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Letter pubs.acs.org/macroletters
Stereoselective Copolymerization of Butadiene and Functionalized 1,3-Dienes Hannes Leicht, Inigo Göttker-Schnetmann, and Stefan Mecking* Chair of Chemical Materials Science, Department of Chemistry, University of Konstanz, 78464 Konstanz, Germany S Supporting Information *
ABSTRACT: [(Mesitylene)nickel(allyl)+][BArF4−] (Ni-1) catalyzes the 1,4-cis selective copolymerization of isoprene and 1,3-butadiene with 15 (RO)3Si-, R2N-, RSO2-, RSO2NH-, and (RO)2B-functionalized 1,3-dienes. Incorporation of the functionalized 1,3-diene occurs very efficiently with high comonomer conversions, as observed, for example, for copolymerizations of 1,3-butadiene with (EtO)3Si(CH2)3C(CH2)CHCH2 (1), PhS(O)2(CH2)3C(CH2)−CH CH2 (7), or (pinB)C(CH2)C(CH3)CH2 (15), while copolymerization rates vary from strongly to slightly decreased when compared to butadiene homopolymerizations.
M
Chart 1. Alkoxysilane Substituted 1,3-Dienes Used in Copolymerizations with BD and IP
etal-catalyzed insertion polymerizations of olefinic monomers are of enormous practical importance for the synthesis of a variety of materials. Polyethylenes, polypropylenes, and cis-1,4-polybutadiene are each produced on scales of many million tons annually. This is due to the ability to control polymer microstructures. A restriction to date is the limited ability to incorporate polar monomers. Fundamental advances have been achieved recently in copolymerizations of ethylene with polar monomers by using less oxophilic late transition metal complexes.1 By contrast, an incorporation of monomers with functional groups in 1,3butadiene (BD) copolymerization is achieved exclusively using anionic or free-radical polymerization techniques.2,3 However, these polymerizations do not allow control over the polymer microstructure. This is particularly notable as on the one hand the unique stereoregularity arising from catalytic butadiene polymerization is essential for elastomer properties and, on the other hand, especially in elastomers, a compatibility with polar fillers like silica or metal surfaces is a ubiquitous issue here. A direct incorporation of (reactive) functionalities during polymerization would be attractive compared to current additional steps of postpolymerization chemistry preceding the vulcanization process. We now report on the first stereoselective insertion copolymerization of 1,3-butadiene (BD) with a variety of polar-functionalized dienes. Trialkoxysilyl-functionalized monomers 1−3 were obtained in purities >97% after a cuprate catalyzed coupling of 3(iodopropyl)trialkoxysilane with 1,3-dien-2-yl-magnesium reagents, while 4 and 6 obtained as an isomeric mixture after hydrosilylation of 2-methyl-but-1-en-3-yne with triethoxysilane in purities >98% (Chart 1, for experimental details, cf. Supporting Information). To probe for the principal viability of functionalization during copolymerization, [(η6-mesitylene)nickel(η3-allyl)+][BArF4−] (Ni-1) was exposed to mixtures of monomer 1 and © XXXX American Chemical Society
1,3-butadiene in CH2Cl2 solution at 0 °C (1:BD ca. 1:25 to 1:41). An observable increase of viscosity occurred within less than 30 min (Table 1, entries 1−1 to 1−3; for isoprene (IP) copolymerizations with selected comonomers, cf. Supporting Information). After isolation by precipitation in methanol the obtained polymers were scrutinized by extensive NMR analysis. In contrast to BD-homopolymers obtained under similar conditions, 1H NMR analysis already indicates the presence of ethoxy (3.82 and 1.18 ppm) and SiCH2 groups (0.63 ppm) in these polymers. The formation of a true random copolymer with a defined microstructure (≥94% 1,4-cis enchainment) was unambiguously proven by GPC and 1D-1H-TOCSY, HSQCTOCSY, and DOSY experiments in addition to HSQC and HMBC-experiments. Furthermore, the polymer of entry 1−3 was analyzed by a 13C−13C-INADEQUATE experiment, clearly indicating the connectivity of the SiCH2-carbon down to the PBD-backbone (cf. Supporting Information). Notably, a major portion of monomer 1 subjected to copolymerizations is consumed and incorporated into the formed copolymer. Under conditions where BD is nearly completely consumed (Table 1, entries 1−1 and 1−2), incorporative consumption of 1 was determined as 89 and 93%, respectively. More striking, even at low BD consumption (ca. 45%, entries 1−3), incorporative consumption of 1 is close to quantitative Received: April 27, 2016 Accepted: June 6, 2016
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DOI: 10.1021/acsmacrolett.6b00321 ACS Macro Lett. 2016, 5, 777−780
Letter
ACS Macro Letters Table 1. (Co)polymerizations of Butadiene and Different Si(OR)3-Substituted Comonomersa entry 1−1 1−2 1−3 1−4 1−5 1−6 1−7 1−8 1−9 1−10 1−11 1−12 1−13
Ni-1 (μmol) f,g
10 + 30 40g 10 + 40f,g 10 + 20 20 + 20f,h 10 + 25f,h 5 + 30f 10h 3i 5 + 2f 4i 4i 5 + 3i
solvent
T (°C)
time (h)
CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 toluene toluene toluene toluene toluene toluene toluene toluene toluene
0 0 0 0 40 30 22 0 22 0 22 22 0
4.0 1.0 4.5 2 3.8 2.0 3.5 0.5 2.8 1.8 2.7 5.5 4.0
BD 6.3 g 6.9 g 6.6 g 6.8 g 25 mL 25 mL 1.05 bar 30 mL 22.5 g 11.0 g 1.05 bar 1.05 bar 1.05 bar
comonomer (mmol)
yield (g)
1 (2.8) 1 (4.7) 1 (4.7)
6.1 7.2 3.2 3.1 12.4 14.6 8.4 8.4 13.5 4.5 11.2 12.6 9.4
3 (3.0) 4 (2.6) 5 (1.1)
1 (0.39) 1 (1.1) 2 (0.58)
comonomer incorp.b (mol %)
comon. conv. (%)
2.2 3.3 7.8
89 93 98
1.2 0.9 0.5
92 96 65
0.4
90
0.5 0.2
95 77
TON
Mnc (g mol−1)
2800 3300 1200 1900 5700 7700 4400 15000 83000 12000 52000 58000 22000
21000 19000 11000 25000 33000 50000 44000 148000 93000 100000 80000 62000 133000
Mw/Mnc
Tg d (°C)
1,4-cis contente (%)
2.8 3.4 2.7 3.4 2.8 3.2 3.6 2.5 3.2 2.4 2.7 2.6 2.3
−96 −94 −93 n.d. −96 −97 −96 −96 −96 −97 −95 −98 n.d.
94 94 94 94 95 95 95 96 95 96 95 95 96
a
Reaction conditions unless noted otherwise: 20 mL solvent, magnetically stirred in a Schlenk tube. bCalculated from 1H NMR. cDetermined by GPC in THF vs PS standards. dDetermined by DSC. eCalculated from 13C NMR spectra. fx + y indicates a successive addition of Ni-1 until polymerization activity was observed by increasing viscosity. gCatalyst batch contained ca. 50% excess BArF4−. hPolymerization in a pressure reactor with mechanical stirring, 30 mL of solvent. i45 mL of solvent.
with 98%. Higher degrees of incorporation (10−37 mol %) and even a homopolymerization of monomer 1 were successfully performed in NMR-tube experiments (cf. Supporting Information). In addition to these high levels of comonomer incorporation, the obtained copolymers exhibit molecular weights similar to those obtained in BD homopolymerizations under otherwise identical conditions as well as a similarly high stereoregularity of ≥94% 1,4-cis units (Table 1, entries 1−1 to 1−3 vs entry 1−4). Higher molecular weight (co)polymers than isolated from polymerizations in methylene chloride were obtained from copolymerization in toluene solution (Table 1, entries 1−5 to 1−13) using different reactor setups and initial batch or continuous BD-feed. Entry 1−12 in Table 1 deserves a special emphasis as high incorporative consumption of 1 (95%) into the copolymer can be combined with high overall activities (58000 TON) under constant BD feed. By comparing polymer yields in Table 1, entries 1−11 and 1−12, however, we note that the copolymerization proceeds somewhat slower than the BD homopolymerization. Copolymerizations with (RO)3Si-modified monomers 2−6 were also successful with (very) high conversions of the comonomer independent of the substitution pattern. These high conversions prompted us to perform in situ NMR copolymerizations, which show that 1 and 4, as well as E/Z-6 are preferentially incorporated compared to BD (Supporting Information).4 Figure 1 shows 1H NMR traces of the copolymerization of 1 with BD at a BD:1:Ni-1 ratio of about 240:24:1 after 5−100 min reaction time at −9 °C. Clearly, the trans-coupled vinylic signal of 1 marked with the * is not detectable anymore after 20 min reaction time, while BD consumption is only about 55%. These results point to copolymerization parameters of kCoMo/kBD > 1 for comonomers 1, 4, and E/Z-6 and are consistent with the observed high comonomer conversions in the reactor polymerizations. Practically more relevant, it enables an efficient copolymerization at low comonomer loadings without wasting comonomer even under constant BD-feed (e.g., entries 1−12 and 1−13). Promising as these results are, some limitations were encountered: (a) Due to impurities in the monomers 1−6
Figure 1. 1H NMR spectra monitoring a BD/1 copolymerization, indicating full conversion of 1 (vinylic signal marked *) after 20 min, while only about 55% BD is consumed (for assignments, cf. Supporting Information).
(97−98% pure), the maximum comonomer: Ni-1 ratio at which copolymerizations proceeded was about 275:1 (entry 1− 12, Table 1) and in some instances, additional Ni-1 had to be added in order to observe formation of copolymers.5 (b) Independent of the presence of a comonomer, effective (co)polymerizations resulting in high overall turnover numbers (TON) require a high BD concentration of at least 20 wt % BD.6 This, however, also limits the percentaged yield due to increasing mass transfer limitations with increasing viscosity of the reaction mixture during the polymerization. (c) Molecular weights of (co)polymers exceeding 100.000 g mol−1 require high BD:Ni-1 ratios, high BD concentrations, and polymerization temperatures below room temperature (Table 1, entries 1−8, 1−9, 1−11, and 1−13 vs all other experiments). Encouraged by these promising results BD homopolymerizations in the presence of model compounds bearing different polar groups were conducted in order to assess the compatibility of Ni-1 toward a broader range of functional groups. While Ni-1 shows no activity in the presence of amides, primary-, secondary-, and aliphatic tertiary amines (e.g., triethyl 778
DOI: 10.1021/acsmacrolett.6b00321 ACS Macro Lett. 2016, 5, 777−780
Letter
ACS Macro Letters
hampered incorporation of the comonomer compared to butadiene. In fact, the opposite is the case; the efficient incorporation of the comonomer allows to generate copolymers with a deliberately chosen composition without wasting excess (unreacted) comonomer, even at low comonomer concentrations and a constantly applied BD feed. However, all copolymers exhibit high number-average molecular weights in the range of 28.000−140.000 g mol−1 and reasonable molecular weight distributions suggesting a single site catalysis. Finally, all copolymers exhibit a virtually identical, defined microstructure of 94−95% 1,4-cis-enchained diene units. The poor or problematic polymerization behavior in copolymerizations with 11, 16, and 17 prompted us to follow copolymerizations with these comonomers by means of 1H NMR spectroscopy. While Ni-1 shows no polymerization activity at all in the presence of 16, even at elevated temperatures, the presence of 11 or 17 is not preventing copolymerization, but causes a significantly reduced polymerization activity: 17 was successfully copolymerized with BD at elevated temperatures (BD:17:Ni-1 = 655:33:1). Although the overall activity was drastically decreased, 17 is enchained faster by Ni-1 than BD, resulting in 46% BD conversion at full consumption of 17 after 7 days at 50 °C. Remarkably, 17 is the only comonomer whose presence in the reaction mixture changes the microstructure of the obtained polymer from a high 1,4-cis content to 83% 1,4-trans units.8 Copolymerization in the presence of 11 shows a similar course, that is, a major part of the BD is consumed only after the comonomer has been fully consumed. This effect is clearly observable for a copolymerization of 11 (BD:11:Ni-1 = 290:10:1), where 11 is completely consumed after 92 h at 22 °C, while only 22% of the BD have been polymerized. Further 56% BD are subsequently polymerized in only 50 h at the same temperature. In conclusion, the cationic nickel complex [(mesitylene)Ni(allyl)+][BArF4−] (Ni-1) enables for the first time the effective copolymerization of alkoxysilyl functionalized dienes with butadiene with high TONs to produce stereoregular copolymers (≥94% 1,4-cis units) with high molecular weights. Due to the extraordinary functional group tolerance of Ni-1, this approach extends to providing for the first time access to highly
amine), polymerizations are possible in the presence of secondary or tertiary aromatic amines (e.g., methylphenylamine or phenylpyrrolidine). Homopolymerization of BD is also observed in the presence of p-butylbenzenesulfonamide. These results prompted us to study 11 different diene comonomers containing polar groups based on B, N, P, and S (Chart 2, for IP copolymerizations with selected comonomers, cf. Supporting Information). Chart 2. Comonomers Synthesized and Used in Copolymerizations with BD and IP
Except for PhNH-functionalized monomer 11 and phosphonate-functionalized 16 and 17, all copolymerizations resulted in the formation of substantial amounts of copolymers (Table 2). A marked decrease in catalyst activity was observed for BD copolymerizations in the presence of monomers 13 and 14 (entries 2−8 and 2−9; probably owing to the substantial amounts of impurities in the used crude products).7 BD copolymerizations in the presence of monomers 7, 8, 9, 10, 12, and 15 proceeded with reasonable catalyst activity when compared to a BD homopolymerization (Table 2, TOF entries 2−2, 2−3, 2−4, 2−5, 2−7, 2−10, and 2−11 vs entry 2−1). Comonomers 7, 8, 9, 10, and 15 can be copolymerized with moderate to very high conversions between 65 (10) and 99% (15) under comparable conditions (entries 2−3, 2−4, 2−5, 2− 10, and 2−11). Lower conversions between 22 (13) and 42% (14) are observed for comonomers 12, 13, and 14. This means, the low comonomer incorporation ratios are not a result of a
Table 2. Copolymerizations with Butadiene and Comonomers Bearing Polar Groups Based on N, S, P, and Ba entry
Ni-1 (μmol)
T (°C)
time (h)
2−1 2−2 2−3 2−4 2−5 2−6 2−7 2−8 2−9 2−10 2−11
5 10 10 10 10 6f 10 10 + 30g 10 + 80g 10 6f
22 22 0 22 22 0 22 22 22 22 0
1 0.5 0.5 1 0.5 4 1.2 2.5 3 1 4
BD 1.05 1.05 1.05 1.05 1.05 1.05 1.05 1.05 1.05 1.05 1.05
bar bar bar bar bar bar bar bar bar bar bar
comonomer (mmol)
yield (g)
7 (0.5) 8 (0.2) 9 (2.0) 10 (1.0) 10 (0.5) 12 (0.21) 13 (0.2) 14 (0.4) 15 (2.06) 15 (0.43)
6.2 4.5 5.6 7.4 6.3 6.2 6.0 2.2 3.3 4.9 13.2
comonomer incorp.b (mol %) 0.48 0.14 1.2 0.56 0.27 0.07 0.11 0.4 1.85 0.175
comon. conv. (%)
TON diene Ni1−
TOF diene Ni1− h−1
Mnc (g mol−1)
Mw/Mn
80 73 82 65 60 37 22 42 81 99
23000 8300 10300 14000 12000 19000 11000 1000 750 9000 41000
23000 16600 20600 14000 24000 4750 9166 n.a.h n.a.h 9000 10250
59000 44000 53000 48000 44000 123000 59000 51000 28000 37000 140000
3.9 2.4 2.9 2.8 3.2 2.2 3.1 2.3 2.2 2.4 1.9
c
Tg d (°C)
1,4-cis contente (%)
−97 −94 −94 −96 −93 −95 −96 −97 −98 −95 −96
95 95 95 95 95 95 95 94 95 95 95
a
Reaction conditions unless noted otherwise: 20 mL toluene, magnetically stirred with a high inertia neodymium magnet in a 100 mL Schlenk tube. Calculated from 1H NMR spectra. cDetermined by GPC in THF vs PS standards. dDetermined by DSC. eCalculated from 13C NMR spectra. f45 mL solvent. gx + y indicates a successive addition of Ni-1 until polymerization activity was observed by increasing viscosity. hSubsequent addition of Ni-1 does not allow for a reasonable TOF calculation. b
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DOI: 10.1021/acsmacrolett.6b00321 ACS Macro Lett. 2016, 5, 777−780
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ACS Macro Letters
(8) The presence of phosphites in nickel allyl catalyzed BD polymerizations also causes a change in the microstructure of the obtained polymer from 1,4-cis to 1,4-trans: Taube, R.; Gehrke, J.-P.; Schmidt, U. J. Organomet. Chem. 1985, 292, 287−296.
stereoregular B-, N-, P-, and S-containing poly(dienes) by insertion polymerization. Remarkably, most of the comonomers studied are very effectively incorporated into the formed polymer, even at low comonomer and high butadiene concentrations.
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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/acsmacrolett.6b00321. NMR spectra, GPC and DSC data, and synthetic procedures (PDF).
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We enjoyed fruitful extensive discussions with Karen Burke, Maggie Flook, and Stephan Rodewald. Financial support by The Goodyear Tire & Rubber Company is gratefully acknowledged. We thank Lars Bolk for DSC and GPC measurements and Patrick Herr for the synthesis and copolymerization of 2.
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REFERENCES
(1) (a) Johnson, L. K.; Mecking, S.; Brookhart, M. J. Am. Chem. Soc. 1996, 118, 267−268. (b) Drent, E.; van Dijk, R.; van Ginkel, R.; van Oort, B.; Pugh, R. I. Chem. Commun. 2002, 744−745. (c) Chen, E. Y. X. Chem. Rev. 2009, 109, 5157−5214. (d) Nakamura, A.; Ito, S.; Nozaki, K. Chem. Rev. 2009, 109, 5215−5244. (2) (a) Beery, M. D.; Rath, M. K.; Sheares, V. V. Macromolecules 2001, 34, 2469−2475. (b) Jing, Y.; Sheares, V. V. Macromolecules 2000, 33, 6255−6261. (c) Sheares, V. V.; Wu, L.; Li, Y.; Emmick, T. K. J. Polym. Sci., Part A: Polym. Chem. 2000, 38, 4070−4080. (d) Sunada, K.; Takenaka, K.; Shiomi, T. J. Appl. Polym. Sci. 2005, 97, 1545−1552. (e) Takenaka, K.; Kawamoto, S.; Miya, M.; Takeshita, H.; Shiomi, T. Polym. Int. 2010, 59, 891−895. (f) Wu, L.; Sheares, V. V. J. Polym. Sci., Part A: Polym. Chem. 2001, 39, 3227−3238. (g) Yang, Y.; Lee, J.; Cho, M.; Sheares, V. V. Macromolecules 2006, 39, 8625−8631. (h) Yang, Y.; Sheares, V. V. Polymer 2007, 48, 105−109. (3) For a (nonstereoselective) catalytic homopolymerization of 2,3bis(4-trifluoroethoxy-4-oxobutyl)-1,3-butadiene, cf. O’Connor, A. R.; Brookhart, M. J. Polym. Sci., Part A: Polym. Chem. 2010, 48, 1901− 1912. For a copolymerization of isoprene with 2-(4-methoxyphenyl)butadiene, cf. Yao, C.; Liu, N.; Long, S.; Wu, C.; Cui, D. Polym. Chem. 2016, 7, 1264−1270. (4) Note that extraction of kinetic data from these NMR experiments is hampered by a strong increase in viscosity of the sample during the experiment and an overlapping of relevant resonances. (5) The nature of these impurities has not been unraveled so far, but a detrimental effect on these copolymerizations of, e.g., 3-(iodopropyl) trialkoxysilane present in monomers 1−3 (ca. 0.2−0.5%) can be excluded on the basis of control polymerizations in presence of these impurities. (6) A saturated BD solution (at 1.05 bar BD) in toluene at 22 °C contains ca. 24−25 wt % BD. (7) Upon isolation, 13 and 14 proved to decompose (probably by [4 + 2] cycloaddition) based on which they were used as heptane stock solutions of the crude reaction products in copolymerization experiments. 780
DOI: 10.1021/acsmacrolett.6b00321 ACS Macro Lett. 2016, 5, 777−780
