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Fullerene-functionalized polycarbonate: synthesis under microwave irradiation and nonlinear optical property.

INTRODUCTION

Fullerene has attracted much attention because of its unique electronic conducting, magnetic, and photophysical properties. However poor solubility and difficulty in processing of fullerene hampers its direct application. Grafting polymer chains on fullerene is gaining particular interest, because this covalent attachment of fullerene to specific polymers may allow combination of the splendid characteristics of fullerene with the outstanding properties of the polymeric matrix to generate new fullerene-based specialty polymeric materials with peculiar physicochemical properties and good processibility. Therefore, several synthetic strategies have been developed for preparing soluble fullerene-containing polymers, and different kinds of fullerene functionalized polymers, including "pearl necklace", "charm bracelet", and "flagellene", have been prepared [1-3]. In recent years, the reaction of fullerene with prefunctionalized polymer to prepare fullerene chemically modified polymers has become popular [4-6]. The preparation of prefunctionalized polymer, however, sometimes requires painstaking synthetic efforts, which in turn limits the application of the method. Therefore, fullerenation by direct reaction between fullerene and preformed polymers, especially those commercially important polymers, may be the most "economic" way for introducing fullerene to polymers.

As an important commercial polymer, polycarbonate (PC) is a typical stepgrowth polymer and also an excellent optical plastic. Tang et al. [7] described their results of direct fullerenation of PC by irradiating a solution of PC and [C.sub.60] at room temperature, using a UV lamp and by warming up a [C.sub.60]/PC solution to 60[degrees]C in the presence of azo-bis-isobutyronitrile (AIBN). Next, they successfully attached [C.sub.60] cages to the aromatic rings of PC by direct reaction of [C.sub.60] with PC, using Al[Cl.sub.3] as catalyst [8]. These methods of direct fullerenation mentioned above, including conventional heating process or irrradiation by a UV lamp, however, usually required a long reaction time (at least 24 h).

Microwave, as a nonconventional source of energy, has become a very popular and useful technology in chemistry and material science. One significant application for microwave irradiation (MI) is in polymer chemistry. MI can be applied to several fields of polymer chemistry, including radical polymerization, step-growth polymerization, ring-opening polymerization, polymer modification, and so on [9-12]. Compared with traditional heating process, MI can significantly accelerate many chemical reactions. As regards the reason of acceleration, some groups attributed it to the "specific microwave effect" [13, 14], others attributed it to the "dielectric heating effect" [15]. Many results [16-18] have shown that, when compared with conventional heating process, reactions under MI have the advantages of higher reaction rate and greater yield within a shorter period of time. Therefore, MI is an effective, less energy exhaustive, and no environmental pollution method.

We once reported the [C.sub.60]-initiated "charge-transfer" bulk polymerization reaction of N-vinylcarbazole under MI (490 W) [19], carrying out the polymerization reaction in the microwave oven was found to be advantageous because of the remarkable decrease in the reaction time and the considerable improvement in yield of poly(N-vinylcarbazole). Recently, we have synthesized the telechelic fullerene [C.sub.60] double-terminated polystyrene and poly(methyl methacrylate) from the difunctional bromo-terminated polymers with [C.sub.60], using CuBr/bipy atom transfer radical addition catalyst system under MI [20]. In this work, we report the synthesis of [C.sub.60]-functionalized PC by direct reaction of [C.sub.60] and PC in the presence of AIBN, using 1,1,2,2-tetrachloroethane as the solvent under MI. The products ([C.sub.60]-PCs) were characterized by gel permeation chromatography (GPC), UV-vis, FTIR, TGA, DSC, [.sup.1]H NMR, and [.sup.13]C NMR. The reaction of [C.sub.60] with PC under MI was monitored by electron spin resonance (ESR) spectroscopy. The nonlinear optical property of [C.sub.60]-PCs in THF was investigated by the open-aperture z-scan technique at 527 nm.

EXPERIMENTAL PROCEDURES

Reagents and Instrumentation

[C.sub.60] (99.9%) was obtained from Yin-Han fullerene High-Tech. Co. Ltd., Wuhan University of China. PC ([M.sub.n] = 17,300; [M.sub.w] = 28,800) was purchased from Aldrich. AIBN was recrystallized from C[H.sub.3]OH, and dried at room temperature under vacuum. 1,1,2,2-Tetrachloroethane was purified by distillation prior to use. All other reagents were used as-received without further purification.

The reactions under MI for synthesis of fullerene-functionalized PC were performed under purified nitrogen in a self-improved domestic microwave oven, WP750L23-6 Galanz with irradiation frequency of 2.45 GHz. The MI power was adjustable to a constant at 300 W.

UV-vis absorption spectra were taken on a HP8452 spectrophotometer. The FTIR spectra were recorded on a nicolet FTIR-5DX spectrometer, using KBr pellets. The molecular weight of the polymers was analyzed by a HP series 1100 gel permeation chromatography (GPC), equipped with Zorbax columns and refractive index /ultraviolet dual-mode detectors. The elution rate of THF was 1 ml/min and standard PSt was used for calibration. [.sup.1]H NMR and [.sup.13]C NMR spectra were recorded with a Brucker-AM 500 apparatus in CD[Cl.sub.3]. Thermogravimetric measurements were made with a Perkin Elemer Pyris 1 DTA-TGA instrument, under nitrogen at a heating rate of 10[degrees]C/min. Glass-transition temperature ([T.sub.g]) was measured on a Perkin Elemer Pyris 1 differential scanning calorimeter at a heating rate of 10[degrees]C/min. Electronic spin resonance (ESR) spectra were obtained on a Bruker ER 200D-SRC ESR spectrometer, and the g values were determined using diphenylpic-rylhydrazine as standard sample.

The nonlinear optical property measurement was carried out using the open-aperture z-scan technique at 527 nm. The light pulse source was the laser beam from a Q-switched Nd:YLF laser with duration of 100 ns, wavelength of 527 nm, and repetition rate of 1 Hz-1 kHz. Part of the beam was used to trigger the Boxcar Integrator. The focal lengths of the focusing lens in front of the sample and the collection lens in front of the detector were 8 and 10 cm, respectively. The solution of [C.sub.60]-PC in THF and [C.sub.60] in toluene were put in quartz cell with a 1-mm path length, and the measurement was done at room temperature. The data detected by a Boxcar integrator was transferred to a personal computer, which also controlled the step motor driving the sample.

Synthesis of Fullerene-Functionalized PC

Microwave Irradiation. In a typical run, 500 mg of PC, a given feed amount of [C.sub.60], and AIBN were introduced into a two-necked round-bottomed flask (150 ml). The charge ratio of [C.sub.60]/PC (wt%) and the concentration of AIBN were shown in Table 1. After the mixture was deoxygenated three times, 10 ml of 1,1,2,2-tetrachloroethane was added via a flamed syringe, and the solid raw materials were allowed to completely dissolve in the solvent. The round-bottomed flask with the mixture was then placed into the self-improved microwave oven equipped with reflux apparatus. The reaction was carried out under nitrogen with a preset microwave power and then stopped at a desired time. After being cooled to room temperature, the solvent was distilled out under vacuum. The solid residue was taken up in THF and filtered. The THF filtrate was concentrated under reduced pressure and precipitated from methanol. The crude product was repeatedly dissolved in THF, and then precipitated with methanol. The purified product was dried under vacuum as brown powder.

Conventional Heating Process. [C.sub.60], AIBN, and PC were introduced into another two-neck flask with the same charge ratio as that for MI. After the mixture was deoxygenated three times, 1,1,2,2-tetrachloroethane was added. The fullerenation was then carried out in an oil bath, under nitrogen at desired temperature. The subsequent procedures are the same as those used for MI.

RESULTS AND DISCUSSION

Fullerenation Reaction

Tang et al. [7] successfully fullerenated PC by heating a [C.sub.60]/PC solution at 60[degrees]C for 24 h, using AIBN as the initiator. However, the fullerenation reaction under conventional heating process needed a long reaction time. MI may act as the efficient measure to enhance the reaction rate and shorten the reaction time.

1,1,2,2-tetrachloroethane was used as solvent at all reactions. When the MI heating started, the reaction mixture solution in flask soon began to reflux. Therefore, actually all the MI assisted addition reaction of [C.sub.60] with PC were well-controlled and proceeded at solvent tetrachloroethane refluxing temperature.

To check whether fullerenation could be induced under MI without the presence of AIBN, we attempted to fullerenate PC by microwave irradiating a 1,1,2,2-tetrachloroethane solution of PC (509.9 mg) and [C.sub.60] (6.0 mg). But the color of the solution changed only a little after 20 min of irradiation at the refluxing temperature, and the [C.sub.60] content of the product was only 0.22%. Since fullerenation efficiency of PC under MI without the initiating agent was poor, 5.7 mg of AIBN was added into the reaction mixture with the same feed ratio of PC and [C.sub.60]. The characteristic purple color of [C.sub.60] turned into dark brown within 1 min of irradiation. It is known that [C.sub.60] reacts with AIBN [21, 22]. If the reaction time was not long enough, the main reaction products were [C.sub.60]-AIBN adducts, which were soluble in methanol and could be removed by repeatedly precipitating THF solution of the reaction product into methanol. Therefore, to incorporate more [C.sub.60] cages into the PC chains, the irradiation time must be long enough. As shown in Table 1 (No. 3 and 4), increasing the reaction time from 6 to 15 min increased the [C.sub.60] content of the product from 3.16 to 4.54%. However, further prolonging the irradiation time will result in a certain extent of heavy multiaddition of PC to [C.sub.60] and possible cross-linking reaction of PC. For this reason, some amount of product was found insoluble in THF, when the irradiation time was more than 25 min.

The extent of fullerenation could be controlled by varying feed ratio of [C.sub.60] and PC (Table 1). Increasing the [C.sub.60]/PC feed ratio increased the [C.sub.60] content of the [C.sub.60]-PCs. As shown in Table 1 (No. 1-3), when the feed ratio was raised from 1.32 to 5.31%, the [C.sub.60] content of the products changed from 0.96 to 3.16%. But the increase of the [C.sub.60] content in the product was less than that of the feed ratio, because all the feed [C.sub.60] molecules did not get attached to the PC chains and some [C.sub.60] molecules reacted with AIBN to produce [C.sub.60]-AIBN adducts.

It was found that the feed amount of AIBN influenced [C.sub.60] content and the solubility of the products. Increasing the addition amount of AIBN from 0.57 mg/ml to 2.06 mg/ml for No.2 (Table 1) just reduced the [C.sub.60] content of the product. To further understand the influence of AIBN on fullerenation of PC, another reaction (No. 5 in Table 1) was carried out with charging amount of AIBN four times as much as that for reaction No. 4. Some amount of insoluble product was found in this reaction. After removing the insoluble product, the [C.sub.60] content of the PC-[C.sub.60] for this reaction was found lower than that for No. 4. The products of all the other experiments shown in Table 1 were completely soluble in THF, with no insoluble gel being found. Therefore, too much amount of AIBN might reduce the extent of fullerenation under MI and at the same time result in formation of some insoluble product.

To compare with conventional thermal heating, the fullerenation reactions were also carried out in oil bath at 60[degrees]C or refluxing for 20 min. The reaction did not proceed at all at 60[degrees]C within 20 min. By heating at 60[degrees]C, the reactions completed at least after 20 h of heating. If the reaction was run at refluxing temperature for 20 min, the [C.sub.60] content of the product was less than 0.5%. Prolonging the reaction time under refluxing for several hours can increase the extent of fullerenation of PC, but unfortunately, most of the products were insoluble due to the uncontrolled multiaddition of PC chain radicals to [C.sub.60] and the possible cross-linking reaction at high temperature and long reaction time. Therefore, when compared with conventional heating process, the addition reaction with the help of MI can proceed in a controlled manner with greatly enhanced reaction rate.

To better understand the radical mechanism of the reaction of [C.sub.60] and MI, we used the in situ ESR technique to monitor the addition reaction. Two samples of reaction mixture solutions (with the same charge ratio of [C.sub.60]/PC (wt%) and the same concentration of AIBN as that of run No. 4 in Table 1), after the MI assisted reaction for 3 min and 10 min, were added to the ESR tubes, respectively, and the tubes were then sealed. The ESR spectra were recorded. At ambient temperature, no ESR signals for either pure PC or fullerene [C.sub.60] were detected, indicating nonexistence of paramagnetic species in PC and [C.sub.60]. As shown in Fig 1, the PC and fullerene [C.sub.60] mixture solution still show no ESR signal before the MI start. After the MI assisted addition reaction for 3 min, the ESR spectrum of the reaction mixture solution showed a weak ESR signal, whose g value and line width ([DELTA][H.sub.PP]) are 2.0027 and 1.0 G, respectively, implying that the concentration of fullerene radical in solution was low. When the reaction time increased to 10 min, the ESR signal intensity was greatly enhanced, showing a strong ESR signal of fullerene radicals in reaction solution. This observation was consistent with the results reported by Stewart and Imrie [22, 23]. It was found that the ESR signal remained even after precipitation and drying of the solid product (Fig 2). Thus, fullerene radicals still existed in the solid polymer samples [24].

Structural Characterization

All the purified products of [C.sub.60]-PCs were brown in color, while the starting parent PC was white suggesting that the fullerenation did occur for PC. The UV detector (330 nm) in the GPC system could not detect any peaks from the starting PC and the product from control experiment No. 6 (Figure 3a). The GPC trace of [C.sub.60]-PCs detected at 330 nm strongly confirmed the covalent bonding between [C.sub.60] cages and the PC chains (Figure 3b and c). All the [C.sub.60]-PCs had higher molecular weights (Table 1) than the starting PC, indicating that multiaddition to [C.sub.60] had taken place [7]. The multiaddition was well under controlled and had not led to cross-linking for the fullerenation reactions (Table 1, No. 1-4), with the result that the [C.sub.60]-PCs were well soluble in common organic solvents such as THF, chloroform, and 1,1,2,2-tetrachloroethane. The [C.sub.60]-PCs from the reaction of 6 min MI gave only one GPC-UV peak, but the products of 15 min irradiation reaction showed two peaks, one peak close to that observed for 6 min reaction time plus a new peak in the high molecular weight region, indicating that heavier multiaddition might have occurred. The amount of heavier multiaddition product, however, should be quite small, because this new UV peak is very weak and the heavier multiaddition product could not even be detected by the RI detector.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

The fullerenated products of PC were further characterized by spectroscopic methods. Figure 4 showed the UV-vis spectra of PC and [C.sub.60]-PC in THF. Comparing with the absorption bands of PC, a new peak at 330 nm was observed for [C.sub.60]-PC, indicating the attaching of [C.sub.60] to PC. Since PC showed nearly no absorption at 330 nm, the [C.sub.60] contents in the [C.sub.60]-PCs could be estimated based on the absorbance peak at 330 nm (Table 1). Unlike the UV-vis absorption spectra, the FTIR spectra of the [C.sub.60]-PCs show less useful information. Because of the relatively low [C.sub.60] content and the overwhelming contributions of polymer chain in the polymer structure, the observed IR spectra shown in Figure 5 for PC and [C.sub.60]-PC were basically similar. Only a very weak band at 528 [cm.sup.-1] could be discovered, which was interpreted as being associated with the characteristic infrared absorption arising from functionalized [C.sub.60] cages. The [.sup.1]H NMR and [.sup.13]C NMR spectra results of the [C.sub.60]-PCs obtained in deuterated chloroform are not significantly different from the spectra of the PC, because the amount of [C.sub.60] incorporated into the PC chains was too small to be detected by NMR. The results of NMR and IR spectra thus indicated the fact that the PC had not suffered from violent degradation during the process of fullerenation under MI but had maintained its basic PC structure.

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

The [C.sub.60]-PCs were thermally stable losing no weight below 350[degrees]C under nitrogen. The glass transition temperatures (Tgs) of the [C.sub.60]-PCs with lower [C.sub.60] content measured by DSC had lower Tg, while the Tg increased when the [C.sub.60] content increased. This phenomenon could be explained as the following: when the [C.sub.60] content is low, the [C.sub.60] cages may act as plasticizers, which could increase the free volumes among the polymer chains, enabling the segmental movements or glass transition at relatively low temperature. However, when the [C.sub.60] content increases, the [C.sub.60] cages may play the part of reinforcers. The incorporation of [C.sub.60] would restrict the movement of polymer chain segments, thus increasing the Tg of the polymer [7].

[FIGURE 5 OMITTED]

[FIGURE 6 OMITTED]

Nonlinear Optical Property

Open-aperture z-scan technique [25, 26] is a very convenient experimental method to measure the nonlinear optical absorption of the materials. In the measurement, the sample is gradually moved through the focus of a lens (along the z-axis) and the total transmittance of the sample as a function of sample position z is measured. The set-up is shown schematically in Figure 6. In our experiment, the pulse light source was the laser beam from a Q-switched Nd:YLF laser with duration of 100 ns, wavelength of 527 nm, and repetition rate of 1 kHz. Part of the beam was used to trigger the Boxcar Integrator. The focal lengths of the focusing lens and the collection lens in front of the detector were 8 and 10 cm, respectively. The solution of sample No. 4 in THF and [C.sub.60] in toluene were put in quartz cell with a 1 mm path length, and the measurement was taken at room temperature. The detected data by the Boxcar was transferred to a personal computer, which also controlled the step motor driving the sample.

The open-aperture z-scan results at 527 nm of [C.sub.60]-PC (sample from Table 1, No. 4) in THF solution with the concentration of 6 mg/ml and [C.sub.60] in toluene solution of 0.27 mg/ml are shown in Figure 7 (a) and (b), respectively. Thus, the concentration of [C.sub.60] in both solutions was kept near the same. As shown in Figure 7, two open-aperture z-scan traces exhibit a reduction in the transmittance at vicinity of the focus of the lens. This behavior means that as the light intensity increases, the transmittance decreases, which indirectly shows the optical limiting effect [27]. This was typical of a photo-induced nonlinear absorption, which may be caused by reversed saturable absorption, two-photon absorption or other processes. All open-aperture z-scan data were fitted using the method described in [25, 26], [T.sub.Norm](z) is given by

[T.sub.Norm](z) = [ln[1 + [q.sub.0](z)]]/[[q.sub.0](z)]] (1)

[FIGURE 7 OMITTED]

where [q.sub.0](z) is given by

[q.sub.0](z) = [q.sub.00]/[1 + (z/[z.sub.0])[.sup.2]] (2)

[z.sub.0] is the diffraction length of the beam and [q.sub.00] = b[I.sub.0][L.sub.eff], where [L.sub.eff]=[1 - exp(- [a.sub.0]L)]/[a.sub.0]. [beta] is the nonlinear absorption coefficient, and [I.sub.0] is the intensity of the light pulse at focus. [L.sub.eff] is known as the effective length of the sample, defined in terms of the linear-absorption coefficient ([[alpha].sub.0]), and the true optical path length of the sample (L).

By fitting the data, the nonlinear absorption coefficient [beta] was determined to be 2.7 X [10.sup.-8] cm/W for sample from Table 1 No. 4 in THF solution and 9.0 X [10.sup.-8] cm/W for [C.sub.60] in toluene solution, respectively. These results show that [C.sub.60]-PCs have nonlinear optical property, the same order as that of [C.sub.60].

CONCLUSION

In this study, fullerenation of PC is successfully carried out by direct reaction of [C.sub.60] and PC in the presence of AIBN, using 1,1,2,2-tetrachloroethane as the solvent under MI. The fullerenation involves a simple one-pot experimental procedure and the direct reaction between [C.sub.60] and a preformed polymer. Furthermore, comparing with conventional heating process (CH), MI could significantly enhance the rate of the fullerenation under identical reaction conditions. The [C.sub.60] content of [C.sub.60]-PCs could be controlled by varying the [C.sub.60]/PC feed ratio and the irradiation time. The [C.sub.60]-PCs are soluble in common organic solvents such as THF and chloroform. The addition reaction of [C.sub.60] with PC under MI was monitored by ESR spectra. The ESR results indicated the existence of the fullerene radicals both in the reaction solution before isolation of the product and in the solid polymer after precipitation and drying in air. The ESR study confirmed that the fullerenation reaction of PC under MI involved a radical mechanism. The nonlinear optical property measurement of [C.sub.60]-PCs in THF was carried out using the open-aperture z-scan technique, and their nonlinear absorption coefficient [beta] was determined. The results show that the [C.sub.60]-PCs have nonlinear optical property close to that of [C.sub.60]. Considering the improved solubility, the [C.sub.60]-PCs synthesized under MI can be applied as effective nonlinear optical materials.

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Huixia Wu, Feng Li, Yanghui Lin, Ruifang Cai

Department of Chemistry, Fudan University, Shanghai 200433, People's Republic of China

Huixia Wu

Department of Chemistry, Shanghai Normal University, Shanghai 200234, People's Republic of China

Rui Tong and Shixiong Qian

Department of Physics, Fudan University, Shanghai 200433, People's Republic of China

Correspondence to: Y. Lin; e-mail: lyanghui@fudan.edu.cn; R. Cai; e-mail: organo@fudan.edu.cn

Contract grant sponsor: National Nature Science Foundation of China; contract grant numbers: 20271013, 10374020.
TABLE 1. The results of AIBN-initiated fullerenation of PC under MI.

 [C.sub.60]/PC AIBN Reaction
No. feed ratio (wt%) (mg/ml) time (min)

1 1.32 0.51 6
2 2.23 0.57 6
3 5.31 0.52 6
4 5.16 0.50 15
5 (c) 5.11 2.00 15
6 (d) -- 0.55 10

 PC-[C.sub.60]
 [M.sub.n] (a) [M.sub.w]/ [C.sub.60] content [T.sub.g]
No. (X[10.sup.4]) [M.sub.n] (a) (wt%) (b) ([degrees]C)

1 2.79 1.64 0.96 96.7
2 2.33 1.97 1.45 108.7
3 3.00 1.72 3.16 150.6
4 2.70 1.73 4.54 151.6
5 (c) 2.48 1.75 2.82 150.2
6 (d) 2.82 1.75 -- 149.7

(a) Determined by GPC (RI detector) on the basis of a polystyrene
calibration.
(b) Estimated by UV analysis at 330 nm.
(c) The insoluble product had been removed.
(d) A control experiment: a PC solution (49.0 mg/ml) without [C.sub.60]
was heated under MI in the presence of AIBN (0.55 mg/ml) for 10 min.
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Date:Apr 1, 2006
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