Article pubs.acs.org/Macromolecules
Synthesis of Well-Defined Unsaturated Polyesters by TransitionMetal-Catalyzed Polycondensation of Bis(diazoacetate)s Hiroaki Shimomoto, Yuji Hara, Tomomichi Itoh, and Eiji Ihara* Department of Material Science and Biotechnology, Graduate School of Science and Engineering, Ehime University, 3 Bunkyo-cho, Matsuyama 790-8577, Japan S Supporting Information *
ABSTRACT: A variety of aliphatic- and aromatic-type unsaturated polyesters (UPs) were prepared by transitionmetal-catalyzed single-component polycondensation of bis(diazoacetate)s under a mild condition. With the secondgeneration Grubbs catalyst, the polycondensation proceeded exclusively through an intermolecular highly cis-selective CC forming coupling of diazo-bearing carbons with N2 release, giving well-defined UPs. The cis-CCs of the resulting polymers could be isomerized quantitatively into trans-CCs with a catalytic amount of diethylamine. Additionally, other metal complexes, the first-generation Grubbs catalyst, rhodium(II) acetate, and copper(II) acetylacetonate, also produced UPs from the bis(diazoacetate)s, with lower stereoselectivities, although an unexpected carbene oligomerization of the monomers partially occurred along with the CC bond-forming coupling.
■
INTRODUCTION Diazocarbonyl compounds are versatile reagents utilized in a variety of reactions, such as cyclopropanation with olefins, X− H insertion (X = C, O, N, etc.), and ylide generation.1,2 In addition, not only for organic synthesis, diazocarbonyl compounds can be used as a monomer for transition-metalcatalyzed polymer synthesis.3−8 In particular, Cu-, Pd-, and Rhmediated polymerization of diazoacetates has been demonstrated to afford polymers bearing a substituent on every main chain carbon atom.4−6 Furthermore, we have recently succeeded in polycondensation of bis(diazocarbonyl) compounds based on their versatile reactivities with various functional groups. Actually, the polycondensation of bis(diazoketone)s with aromatic diols and dicarboxylic acids was found to produce unique three-component poly(ether ketone)s7 and poly(ester ether ketone)s,8 respectively, based on insertion of carbenes derived from diazoketones into O−H groups accompanied by unexpected incorporation of a ringopened cyclic ether used as a solvent. Transition-metal-catalyzed CC forming coupling of diazocarbonyl compounds is also carbene-based reaction,9 which is occasionally encountered as an unwanted side reaction of diazocarbonyl decomposition. Effective utilization of the alkene formation for the polymer synthesis will provide a new type of single-component polycondensation of bis(diazocarbonyl) compounds (Scheme 1A).10,11 It should be noted that the resulting polymers are classified as unsaturated polyesters (UPs),12 which contain a reactive double-bond bearing ester carbonyl carbons on both sides in every repeating unit. UPs have been used as thermosetting resins by cross© 2013 American Chemical Society
Scheme 1. Polymerization Routes for the Synthesis of Unsaturated Polyesters: (A) This Work and (B) Conventional Methods
linking with a vinyl monomer13 and thus are of great importance for industrial applications. A common method for UP synthesis is polycondensation of maleic anhydride with a diol accompanied by ring-opening of the former monomer (Scheme 1Ba). However, it requires high temperature and long reaction periods and often affords illdefined products with low molecular weights. Catalytic chaingrowth copolymerization of maleic anhydride with epoxides is another method for the synthesis of UPs (Scheme 1Bb),14−21 Received: May 20, 2013 Revised: June 19, 2013 Published: June 28, 2013 5483
dx.doi.org/10.1021/ma401047x | Macromolecules 2013, 46, 5483−5487
Macromolecules
Article
Table 1. Polycondensation of M1−4 Using C1−4a
although this system also suffered from low reactivity and undesirable side reactions. Recently, Coates et al. reported a chromium(III)−salen complex capable of copolymerizing maleic anhydride with epoxides under mild conditions to afford UPs with higher molecular weights and with cis-selective CCs.22 This excellent catalyst can give well-defined UPs with a variety of functional groups. However, the functional groups are inevitably located in a position lateral to the backbone and the main-chain structure [poly(propylene maleate) framework] cannot be changed on the ground that the copolymerization involves ring-opening of epoxides. On the other hand, with our proposed method (Scheme 1A), UPs with a variety of functional groups in the backbone may be obtained by single-component polycondensation of bis(diazoacetate)s, which can be readily prepared from the corresponding diols,23 including aromatic alcohols. UPs derived from aromatic-type diazoacetates would give cross-linked products with high mechanical property after copolymerization with vinyl monomers. Very recently, Liu et al. reported polycondensation of aliphatic-type bis(diazoacetate)s using Cu catalysts with nonstereoselectivity in the CC foramtion.11 Independently from them, we have studied polycondensation of aliphatic-type as well as aromatic-type bis(diazoacetate)s using various transition-metal catalysts (Ru, Rh,10 and Cu). In particular, it was found that the second-generation Grubbs catalyst can afford well-defined UPs with highly cis-selective CCs in the main chain. Herein, we report the well-defined UP synthesis and the difference in catalytic performance of Ru-, Rh-, and Cu-based complexes on the polycondensation with respect to the resulting polymer structures.
entry
catalyst
monomer
yieldd (%)
Mn
Mw/Mn
cis:transe
1 2 3 4 5b 6 7 8c 9 10 11 12 13 14 15 16 17
C1
M1 M2 M3 M4
C2
M1 M2 M3 M4 M1 M2 M3 M4 M1 M2 M3 M4
45 39 61 38 46 46 40 − 49 22 21 42 59 18 31 65 56
2300 3100 6900 9100 19800 4500 4500
1.71 2.08 2.18 1.97 2.15 2.10 12.2
86:14 85:15 94:6 − − 79:21 75:25
18600 2200 1900 22100 5800 5400 3700 8200 6000
3.13 1.51 2.01 1.64 1.62 1.51 2.59 4.56 2.77
− 42:58 43:57 44:56 − 32:68 32:68 35:65 −
C3
C4
a
[Catalyst] = 2.0 mM, [M1−4]/[C1−4] = 100/1, in CH2Cl2 (3.0 mL) at 30 °C for 17 h. bPolymerization in 1,1,2,2-tetrachloroethane at 80 °C. cPartially insoluble to CHCl3 for recycling GPC. dAfter purification with preparative GPC to remove lower molecular weight oligomers. eUnable to determine for polyM4′ due to overlapping peaks.
purification with preparative recycling GPC to remove lower molecular weight oligomers. The molecular weights increased with increasing the reaction temperature, although longer reaction time did not result in significant increase of the molecular weights. For the polycondensation of M4, raising the reaction temperature to 80 °C afforded a polymer with Mn = 19 800 in 46% (entry 5). The polymer structures were confirmed by 1H NMR spectroscopy (Figure 1). In each spectrum, the signals corresponding to the expected repeating unit that should be generated via the coupling of diazocarbonyl groups accompanied by N2 release were clearly observed, and no other significant peaks were present. In addition, for polyM1′−3′, separated signals for the cis (δ = 6.2−6.3 ppm) and trans (δ = 6.8−6.9 ppm) vinyl protons enabled us to determine the stereoregularity; cis-CC was preferentially formed (cis:trans = 86:14 for polyM1′, 85:15 for polyM2′, and 94:6 for polyM3′).25 The cis selectivities were equal to or slightly lower than those of homocoupling products obtained by the corresponding “monofunctional” alkyl diazoacetates (cis:trans = 98:2 for ethyl diazoacetate, 86:14 for n-butyl diazoacetate, and 95.5:4.5 for cyclohyexyl diazoacetate).24b To explore the catalyst scope of this condensation polymerization, we examined a variety of transition-metal catalysts. It is known that the first-generation Grubbs catalyst (C2) shows high reactivity (but low stereoselectivity) compared to C1 for the coupling of alkyl diazoacetates.24 Polycondensation of M1− 4 using C2 was thus conducted under the same conditions as using C1, giving polymers with higher molecular weights (Table 1, entries 6−9). However, the stereoselectivity of CC double bond was lower than those of obtained using C1 (cis:trans = 79:21 for polyM1′ and 75:25 for polyM2′). Aside from the Grubbs catalysts, other transition-metal complexes may be able to use for the polycondensation of bis(diazocarbonyl) compounds, since it is known that various transition-metal complexes (e.g., ruthenium,26−29 osmium,30
■
RESULTS AND DISCUSSION First, we examined polycondensation of a series of bis(diazocarbonyl) compounds M1−4 (see Scheme 2) using the Scheme 2. Synthesis of Unsaturated Polyesters by Polycondensation of Bis(diazoacetate)s (M1−4) Using Transition-Metal Catalysts (C1−4)
second-generation Grubbs catalyst (C1), which is recently reported to catalyze homocoupling of “monofunctional” alkyl diazoacetates in high yields and with high stereoselectivity.24 Polycondensation of M1−4 with C1 ([M1−4]/[C1] = 100/1) was conducted in CH2Cl2 at 30 °C for 17 h (Table 1, entries 1−4). Aliphatic-type bis(diazoacetate)s M1−3 as well as aromatic-type bis(diazoacetate) M4 produced polymeric products with GPC-estimated Mns of 2300−9100, after 5484
dx.doi.org/10.1021/ma401047x | Macromolecules 2013, 46, 5483−5487
Macromolecules
Article
C2−4 are attributed to the formation of an unexpected structure, as described below. To examine the polymer structure, a model reaction for the polycondensation was conducted using hexyl diazoacetate M5 [monofunctional counterpart to aliphatic-type bis(diazoacetate) (M1−3)] and phenyl diazoacetate M6 [monofunctional counterpart to aromatic-type bis(diazoacetate) (M4)], as shown in Scheme 3. As a result, C1 gave desirable dimers Scheme 3. Model Reaction Using Hexyl Diazoacetate (M5) and Phenyl Diazoacetate (M6)
Table 2. Model Reaction Using Hexyl Diazoacetate (M5) and Phenyl Diazoacetate (M6)a yield (%)
1
Figure 1. H NMR spectra of polyM1′ (A, entry 1 in Table 1), polyM2′ (B, entry 2 in Table 1), polyM3′ (C, entry 3 in Table 1), and polyM4′ (D, entry 4 in Table 1).
entry
catalyst
monomer
dimer
oligomer
cis:trans in dimer
1 2 3 4 5 6 7 8
C1
M5 M6 M5 M6 M5 M6 M5 M6
92 94 62 75 63 38 67 55
0 0 12 6 12 11 12 4
93:7 69:31 85:15 87:13 45:55 42:58 36:64 45:55
C2 C3 C4
a [Catalyst] = 2.0 mM, [M5,6]/[C1−4] = 100/1, in CH2Cl2 (3.0 mL) at 30 °C for 17 h.
nickel,31 rhodium,32,33 copper33,34) have been shown to effect the coupling of diazocarbonyl compounds. In fact, rhodium(II) acetate, C3, produced polymeric products (Mn = 1900−22 100) in 21−59% yields with poor stereoselectivity (cis:trans = 42:58 to 44:56 for polyM1′−3′), and an inexpensive Cu catalyst, copper(II) acetylacetonate (C4), also gave polymers (Mn = 3700−8200) in 18−65% yields (cis:trans = 32:68 to 35:65 for polyM1′−3′) (Table 1, entries 10−17). As mentioned in the Introduction, Liu and co-workers11 conducted polycondensation of aliphatic-type bis(diazoacetate)s including M2 using various Cu catalysts [C4, copper(II) trifluoromethanesulfonate, and copper(II) hexafluoroacetylacetonate] in toluene at 100 °C. UPs with molecular weights in the range of Mn = 2900−35 400 were obtained in 29.0−78.8% yields. The stereoselectivity of CC double bond was low and varied considerably among conditions such as polymerization time and the ratio of monomer and catalyst (cis:trans = 23.9:76.1 to 68.1:31.9). As mentioned above, it was revealed that Rh and Cu catalysts can also produce UPs, with lower stereoselectivity. However, in the 1H NMR spectra of polymers obtained by these catalysts (C2−4), signal intensities for vinyl protons were obviously smaller than the theoretical values calculated based on the other repeating units (C2: 62−79%; C3: 57−74%; C4: 46−83%) after considering the defect in terminal structures, while the vinyl content values for the polymers obtained by C1 were close to the theoretical value (82−95%). The lower values for
[dimer(M5′,6′)] in almost quantitative yields (Table 2, entries 1 and 2), where CC bond-forming coupling was successfully proceeded. The stereoselectivity of dimerM5′ and dimerM6′ was determined from the relative signal intensity between cisand trans-alkene in the 1H NMR spectrum (Figure S1) and gravimetry of the isolated stereoisomers with recycling GPC,35 respectively. On the other hand, C2−4 produced dimers in 38−75% yields, along with a small amount of low-molecularweight oligomers (entries 3−8). The product oligomer obtained in the model reaction using M5 and C3 (Mn = 940, Mw/Mn = 1.17) was analyzed by MALDI-TOF-MS spectrometer (Figure S3). In the spectrum, a series of peaks separated by the formula weight of the repeating unit derived from M5 were observed (M5′, CHCO2C6H13, m/z = 142), indicating the byproducts were C−C main chain oligomers bearing a hexyloxy substituent on each main-chain carbon atom (oligoM5′), as is the case in Cu-, Pd-, and Rh-mediated polymerization of diazoacetates.4−6 The 1H NMR spectrum of the product also supported the formation of such oligomer (Figure S4). These results of the model reaction suggest that polycondensation of bis(diazoacetate)s using C2−4 produced UPs with a slight irregularity of the distribution of unsaturated bond, where the C−C single bond formation competitively occurred along with 5485
dx.doi.org/10.1021/ma401047x | Macromolecules 2013, 46, 5483−5487
Macromolecules
■
the CC bond-forming coupling. Three or more sequential C−C single bond formations result in a branched structure. Next, we investigated thermal properties of the resulting polymers. Thermogravimetric analysis showed that aromatictype UPs had a somewhat higher decomposition temperature than those of aliphatic-type UPs (Scheme S5). We also measured glass transition temperature (Tg); the Tg of polyM4′ was 76 °C, which was higher than that of polyM2′ (Tg = 39.4 °C).11 These results indicated that incorporation of aromatic groups in polymer backbone was effective to raise decomposition temperature and Tg. Finally, we examined cis−trans isomerization of the obtained UPs.19,22 It is known that maleic esters can be converted into the fumaric form by means of some amine compounds as a catalyst.36 Thus, catalytic isomerization with diethylamine was conducted in CDCl3 at room temperature for 48 h. Figure 2
AUTHOR INFORMATION
Corresponding Author
*Phone +81-89-927-8547; e-mail
[email protected] (E.I.). Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS The authors thank Venture Business Laboratory in Ehime University for its assistance in NMR measurements. This work was partially supported by the Ehime Industrial Promotion Foundation.
■
REFERENCES
(1) Doyle, M. P.; Mckervey, M. A.; Ye, T. Modern Catalytic Methods for Organic Synthesis with Diazo Compounds; John Wiley & Sons: New York, 1998. (2) Zhang, Z.; Wang, J. Tetrahedron 2008, 64, 6577−6605. (3) Imoto, M.; Nakaya, T. J. Macromol. Sci., Rev. Macromol. Chem. 1972, C7, 1−48. (4) Liu, L.; Song, Y.; Li, H. Polym. Int. 2002, 51, 1047−1049. (5) Ihara, E. Adv. Polym. Sci. 2010, 231, 191−231. (6) (a) Jellema, E.; Jongerius, A. L.; Reek, J. N. H.; de Bruin, B. Chem. Soc. Rev. 2010, 39, 1706−1723. (b) Franssen, N. M. G.; Walters, A. J. C.; Reek, J. N. H.; de Bruin, B. Catal. Sci. Technol. 2011, 1, 153−165. (7) Ihara, E.; Saiki, K.; Goto, Y.; Itoh, T.; Inoue, K. Macromolecules 2010, 43, 4589−4598. (8) Ihara, E.; Hara, Y.; Itoh, T.; Inoue, K. Macromolecules 2011, 44, 5955−5960. (9) In ref 1, pp 624−633. (10) Hara, Y.; Shimomoto, H.; Itoh, T.; Ihara, E. Polym. Prepr. Jpn. 2012, 61, 2311−2322. (11) Xiao, L.; Li, Y.; Liao, L.; Liu, L. New J. Chem. 2013, 37, 1874− 1877. (12) Nava, H. In Kirk-Othmer Encyclopedia of Chemical Technology, 5th ed.; Wiley: New York, 2004; Vol. 20, pp 95−119. (13) Penczek, P.; Czub, P.; Pielichowski, J. Adv. Polym. Sci. 2005, 184, 1−95. (14) Fischer, R. F. J. Polym. Sci. 1960, 44, 155−172. (15) Schaefer, J.; Katnik, R. J.; Kern, R. J. J. Am. Chem. Soc. 1968, 90, 2476−2480. (16) Hsieh, H. L. J. Macromol. Sci., Chem. 1973, A7, 1525−1535. (17) (a) Kuran, W.; Nieslochowski, A. Polym. Bull. 1980, 2, 411−416. (b) Kuran, W.; Nieslochowski, A. J. Macromol. Sci., Chem. 1981, A15, 1567−1575. (18) (a) Fang, J. H.; Chen, X. H.; Zhang, Y. F.; Shen, Z. Q. Chin. J. Catal. 1994, 15, 45−49. (b) Fang, J. H.; Zhang, Z. Y. Acta Polym. Sin. 2001, 2, 254−256. (19) Takenouchi, S.; Takasu, A.; Inai, Y.; Hirabayashi, T. Polym. J. 2002, 34, 36−42. (20) Hua, Z.; Qi, G.; Chen, S. J. Appl. Polym. Sci. 2004, 93, 1788− 1792. (21) Suh, H. S.; Ha, J. Y.; Yoon, J. H.; Ha, C.-S.; Suh, H.; Kim, I. React. Funct. Polym. 2010, 70, 288−293. (22) DiCiccio, A. M.; Coates, G. W. J. Am. Chem. Soc. 2011, 133, 10724−10727. (23) Toma, T.; Shimokawa, J.; Fukuyama, T. Org. Lett. 2007, 9, 3195−3197. (24) (a) Hodgson, D. M.; Angrish, D. Chem. Commun. 2005, 4902− 4904. (b) Hodgson, D. M.; Angrish, D. Chem.Eur. J. 2007, 13, 3470−3479. (25) Longer reaction period (72 h) produced no difference in cis content for the polymerization of M4 using C1 as a catalyst. This result suggests that cis−trans isomerization did not occur in the course of polymerization using Ru catalysts. (26) Collman, J. P.; Rose, E.; Venburg, G. D. J. Chem. Soc., Chem. Commun. 1993, 934−935.
Figure 2. 1H NMR spectra of before (upper) and after (lower) the cis−trans isomerization reaction of polyM1′ obtained by C1 (entry 1 in Table 1) using diethylamine in CDCl3.
represents the 1H NMR spectra of polyM1′ obtained by C1 before (upper) and after (lower) the isomerization. After the reaction, the intensity of a peak at about 6.3 ppm assignable to cis-CC completely disappeared, and the intensity of a peak at about 6.9 ppm assignable to trans-CC increased. Additionally, the ratio of cis−trans configuration could be optionally controlled by changing isomerization reaction time. In conclusion, we have demonstrated a new method for the synthesis of UPs through a transition-metal-catalyzed singlecomponent polycondensation of bis(diazoacetate)s under a mild condition. The reaction using the second-generation Grubbs catalyst afforded well-defined UPs, with highly cisselective CC formation. The cis-CC could be isomerized quantitatively into trans-CC with a catalytic amount of diethylamine, giving well-defined UPs with only trans-CC. The obtained UPs could be used as a cross-linking agent for UP resin.37 Further study (e.g., expansion of the catalyst and substrate scope, copolymerization, and postpolymerization modification) is under way in our laboratory.
■
Article
ASSOCIATED CONTENT
S Supporting Information *
Experimental details and model reaction results. This material is available free of charge via the Internet at http://pubs.acs.org. 5486
dx.doi.org/10.1021/ma401047x | Macromolecules 2013, 46, 5483−5487
Macromolecules
Article
(27) (a) Baratta, W.; Zotto, A. D.; Rigo, P. Chem. Commun. 1997, 2163−2164. (b) Baratta, W.; Zotto, A. D.; Rigo, P. Organometallics 1999, 18, 5091−5096. (c) Zotto, A. D.; Baratta, W.; Verardo, G.; Rigo, P. Eur. J. Org. Chem. 2000, 2795−2801. (28) Graban, E.; Lemke, F. R. Organometallics 2002, 21, 3823−3826. (29) Li, G.-Y.; Che, C.-M. Org. Lett. 2004, 6, 1621−1623. (30) Woo, L. K.; Smith, D. A. Organometallics 1992, 11, 2344−2346. (31) Bock, H.; Wolf, H. P. J. Chem. Soc., Chem. Commun. 1990, 690− 692. (32) Lee, E.; Lee, D. S.; Lee, K. H.; Choi, I. S. Bull. Korean Chem. Soc. 1992, 13, 470−471. (33) Doyle, M. P.; Hu, W.; Phillips, I. M.; Wee, A. G. W. Org. Lett. 2000, 2, 1777−1779. (34) Kulkowit, S.; McKervey, M. A. J. Chem. Soc., Chem. Commun. 1978, 1069−1070. (35) The cis/trans ratio of dimerM6′ could not be determined from the signal intensity in the 1H NMR spectrum due to the signal overlap. Figure S2 shows the 1H NMR spectra of cis- and trans-alkene, which were isolated by recycling GPC. (36) Fryhle, C. B.; Rybak, C. M.; Pulley, K. E. J. Chem. Educ. 1991, 68, 1050−1053. (37) Dissolution in methyl methacrylate of the obtained UPs followed by radical copolymerization afforded products insoluble in organic solvents.
5487
dx.doi.org/10.1021/ma401047x | Macromolecules 2013, 46, 5483−5487
