Cis-Specific Chain Transfer Ring-Opening Metathesis Polymerization

Oct 24, 2017 - Department of Chemistry, Faculty of Science and Engineering, Tokyo Metropolitan University, 1-1 minami Osawa, Hachioji, Tokyo 192-0397,...
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Cis-Specific Chain Transfer Ring-Opening Metathesis Polymerization Using a Vanadium(V) Alkylidene Catalyst for Efficient Synthesis of End-Functionalized Polymers Kotohiro Nomura* and Xiaohua Hou Department of Chemistry, Faculty of Science and Engineering, Tokyo Metropolitan University, 1-1 minami Osawa, Hachioji, Tokyo 192-0397, Japan S Supporting Information *

ABSTRACT: Highly efficient synthesis of end-functionalized polymers has been achieved by combined cis-specific ring-opening metathesis polymerization (ROMP) of norbornene with terminal olefins (1-hexene, allyltrimethylsilane (ATMS), etc.) as the chain transfer (cross metathesis) agent at 80 °C in the presence of V(CHSiMe3)(N-2,6-Cl2C6H3)[OC(CF3)3](PMe3)2. The ROMP proceeds with high activity and high cis selectivity (97−98%) upon addition of PMe3, and the Mn values could be controlled by varying the concentration of terminal olefins; the activity at 80 °C was not affected by the ATMS concentrations (5.0−50.0 mol %, TOF 1860−1890 min−1 ((1.12−1.13) × 105 h−1)). chain transfer ROMP proceeds with 1 at 80 °C, affording ringopened polymers with well-defined chain ends (Scheme 1).10

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lefin metathesis, mediated by a metal carbene (alkylidene) catalyst, is known as a useful method for efficient carbon−carbon bond formation and has been applied to the synthesis of various organic compounds, polymers, and advanced materials.1 In particular, ring-opening metathesis polymerization (ROMP) has been employed for the synthesis of polymeric functional materials.1,2 Both ruthenium carbene (Grubbs type) and molybdenum alkylidene (Schrock type) catalysts are the known successful examples;1 Z-selective (cisspecific) ROMP has also been achieved recently by certain catalysts.1b,c,3−5 (Imido)vanadium(V) alkylidene complexes have also been effective catalysts for the ROMP of cyclic olefins;6,7 in particular, V(CHSiMe3)(N-2,6-Cl2C6H3)(OC6F5)(PMe3)2 exhibited remarkable activities for the ROMP of norbornene (NBE) and its derivatives.7a,b The ROMP of NBE proceeded in a living manner, affording ultrahigh-molecular-weight polymers with low PDI (Mw/Mn) values.7b The complex also initiated ROMP of cyclooctene,7b,c and the activity increased at high temperature (50−100 °C).7b,c Moreover, the cis-specific ROMP could be achieved by the alkoxo analogues V(CHSiMe3)(N2,6-Cl2C6H3)[OC(CF3)3](PMe3)2 (1),7b and both the activity and the selectivity increased upon addition of PMe3 even at 80 °C.7b Notably, this is still a rare demonstration of thermally robust and highly cis specific metathesis reactions (>98% at 80 °C).7b It also turned out that the ROMPs in the presence of 1hexene8 by the above (dichlorophenylimido)vanadium(V) alkylidenes proceeded at 25 °C,7b and the Mn values could be controlled by the degree of chain transfer (cross metathesis). We thus herein demonstrate the combined ROMP with chain transfer with terminal olefins;9 in particular, in this work we wish to demonstrate that highly efficient cis-specific (>98%) © XXXX American Chemical Society

Scheme 1. Synthesis of End-Functionalized Polymers by Combined Cis Specific Ring-Opening Metathesis Polymerization (ROMP) of Norbornene (NBE) with Chain Transfer (Cross Metathesis) of Terminal Olefins

The ROMPs of NBE in the presence of 1-hexene as the chain transfer (cross metathesis, CM) agent (CTA)7b,9,11 were conducted in the presence of 1 at 25 or 80 °C, and the results are summarized in Table 1.11 As reported previously,7b the activity (in the absence of 1-hexene) increased upon addition of PMe3 or at high temperature (runs 1−4, 25 °C vs 80 °C), and the resultant polymers possessed high molecular weights with narrow molecular weight distributions (low PDI values), demonstrated previously as a living nature.7b The olefinic double bonds in the resultant polymers showed high cis selectivity (97−98%). It turned out in the ROMP at 25 °C (in the presence of PMe3) that the activity slightly decreased upon addition of 1-hexene (Figure S1 in the Supporting Received: September 5, 2017

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DOI: 10.1021/acs.organomet.7b00675 Organometallics XXXX, XXX, XXX−XXX

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Table 1. ROMP of Norbornene (NBE) by V(CHSiMe3)(N-2,6-Cl2C6H3)[OC(CF3)3](PMe3)2 (1) in the Presence of 1-Hexene (C6′)a run 1f 2f 3f 4f 5f 6 7 8 9 10 11 12 13 14 15

PMe3/equiv

C6′b/mol %

temp/°C

TONc

TOFc/min−1

10−4Mnd

Mw/Mnd

cise/%

3.0 3.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 30.0 100

25 25 80 80 25 25 25 25 80 80 80 80 80 80 80

3610 6270 4530 6870g 5380 5060 3010 4140 1790 3200 4350 4280 4570 2470 1410

1200 2090 1510 2290 1790 1690 1000 1380 596 1070 1450 1430 1520 823 469

133 142 121 128 24.8 18.1 8.67 7.64 6.28 4.80 3.90 3.72 4.06 2.12 0.95

1.17 1.23 1.28 1.25 1.83 2.16 1.67 1.79 1.51 1.74 2.04 1.87 2.03 1.80 1.57

98 98 98 97 98

3.0 3.0 3.0 10 10 3.0 10 50 100 10 10

94 97 98 98 98

Reaction conditions: complex 1 0.3 μmol, NBE 200 mg (2.12 mmol), benzene 4.8 mL (initial NBE concentration 0.44 mmol/mL), 3 min. bInitial concentration in mol % to NBE. cTON (turnovers) = (NBE reacted (mmol))/(V (mmol)), TOF = TON/time. dGPC data in THF vs polystyrene standards (Mn in g/mol). eCis percentage (%) estimated by 1H NMR spectra.11 fReported data in ref 7b. gYield >97%. a

Information),11 although the polymerization reached completion over longer reaction times.7b,11 The Mn values decreased upon increasing the 1-hexene concentration with broadening of the Mw/Mn values, clearly demonstrating that a certain degree of chain transfer (CM) reaction proceeded under these conditions (Figure S1).11 The catalyst 1 still showed high cis selectivity. Importantly, the activity in the presence of 1-hexene increased at 80 °C (e.g. TOF = 1380 min−1 (at 25 °C, run 8) vs 1450 (at 80 °C, run 11), upon addition of PMe3), whereas the activity decreased if the polymerization was conducted in the absence of PMe3 (Table 1, run 7 vs run 9).12 Both the activity and the Mn value were unchanged by varying the PMe3/ V molar ratios (10−100, runs 11−13) under these conditions. Moreover, the Mn values in the polymers prepared at 80 °C (in the presence of 1-hexene) were lower than those prepared at 25 °C, suggesting that the degree of chain transfer (CM) was affected by the polymerization temperature (Mn = 76400 (run 8) vs Mn = 37200−40600 (runs 11−13)). The resultant polymers showed high cis selectivity (97−98%), although a slight decrease in the selectivity was observed if the ROMP was conducted without addition of PMe3 (94%, run 9).12 It was revealed that resonances ascribed to protons in the vinyl (−CHCH2) group were clearly observed in the 1H NMR spectrum for the resultant polymer (sample, run 13, Figure S2 in the Supporting Information), and the Mn value estimated on the basis of integration ratio (with olefinic protons) was 23500, which is rather low in comparison to that measured by GPC versus polystyrene standards (Mn = 40600); this is generally observed in the ring-opened poly(NBEs).11 Table 2 summarizes the results in the ROMPs upon the presence of various terminal olefins such as 1-octene, allyltrimethylsilane (ATMS), vinylcyclohexane (VCH), 4vinyl-1-cyclohexene (VCHE), and 3,3-dimethyl-1-butene (DM1B) conducted at 80 °C in the presence of PMe3.11 Note that the activity in the ROMP in the presence of ATMS at 80 °C was not affected by changes in the ATMS concentrations (5.0−50.0 mol %), whereas the Mn values could be controlled by varying the concentration (runs 17−22) as well as the polymerization temperature (run 16 vs run 17).

Table 2. ROMP of Norbornene (NBE) by V(CHSiMe3)(N2,6-Cl2C6H3)[OC(CF3)3](PMe3)2 (1) in the Presence of Chain Transfer Agents (CTA) at 80 °Ca run

CTA/mol %

TONb

TOFb/ min−1

10−4Mnc

16 17 18 19 20 21 22 11 23 24 25 26 27

ATMS (5.0)e ATMS (5.0) ATMS (10.0) ATMS (30.0) ATMS (30.0) ATMS (50.0) ATMS (50.0) 1-hexene (10.0) 1-octene (10.0) VCH (10.0) VCHE (10.0) DM1B (10.0) cis-4-octene (10.0) cis-stilbene (10.0) cis-DC2B (10.0) cis-DC2B (10.0)e

4890 5490 5590 5740 6970f 5590 5660 4350 2940 6660 5490 5450 6620

1630 1830 1860 1910 1390 1860 1890 1450 980 2220 1830 1820 2210

42.7 15.2 8.73 3.45 3.35 2.16 1.94 3.90 5.14 9.85 9.97 104 148

1.78 2.46 2.43 2.46 2.24 2.16 2.43 2.04 1.68 2.35 1.95 2.56 2.58

5810

1940

145

2.48

98

2120 3970

708 1320

105 81.5

1.78 1.99

99

28 29 30

Mw/Mnc cisd/%

98 98 99 97 98 98 98 98 98 98

a Reaction conditions: complex 1 0.3 μmol, NBE 200 mg (2.12 mmol), benzene 4.8 mL (initial NBE concentration 0.44 mmol/mL), 80 °C, 3 min. Abbreviations: ATMS = allyltrimethylsilane, VCH = vinylcyclohexane, VCHE = 4-vinyl-1-cyclohexene, DM1B = 3,3-dimethyl-1butene, cis-DC2B = cis-1,4-dichloro-2-butene. bTON (turnovers) = (NBE reacted (mmol))/(V (mmol)), TOF = TON/time. cGPC data in THF vs polystyrene standards (Mn in g/mol). dCis percentage (%) estimated by 1H NMR spectra.11 ePolymerization at 25 °C. fReaction 5 min, yield >98%.

The reaction reached completion after 5 min without a change in the Mn value (run 20), and the results are reproducible (runs 21 and 22). The activity at 25 °C, however, became negligible if the ROMP was conducted at 25 °C without addition of PMe3 under rather high ATMS concentration (10.0 mol %, shown in the Supporting Information).11 B

DOI: 10.1021/acs.organomet.7b00675 Organometallics XXXX, XXX, XXX−XXX

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demonstration of “chain transfer cis-specific ROMP at high temperature” but also a limited example of precise and efficient catalytic synthesis of end-functionalized ROMP polymers. We are exploring the utility of this method for application of the synthesis of grafting polymers (via grafting from/to approach) and other cross metathesis reactions, including the development of more active catalysts (that would also enable tacticity control).

Importantly, as shown in Figure 1 (Table 2, run 18; other data are shown in Figures S3−S5 in the Supporting

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.7b00675. Experimental details including additional polymerization results and selected NMR spectra for resultant polymers (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail for K.N.: [email protected]. ORCID Figure 1. 1H NMR spectrum (in CDCl3 at 25 °C) for poly(NBE) (Table 2, run 18, conducted at 80 °C, allyltrimethylsilane (ATMS) 10 mol %, PMe3/V = 10 (molar ratio)). The satellite (SSB, spinning side band) for the cis olefinic double bond is denoted with #.

Kotohiro Nomura: 0000-0003-3661-6328 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This project was partially supported by a Grant-in-Aid for Scientific Research on Innovative Areas (“3D Active-Site Science”, No. 26105003) from The Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan, and a Grant-in-Aid for Scientific Research (B) from the Japan Society for the Promotion of Science (JSPS, No. 15H03812). X.H. acknowledges the Tokyo Metropolitan government (Asian Human Resources Fund) for a predoctoral fellowship. X.H. and K.N. express their thanks to Profs. K. Tsutsumi, S. Komiya, A. Inagaki, and Dr. S. Sueki (Tokyo Metropolitan University) for discussions.

Information),11 resonances corresponding to protons in the SiMe3 and vinyl group (−CHCH2) were clearly observed and the integration ratios (9:3 SiMe3:−CHCH2 in the resultant polymers with various Mn values)11 strongly suggest that the resultant polymers possessed both SiMe3 and vinyl groups at each polymer chain end.13,14 Similarly, the ROMPs in the presence of VCH, VCHE, and 1-octene afforded polymers with Mn values that are relatively similar to those obtained in the presence of ATMS, 1-hexene. Moreover, protons corresponding to the vinyl group were also observed by their 1H NMR spectra (and the Mn values estimated by the integration ratios showed similar trends).11,14 The results thus suggest that the chain transfer ROMP (tandem ROMP and CM or combined ROMP with CM) proceeded in the presence of these olefins. In contrast, as is also shown in Table 2, the ROMPs in the presence of DM1B (tertbutylethylene) gave high-molecular-weight polymer (run 26), and use of cis-4-octene, cis-stilbene, and cis-1,4-dichloro-2butene15 also afforded high-molecular-weight polymers with rather broad molecular weight distributions. These results suggest that the chain transfer (CM) reactions did not occur efficiently under these conditions.15 The results thus demonstrate that terminal olefins with less steric bulk would be used as the chain transfer (CM) agents in this catalysis. We have shown that an efficient “cis-specific” chain transfer ROMP has been achieved even at 80 °C by V(CHSiMe3)(N2,6-Cl2C6H3)[OC(CF3)3](PMe3)2 (1) in the presence of terminal olefins (1-hexene, ATMS, VCH, etc.) as the chain transfer (cross metathesis) agents, to afford well-defined endfunctionalized polymers catalytically. The Mn value could be controlled by varying the concentration of the terminal olefin (and the polymerization temperature), and addition of PMe3 plays a role in increasing the activity while maintaining the high cis selectivity of the olefinic double bonds in the resultant polymers. As far as we know, this is not only the first

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REFERENCES

(1) For selected books, see: (a) Handbook of Metathesis; Grubbs, R. H., Ed.; Wiley-VCH: Weinheim, Germany, 2003. (b) Olefin Metathesis: Theory and Practice; Grela, K., Ed.; Wiley: Hoboken, NJ, 2014. (c) Handbook of Metathesis, 2nd ed.; Grubbs, R. H., Wenzel, A. G., O’Leary, D. J., Khosravi, E., Eds.; Wiley-VCH: Weinheim, Germany, 2015. (2) For selected reviews, see ref 1 and: (a) Buchmeiser, M. R. Chem. Rev. 2000, 100, 1565−1604. (b) Nomura, K.; Abdellatif, M. M. Polymer 2010, 51, 1861−1881. (c) Leitgeb, A.; Wappel, J.; Slugovc, C. Polymer 2010, 51, 2927−2946. (d) Buchmeiser, M. R. Macromol. Symp. 2010, 298, 17−24. (e) Mutlu, H.; de Espinosa, L. M.; Meier, M. A. R. Chem. Soc. Rev. 2011, 40, 1404−1445. (3) For selected reports of Z-selective ROMP by Mo and W catalysts, see ref 1 and: (a) Flook, M. M.; Jiang, A. J.; Schrock, R. R.; Muller, P.; Hoveyda, A. H. J. Am. Chem. Soc. 2009, 131, 7962−7963. (b) Flook, M. M.; Gerber, L. C. H.; Debelouchina, G. T.; Schrock, R. R. Macromolecules 2010, 43, 7515−7522. (c) Marinescu, S. C.; Ng, V. W. L.; Lichtscheidl, A. G.; Schrock, R. R.; Müller, P.; Takase, M. K. Organometallics 2012, 31, 6336−6343. (d) Jeong, H.; Kozera, D. J.; Schrock, R. R.; Smith, S. J.; Zhang, J.; Ren, N.; Hillmyer, M. A. Organometallics 2013, 32, 4843−4850. (e) Forrest, W. P.; Weis, J. G.; John, J. M.; Axtell, J. C.; Simpson, J. H.; Swager, T. M.; Schrock, R. R. J. Am. Chem. Soc. 2014, 136, 10910−10913. (f) Autenrieth, B.; Jeong, H.; Forrest, W. P.; Axtell, J. C.; Ota, A.; Lehr, T.; Buchmeiser, M. R.; C

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Organometallics Schrock, R. R. Macromolecules 2015, 48, 2480−2492. (g) Autenrieth, B.; Schrock, R. R. Macromolecules 2015, 48, 2493−2503. (h) Gonsales, S. A.; Kubo, T.; Flint, M. K.; Abboud, K. A.; Sumerlin, B. S.; Veige, A. S. J. Am. Chem. Soc. 2016, 138, 4996−4999. (i) Jang, E. S.; John, J. M.; Schrock, R. R. ACS Cent. Sci. 2016, 2, 631−636. (j) Lienert, C.; Frey, W.; Buchmeiser, M. R. Macromolecules 2017, 50, 5701−5710. (4) For selected reports of Z-selective ROMP by Ru catalysts, see ref 1c and: (a) Keitz, B. K.; Fedorov, A.; Grubbs, R. H. J. Am. Chem. Soc. 2012, 134, 2040−2043. (b) Rosebrugh, L. E.; Marx, V. M.; Keitz, B. K.; Grubbs, R. H. J. Am. Chem. Soc. 2013, 135, 10032−10035. (c) Khan, R. K. M.; Torker, S.; Hoveyda, A. H. J. Am. Chem. Soc. 2013, 135, 10258−10261. (d) Mikus, M. S.; Torker, S.; Xu, C.; Li, B.; Hoveyda, A. H. Organometallics 2016, 35, 3878−3892. (5) (a) Schrock, R. R. Dalton Trans. 2011, 40, 7484−7495. (b) Schrock, R. R. Acc. Chem. Res. 2014, 47, 2457−2466. (6) (a) Yamada, J.; Nomura, K. Organometallics 2005, 24, 2248− 2250. (b) Nomura, K.; Onishi, Y.; Fujiki, M.; Yamada, J. Organometallics 2008, 27, 3818−3824. (c) Nomura, K.; Suzuki, K.; Katao, S.; Matsumoto. Organometallics 2012, 31, 5114−5120. (d) Nomura, K.; Bahuleyan, B. K.; Tsutsumi, K.; Igarashi, A. Organometallics 2014, 33, 6682−6691. (7) (a) Hou, X.; Nomura, K. J. Am. Chem. Soc. 2015, 137, 4662− 4665. (b) Hou, X.; Nomura, K. J. Am. Chem. Soc. 2016, 138, 11840− 11849. (c) Chaimongkolkunasin, S.; Hou, X.; Nomura, K. J. Polym. Sci., Part A: Polym. Chem. 2017, 55, 3067−3074. (8) For examples of ROMP of cyclic olefins in the presence of 1hexene and 1-octene, see: (a) Hayano, S.; Kurakata, H.; Tsunogae, Y.; Nakayama, Y.; Sato, Y.; Yasuda, H. Macromolecules 2003, 36, 7422− 7431. (b) Hayano, S.; Tsunogae, Y. Macromolecules 2006, 39, 30−38. (c) Hayano, S.; Nakama, Y. Macromolecules 2014, 47, 7797−7811. (9) For examples of ROMP in the presence of a chain transfer agent, see: (a) Crowe, W. E.; Mitchell, J. P.; Gibson, V. C.; Schrock, R. R. Macromolecules 1990, 23, 3534−3536. (b) Hillmyer, M. A.; Grubbs, R. H. Macromolecules 1993, 26, 872−874. (c) Hillmyer, M. A.; Grubbs, R. H. Macromolecules 1995, 28, 8662−8667. (d) Hillmyer, M. A.; Nguyen, S. T.; Grubbs, R. H. Macromolecules 1997, 30, 718−721. (e) Morita, T.; Maughon, B. R.; Bielawski, C. W.; Grubbs, R. H. Macromolecules 2000, 33, 6621−6623. (f) Bielawski, C. W.; Benitez, D.; Morita, T.; Grubbs, R. H. Macromolecules 2001, 34, 8610−8618. (g) Matson, J. B.; Virgil, S. C.; Grubbs, R. H. J. Am. Chem. Soc. 2009, 131, 3355−3362. (h) Martinez, H.; Hillmyer, M. A. Macromolecules 2014, 47, 479−485. (i) Diallo, A. K.; Annunziata, L.; Fouquay, S.; Michaud, G.; Simon, F.; Brusson, J. M.; Guillaume, S. M.; Carpentier, J. F. Polym. Chem. 2014, 5, 2583−2591. (j) Diallo, A. K.; Michel, X.; Fouquay, S.; Michaud, G.; Simon, F.; Brusson, J. M.; Carpentier, J. F.; Guillaume, S. M. Macromolecules 2015, 48, 7453−7465. (k) Nagarkar, A. A.; Kilbinger, A. F. M. Nat. Chem. 2015, 7, 718−723. (10) Part of these data were presented by K.N. at the 3rd International Conference on Molecular & Functional Catalysis 2017 (ICMFC-3), Singapore, March, 2017. (11) The general experimental procedure, additional polymerization data, and selected NMR spectra, including estimation of Mn values on the basis of integration ratio, are given in the Supporting Information. (12) In our previous report,7a,b we observed a fast equilibrium of PMe3 coordination and dissociation in solution and assumed coordination of at least one PMe3 for exhibiting the high cis specificity to form the proposed intermediate. Moreover, we also assume that PMe3 is also necessary to stabilize this electron-deficient vanadium alkylidene.7b,c (13) The Mn values on the basis of integration ratio (expressed as Mn(NMR)) in the resultant polymer in the presence of ATMS were 47100 (Mn(GPC) = 87300, run 18), 20000 (Mn(GPC) = 34500, run 19), and 14700 (Mn(GPC) = 19400, run 22), respectively. These results clearly indicate that the Mn value could be controlled by an introduction of ATMS as the chain transfer agent. The results are shown in the Supporting Information. (14) One reviewer commented that a possibility of formation of polymers having two vinyl and/or SiMe3 groups (ROMP polymers with same chain ends) cannot be discounted even in small amount.

Further confirmation of the selectivity will be done by a postmodification (grafting) approach or by model cross metathesis reactions in the near future. (15) Significant decreases in the activity (both TOF and TON values after 3 min) were not observed when the ROMPs were conducted in the presence of cis-1,4-dichloro-2-butene (DCB) at 25 °C, affording ultrahigh-molecular-weight polymers (more data are shown in Table S2 in the Supporting Information). Although the activity decreased at 80 °C (run 29), the results suggest that DCB did not affect (disturb) the activity and the chain transfer (CM) did not take place at 25 °C.

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DOI: 10.1021/acs.organomet.7b00675 Organometallics XXXX, XXX, XXX−XXX