. Meanwhile, the Mn of the obtained poly(2-VP) increased accordingly with the monomer-to-catalyst ratio, reaching up to Mn = 178000 g/mol (run 4, Table 1). The resulting initiator efficiency (I*) was calculated to be 58%. Moving to catalyst 3, we observed an poly(2-VP) with a much lower Mn of 1.21×104 g/mol (thus a much higher I* of 140 %) and a lower PDI of 1.03 (run 9, Table 1). The molecular weight


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distribution typically ranged from 1.0 to 1.6, which is consistent with the single-site nature of the active species. Table 1. Results of 2-vinylpyridine (2-VP) polymerization using catalysts 1–3a

No. Cat. [2-VP]/[cat.] Solvent Time Conv. TOF I* Mn×10-4 Mw/Mnc Pmd Tg/Tm b

ratio

(min) (%)

-1

∆H -1 e (Jg ) ( C)

-1 c

o

(h ) (%) (gmol )

e

1

1

200

Toluene 40

100

300 22

9.56

1.36 0.96 94/205

2

1

400

Toluene 60

100

400 34

12.4

1.33 0.96 91/204 17

3

1

800

Toluene 100

98.6

473 53

15.5

1.38 0.97 91/205 17

4

1

1000

Toluene 125

98.1

471 58

17.8

1.32 0.96 91/205 18

5

1

200

CH2Cl2 70

100

171 37

5.76

1.52 0.96 91/204

9

6

1

200

82.1

164 16

11.1

2.18 0.63

—

—

7

1

200

Pyridine 1440 48.9

16

6.36

1.51 0.63

—

—

8

2

200

Toluene 30

100

400 32

6.66

1.58 0.89 95/198

3

3

200

Toluene 120

80

80 140

1.21

1.03 0.85 91/199

3

200

Toluene 720

50.7

9

3.40

1.59 0.52 109/

—

9 f

10 AIBN

THF

60

4

32

9

a

Reaction conditions: room temperature (~20 °C), toluene, [2-VP] = 1.0 M, and 10 µmol of yttrium complex; b2-VP conversions were analyzed by 1H NMR spectroscopy; c I*=Mn(calcd)/Mn(exptl), where Mn(calcd) = MW(M)×[M]/[I]×conversion (%) + MW (chain-end groups). cThe molecular weight (Mn) was obtained by Gel Permeation Chromatography (GPC) in THF relative to PS; dPm is the probability of meso linkages between monomer units and is determined by 13C NMR spectroscopy; eDetermined by DSC; f 70 °C. Significantly, the 2-VP polymerization is also highly stereospecific, producing it-poly(2-VP) with a high isotacticity of Pm = 0.63−0.97 (Table 1) and 2[rr]/[mr] = 1 (Table S4), indicative of the enantiomorphic-site controlled mechanism. The microstructural analysis of poly(2-VP) with catalysts 1–3 revealed that the structure of the ligand also significantly influenced the stereoregularity of the growing polymer chain (run 1, 8, and 9, Table 1). Variation in the bridge from (CH2)2 to (CH2)4 evidently improved the Pm value (from 0.85 to 0.97). Catalyst 1 with the (CH2)4 bridge was shown to be the most isospecific catalyst of the series (which also represents the most stereoselective catalyst known in the literature) for the polymerization of 2-VP, affording a maximum Pm value of 0.97 (Figure 2). The results from differential scanning calorimetry (DSC)


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analysis of the polymer with Pm = 0.97 (run 4, Table 1) showed a Tm value of 205 °C and a ∆H of 18 J/g, further confirming the formation of the high isotactic polymer (Figure 3). An onset degradation temperature (Td) value of approximately 350 °C was observed by thermogravimetric analysis (TGA) (Figure S30), showing the high thermal stability of this isotactic poly(2-VP). The wide-angle X-ray diffraction (WAXD) patterns showed five characteristic diffraction peaks at 2θ of 11.4°, 17.6°, 18.0°, 19.8°, and 27.7°, which were attributed to the isotactic poly(2-VP) sequences, and sharper than the atactic poly(2-VP) counterpart (Figure 4). Changing solvent polarity did not noticeably affect the isotacticity of poly(2-VP) (run 1 vs. 5, Table 1). However, the coordinating solvent significantly affected the tacticity of the resulting poly(2-VP). Thus, polymerization in both THF and pyridine lost stereoselectivity and yielded an essentially atactic polymer (runs 6 and 7, Table 1), presumably because of competing solvent coordination to the metal center with monomer coordination.

Figure 2. 13C NMR spectrum of isotactic poly(2-VP) (run 3, Table 1) in CD3OD

Figure 3. Differential scanning calorimetry profiles of various poly(2-VP) produced by AIBN


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(run 10, Table 1) (a) and complex 1 (run 4, Table 1) (b)

Figure 4. Wide-angle X-ray diffraction profiles of various poly(2-VP) produced by AIBN (a) and complex 1 (b) (The samples were crystallized isothermally at 160 °C for 2 h)

Initiation and propagation in 2-VP polymerization 1

H NMR spectroscopy was employed to investigate the initiation and propagation in 2-VP

polymerization catalyzed by complex 1. The 1H NMR spectra of the reaction of complex 1 with 2 equiv. of 2-VP in C6D6 at 8 °C showed two sets of characteristic traces of Y–CH2SiMe3 (–0.55 and –0.98 ppm for complex 1; –0.45 and –1.09 ppm for 2-VP coordinated complex 1), without peak of detectable amount of the 2-VP polymer. This result suggests that the coordination of 2-VP to the yttrium center to replace the coordinated THF molecule is the first step of the polymerization process and is reversible. The polymerization was observed when the reaction temperature was increased to 20 °C. This process consumed all monomers in 20 min, but the reaction only led to the consumption of ~10% of complex 1 (Figure 5). The 1H NMR spectra revealed that a trace of the initiator was always found even when a large excess (40 equiv.) of 2-VP was added (SI, Figure S31). This result demonstrates that the initiation step of the polymerization reaction (2-VP insertion into the Y–C bond) is much slower than chain propagation (Figure 6). The end-group analysis of oligomeric poly(2-VP) showed a shift in the SiMe3 group (0.06 ppm) (Figure 7). This result indicates that the initial step of polymerization proceeds by coordination insertion of the monomer into the Y–CH2SiMe3 bond of catalyst 1, and 2,1-insertion of 2-VP, similar to the styrene polymerization reaction and 2-VP insertion into a Zr–


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C or Y–C bond.16. Furthermore, the MALDI−TOF mass spectrum of an oligomer also showed the formation of Me3SiCH2-terminated poly(2-VP) resulting the

from

the

initial

insertion

of

CH2SiMe3 group into the 2-VP molecule. Linear plots of m/z values of the peaks versus the

number of 2-VP repeat units show the slope corresponding to the molar mass of 2-VP and the intercept corresponding to the sum of the masses of H+ (NH4+) and the Me3SiCH2 end group (Figure 8).

Figure 5. 1H NMR (C6D6) of (a) [1]/[2-VP] = 1:2, 1 min, 8 oC; (b) [1]/[2-VP] = 1:2, 10 min, 20 o

C; (c) [1]/[2-VP] = 1:2, 20 min, 20 oC.

Figure 6. Proposed initiation and propagation in 2-VP polymerization catalyzed by complex 1

Figure 7. The 1H NMR spectrum of poly(2-VP) (400 MHz, CDCl3, 20 °C) produced by complex


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1. The propagation in 2-VP polymerization was investigated by a kinetic experiment. The 2-VP polymerization kinetic experiments used the [2-VP]0/[complex 1]0 ratio varying from 50 to 400 in toluene at 20°C. Moreover, the conversion data (< 90% conversion) from the early stage of polymerization were used to minimize the effects of potential side reactions at the later stage ofthe polymerization. The results showed a first-order dependence on [2-VP] (Figure 9). Furthermore, a plot of ln(kapp) versus ln[Y] was fit to a straightline (R2 = 0.990) with a slope of 1.02 (Figure 10). Thus, thekinetic order with respect to [Y] is also a first-order. These results suggest a monometallic polymerization process, analogous to the monometallic mechanism established for the 2-VP polymerization by 2-methoxyethylaminobis(phenolate)yttrium catalysts.12b

Figure 8. Structure of 2-VP oligomer and its MALDI-TOF spectrum.

Figure 9. Plots of ln([M]0/[M]t) versus time for the 2-VP polymerization using complex 1 with


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varied concentrations:[complex 1]0 = 2.5 mM (green ▲), 5 mM (blue ◆), 10 mM (red ●), 20 mM (black ■) (reaction conditions: toluene, 20 °C, [2-VP]0 = 1.0 M).

Figure 10. Plot of ln(kapp) versus ln[Y] for the 2-VP polymerization catalyzed by complex 1 (reaction conditions: toluene, 20 °C, [2-VP]0 =1.0 M). We performed computational studies of this polymerization reaction using a B3LYP density functional procedure to understand the initiation, propagation, and stereocontrol mechanism governing this polymerization reaction.17 The postulated mechanism of the yttrium-catalyzed 2-VP polymerization including both initiation （Figure 11） and propagation processes (Figure 12) is shown. The geometry of catalyst 1 was first optimized during the initiation process. This step was followed by the coordination of a first 2-VP unit on either the re-face or si-face through bonding of the Y center with the carbon atom. The THF molecule in this process was displaced, forming either the intermediate INT1-Re or INT1-Si, respectively (N-Y: 2.636 Å for INT1-Re and 2.575 Å for INT1-Si). In the third step, the intramolecular nucleophilic attack by the Y–CH2SiMe3 onto the vinyl group of 2-VP, formed a tetrahedral transition state TS1-Si or TS1-Re. The re-face coordination of 2-VP to catalyst 1 to produce INT2-Re was favored because the transition-state TS1-Re was 2.0 kcal/mol lower in energy than the transition state TS1-Si (26.7 kcal/mol). In particular, special attention was paid to the propagation process that determined the stereoselectivity. We analyzed the isotactic selectivity of the process by studying the addition of two successive 2-VP units at the Y center of INT2-Re. Contrary to the coordination of the first 2-VP, the si-face of second 2-VP to INT2-Re was favored because the transition-state TS2-Si was 15.2 kcal/mol lower in energy than the transition state TS2-Re. This pathway is confirmed further by the energy profiles for coordination of a third 2-VP to INT4-Si, thus generating an isotactic


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unit which needs to overcome 13.0 kcal/mol of energy (TS3-Si) compared with 27.9 kcal/mol for the re-face coordination to INT4-Si to form the syndiotactic unit. The resulting poly(2-VP) should display a high isotactic selectivity because the calculation showed the si-face coordination of 2-VP to Y–C bond over its re-face. The initiation step of the reactionis was much slower than chain propagation, because the transition-states TS1 (24.7 kcal/mol) (for initiation process) had higher in energy than the transition-states TS2 (19.1 kcal/mol) and TS3 (13.0 kcal/mol) (for propagation process). This result is congruous with above control experiment.

Figure 11. Energy profiles for the initiation process (first 2-VP insertion into the Y–CH2SiMe3 bond)


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Figure 12. Energy profiles for the propagation process and generation of stereoregular poly(2-VP) In summary, we successfully developed a highly active yttrium catalyst with the bis(phenolate) ether ligand to effect the isopecific polymerization of 2-VP to produce highly isotactic poly(2-VP) with Pm = 0.97 and high–Mn. Kinetic and computational studies provided to a mechanistic understanding of the polymerization and stereocontrol process. The initial step of polymerization proceeded by a coordination insertion of the 2-VP on the re-face through the bonding of the Y center with the carbon atom of catalyst 1, and 2,1-insertion of 2-VP. Chain initiation was much slower than chain propagation, resulting in the production of high-Mn polymer. The formation of an isotactic polymer originated from the directed si-face coordination of 2-VP to Y–C bond of yttrium immediate. Further studies are focused on developing new rare-earth catalyst systems that exhibit high activity, excellent iso-selectivity, and living fashion for the polymerization of 2-VP.

Supporting Information. Supporting Information is available free of charge on the ACS Publications website. Complete experimental details and additional characterization data including Figures S1–S31 and Tables S1–S5 were showed in supporting information. AUTHOR INFORMATION Corresponding Author *[email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (Nos. 21274015 and 21574016), Program for Liaoning Excellent Talents in University (LJQ2015025) and the Fundamental Research Funds for the Central Universities (DUT14LK28), the Chang Jiang Scholars Program (No. T2011056) from Ministry of education, People’s Republic of China.


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37, 160−162. (5) Altenbuchner, P. T.; Adams, F.; Kronast, A.; Herdtweck, E.; Pöthigb, A.; Rieger, B. Polym. Chem. 2015, 6, 6796−6801. (6) Kaneko, H.; Nagae, H.; Tsurugi, H.; Mashima, K. J. Am. Chem. Soc. 2011, 133, 19626−19629. (7) Chen, E. Y.-X. Chem. Rev. 2009, 109, 5157−5214. (8) a) Zhang, Y.; Ning, Y.; Caporaso, L.; Cavallo, L.; Chen, E. Y.-X. J. Am. Chem. Soc. 2010, 132, 2695−2709. b) Hu, Y.; Miyake, G. M.; Wang, B.; Cui, D.; Chen, E. Y.-X. Chem. Eur. J. 2012, 18, 3345−3354. c) Vidal, F.; Gowda, R. R.; Chen, E. Y.-X. J. Am. Chem. Soc. 2015, 137, 9469−9480. (9) a) Miyake, G. M.; Caporaso, L.; Cavallo, L.; Chen, E. Y.-X. Macromolecules 2009, 42, 1462−1471. b) Miyake, G. M.; DiRocco, D. A.; Liu, Q.; Oberg, K. M.; Bayram, E.; Finke, R. G.; Rovis, T.; Chen, E.Y.-X. Macromolecules 2010, 43, 7504−7514. (10) Chen, X.; Caporaso, L.; Cavallo, L.; Chen, E. Y.-X. J. Am. Chem. Soc. 2012, 134, 7278−7281. (11) a) Liu, D.; Yao, C.; Wang, R.; Wang, M.; Wang, Z.; Wu, C.; Lin, F.; Li, S.; Wan, X.; Cui, D. Angew. Chem. Int. Ed. 2015, 54, 5205−5209. b) Liu, D.; Wang, R.; Wang, M.; Wu, C.; Wang, Z.; Yao, C.; Liu, B.; Wan, X.; Cui, D. Chem. Comm. 2015, 51, 4685−4688. (12) a) Zhang, N.; Salzinger, S.; Soller, B. S.; Rieger, B. J. Am. Chem. Soc. 2013, 135, 8810−8813. b) Altenbuchner, P. T.; Soller, B. S.; Kissling, S.; Bachmann, T.; Kronast, A.; Vagin, S. I.; Rieger, B. Macromolecules 2014, 47, 7742−7749. (13) Kiesewetter, E. T.; Randoll, S.; Radlauer, M.; Waymouth, R. M. J. Am.Chem. Soc. 2010, 132, 5566−5567. (14) Data for the X-ray crystal structure of complex 1: C56H67O5SiY·(n-hexane), Mw = 1023.27, T = 293 K, crystal size 0.16×0.13×0.11 mm3, triclinic, P-1, a = 12.0346(4), b = 14.3451(5), c = 16.9307(5), α = 101.182(2), β = 90.368(2), γ = 96.000(2), Z = 2, U = 2850.61(16) Å3, ρ = 1.192 gcm-1, µ = 1.090 mm-1,10022 collected, 7173 unique (Rint = 0.0388), final R1 = 0.0668, wR2


ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

[I>2s(I)]=0.1697. CCDC 965000 contains the supplementary crystallographic data for this paper (accessed October 7, 2013). These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www. ccdc.cam.ac.uk/data_ request/cif. (15) The sum of bond angles O(1)−Y−O(3) of 85.51(12), O(3)−Y−O(4) of 72.46(12), O(4)−Y−C(53) of 96.99(16), and C(53)−Y−O(1) of 105.01(16) is 359.97 degree. (16) a) Pellecchia, C.; Longo, P.; Grassi, A.; Ammendola, P.; Zambelli, A.; Makromol. Chem. Rapid Commun.1987, 8, 277−279. b) Zambelli, A.; Pellecchia, C.; Oliva, L.; Longo, P.; Grassi, A. Makromol. Chem. 1991, 192, 223−231. c) Guram, A. S.; Jordan, R. F. Organometallics 1991, 10, 3470−3479. (17) All calculations were performed with Gaussian 09 suite of the programs.

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ACS Catalysis

Title:Highly Isotactic and High-molecular-weight Poly(2-vinyl-pyridine) by Coordination Polymerization with Yttrium Bis(phenolate) Ether Catalysts Authors:Tie-Qi Xu,* Guan-Wen Yang, and Xiao-Bing Lu


Highly Isotactic and High-Molecular-Weight Poly(2-vinylpyridine) by

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