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The synthesis of CDA-g-PMMA copolymers through atom transfer radical polymerization
Polymer 45 (2004) 7091–7097 www.elsevier.com/locate/polymer
The synthesis of CDA-g-PMMA copolymers through atom transfer radical polymerization Dawa Shen, Yong Huang* State Key Laboratory of Polymer Physics and Chemistry, Joint Laboratory of Polymer Science and Material, Institute of Chemistry, Chinese Academy of Science, Beijing 100080, People’s Republic of China Received 22 June 2004; received in revised form 20 August 2004; accepted 23 August 2004
Abstract Graft copolymer of cellulose diacetate (CDA) and PMMA was synthesized through atom transfer radical polymerization (ATRP). The residual hydroxyl groups on the diacetate cellulose reacted with 2-bromoisobutyryl bromide to yield 2-bromoisobutyryl groups known to be an efficient initiator for ATRP. Then the functional CDA was used as macroinitiator in the ATRP of MMA. The polymerization was carried out in the system of PMDETA/CuBr/1,4-dioxane under 70 8C. Kinetic study indicated that the polymerization is first order. Copolymers were characterized by 1H NMR and GPC. The molecular weight increased without any trace of the macroinitiator, and the polydispersities were low. q 2004 Published by Elsevier Ltd. Keywords: Graft copolymer; Cellulose diacetate; Atom transfer radical polymerization
1. Introduction Cellulose is the most abundant organic raw material in the world and it has been widely studied during the past decades because it is a biodegradable material and a renewable resource. Modification of cellulose by graft copolymerization provides a significant route to alter the physical and chemical properties [1] and cellulose has been graft copolymerized using various techniques in the past decades [2–10]. Most of them are based on a ‘grafting from process’, in which the radicals are formed on the polymer backbone either by various chemical initiator or by irradiation and the free radical polymerization of vinyl monomers occurs. In these methods, the chain scission may occur and it is impossible to predetermine the length of the graft chains from the cellulose backbone because these methods are not controlled.
* Corresponding author. Tel.: C86-1082618573; fax: C86-1062599373. E-mail address: [email protected] (Y. Huang). 0032-3861/$ - see front matter q 2004 Published by Elsevier Ltd. doi:10.1016/j.polymer.2004.08.042
Atom transfer radical polymerization (ATRP) has been independently discovered by Matyjaszewski and Sawamoto in 1995 [11–13], which is a robust and versatile technique to accurately control the chain length and polydispersity of the polymer, and could be used to synthesize well-defined copolymers. It is generally believed that the living/controlled nature of ATRP is due to the relatively low radical concentration in the reaction system, which suppress the termination relative to propagation. Graft copolymerization of vinyl monomer onto the cellulose and other natural polymers using ATRP has only recently attracted interest [14–17]. This technique could potentially provide a new way to synthesize copolymers with well-defined structure. It would enable a wide variety of molecular designs to afford some novel types of tailored hybrid materials. In this study, ATRP was used to synthesize the graft copolymer of cellulose diacetate (CDA) with PMMA. In the synthesis process, the cellulose substituent has been modified to serve as a macro-initiator that initiates the ATRP of MMA. The copolymers were characterized with GPC and 1H NMR and the living nature of the polymerization were discussed.
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2. Experimental 2.1. Materials Methyl methacrylate (Beijing Chemical Engineering Plant) was dried over anhydrous magnesium sulfate and then, distilled under reduced pressure before used. In order to remove the copper(II), copper(I)bromide (CuBr) (Shanghai Zhenxing Chemical Regent Factory) was stirred in glacial acetic acid, filtered, and washed with acetone three times. The solid was dried under vacuum at room temperature overnight. Pyridine was dried over anhydrous magnesium sulfate and filtered just prior to use. 1,4-Dioxane was purified with active alumina and then, distilled under reduced pressure. N, N, N 0 , N 00 , N 00 -pentamethyldiethylenetriamine (PMDETA) (Aldrich, 98%), 2-bromoisobutyryl bromide (Aldrich, 98%) and cellulose diacetate (CDA) (Fluka, 40% acetyl group) were used as received. All other solvents were dried by anhydrous magnesium sulfate. 2.2. Characterization Monomer conversion was obtained gravimetrically. Molecular weights and molecular weight distributions of the CDA macro-initiator, graft copolymers, and graft chains were measured by gel permeation chromatography (GPC), on a system equipped with a Waters 515 pump, three columns (Styragel HR1, Styragel HR3 and Styragel HT4) and a 2410 differential refractometer detector. The eluant was THF and the flow rate was 1 ml/min. Monodisperse polystyrene were used as the standard to generate the calibration curve. 1H NMR analysis was carried out with a Bruker DMX 300 NMR spectrometer and CDC13 was used as solvent. FT-IR spectra recorded on a Bruker-Equinox 55 FT-IR spectrometer were obtained by dissolution of the polymer in CHCl3 and casting the films onto a NaCl plates. The films were dried in vacuum at room temperature for 48 h. 2.3. Synthesis of the macro-initiator The preparation of CDA macro-initiator was carried out according to the procedure shown in Scheme 1. In a
500 ml three-necked round-bottom flask, CDA (4.9272 g, 20 mmol) was dissolved in the mixture of 100 ml dry THF and 5 ml pyridine with magnetic stirring. And then, 2-bromoisobutyryl bromide (2.3027 g, 10 mmol) in 20 ml dry THF was slowly dropped into the solution at 0 8C in an ice/water bath. The reaction mixture was further stirred at room temperature overnight. Then, the mixture was diluted with 100 ml THF. After removing the salt by filtration, the solvent was removed by rotary evaporation. The crude product was purified by repeated dissolution in THF and precipitation in methanol and then dried for 48 h at 30 8C in vacuum. 2.4. Synthesis of copolymers The macro-initiator (0.1695 g, 0.25 mmol initiating site), PMDETA (0.0437 g, 0.25 mmol) and CuBr (0.0181 g, 0.125 mmol) were mixed in a 100 ml threenecked round-bottom flask equipped with a magnetic stirring bar. After sealing it with a rubber septum, the flask was degassed and back-filled with nitrogen, which was repeated three times. Deoxygenated 1,4-dioxane (2.2145 g, 25 mmol) was added in the system to dissolve the macro-initiator. After the macro-initiator had been dissolved, the methyl methacrylate (7.5122 g, 75 mmol) was added. The reaction mixture was degassed by three freeze-pump-thaw cycles after stirring for 10 min and then, the flask was immersed in an oil bath thermostated at 70 8C. At timed intervals, samples were withdrawn from the flask using degassed syringes to determine monomer conversion and molecular weight. 2.5. Hydrolysis of the CDA backbone One gram copolymer was dissolved in the mixture of 30 ml THF and 20 ml acetone and then, 4 ml 70% H2SO4 was added. The solution refluxed at boiling point for 8 h. The residual polymer was participated into methanol and dried in vacuum at 50 8C.
Scheme 1. Synthesis of CDA-g-PMMA.
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Scheme 2. Formula describing the relationship of DS and content of acetyl.
3. Results and discussion 3.1. Synthesis of the macro-initiator The content of the acetyl group in the CDA backbone is 40%, which means that the substitution degree of the acetyl is 2.37, calculated according to the formula describing the relationship between the acetyl content and the degree of substitution shown in Scheme 2. The degree of the acetyl substitution is consistent with that obtained from the 1H NMR spectrum (Fig. 1) by comparing the integral areas of the signals for the methyl proton of acetyl group around dZ 2.00 ppm to those for the proton on the glucose ring around dZ3.00–5.50 ppm. Therefore, there are still some hydroxyl groups on the CDA backbone, which could be used to react with 2-bromoisobutyryl bromide to obtain the functional CDA. Comparing the FT-IR spectrum of the product to that of the parent CDA (Fig. 2), the stretching peak of hydroxyl groups in the glucose rings around 3500 cmK1 becomes smaller after the reaction of the CDA with the 2-bromoisobutyryl bromide, which means that some of the hydroxyl groups were substituted by the bromoisobutyryl groups. The introduction of the bromoisobutyryl group is
Fig. 2. FT-IR spectra before and after reacted with 2-bromoisobutyryl bromide.
further confirmed by the appearance of the methyl proton signal at dZ1.67 ppm, next to the broad proton signal of the parent cellulose ester at dZ3.0–5.5 ppm in the 1H NMR spectrum (Fig. 3). Substitution value of the bromoisobutyryl group (DBr) can be determined from the ratio of the integral areas of the signals between the methyl proton of bromoisobutyryl group around dZ1.67 and the methyl protons of glucose ring around dZ3.00–5.5 ppm and it is found that DBr of the CDA with 2-bromoisobutyryloxy groups is approximately 0.43.
Fig. 1. 1H NMR spectrum of CAD.
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Fig. 3. 1H NMR spectrum of the macro-initiator.
The data in Table 1 shows the effect of reaction time and the stoichiometric ratio of the reactant on the substitution degree of the bromoisobutyryl group. It can be seen that increasing the ratio of 2-bromoisobutyryl bromide to CDA and extending the reaction time may raise the degree of substitution of bromoisobutyryl group (DBr). But it is difficult to substitute all of the residual hydroxy group, because the acylation reaction occurs in preference to C-6 position, and it is difficult to substitute the C-2-OH and C-3OH thoroughly under the reactive condition. 3.2. Copolymerization It is well known that ethyl 2-bromoisobutyrate is an excellent initiator for ATRP with high initiating efficiency and, therefore, the functional CDA that DBr is 0.43 was used to initiate ATRP of methyl methyacrylate to prepare CDAg-PMMA graft copolymers. Because the substitution of 2-bromo isobutyryl is 0.43, namely there is one initiating site every two glucose rings, the polymerization is a densely graft polymerization in reality, in which the radical–radical coupling termination is prone to occur much more than the general ATRP [18–21]. In order to suppress the termination, a dilute system must be used and the polymerization should be stop at low monomer conversion (O15%). Table 2 lists a series of experiments using DAC-Br as the macroinitiator for different reaction conditions in the polymerization of
MMA. In addition, lower temperature and lower [Cu(I)] were also used to reduce the radical concentration during the polymerization, which results in the narrow distribution of the molecular weight. The GPC spectra of the resulting graft copolymers at different MMA conversion are shown in Fig. 4. It can be seen that the molecular weight grows without any trace of the CDA macro-initiator, indicating that efficient initiation has taken place. The plot of Mn and Mn/Mw vs. conversion is displayed in Fig. 5. The number average molecular weight increased with monomer conversion and that the polydispersity decreased during the polymerization process. It can be calculated from Fig. 7 that the molar ratio of MMA unite to glucose unite is 15:1. As a result, the well-defined graft chain affected the polydispersity of the copolymer more and more along with the polymerization proceeding. But the plot of Mn vs. the monomer conversion is not a linear one, which suggests that the molecular weight of the
Table 1 Effect of reactive condition on DSBr Number
CDA:2-bromoisobutyryl bromide (mol)
Time (h)
DSBr
1 2 3 4
2:1 2:1 1:2 1:2
12 24 12 24
0.43 0.506 0.499 0.508
Fig. 4. GPC chromatograms for the graft copolymer initiated by CDA-Br at various reaction times.
D. Shen, Y. Huang / Polymer 45 (2004) 7091–7097
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Table 2 Reaction conditions for ATRP of MMA from DAC-Br Number
[M]:[I]a:[CuBr]: [PMDETA]:[dioxane]
Temp (8C)
Time (h)
Conv (%)
Mn
Mw/Mn
1 2 3 4 5
50:1:1:1.5:30 150:1:1:1.5:300 300:1:1:1:100 300:1:0.5:1:100 300:1:0.5:1:100
80 80 70 70 80
1 55 8 8 8
Gelled 4 5.4 4 5.1
93,648 56,339 45,633 63,500
1.99 1.45 1.37 1.45
a
[I] is defined as the molar of Br in the macroinitiator determined from the 1H NMR spectrum (mass of sample/unite molecular weight of DAC-Br).
copolymer determined by GPC may be not totally same as the real one because the copolymer is a non-linear polymer and molecular weight is determined vs. the linear polystyrene standard.
The preceding results suggest that the polymerization is a well-controlled and living process. This can be further confirmed by the kinetic study. A linear semilogarithmic plot of monomer conversion vs. time is shown in Fig. 6. It is
Fig. 5. Plot of the Mn and Mw/Mn vs. conversion for the polymerization.
Fig. 6. Semilogarithmic plot of monomer conversion vs. time.
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Fig. 7. 1H NMR spectrum of the last graft copolymer sample.
Fig. 8. 1H NMR spectrum of the polymer after hydrolyzing the copolymer.
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4. Conclusions Cellulose diacetate has been modified by reacting the residual hydroxyl groups with 2-bromoisobutyryl bromide. The degree of the substitution of 2-bromoisobutyryl bromide was determined by 1H NMR. The modified cellulose diacetate then was grafted with methyl methacrylate through ATRP. Kinetic study indicated that the polymerization is living/controlled. The cleaved PMMA side chains have a low polydispersity. This shows that well-defined graft copolymer of CDA and PMMA can be successfully prepared via atom transfer radical polymerization.
Acknowledgements The financial support by Chinese Academy of Sciences (Grant No.KJCX2-SW-H07) is greatly appreciated. Fig. 9. GPC spectrum before and after hydrolysis.
clear that the variation of ln½M0 =½Mt is linear with time, which indicates a stable radical concentration in the system during the polymerization when monomer conversion is low. The 1H NMR spectrum of the last copolymer sample is shown in Fig. 7, the signal at dZ3.6 is ascribed to the methyl proton of –OCH3 of PMMA. The degree of polymerization of PMMA side chain is determined as 35 according to the ratio of the integral area of the methyl proton of –OCH3 to that of the proton on the glucose ring. In order to determine the real molecule weight of the PMMA side chain, the CDA-g-PMMA of the last sample was hydrolyzed in the mixture of 70% sulfuric acid and THF/acetone at boiling point for 8 h. The residual polymer was characterized with GPC and 1H NMR. In the 1H NMR spectrum (Fig. 8), the signal ascribed to the protons of CDA backbone disappeared, indicating that the CDA backbone has been hydrolyzed thoroughly. GPC trace of the polymer before and after hydrolysis is shown in Fig. 9. The number average molecular weight of the hydrolyzed product is 3150, which is in accordance with the result calculated from the 1H NMR spectrum. And the polydispersity is very low. As a result, the graft copolymer is verified having welldefined PMMA side chains.
References [1] Hebeish A, Guthrie JT. The chemistry and technology of cellulosic copolymers. Berlin: Springer; 1981. [2] Mino G, Kaizerman S. J Polym Sci 1958;31:242. [3] Geacintoc N, Stanett V, Abrahamson EW, Hermans JJ. J Appl Polym Sci 1960;3:54. [4] Richards GN. J Appl Polym Sci 1961;5:539. [5] Epstein JA, Bar-Nun A. J Polym Sci 1964;2:27. [6] Ranga Rao S, Kapur SL. J Appl Polym Sci 1969;13:2619. [7] Kubota H, Ogiwara Y. J Appl Polym Sci 1970;14:2879. [8] Ra˚nby B, Zuchowska D. Polym J 1987;19:623. [9] Lu J, Yi M, Li JQ, Ha HF. J Appl Polym Sci 2001;81:3578. [10] Gupta KC, Khandekar K. Biomacromolecules 2003;4:758. [11] Wang J-S, Matyjaszewski K. J Am Chem Soc 1995;117:5614. [12] Wang J-S, Matyjaszewski KK. Macromolecules 1995;28:7901. [13] Kato M, Kamigaito M, Sawamoto M, Higashimura T. Macromolecules 1995;28:1721. [14] Carlmark A, Malmstrom EE. Biomacromolecules 2003;4:1740. [15] Carlmark A, Soderberg S, Malmstrom E. Polym Prep 2002;14:57. [16] Carlmark A, Malmstrom EE. J Am Chem Soc 2002;124:900. [17] El Tahlawy K, Hudson SM. J Polym Sci 2003;89:901. [18] Beers KL, Gaynor SG, Matyjaszewski K. Macromolecules 1998;31: 9413. [19] Borner HG, Beers K, Matyjaszewski K. Macromolecules 2001;34: 4375. [20] Matyjaszewski K. Polym Int 2003;52:1559. [21] Qin S, Matyjaszewski K, Xu H, Sheiko SS. Macromolecules 2003;36: 605.
The synthesis of CDA-g-PMMA copolymers through atom transfer radical polymerization Dawa Shen, Yong Huang* State Key Laboratory of Polymer Physics and Chemistry, Joint Laboratory of Polymer Science and Material, Institute of Chemistry, Chinese Academy of Science, Beijing 100080, People’s Republic of China Received 22 June 2004; received in revised form 20 August 2004; accepted 23 August 2004
Abstract Graft copolymer of cellulose diacetate (CDA) and PMMA was synthesized through atom transfer radical polymerization (ATRP). The residual hydroxyl groups on the diacetate cellulose reacted with 2-bromoisobutyryl bromide to yield 2-bromoisobutyryl groups known to be an efficient initiator for ATRP. Then the functional CDA was used as macroinitiator in the ATRP of MMA. The polymerization was carried out in the system of PMDETA/CuBr/1,4-dioxane under 70 8C. Kinetic study indicated that the polymerization is first order. Copolymers were characterized by 1H NMR and GPC. The molecular weight increased without any trace of the macroinitiator, and the polydispersities were low. q 2004 Published by Elsevier Ltd. Keywords: Graft copolymer; Cellulose diacetate; Atom transfer radical polymerization
1. Introduction Cellulose is the most abundant organic raw material in the world and it has been widely studied during the past decades because it is a biodegradable material and a renewable resource. Modification of cellulose by graft copolymerization provides a significant route to alter the physical and chemical properties [1] and cellulose has been graft copolymerized using various techniques in the past decades [2–10]. Most of them are based on a ‘grafting from process’, in which the radicals are formed on the polymer backbone either by various chemical initiator or by irradiation and the free radical polymerization of vinyl monomers occurs. In these methods, the chain scission may occur and it is impossible to predetermine the length of the graft chains from the cellulose backbone because these methods are not controlled.
* Corresponding author. Tel.: C86-1082618573; fax: C86-1062599373. E-mail address: [email protected] (Y. Huang). 0032-3861/$ - see front matter q 2004 Published by Elsevier Ltd. doi:10.1016/j.polymer.2004.08.042
Atom transfer radical polymerization (ATRP) has been independently discovered by Matyjaszewski and Sawamoto in 1995 [11–13], which is a robust and versatile technique to accurately control the chain length and polydispersity of the polymer, and could be used to synthesize well-defined copolymers. It is generally believed that the living/controlled nature of ATRP is due to the relatively low radical concentration in the reaction system, which suppress the termination relative to propagation. Graft copolymerization of vinyl monomer onto the cellulose and other natural polymers using ATRP has only recently attracted interest [14–17]. This technique could potentially provide a new way to synthesize copolymers with well-defined structure. It would enable a wide variety of molecular designs to afford some novel types of tailored hybrid materials. In this study, ATRP was used to synthesize the graft copolymer of cellulose diacetate (CDA) with PMMA. In the synthesis process, the cellulose substituent has been modified to serve as a macro-initiator that initiates the ATRP of MMA. The copolymers were characterized with GPC and 1H NMR and the living nature of the polymerization were discussed.
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2. Experimental 2.1. Materials Methyl methacrylate (Beijing Chemical Engineering Plant) was dried over anhydrous magnesium sulfate and then, distilled under reduced pressure before used. In order to remove the copper(II), copper(I)bromide (CuBr) (Shanghai Zhenxing Chemical Regent Factory) was stirred in glacial acetic acid, filtered, and washed with acetone three times. The solid was dried under vacuum at room temperature overnight. Pyridine was dried over anhydrous magnesium sulfate and filtered just prior to use. 1,4-Dioxane was purified with active alumina and then, distilled under reduced pressure. N, N, N 0 , N 00 , N 00 -pentamethyldiethylenetriamine (PMDETA) (Aldrich, 98%), 2-bromoisobutyryl bromide (Aldrich, 98%) and cellulose diacetate (CDA) (Fluka, 40% acetyl group) were used as received. All other solvents were dried by anhydrous magnesium sulfate. 2.2. Characterization Monomer conversion was obtained gravimetrically. Molecular weights and molecular weight distributions of the CDA macro-initiator, graft copolymers, and graft chains were measured by gel permeation chromatography (GPC), on a system equipped with a Waters 515 pump, three columns (Styragel HR1, Styragel HR3 and Styragel HT4) and a 2410 differential refractometer detector. The eluant was THF and the flow rate was 1 ml/min. Monodisperse polystyrene were used as the standard to generate the calibration curve. 1H NMR analysis was carried out with a Bruker DMX 300 NMR spectrometer and CDC13 was used as solvent. FT-IR spectra recorded on a Bruker-Equinox 55 FT-IR spectrometer were obtained by dissolution of the polymer in CHCl3 and casting the films onto a NaCl plates. The films were dried in vacuum at room temperature for 48 h. 2.3. Synthesis of the macro-initiator The preparation of CDA macro-initiator was carried out according to the procedure shown in Scheme 1. In a
500 ml three-necked round-bottom flask, CDA (4.9272 g, 20 mmol) was dissolved in the mixture of 100 ml dry THF and 5 ml pyridine with magnetic stirring. And then, 2-bromoisobutyryl bromide (2.3027 g, 10 mmol) in 20 ml dry THF was slowly dropped into the solution at 0 8C in an ice/water bath. The reaction mixture was further stirred at room temperature overnight. Then, the mixture was diluted with 100 ml THF. After removing the salt by filtration, the solvent was removed by rotary evaporation. The crude product was purified by repeated dissolution in THF and precipitation in methanol and then dried for 48 h at 30 8C in vacuum. 2.4. Synthesis of copolymers The macro-initiator (0.1695 g, 0.25 mmol initiating site), PMDETA (0.0437 g, 0.25 mmol) and CuBr (0.0181 g, 0.125 mmol) were mixed in a 100 ml threenecked round-bottom flask equipped with a magnetic stirring bar. After sealing it with a rubber septum, the flask was degassed and back-filled with nitrogen, which was repeated three times. Deoxygenated 1,4-dioxane (2.2145 g, 25 mmol) was added in the system to dissolve the macro-initiator. After the macro-initiator had been dissolved, the methyl methacrylate (7.5122 g, 75 mmol) was added. The reaction mixture was degassed by three freeze-pump-thaw cycles after stirring for 10 min and then, the flask was immersed in an oil bath thermostated at 70 8C. At timed intervals, samples were withdrawn from the flask using degassed syringes to determine monomer conversion and molecular weight. 2.5. Hydrolysis of the CDA backbone One gram copolymer was dissolved in the mixture of 30 ml THF and 20 ml acetone and then, 4 ml 70% H2SO4 was added. The solution refluxed at boiling point for 8 h. The residual polymer was participated into methanol and dried in vacuum at 50 8C.
Scheme 1. Synthesis of CDA-g-PMMA.
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Scheme 2. Formula describing the relationship of DS and content of acetyl.
3. Results and discussion 3.1. Synthesis of the macro-initiator The content of the acetyl group in the CDA backbone is 40%, which means that the substitution degree of the acetyl is 2.37, calculated according to the formula describing the relationship between the acetyl content and the degree of substitution shown in Scheme 2. The degree of the acetyl substitution is consistent with that obtained from the 1H NMR spectrum (Fig. 1) by comparing the integral areas of the signals for the methyl proton of acetyl group around dZ 2.00 ppm to those for the proton on the glucose ring around dZ3.00–5.50 ppm. Therefore, there are still some hydroxyl groups on the CDA backbone, which could be used to react with 2-bromoisobutyryl bromide to obtain the functional CDA. Comparing the FT-IR spectrum of the product to that of the parent CDA (Fig. 2), the stretching peak of hydroxyl groups in the glucose rings around 3500 cmK1 becomes smaller after the reaction of the CDA with the 2-bromoisobutyryl bromide, which means that some of the hydroxyl groups were substituted by the bromoisobutyryl groups. The introduction of the bromoisobutyryl group is
Fig. 2. FT-IR spectra before and after reacted with 2-bromoisobutyryl bromide.
further confirmed by the appearance of the methyl proton signal at dZ1.67 ppm, next to the broad proton signal of the parent cellulose ester at dZ3.0–5.5 ppm in the 1H NMR spectrum (Fig. 3). Substitution value of the bromoisobutyryl group (DBr) can be determined from the ratio of the integral areas of the signals between the methyl proton of bromoisobutyryl group around dZ1.67 and the methyl protons of glucose ring around dZ3.00–5.5 ppm and it is found that DBr of the CDA with 2-bromoisobutyryloxy groups is approximately 0.43.
Fig. 1. 1H NMR spectrum of CAD.
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Fig. 3. 1H NMR spectrum of the macro-initiator.
The data in Table 1 shows the effect of reaction time and the stoichiometric ratio of the reactant on the substitution degree of the bromoisobutyryl group. It can be seen that increasing the ratio of 2-bromoisobutyryl bromide to CDA and extending the reaction time may raise the degree of substitution of bromoisobutyryl group (DBr). But it is difficult to substitute all of the residual hydroxy group, because the acylation reaction occurs in preference to C-6 position, and it is difficult to substitute the C-2-OH and C-3OH thoroughly under the reactive condition. 3.2. Copolymerization It is well known that ethyl 2-bromoisobutyrate is an excellent initiator for ATRP with high initiating efficiency and, therefore, the functional CDA that DBr is 0.43 was used to initiate ATRP of methyl methyacrylate to prepare CDAg-PMMA graft copolymers. Because the substitution of 2-bromo isobutyryl is 0.43, namely there is one initiating site every two glucose rings, the polymerization is a densely graft polymerization in reality, in which the radical–radical coupling termination is prone to occur much more than the general ATRP [18–21]. In order to suppress the termination, a dilute system must be used and the polymerization should be stop at low monomer conversion (O15%). Table 2 lists a series of experiments using DAC-Br as the macroinitiator for different reaction conditions in the polymerization of
MMA. In addition, lower temperature and lower [Cu(I)] were also used to reduce the radical concentration during the polymerization, which results in the narrow distribution of the molecular weight. The GPC spectra of the resulting graft copolymers at different MMA conversion are shown in Fig. 4. It can be seen that the molecular weight grows without any trace of the CDA macro-initiator, indicating that efficient initiation has taken place. The plot of Mn and Mn/Mw vs. conversion is displayed in Fig. 5. The number average molecular weight increased with monomer conversion and that the polydispersity decreased during the polymerization process. It can be calculated from Fig. 7 that the molar ratio of MMA unite to glucose unite is 15:1. As a result, the well-defined graft chain affected the polydispersity of the copolymer more and more along with the polymerization proceeding. But the plot of Mn vs. the monomer conversion is not a linear one, which suggests that the molecular weight of the
Table 1 Effect of reactive condition on DSBr Number
CDA:2-bromoisobutyryl bromide (mol)
Time (h)
DSBr
1 2 3 4
2:1 2:1 1:2 1:2
12 24 12 24
0.43 0.506 0.499 0.508
Fig. 4. GPC chromatograms for the graft copolymer initiated by CDA-Br at various reaction times.
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Table 2 Reaction conditions for ATRP of MMA from DAC-Br Number
[M]:[I]a:[CuBr]: [PMDETA]:[dioxane]
Temp (8C)
Time (h)
Conv (%)
Mn
Mw/Mn
1 2 3 4 5
50:1:1:1.5:30 150:1:1:1.5:300 300:1:1:1:100 300:1:0.5:1:100 300:1:0.5:1:100
80 80 70 70 80
1 55 8 8 8
Gelled 4 5.4 4 5.1
93,648 56,339 45,633 63,500
1.99 1.45 1.37 1.45
a
[I] is defined as the molar of Br in the macroinitiator determined from the 1H NMR spectrum (mass of sample/unite molecular weight of DAC-Br).
copolymer determined by GPC may be not totally same as the real one because the copolymer is a non-linear polymer and molecular weight is determined vs. the linear polystyrene standard.
The preceding results suggest that the polymerization is a well-controlled and living process. This can be further confirmed by the kinetic study. A linear semilogarithmic plot of monomer conversion vs. time is shown in Fig. 6. It is
Fig. 5. Plot of the Mn and Mw/Mn vs. conversion for the polymerization.
Fig. 6. Semilogarithmic plot of monomer conversion vs. time.
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Fig. 7. 1H NMR spectrum of the last graft copolymer sample.
Fig. 8. 1H NMR spectrum of the polymer after hydrolyzing the copolymer.
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4. Conclusions Cellulose diacetate has been modified by reacting the residual hydroxyl groups with 2-bromoisobutyryl bromide. The degree of the substitution of 2-bromoisobutyryl bromide was determined by 1H NMR. The modified cellulose diacetate then was grafted with methyl methacrylate through ATRP. Kinetic study indicated that the polymerization is living/controlled. The cleaved PMMA side chains have a low polydispersity. This shows that well-defined graft copolymer of CDA and PMMA can be successfully prepared via atom transfer radical polymerization.
Acknowledgements The financial support by Chinese Academy of Sciences (Grant No.KJCX2-SW-H07) is greatly appreciated. Fig. 9. GPC spectrum before and after hydrolysis.
clear that the variation of ln½M0 =½Mt is linear with time, which indicates a stable radical concentration in the system during the polymerization when monomer conversion is low. The 1H NMR spectrum of the last copolymer sample is shown in Fig. 7, the signal at dZ3.6 is ascribed to the methyl proton of –OCH3 of PMMA. The degree of polymerization of PMMA side chain is determined as 35 according to the ratio of the integral area of the methyl proton of –OCH3 to that of the proton on the glucose ring. In order to determine the real molecule weight of the PMMA side chain, the CDA-g-PMMA of the last sample was hydrolyzed in the mixture of 70% sulfuric acid and THF/acetone at boiling point for 8 h. The residual polymer was characterized with GPC and 1H NMR. In the 1H NMR spectrum (Fig. 8), the signal ascribed to the protons of CDA backbone disappeared, indicating that the CDA backbone has been hydrolyzed thoroughly. GPC trace of the polymer before and after hydrolysis is shown in Fig. 9. The number average molecular weight of the hydrolyzed product is 3150, which is in accordance with the result calculated from the 1H NMR spectrum. And the polydispersity is very low. As a result, the graft copolymer is verified having welldefined PMMA side chains.
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