Highly Active ansa-(Fluorenyl)(amido)titanium-Based Catalysts with

Aug 11, 2017 - Graduate School of Engineering, Hiroshima University, Higashi-Hiroshima 739-8527, Japan .... Number-average polymerization degree. ...
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Highly Active ansa-(Fluorenyl)(amido)titanium-Based Catalysts with Low Load of Methylaluminoxane for Syndiotactic-Specific Living Polymerization of Propylene Yanjie Sun,† Bo Xu,† Takeshi Shiono,*,‡ and Zhengguo Cai*,† †

State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Materials Science and Engineering, Donghua University, Shanghai 201620, People’s Republic of China ‡ Graduate School of Engineering, Hiroshima University, Higashi-Hiroshima 739-8527, Japan S Supporting Information *

ABSTRACT: ansa-Dimethylsilylene(fluorenyl)(amido)titanium complexes bearing various electron-donating substituents are synthesized and applied to propylene polymerization by the use of MMAO/2,6-di-tert-butyl-4-methylphenol as a cocatalyst. The complexes containing adamantylamido ligands (1a,b) show unexpectedly high activity (31150 kg of polymer (mol of Ti)−1 h−1) with a low Al:Ti ratio of 20. The catalysts also promote syndiotactic-specific living polymerization to produce propyleneethylene block copolymers.

S

modifications. A further study to clarify the substituent effects by systematically tuning the electronic and/or steric properties of the substituent would be necessary to establish the strategies for designing high-performance CGCs. On the other hand, the high production cost of single-site catalysts is partially attributable to the requirement of huge amounts of methylaluminoxane (MAO) for high activity. In this paper, we synthesized a series of ansa-dimethylsilylene(fluorenyl)(amido)dimethyltitanium complexes bearing different electron-donating substituents on the ligands, i.e., an adamantyl group on the amido ligand and/or a methoxy group on the fluorenyl ligand (1a,b and 2a,b), and found that complexes 1a,b conducted living polymerization of propylene in very high activities activated by a small amount of modified MAO (MMAO)/2,6-di-tert-butyl-4-methylphenol (BHT). The titanium complexes were synthesized in good yields by a one-pot reaction of the corresponding ligand with 4 equiv of methyllithium and 1 equiv of TiCl4.14 The 1H NMR spectra of all the complexes showed that the methyl groups bonded to Ti and Si atoms are equivalent, respectively, indicating a Cs-symmetric nature of the complexes in solution. The molecular structures of 1a and 2a,b determined by single-crystal X-ray analysis are displayed in Figure 1. The distances between the titanium center and five-membered carbon rings in 1a and 2a,b are very close to those in B1 and B2, where the fluorenyl ligand is coordinating to the titanium in a η3 mode (Table 1). Miller et al. evaluated the hapticity of the fluorenyl ligand of zirconium complexes by comparing the C−C bond lengths of the five-membered ring (δ = (a + b − c − d)/2) (Table1).15 The similar values of hapticity (0.025−0.032 Å) in our titanium complexes also testified to the tendency for the η3 form. In addition, the Tolman cone angles16

ince the discovery of homogeneous Ziegler−Natta catalysts based on group 4 metallocenes,1 much effort has been devoted to the development of single-site catalysts in academia and industry.2 Among them, “constrained geometry catalysts” (CGCs)3 have especially attracted attention due to their excellent copolymerization ability,4−9 stereospecificity,10−12 and living polymerization characteristics.13 For example, the introduction of an alkyl substituent to the fluorenyl ligand of fluorenylamido-ligated zirconium dichloride (A1 and A2) improved the activity and syndiotactic specificity (syn-specificity), and sterically expanded complex A4 showed highest synspecificity to produce highly syndiotactic syn-polypropylene (PP) with a pentad (rrrr) value of 0.99.10

Our previous research focused on the corresponding dimethyltitanium complexes B1 and B2.13 The catalytic systems conducted homo- and copolymerization of ethylene, propylene, higher α-olefins, and norbornene in a living manner. The introduction of a tert-butyl group on the fluorenyl ligand improved the activity regardless of the position of the tert-butyl substituent, but the 3,6-positions were more effective than the 2,7-positions for the improvement of syn-specificity. As described above, the catalytic abilities of the fluorenylamido-ligated group 4 complexes have been improved by ligand © XXXX American Chemical Society

Received: June 6, 2017

A

DOI: 10.1021/acs.organomet.7b00415 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

exists in the MMAO/BHT system. Therefore, the polymerization was conducted with the dMMAO system by adding the same amount of iBu2Al(OC6H2tBu2Me) (entry 5). The observed activity was similar to that of the MMAO/BHT system. The improved activity in the presence of iBu2Al(OC6H2tBu2Me) can be attributed to the efficient separation of the active ion pair caused by the interaction of an electron-deficient iBu2Al(OC6H2tBu2Me) with the counteranion.13a The polymerization by 1a,b proceeded so quickly that the stirring was stopped within 12 s. We therefore evaluated the polymerization activity by reducing the Al:Ti ratio in these catalysts. To our surprise, 1b showed a very high activity of ∼31000 kg of polymer (mol of Ti)−1 h−1 even with an Al:Ti ratio of 20 (entry 10, Table 2). For comparison, propylene polymerization was conducted with B1 and B2 under the same conditions (entries 13 and 14). The activities were 1 order of magnitude lower than those of 1a,b. The high activity of 1 can be ascribed to the decreased electrophilicity of the Ti cation caused by the electron-donating adamantyl group on the amido ligand, which enhances the separation of the counteranion. To the best of our knowledge, this is the most highly active single-site catalyst with such a low Al:Ti ratio of MAO.19 To shed light on the effect of electron-donating ligands, we also synthesized complexes 2a,b by the introduction of more strongly electron donating methoxy groups at the 3,6-positions and conducted propylene polymerization (entries 15 and 16, Table 2). The activities of 2a,b activated by MMAO/BHT with Al:Ti = 200 were comparable to those of B1 and B2 activated by MMAO/BHT with Al:Ti = 20. Complexes 1, 2, and B activated by MMAO/BHT gave highmolecular-weight PPs (Mn > 100,000) with narrow molecular weight distributions, and the numbers of polymer chains (N) were almost the same and were about 60−70% of the Ti complex used regardless of the change in the Al:Ti ratios from 200 to 20. Thus, the propagation rates of these complexes were evaluated by the turnover frequency determined from the number-average polymerization degree (Pn) and the polymerization time (tp) on the assumption that the polymerization proceeds in a living manner (Table 2). The Pn/tp values of 1a,b with an Al:Ti ratio of 20 were approximately 1 order of magnitude higher than those of B1 and B2 with the same Al:Ti ratio and those of 2a,b with an Al:Ti ratio of 200, indicating that the substitution of tert-butyl on the adamantyl group remarkably enhanced the propagation rate, while the introduction of methoxy groups on the fluorenyl ligand dramatically suppressed the propagation rate. The low N and Pn/ tp values of 1b with an Al:Ti ratio of 10 indicated that a certain

Figure 1. Molecular structures of 1a and 2a,b. Hydrogen atoms are omitted for clarity.

of the amino ligand (θ value in Table1) indicate that 1 and B possess similar steric environments around the cationic Ti center. These results strongly encouraged us to investigate the true electronic effect of the substituent on the ligand in propylene polymerization with these fluorenylamido-ligated titanium complexes. Propylene polymerization was conducted with 1 activated by trialkylaluminum-free dried MMAO (dMMAO). In comparison to the results with complexes B1 and B2 reported previously,17 the adamantyl substituted complexes 1a,b showed 5 times higher activity with an Al:Ti ratio of 200 (entries 1 and 2, Table 2). The modification of trialkyaluminum in MMAO with 2,6-di-tertbutyl-4-methylphenol (BHT) reported previously18 resulted in an increase in the activity (up to ∼56000 kg of polymer (mol of Ti)−1 h−1) by 1 order of magnitude (entries 6 and 7). The difference between dMMAO and MMAO/BHT is that iBu2Al(OC6H2tBu2Me) derived from the reaction of iBu3Al and BHT

Table 1. Selected Bond Lengths (Å) and Angles (deg) of Related Complexes

a

param

1a

2a

2b

B1

B2

Ti−C(1) Ti−C(3) Ti−C(5) Ti−C(4) Ti−C(2) C(1)−C(3) (a) C(1)−C(2) (b) C(3)−C(5) (c) C(2)−C(4) (d) δa = (a + b − c − d)/2 θb = 2/3(θ1 + θ2 + θ3)

2.236(13) 2.436(13) 2.640(14) 2.624(14) 2.382(14) 1.4531(18) 1.4542(19) 1.4277(18) 1.4520(19) 0.027 71

2.227(3) 2.375(3) 2.586(3) 2.595(3) 2.393(3) 1.450(4) 1.453(4) 1.420(4) 1.431(4) 0.026 71

2.248(5) 2.380(5) 2.572(4) 2.622(4) 2.426(5) 1.461(7) 1.455(7) 1.437(7) 1.416(7) 0.032 70

2.236(2) 2.470(2) 2.663(2) 2.620(2) 2.366(3) 1.458(5) 1.451(5) 1.436(5) 1.448(5) 0.025 66

2.247(3) 2.422(3) 2.619(3) 2.609(3) 2.401(3) 1.462(7) 1.447(7) 1.424(7) 1.436(7) 0.025 65

Parameter of hapticity of the fluorenyl ligand. bTolman cone angle of amido groups. B

DOI: 10.1021/acs.organomet.7b00415 Organometallics XXXX, XXX, XXX−XXX

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Organometallics Table 2. Results of Propylene Polymerization with Titanium Complexesa entry

cat.

cocat.

Al:Ti

time (s)

yield (g)

activityb

Mnc (104)

Mw/Mnc

Nd (μmol)

Pne/tp (s−1)

f

1a 1b B1 B2 1a 1a 1b 1b 1b 1b 1b 1a B1 B2 2a 2b

dMMAO dMMAO dMMAO dMMAO dMMAO MMAO/BHT MMAO/BHT MMAO/BHT MMAO/BHT MMAO/BHT MMAO/BHT MMAO/BHT MMAO/BHT MMAO/BHT MMAO/BHT MMAO/BHT

200 200 200 200 200 200 200 100 50 20 10 20 20 20 200 200

60 60 540 420 15 10 12 12 16 17 120 20 60 60 60 75

1.78 1.81 2.36 2.14 1.89 1.55 1.88 1.07 1.41 1.47 0.33 1.57 0.65 0.70 0.77 0.67

5340 5430 787 916 45360 55760 56520 31980 31820 31150 990 28420 3890 4200 4620 3230

16.2 18.2 20.2 16.9

1.49 1.39 1.68 1.45

11 11 12 13

64.3 72.2 8.90 9.58

26.0 32.8 15.1 19.5 22.9 18.3 24.2 10.6 12.6 12.2 10.8

1.47 1.27 1.43 1.34 1.38 1.40 1.47 1.41 1.42 1.40 1.37

6 6 7 7 6 2 6 6 6 6 6

1 2f 3g 4g 5h 6 7 8 9 10 11 12 13 14 15 16

619 651 300 290 321 36.3 288 42.0 50.0 48.4 34.3

Polymerization conditions unless specified otherwise: heptane 30 mL, Ti 10 μmol, propylene 1 atm, 0 °C. bActivity in kg of PP (mol of Ti)−1 h−1. Number-average molecular weight and molecular weight distribution determined by GPC using universal calibration. dNumber of polymer chains calculated from yield and Mn. eNumber-average polymerization degree. fSame polymerization conditions as in ref 17; Ti 20 μmol. gData taken from ref 7a. h140 μmol of Bu3Al/BHT added. a c

GPC curves of PP-b-PP and PP-b-PE shifted to a higher molecular weight region without any other peaks or shoulders, respectively. The 13C NMR resonances corresponding to the each homopolymer (PE and PP) were observed without any isolated propylene subunits (Figure S1 in the Supporting Information), indicating the living polymerization and the formation of a block copolymer with this catalytic system. The 13C NMR spectra of PPs obtained with each complex did not show any regioirregular units, indicating the high regiospecificity of these catalysts regardless of the substituent on the fluorenyl and amido ligand (see the Supporting Information). The steric pentad distributions of the PPs were evaluated from the methyl region of the 13C NMR spectra (Table 3). The results indicate that the PPs obtained were syndiotactic and the syn-specific propagation proceeded via an enantiomorphic-site-controlled mechanism.20 The rrrr value was slightly decreased by the replacement of a tert-butyl amino group by an adamantyl amino group (1a (0.67) < B1 (0.69); 1b (0.83) < B2 (0.86)) due to the increase in the rmrr value arising from the “chain migration” without monomer insertion, although the rmmr values arising from the “monomer mis-insertion” were slightly decreased. The high rmrr value of 1 in comparison with that of 2 can be attributed to the increase in polymerization temperature during the polymerization due to the high activity of 1. Similar phenomena were observed in the syn-specific propylene polymerization with Cs-symmetric zirconium catalysts.21 The rrrr contents of 2a,b were close to that of

amount of MMAO is necessary for the activation of the complex and the separation of active ion pair. The livingness of the propylene polymerization with 1a was then confirmed by block copolymerization in a batch-type operation (Figure 2). In each step, the conversions were almost

Figure 2. Polymerization results and GPC curves obtained in block copolymerization.

quantitative and the Mn values increased with the constant N values kept constant, although the molecular weight distributions were slightly larger than that of an ideal living polymerization system. The broader molecular weight distributions can be attributed to the poor solubility of polymers in heptane decreasing the homogeneity of the polymerization system. The

Table 3. Stereosequence Distributions for Sample Entries 6, 7, 15, and 16 in Table 2a

a

cat.

mmmm

mmmr

rmmr

mmrr

mmrm + rmrr

rmrm

rrrr

mrrr

mrrm

1a 1b 2a 2b B1 B2

0.00 0.00 0.00 0.00 0.00 0.00

0.01 0.00 0.00 0.00 0.00 0.00

0.03 0.01 0.01 0.01 0.04 0.02

0.06 0.02 0.03 0.03 0.08 0.03

0.07 0.04 0.04 0.05 0.05 0.02

0.02 0.00 0.00 0.00 0.02 0.00

0.67 0.83 0.83 0.80 0.69 0.86

0.14 0.10 0.09 0.11 0.12 0.07

0.00 0.00 0.00 0.00 0.00 0.00

Determined by by 13C NMR spectroscopy. bData taken from ref 5. C

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2002, 41, 2236−2257. (h) Gibson, V. C.; Spitzmesser, S. K. Chem. Rev. 2003, 103, 283−315. (i) Makio, H.; Terao, H.; Iwashita, A.; Fujita, T. Chem. Rev. 2011, 111, 2363−2449. (l) Mu, H.; Pan, L.; Song, D.; Li, Y. Chem. Rev. 2015, 115, 12091−12137. (3) (a) Shapiro, P. J.; Bunel, E.; Schaefer, W. P.; Bercaw, J. E. Organometallics 1990, 9, 867−869. (b) Okuda, J.; Schattenmann, F. J.; Wocadlo, S.; Massa, S. W. Organometallics 1995, 14, 789−795. (c) McKnight, A. L.; Waymouth, R. M. Chem. Rev. 1998, 98, 2587− 2598. (4) Xu, G. Macromolecules 1998, 31, 2395−2402. (5) (a) Irwin, L. J.; Reibenspies, J. H.; Miller, S. A. J. Am. Chem. Soc. 2004, 126, 16716−16717. (b) Schwerdtfeger, E. D.; Miller, S. A. Macromolecules 2007, 40, 5662−5668. (c) Schwerdtfeger, E. D.; Irwin, L. J.; Miller, S. A. Macromolecules 2008, 41, 1080−1085. (d) Schwerdtfeger, E. D.; Price, C. J.; Chai, J.; Miller, S. A. Macromolecules 2010, 43, 4838−4842. (e) Chai, j.; Abboud, K. A.; Miller, S. A. DaltonTrans. 2013, 42, 9139−9147. (6) Jung, H. Y.; Hong, S. D.; Jung, M. W.; Lee, H.; Park, Y. W. Polyhedron 2005, 24, 1269−1273. (7) Kirillov, E.; Razaci, A.; Carpentier, J. F. J. Mol. Catal. A: Chem. 2006, 249, 230−235. (8) (a) Na, S. J.; Wu, C. J.; Yoo, J.; Kim, B. E.; Lee, B. Y. Macromolecules 2008, 41, 4055−4057. (b) Yu, S. T.; Na, S. J.; Lim, T. S.; Lee, B. Y. Macromolecules 2010, 43, 725−730. (9) Nakayama, Y.; Sogo, Y.; Cai, Z.; Shiono, T. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 1223−1229. (10) (a) Razavi, A.; Thewalt, U. J. Organomet. Chem. 2001, 621, 267− 276. (b) Busico, V.; Cipullo, R.; Cutillo, F.; Talarico, G.; Razavi, A. Macromol. Chem. Phys. 2003, 204, 1269−1274. (11) Irwin, L. J.; Miller, S. A. J. Am. Chem. Soc. 2005, 127, 9972−9973. (12) (a) Cai, Z.; Su, H.; Nakayama, Y.; Shiono, T.; Akita, M. J. Organomet. Chem. 2014, 770, 136−141. (b) Tanaka, R.; Chie, Y.; Cai, Z.; Nakayama, Y.; Shinon, T. J. Organomet. Chem. 2016, 804, 95−100. (13) (a) Shiono, T. Polym. J. 2011, 43, 331−351. (b) Cai, Z.; Su, H.; Shiono, T. Chin. J. Polym. Sci. 2013, 31, 541−549. (14) Nishii, K.; Hagihara, H.; Ikeda, T.; Akita, M.; Shiono, T. J. Organomet. Chem. 2006, 691, 193−201. (15) (a) Irwin, L. J.; Reibenspies, J. H.; Miller, S. A. Polyhedron 2005, 24, 1314−1324. (b) Chai, J.; Abboud, K. A.; Miller, S. A. Dalton Trans. 2013, 42, 9139−9147. (16) Tolman, C. A. J. Am. Chem. Soc. 1974, 96, 53−60. (17) Cai, Z.; Ikeda, T.; Akita, M.; Shiono, T. Macromolecules 2005, 38, 8135−8139. (18) (a) Busico, V.; Cipullo, R.; Cutillo, F.; Friederichs, N.; Ronca, S.; Wang, B. J. Am. Chem. Soc. 2003, 125, 12402−12403. (b) Cipullo, R.; Busico, V.; Fraldi, N.; Pellecchia, R.; Talarico, G. Macromolecules 2009, 42, 3869−3872. (c) Descour, C.; Sciarone, T. J. J.; Cavallo, D.; Macko, T.; Kelchtermans, M.; Korobkov, I.; Duchateau, R. Polym. Chem. 2013, 4, 4718−4729. (d) Tanaka, R.; Suenaga, T.; Cai, Z.; Nakayama, Y.; Shiono, T. J. Polym. Sci., Part A: Polym. Chem. 2014, 52, 267−271. (19) Shoken, D.; Sharma, M.; Botoshansky, M.; Tamm, M.; Eisen, M. S. J. Am. Chem. Soc. 2013, 135, 12592−12595. (20) (a) Gladysz, J. A. Chem. Rev. 2000, 100, 1167−1682. (b) Ewen, J. A.; Jones, R. L.; Razavi, A.; Ferrara, J. D. J. Am. Chem. Soc. 1988, 110, 6255−6256. (21) (a) Ewen, J. A.; Elder, M. J.; Jones, R. L.; Curtis, S.; Cheng, H. N. Stud. Surf. Sci. Catal. 1990, 56, 439−482. (b) Veghini, D. L.; Henling, M.; Burkhardt, T. J.; Bercaw, J. E. J. Am. Chem. Soc. 1999, 121, 564−573. (c) Chen, M.-C.; Marks, T. J. J. Am. Chem. Soc. 2001, 123, 11803− 11804.

1b, indicating that the steric effect of 3,6-dimethoxy substituents is almost the same as that of 3,6-di-tert-butyl groups. In conclusion, high-performance fluorenylamido-ligated titanium catalysts were developed by the introduction of an electron-donating adamantyl substituent on the amido ligand, which realized highly active catalysts with a very low amount of MMAO for propylene polymerization. Complexes 1a,b showed remarkably high activity (∼31000 kg of polymer (mol of Ti)−1 h−1) with an Al:Ti ratio of 20. The catalytic systems conducted living polymerization of propylene to produce propyleneethylene block copolymer with a narrow molecular weight distribution. The results should be valuable to the ligand design of a transition-metal complex and the choice of a rational cocatalyst for improving the performance of single-site catalysts.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.7b00415. Experimental procedures including synthesis and characterization of the complexes, X-ray diffraction information, polymerization data, and polymer characterization (PDF) Accession Codes

CCDC 1541370−1541372 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail for T.S.: [email protected]. *E-mail for Z.C.: [email protected]. ORCID

Takeshi Shiono: 0000-0002-1118-9991 Zhengguo Cai: 0000-0001-5784-3920 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21174026), the Program for New Century Excellent Talents in University, the Program for Professor of Special Appointment (Eastern Scholar) at the Shanghai Institutions of Higher Learning, the “Shu Guang” project supported by the Shanghai Municipal Education Commission and the Shanghai Education Development Foundation, and the Fundamental Research Funds for the Central Universities. We thank Tosoh-Finechem Co. for a generous donation of MMAO.

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REFERENCES

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DOI: 10.1021/acs.organomet.7b00415 Organometallics XXXX, XXX, XXX−XXX