J. Phys. Chem. 1995,99, 17302-17304
17302
Grafting of Buckminsterfullerene onto Polydiene: A New Route to Fullerene-Containing Polymers Liming Dai,* Albert W. H. Mau, Hans J. Griesser, and Tom H. Spurling Division of Chemicals and Polymers, CSIRO, Private Bag IO, Rosebank MDC, Clayton, Vic. 3169, Australia
John W. White Research School of Chemistry, The Australian National University, Canberra, ACT 0200, Australia Received: September 12, 1995@
Buckminsterfullerenes have been covalently attached along polydiene chains via a lithiation reaction. The highly processable (Le., soluble and fusible) fullerene-functionalized polymeric materials thus obtained could facilitate the use of fullerenes for novel applications.
A tractable (soluble and fusible) form of buckminsterfullerene would, in principle, open up possibilities for novel applications,’ which otherwise are limited by its poor processability.2 The large-scale synthesis of fullerenes3 has made c60 readily available, and chemical modification of “buckyballs” has since attracted considerable a t t e n t i ~ n . ~ -An ’ ~ important purpose of chemical modification is to render fullerenes into processable forms by, for example, covalently bonding them to tractable polymer chains to produce fullerene-containing polymers with fullerenes either as pendant groups or as constituent units of the polymer backbones. A few chemical routes to such materials have been devised. Examples include the chemical or photopolymerization of C60,15the amine addition of amino polymers into fullerene double bonds,I6 the cycloaddition reaction of functionalized polymers with C60,17.18 the addition of living poly(styry1)lithium chains (lithiated at one end only) onto c 6 0 to form c6f~(Ps)Xwith X ranging from 1 to 10 depending on the initial ratio of PS carbanion to &,lo and the grafting of c60 onto nucleophilic lithiated polyethylene surf a c e ~ . ’The ~ ease with which the allylic hydrogen atoms of a diene polymer can be substituted by lithium ions through lithiation of the polymer chains by sec-butyllithium (sec-BuLi), in the presence of tetramethylethylenediamine (TMEDA),*O prompted us to investigate the lithiation of polydienes, such as polyisoprene and polybutadiene, followed by addition of fullerenes to the lithiated polymer chains according to the reaction mechanism shown in Scheme 1. The C&imctionalized polydiene materials with multiple pendant buckyballs dispersed along their polymer backbones thus prepared are highly soluble and fusible, so that they can be spin-coated, solvent cast or melt extruded. In a typical experiment, we carried out the reactions shown in Scheme 1 by dissolving 100 mg of cis-1,4-polyisoprene (anionically synthesized as described in previous publications21-23) or cis-1,4-polybutadiene (98% cis, M , = 2 500 000 glmol, Aldrich) in 10 mL of dry cyclohexane under an argon atmosphere. Then, a stoichiometric amount of sec-BuLi (supplied in cyclohexane, Aldrich), required for ca. 2.5% of the monomer units in the polymer chains to be lithiated,24was added under stirring, and to the stirred solution TMEDA was subsequently injected at a 1:l molar ratio with respect to sec-BuLi. Several minutes later, the color of the reaction mixture started to change from pale-yellow to dark-red through reddish brown
@
To whom correspondence should be addressed. Abstract published in Aduance ACS Absrracts, November 1, 1995.
0022-365419512099-17302$09.0010
SCHEME 1: Lithiation of Polydienes Followed by Reaction with Fullerene
-
CH2-CR=CH-CHVCH2-CR=CH-CHV
7-=
\N
!=:
H CR CH CHZ-CH2-CR=CH-CH-
Li
-
vv CH -CR=CH-CH,-
I
I
Li
c@ CH2-CR=CH-CH-
(R = H or CH3; \N = polydiene segments)
as the reaction progressed, which was reflected by a continuous red-shift in the UV/vis (Hewlett-Packard HP-8451A) spectrum (Figure la-d), indicating the occurrence of lithiation of polydienes in cyclohexane. The reaction mixture was further stirred at room temperature for about 2 h before 10 mg of C60 (Aldrich) predissolved in toluene was added. Consequently, a color change from dark-red to dark brown was observed. Thereafter, the fullerene-functionalized polymeric adduct was quenched and precipitated by addition of methanol. The C ~ O grafted polymer was then redissolved into THF and followed by centrifugation to remove unreacted “buckyballs”, if any, as the solubility of free fullerenes in THF is reported to be negligibly The UVlvis spectrum of the resulting brasscolored solution in THF (curve 0, together with the corresponding spectrum of pure Cm (curve e), are shown in the inset of Figure 1. Comparison of curve f with curve e shows the disappearance of the characteristic peaks of fullerene at 213 and 329 nm,2 which is accompanied by the appearance of several new absorption bands, suggesting the formation of fullerenefunctionalized polydiene chains. A similar decrease in the absorption at 329 nm observed for calixfullerene was attributed to the isolation of C ~ by O the intramolecularly linked calix[ 81arene,26and the appearance of peaks at 213,248,257,308, and 326 nm had been taken as evidence for monosubstitution of fullerenes.6.16.27The other newly appearing weak absorption
0 1995 American Chemical Society
Letters
J. Phys. Chem., Vol. 99, No. 48, 1995 17303
44 000 g/mol (curve a). Curve b, however, shows the appear-
Wavelength /(nm) Figure 1. UV/vis absorption spectra recorded in situ for (a) the pristine polybutadiene in cyclohexane; (b) the solution (a) upon addition of sec-BuLbTMEDA; (c) the solution (a) after the lithiation for ca. 1 h; (d) the solution (a) after the lithiation for ca. 3 h. Inset shows the corresponding spectra for (e) Cm in cyclohexane and (f) the purified Cm-grafted polybutadiene diluted in THF. 1
-
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m
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C 0
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,
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..
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,
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Molecular w e i g h t /(g.mol-') Figure 2. GPC chromatograms of (a) the pristine cis-1,Cpolyisoprene in THF recorded by the refractive index detector; (b) the fullerenegrafted polyisoprene in THF recorded by the refractive index detector.
bands seen in Figure lf, however, may indicate the occurrence of slightly higher degrees of addition onto traces of Cm. The grafting reaction shown in Scheme 1 was also confirmed by gel permeation chromatography (GPC, Waters Associates) measurements. Figure 2 shows the GPC chromatograms for pristine polyisoprene (curve a) and the Cm-grafted polyisoprene (curve b) recorded with a refractive index (RI) detector. As expected, the anionically polymerized polyisoprene has a very narrow molecular weight distribution (Mw/Mn= 1.1) with a weight-average molar mass (polystyrene equivalent) M, =
ance of a shoulder at higher molecular weight while the peak corresponding to that of curve a remained unshifted. As a result, new values of Mw = 95 000 g/mol and M,/M,, = 1.8 were obtained for the modified polymer. A similar change in GPC chromatograms has previously been observed for the cycloaddition of Cm onto azido-substituted polystyrenes with monoaddition being the dominant process,I7 and the reasonably small value of the polydispersity (Le., MwIM,, = 1.8) for the c60grafted polyisoprene suggests that cross-linking, if any, is insignificant. However, one should note that GPC measures the hydrodynamic volume of a macromolecular chain rather than its absolute molar mass. From the GPC results, therefore, a calculation of the molar percentage of Cm in the host polymeric chains cannot be made without detailed information on changes in the polymer conformation upon the grafting reaction.I0 Nevertheless, it was estimated from thermal gravimetric analysis (TGA, Mettler TG50)I7 that nearly all of the buckyballs reacted quantitatively resulting in an about 10% (w/w) incorporation of Cm into either polyisoprene or polybutadiene, and that a maximum of ca. 50% (w/w) Cm incorporation into polyisoprene chains had been recorded in a separate experiment (vide infra). The corresponding GPC result measured by an ultraviolet/visible (W/vis) detector at 1 = 326 nm, a wavelength at which only the fullerene absorbs,21c o n f i i s that the higher molecular weight species associated with the shoulder in curve b of Figure 2 correspond to the buckyball-grafted polyisoprene chains, as only the species of higher molar masses were observed in the UV/vis chromatogram. As a control, GPC measurements were also made on the lithiated, fullerene-freepolyisoprene, after it had been quenched by MeOH in the same manner as for the buckyball-grafted polyisoprene. No change in the molecular weight with respect to the polyisoprene precursor was found. Therefore, the differences between curves a and b in Figure 2 can be unambiguously attributed to the grafting reaction of Cm onto the polymer chains. Further evidence for the grafting reaction between fullerenes and the lithiated polydienes was obtained by nuclear magnetic resonance (NMR, Bruker AC-200) measurements. We observed a downfield shift for the IH NMR peaks of the allylic hydrogen atoms in the cis-1,Cpolyisoprene and cis-1,4-polybutadiene from 2.06 and 2.08 ppm, respectively?1q28to the region between 2.20 and 2.65 ppm for the corresponding resonances of the Cmgrafted polymers, consistent with an electron-withdrawing influence from the grafted buckyball^.^,^^ In addition, a weak band in the range 5.70-7.20 ppm was observed, presumably arising from the newly formed fullerene protons.6 To assess the thermal processability of the polymer derivatives of fullerenes, differential scanning calorimetry (Mettler, DSC 30) was made on the Cm-grafted cis-1,4-polybutadiene (10% w/w) and Cm-grafted cis- 1,4-polyisoprene (50% w/w), respectively. The DSC thermogram for the unreacted cis-1,C polybutadiene (Figure 3A, curve a) shows a glass transition temperature, Tg, at ca. -100 "C, together with an exothermic peak at -60 "C arising from crystallization and an endothermic peak at 0 "C corresponding to the melting point of the crystalline structure, in good agreement with literature data.30 The DSC result for the Cm-grafted cis- 1P-polybutadiene chains (Figure 3A, curve b) also shows similar thermal properties, although the temperature range between crystallization and melting becomes narrower (ca. -50 to -10 "C) with a much weaker recrystallization transition. The Tg shifts to ca. -90 "C, indicating that the chain rigidity increases slightly, but the final product remains thermally processable. Figure 3B, however, shows a much greater up-shift in Tg,from ca. -65 "C (Figure
17304 J. Phys. Chenz., Vol. 99, No. 48, I995
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I
Letters
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-100 A
T
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"
-100
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Figure 3. DSC thermograms of (A) cw1,4-polybutadiene: (a) the unreacted polymer: (b) Ca-grafted (10% w/w); (B) cis- 1.4-polyisoprene: (a) the unreacted polymer; (b) &-grafted (50% wiw). Scanning rate. 10 "C/min.
3B, curve a) characteristic of pristine cis- 1,4-polyisoprene3' to ca. 10 "c for the (&-grafted cis-1,4-polyisoprene (50% w/w) chains with a higher percentage of C60. While this work was in progress, the cycloaddition of c60 to azido-substituted polystyrene for multiple pendant grafting of buckyballs was published and a similar increase in T, was reported for the polystyrene chains upon grafting with C~O.'' In summary, we have demonstrated that fullerene-functionalized polymers with multiple pendant buckyballs dispersed along their polymer backbones can be obtained through the lithiation of polydienes followed by addition of c 6 0 to the lithiated polymer chains. The highly soluble and fusible materials thus prepared may open up novel applications for the buckyballs. Furthermore, the method described in this communication is potentially useful for chemical grafting of fullerenes onto a wide range of small organic molecules and polymer chains, as various polymers including poly~tyrene,~? p o l y ~ u l f o n eand , ~ ~diphenylmethyl-substituted polyethylene3? are also known to be susceptible to lithium substitution, and the lithiation reaction is regarded as an exceptionally general method for preparation of organolithium corn pound^.^^ Such experiments are currently in progress. Acknowledgment. We thank Drs. G . Hawthorne and R. Chatelier for assistance with the GPC analyses. References and Notes (1) See for example: Hebard. A. F. Annu. Reu. Mater. Sci. 1993. 23. 159. Baum. R. M. Chem. Eng. News 1993, Nov 22, 8. (2) Kroto, H. W.: Allaf. A. W.: Balm, S. P. Chem. Ret'. 1991. 91. 1213.
(3) Kratschmer, W.: Lamb. L. D.: Fostiropolous, K.; Huffman, D. R. Nature 1990, 347, 354. (4)Wudl. F. Arc. Chem. Res. 1992, 25, 157. Hirsh, A. Adu. Mater. 1993, 5 , 859. (5) Hirsch. A,: Li, Q.: Wudl. F. Angew. Chem.. Int. Ed. Engl. 1991. 30. 1309. (6) Hirsch. A,: Soi. A,: Karfunkel. H. R. Angew. Chem.. lnt. Ed. EngI. 1992. 31, 766. (7) Fagan, P. J.: Krusic. P. J.; Evans, D. H.: Lerke, S. A,: Johnston. E. J . Am. Chem. SOC. 1992, 114. 9697. 18) Lo). D. A,: Assink, R. A. J . A m . Chem. Soc. 1992, 114, 3977. (9) Seshadri, R.: Govindaraj, A,; Nagarajan, R.: Pradeep, T.; Rao, C. N. R. Tetrahedron Lett. 1992. 33, 2069. (10) Samulski. E. T.: DeSimone. J. M.; Hunt, M. 0..Jr.: Menceloglu, Y. Z.: Jarnagin, R. C.: York. G. A.; Labat, K. B.: Wang. H. Chem. Mater. 1992. 4. 1153. (11) Fagan. P. J.: Krusic. P. J.: McEwen, C. N.: Lazar, J.: Parker, D. H.: Herron. N.: Wasserman, E. Science 1993, 262. 404 and references therein. (12) Prato, M.: Chan. Q.: Wudl, F. J . Am. Chem. Soc. 1993, 115, 1148. (13) Patil. A. 0.: Schriver, G. W.; Carstensen, B.: Lundberg. R. D. Poljm. Bull. 1993. SO. 187. (14) Fullagar, W. K.; Gentle, I. R.: Haeth, G. A,: White, J. W. J . Chem. Sor.. Chem. Commun. 1993. 525. Hawker, C. J.: Saville. P. M.; White, J. W. J . Org. Chem. 1994, 59. 3503. (15) See for example: Hassanien. A,; Mrzel. T.: Venturini, P.; Wudl. F.; Mihailovic. D.: Gasperic. J.: Kralj, B.; Zigon, D.: Milicev, S.: Demsar, A. In Electronic Properties of Fullerenes: Springer Ser. Solid-Srate Sei. 1993. I 17. 3 16. Zhang, N.; Schricker. S . R.; Wudl, F.: Prato, M.; Maggini, M.;Scorrano, G . Chem. Mater. 1995, 7 , 441. (16) Geckeler, K. E.; Hirsch, A. J . Am. Chem. Soc. 1993. 115. 3850. (17) Hawker, C. J. Macromolecules 1994, 27, 4836. 118) Guhr. K. I.: Greaves. M. D.: Rotello, V. M . J . Am. Chem. Soc. 1994. 116. 5997. (19) Bergbreiter, D. E.: Gray, H. N. J , Chem. Soc.. Chem. Commun. 1993. 645. (20) TMEDA is known to make the metalation of diene polymers more effective. See for example: Minoura. Y.; Shiina, K.: Harada, H. J . Polym. Sci.. AI 1986, 6. 559. Bolognesi, A.; Catellani, S.; Destri, S.: Porzio. W. Poljmer 1986. 27, 1128. Stowell, J. A,: Amass. A. J.: Beevers. M. S.; Farren. T. R. Polymer 1989. 30, 195. (21) Dai. L.: White, J. W. Polyner 1991. 32, 2120. (22) Dai. L.: White, J. W. J . P o l y . Sci.. Poljm. Phys. Ed. 1993, 31, 3 . (23) Dai, L. J . Phys. Chem. 1992. 96, 6469. 124) Although a higher degree of lithiation and hence a higher grafting degree of c60. can be readily achieved by increasing the molar ratio of sec-BuLi to polydienes. a low degree of lithiation was chosen. for the given amount of C ~ Tto) . limit the molar ratio of lithium functional groups to C ~ O in order to minimize multiple addition onto any single C a entity.'" ( 2 5 ) Patil. A . 0.;Schriver. G . W.: Lundberg. R. D. ACS Polym. Prepr. 1993. 34. 592. (26) Takeshita. M.: Suzuki. T.; Shinkai. S. J . Chem. Soc., Chem. Commun. 1994. 2587. ( 2 7 ) Suzuki, T.: Li. Q.; Khemani. K. C.: Wudl, F.: Almarsson, 0. Science 1991, 254, 1186. 128) Dai, L.: Mau, A. W. H.; Griesser, H. J.: Winkler, D. A. Macromolecules 1994, 27, 6728. (29) Hirsch. A.: Li. Q.; Wudl. F. Angew. Chem., Int. Ed. Engl. 1991, SO. 1309. (30) See for example: Brandrup, J.: Immergut, E. H. Polymer Handbook; John Wiley: New York, 1966. Cheng. T. L.: Su. A. C. Macromolecules 1993, 26. 7161. (31) See for example: Brandmp, J.: Immergut, E. H. Poljmer Handbook: John Wiley: New York, 1966. Maurer, J. In Analytical Calorimetry; Porter. R. S.. Johnson, J. F., Eds.: Plenum Press: New York, 1968. Sircar, A. K.: Lamond. T. G. Rubber Chem. Technol. 1975, 48, 301. Widmaier, J. M.: Meyer, G. C. Rubber Chem. Technol. 1981, 54. 940. (32) Chalk, A. J. J . Polym. Sci., Part E 1968, 6 , 649. Plate, N. A,: Jampolskaya, M. A.: Davydova, S . L.; Kargin. V. A. J . Polym. Sei., Parr C 1969. 22. 537. Tomita, H.: Register, R. A. Macromolecule:, 1993. 26, 2791 and references therein. ( 3 3 ) Guiver. M. D.; Robertson. G. P. Macromolecules 1995, 28, 294. (34) Bergbreiter, D. E.: Gray. H. N.: Srinivas. B. Macromolecules 1993. 26. 3245. (35) Mudryk. B.: Cohen, T. J . A m . Chem. Soc. 1993, 115. 3855. JP952680B
