Macromolecules 2000, 33, 6621-6623
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Notes A Ring-Opening Metathesis Polymerization (ROMP) Approach to Carboxyl- and Amino-Terminated Telechelic Poly(butadiene)s Takeharu Morita, Bob R. Maughon, Christopher W. Bielawski, and Robert H. Grubbs* Arnold and Mabel Beckman Laboratories of Chemical Synthesis, Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125 Received January 4, 2000 Revised Manuscript Received June 26, 2000
Over the past several years there has been an increasing interest in the synthesis of polymers with low glass transition temperatures and reactive functional groups at each end of a given polymeric chain. They have found use as agents capable of modifying the thermal and mechanical properties of condensation polymers, in the formation of polymeric networks, and as components in the synthesis of block copolymers.1-3 Telechelic hydroxyl,4-6 amino,7-9 and carboxyl10,11 poly(butadiene)s1 (PBD)s are excellent examples of such polymers that have been employed in the above applications. While these telechelic polymers can be synthesized through the radical or anionic polymerization of 1,3-butadiene using the appropriate initiating or terminating agents, there are several drawbacks. The average number of functional groups per polymer chain (Fn) can deviate greatly from 2 and depends heavily on experimental conditions, which are often demanding.4-10 Furthermore, varying amounts of 1,2-linkages are introduced during the polymerization, limiting the elastomeric potential of the resulting polymer.3 In addition to radical and anionic routes to telechelic PBDs, several approaches involving olefin metathesis have been explored.12 In the presence of a difunctional acyclic olefin, metathesis degradation of high molecular weight PBD13-16 and the ROMP of cyclooctadiene (COD)14,17-19 have been successfully employed. The inclusion of an acyclic olefin is critical to the whole process, because it effectively acts as a chain transfer agent (CTA) that not only regulates molecular weight but ultimately determines the functional group that is placed at the end of the polymer chains.12 The extent of functional group diversity that can be incorporated into the CTA is limited by the metathesis catalyst stability in the presence of such groups. Classical ill-defined catalysts such as WCl6/SnMe420-22 typically require high catalyst loadings and undesired vinyl end groups are found23,24 when functionalized CTAs are employed. The development of well-defined tungsten25 and molybdenum26-28 alkylidene complexes not only helped broaden the range of functional group tolerance but also aided in understanding the nature of chain transfer that occurs between growing polymer chains
and the CTA.12,29,30 It was discovered that neighboring functional groups deactivate the catalyst through coordination or polarization effects. Thus, generally two or more methylene spacers between the olefin and any functionality in the CTA are necessary.29,31,32 Rutheniumbased catalysts, such as (PCy3)2Cl2RudCHPh (1),33 display a high tolerance to a vast array of functional groups including esters, ketones, alcohols, and amides, and in general, only one methylene spacer between the olefin and any functional group is required.34-36 This tolerance has allowed more freedom in the choice of functionality that can be incorporated in the CTA and thus at the end of the polymer chains. Recently, we reported the synthesis of hydroxylterminated telechelic PBDs via the ruthenium-catalyzed ROMP of COD in the presence of cis-1,4-bis(acetoxy)2-butene followed by a post-polymerization deprotection step.19 In this paper, we extend this approach to carboxyl- and amino-terminated telechelic PBDs by employing appropriately functionalized CTAs (Scheme 1). These polymers contained a 1,4-PBD backbone exclusively and exhibited a Fn near 2, and their molecular weights were controllable up to 10 kDa. Ideally, CTAs bearing the desired functionality would be employed directly, as this circumvents the need for any post-polymerization transformations. However, while ruthenium catalyst 1 is tolerant of functional groups such as amines and carboxylic acids, we found that in the presence of 3-hexenedioic acid or 1,4-diamino-2butene, its activity is diminished over the time scale of the polymerization. This suppressed CTA incorporation and gave low yields of telechelic polymer. Hence, CTA’s bearing protected carboxylic acids and amines were employed. The protected carboxyl CTA, cis-1,4-bis(2-tertbutoxycarbonyl)-2-butene (2), was easily synthesized via addition of commercially available cis-2-butene-1,4-diol to tert-butyl acrylate under basic conditions (92% yield).37 The protected amino CTA, cis-1,4-di-tert-butyl-2-butene1,4-dicarbamate (3), was synthesized in two steps as illustrated in Scheme 2. Excess potassium phthalimide was reacted with cis-1,4-dichloro-2-butene to afford the known38 intermediate cis-1,4-diphthalimido-2-butene (8). Purification of 8 was not required, and treatment of this compound with HOAc/HCl gave cis-1,4-diamino2-butene‚2HCl39 (9) as a crystalline solid in 72% overall yield. Introduction of the carbamate group was achieved by the addition of triethylamine to a methanolic solution of di-tert-butyl dicarbonate and 9, affording 3 in 96% yield.40 Treatment of either 2 or 3 with formic acid quantitatively provided cis-(4-carboxymethoxy-but-2enyloxy)acetic acid (6) or cis-1,4-diamino-2-butene38 (7), respectively. The ROMP of COD in the presence of CTA 2 or 3 resulted in the corresponding bis-functionalized telechelic PBD 4 or 5 as shown in Scheme 1. The polymerizations were initiated with 1 using a [COD]/[1] ratio of 4000:1 and run for 24 h at 45 °C. Telechelic PBDs
10.1021/ma000013x CCC: $19.00 © 2000 American Chemical Society Published on Web 08/04/2000
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Notes
Macromolecules, Vol. 33, No. 17, 2000 Scheme 1
Scheme 2
Figure 2. (a) Plot of monomer consumption, CTA incorporation, and percent trans olefin in the polymer backbone vs time. Conversions were monitored using gas chromatography. Olefin content was determined by 1H NMR spectroscopy. (b) Plot of molecular weight and PDI vs time. The Mn and PDI values were determined by GPC and are reported relative to PS standards. A 0.5 correction factor was applied.43 Table 1. Synthesis of Telechelic PBDs 4 and 5 with Various Molecular Weightsa polymer [COD]/[CTA] % yieldb Mn (NMR)c Mn (GPC)d PDId 4 4 4 4 5 5 5 5
15 30 50e 70f 15 30 50e 70f
78 86 88 85 75 86 88 85
1700 3100 5600 8500 1800 3000 5600 8000
2300 4000 5100 7100 2200 3300 5800 7700
1.59 1.86 2.02 2.06 1.70 2.09 2.05 2.06
a Polymerizations were run neat at 45 °C for 24 h with [COD]/ [1] ) 4000. For polymer 4, CTA ) 2; for polymer 5, CTA ) 3. b Isolated yields after purification. c Determined by 1H NMR analysis assuming Fn ) 2.0. d Determined by gel permeation chromatography in CH2Cl2 relative to monodispersed poly(styrene) standards. A correction factor of 0.5 was applied.39 e To ensure better mixing, [COD]0 ) 4.9 M in toluene was used. f [COD]0 ) 4.3 M in toluene was used.
Figure 1. Dependence of the observed Mn values on the [COD]/[CTA] ratio for the bis-functionalized telechelic PBDs 4 (closed circles) and 5 (open circles). The Mn values were determined by 1H NMR analysis assuming Fn ) 2.0 (see Table 1).
with a variety of molecular weights were synthesized (Table 1) and a linear relationship between Mn (NMR) and the [COD]/[CTA] ratio was observed (Figure 1) for both CTAs. As previously observed in similar sys-
tems,14,17-19 the PBDs contained approximately 35% trans olefin and only 1,4-linkages were found.41 The stereochemistry of the polymer end groups were determined to be approximately 70% trans olefin geometry. Monomodal MW distribution of polymers with PDIs ranging from 1.59 to 2.09 were obtained in yields ranging between 71% and 88%. Both 1H and 13C NMR spectroscopy supported a Fn near 2.0 and no terminal methylene resonances were observed.42 Good agreement between the molecular weights obtained by NMR analysis and GPC provide further evidence for the difunctionality (Fn ≈ 2.0) of these telechelic polymers.43 To help provide a better understanding of the polymerization systems described above, the bulk polymerization of COD in the presence of 2 or 3 was monitored over time using a combination of gas chromatography, GPC, and 1H NMR spectroscopy. As shown in Figure 2, for an initial COD/1 ) 4000 and COD/2 ) 20, the rate of COD consumption was slightly higher than the rate of CTA incorporation. This is reflected in the molecular weight and polydispersity of the resulting polymer as both appeared to increase over the early stages of the polymerization. During the later stages, the continuing incorporation of CTA gradually reduced the polymer’s molecular weight. The trans olefin content in the polymer backbone (∼35%) remained relatively constant throughout the polymerization. Similar observations were obtained when 3 or cis-1,4-bis(acetoxy)-2-butene19 was used as the CTA. The ester groups on PBDs 4 (Mn 1700-8500) and 5 (Mn 1800-8000) were removed with excess formic acid to give the corresponding bis(carboxyl)-functionalized telechelic PBDs 6 or bis(amino)-functionalized telechelic PBDs 7 (Scheme 1). The efficiency of deprotection depended on the molecular weight of the starting material (Table 2). While relatively low molecular weight polymers (Mn < 8000) were deprotected with high conversion, their higher molecular weight coun-
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Notes
Table 2. Synthesis of Telechelic PBDs 6 and 7 via Deprotection of PBDs 4 and 5 protected deprotected polymer Mn (NMR)a polymer % conversionb Mn (NMR)a 4 4 4 4 5 5 5 5
1700 3100 5600 8500 1800 3000 5600 8000
6 6 6 6 7 7 7 7
100 96 87 18 100 96 85 15
1500 2800 5500 8300 1600 2800 5300 7700
a Molecular weights determined by 1H NMR analysis assuming Fn ) 2.0. b Determined by 1H NMR analysis.
terparts were only moderately soluble in formic acid/ CH2Cl2 solutions, thus limiting the extent of deprotection. A Fn near 2 for the lower molecular weight materials was supported by 1H and 13C NMR analysis.42 Although cis/trans isomerization is known to occur under acidic conditions, no change in the polymer microstructure was observed. In conclusion, the ROMP of COD in the presence of an appropriately difunctionalized acyclic chain transfer agent bearing either protected amino or carboxyl groups afforded the respective end-functionalized PBDs. The polymerizations were initiated with (PCy3)2Cl2RudCHPh and molecular weight could be controlled through varying the initial COD/CTA ratio. Postpolymerization deprotection under mild conditions provided amino and carboxyl end-terminated telechelic PBDs. The polymers synthesized via this approach possess a 1,4-PBD microstructure (ca. 65% cis olefin) exclusively and the average number of functional groups per polymer chain was found to be near 2. In conjunction with previous results detailing the synthesis of hydroxyl-terminated PBDs via various metathetical routes, this report demonstrates that telechelic PBDs bearing the three most common functionalities can be readily prepared through olefin metathesis. Acknowledgment. Funding for this research was provided by the National Science Foundation and the Bayer Corp. T.M. gratefully acknowledges the Sekisui Chemical Co. for financial support. C.W.B. thanks the NSF for a predoctoral fellowship. References and Notes (1) Goethals, E. J. Telechelic Polymers: Synthesis and Applications; CRC Press: Boca Raton, FL, 1989. (2) Van Caeter, P.; Goethals, E. J. TRIP 1995, 3, 227. (3) Odian, G. Principles of Polymerizations, 3rd ed.; WileyInterscience: New York, 1991. (4) Brosse, J.-C.; Derouet, D.; Epaillard, F.; Soutif, J.-C.; Legeay, G.; Dusek, K. Adv. Polym. Sci. 1987, 81, 167. (5) Schnecko, G.; Degler, H.; Dongowski, R.; Caspary, G.; Angerer, S.; Ng, T. Angew. Makromol. Chem. 1978, 70, 9. (6) Kanakavel, M. Makromol. Chem. 1987, 188, 845. (7) Xu, J.; Dimonie, V. L.; Sudol, E. D.; El-Aasser, M. S. J. Polym. Sci., A-1 1995, 33, 1353. (8) Heitz, W.; Ball, P.; Lattekamp, M. Z. Kautschuk Gummi Asbest Kunstst. 1981, 34, 459. (9) Hinney, H. R.; Baghdadchi, J. U.S. Pat. 4,658,062, 1987. (10) Evans, W.; Baxendale, J. Trans. Faraday Soc. 1946, 42, 140. (11) Reed, S. F. J. Polym. Sci., A-1 1971, 9, 2147. (12) Ivin, K. J.; Mol, J. C. Olefin Metathesis; Academic Press: London, 1997. (13) Hummel, K. Pure Appl. Chem. 1982, 54, 351. (14) Chung, T. C.; Chasmawala, M. Macromolecules 1992, 25, 5137.
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(15) Wagener, K. B.; Marmo, J. C. Macromol. Rapid Commun 1995, 16, 557. (16) Tamura, H.; Maeda, N.; Matsumoto, R.; Nakayama, A.; Hayashi, H.; Ikushima, K.; Kuraya, M. J. Macromol. Sci.s Pure Appl. Chem. 1999, A36, 1153. (17) Cramail, H.; Fontanille, M.; Soum, A. J. Mol. Catal. 1991, 65, 193. (18) Hillmyer, M. A.; Grubbs, R. H. Macromolecules 1993, 26, 872. (19) Hillmyer, M. A.; Nguyen, S. T.; Grubbs, R. H. Macromolecules 1997, 30, 718. (20) Otton, J.; Colleuille, Y.; Varagnat, J. J. Mol. Catal. 1980, 8, 313. (21) Reyx, D.; Campistron, I.; Hamza, J. J. Mol. Catal. 1986, 36, 101. (22) Chung, T. C.; Chasmawala, M. Macromolecules 1991, 24, 3718. (23) Nubel, P. O.; Yokelson, H. B.; Lutman, C. A.; Bouslog, W. G.; Behrends, R. T.; Runge, K. D. J. Mol. Catal. A 1997, 115, 43. (24) Grubbs, R. H.; Hoppin, C. R. J. Chem. Soc., Chem. Commun. 1977, 634. (25) Johnson, L. K.; Virgil, S. C.; Grubbs, R. H.; Ziller, J. W. J. Am. Chem. Soc. 1990, 112, 5384. (26) Schrock, R. R.; Murdzek, J. S.; Bazan, G. C.; Robbins, J.; DiMare, M.; O’Regan, M. J. Am. Chem. Soc. 1990, 112, 3875. (27) Bazan, G. C.; Khosravi, E.; Schrock, R. R.; Feast, W. J.; Gibson, V. C.; O’Regan, M. B.; Thomas, J. K.; Davis, W. M. J. Am. Chem. Soc. 1990, 112, 8378. (28) Bazan, G. C.; Oskam, J. H.; Cho, H.-N.; Park, L. Y.; Schrock, R. R. J. Am. Chem. Soc. 1991, 113, 6899. (29) Hillmyer, M. A.; Grubbs, R. H. Macromolecules 1995, 28, 8662. (30) Hillmyer, M. A. Ph.D. Thesis, California Institute of Technology, 1995. (31) Wagener, K. B.; Brzezinska, K. Macromolecules 1991, 24, 5273. (32) Patton, J. T.; Boncella, J. M.; Wagener, K. B. Macromolecules 1992, 25, 3862. (33) Schwab, P.; Grubbs, R. H.; Ziller, J. W. J. Am. Chem. Soc. 1996, 115, 110. (34) Hillmyer, M. A.; Laredo, W. R.; Grubbs, R. H. Macromolecules 1995, 28, 6311. (35) Maughon, B. R.; Grubbs, R. H. Macromolecules 1996, 29, 5765. (36) Maughon, B. R.; Grubbs, R. H. Macromolecules 1997, 30, 3459. (37) Representative spectroscopic data for CTA 2. 1H NMR (300 MHz, CDCl3): δ 5.65 (t, J ) 4.3 Hz, 2H), 4.01 (d, J ) 4.3 Hz, 4H), 3.61 (t, J ) 7.0 Hz, 4H), 2.44 (t, J ) 7.0 Hz, 4H), 1.40 (s, 18H). 13C NMR (75 MHz, CDCl3): δ 170.09, 128.73, 79.69, 66.04, 65.30, 35.67, 27.50. HR-MS exact mass: calculated for [M + 1]+ C18H32O6, 345.2278; found, 345.2265. (38) (a) Marquez, V. E.; Liu, P. S.; Kelley, J. A.; Driscoll, J. S. J. Org. Chem. 1980, 45, 485. (b) Feigenbaum, A.; Lehn, J. M. Bull. Soc. Chim. Fr. 1973, 198. (39) He, Z.; Nadkarni, D. V.; Sayre, L. M.; Greenaway, F. T. Biochim. Biophys. Acta 1995, 1253, 117. (40) Representative spectroscopic data for CTA 3. 1H NMR (300 MHz, CDCl3): δ 5.50 (t, J ) 4.7 Hz, 2H), 4.84 (bs, 2H), 3.77 (t, J ) 6.3 Hz, 4H), 1.44 (s, 18H). 13C NMR (75 MHz, CDCl3): δ 155.86, 128.79, 79.31, 37.03, 28.33. HR-MS exact mass: calculated for [M + 1]+ C14H26N2O4, 287.1970; found, 287.1966. (41) The glass transition temperatures (Tg) of the telechelic polymers synthesized in this work were found to occur near -105 °C using differential scanning calorimetry (DSC). This provides further evidence for a very high content of 1,4-PBD linkages in the polymer backbone.3. (42) Monofunctionalized polymers were independently synthesized using terminal olefin CTAs. The resonances of functionalized termini and terminal methylenes were clearly present in 1H and 13C NMR spectra. Morita, T.; Bielawski, C.; Grubbs, R. H. Unpublished results. (43) A correction factor of 0.5 was applied in order to obtain more reliable molecular weight information. This value has been previously reported; see: Wu, Z.; Wheeler, D. R.; Grubbs, R. H. J. Am. Chem. Soc. 1992, 114, 146. Katz, T. J.; McGinnbis, J.; Altus, C. J. Am. Chem. Soc. 1976, 98, 606.
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