Diblock Copolymer of Bacterial Cellulose and Poly(methyl methacrylate)

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Diblock Copolymer of Bacterial Cellulose and Poly(methyl methacrylate) Initiated by Chain-End-Type Radicals Produced by Mechanical Scission of Glycosidic Linkages of Bacterial Cellulose Masato Sakaguchi,*,† Takeshi Ohura,‡ Tadahisa Iwata,§ Shuhei Takahashi,† Shuji Akai,| Toshiyuki Kan,| Hisao Murai,⊥ Motoyasu Fujiwara,# Osamu Watanabe,∇ and Mamiko Narita∇ Institute for Environmental Science, University of Shizuoka, 52-1, Yada, Suruga-ku, Shizuoka 422-8526, Japan, Faculty of Agriculture, Meijo University, 1-501, Tempaku-ku, Nagoya 468-8502, Japan, Department of Biomaterial Sciences, Graduate School of Agricultural and Life Sciences, The University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 133-8657, Japan, Graduate School of Pharmaceutical Sciences, University of Shizuoka, 52-1, Yada, Suruga-ku, Shizuoka 422-8526, Japan, Department of Chemistry, Faculty of Science, Shizuoka University, 836, Ohya, Suruga-ku, Shizuoka 422-8529 Japan, Institute for Molecular Science, National Institutes of Natural Sciences, Nishigonaka 38, Myoudaiji, Okazaki 444-8585, Japan, and Toyota Central Research and Development Laboratories, Inc., 41-Yokomichi, Nagakute, Aichi 480-1192, Japan Received July 30, 2010; Revised Manuscript Received September 12, 2010

Bacterial cellulose (BC) was mechanically fractured in vacuum at 77 K; this resulted in the scission of the β-1,4 glycosidic linkages of BC. The chain-end-type radicals (mechanoradicals) generated from the scissions were assigned by electron spin resonance (ESR) spectral analyses. A diblock copolymer of BC and poly(methyl methacrylate) (BC-block-PMMA) was produced by the mechanical fracture of BC with MMA (methyl methacrylate) in vacuum at 77 K. Radical polymerization of MMA was initiated by the mechanoradicals located on the BC surface. The BC surface was fully covered with the PMMA chains of the BC-block-PMMA. Novel modification of the BC surface with the BC-block-PMMA was confirmed by spectral analyses of ESR, Fourier-transform infrared, 1H NMR, and gel permeation chromatography.

1. Introduction There is an urgent need to develop a sustainable society that does not face problems such as resource depletion and carbon dioxide emission. Cellulose is the most abundant biological resource on Earth and has been used by humans since ancient times; for example, it has been used to make paper, as a building material and as a textile. Thus, cellulose should be utilized on a wider scale in order to realize a sustainable society. Chemical modifications of cellulose have been limited to the hydroxide groups on the glucopyranose ring (i.e., side chain modification) because the β-1,4 glycosidic linkages that form the main chain of cellulose are very robust. On the other hand, modification of the main chain of cellulose, which is a process that involves diblock copolymerization, may result in novel uses of this compound. Recently, the synthesis of diblock polysaccharides by end-chain modification via atom transfer radical polymerization using macroinitiators has been reported.1,2 The syntheses of macroinitiators at the end of the cellulose chain are an essential part of this diblock copolymerization, and these syntheses are difficult. It is expected that the development of a novel method for synthesizing a diblock copolymer of cellulose * To whom correspondence should be addressed. Phone and Fax: +8154-264-5786. E-mail: [email protected]. † Institute for Environmental Science, University of Shizuoka. ‡ Meijo University. § The University of Tokyo. | Graduate School of Pharmaceutical Sciences, University of Shizuoka. ⊥ Department of Chemistry, Shizuoka University. # National Institutes of Natural Sciences. ∇ Toyota Central Research and Development Laboratories.

will result in the development of radical polymerization based on the use of radicals located at the end of the cellulose chain. Our aim was to synthesize the diblock copolymer of cellulose via radical polymerization initiated by radicals located at the end of the cellulose chain, which are produced by the mechanical scission of the β-1,4 glycosidic linkage. It has been widely reported3-6 that mechanical destruction of cellulose results in the production of radicals. In these studies, the mechanical fracture of cellulose was carried out at room temperature and in an air atmosphere. The radicals are unstable at room temperature and in an air atmosphere, and they are converted into another chemical species through a side reaction.7 Furthermore, the radical that is located at the end of the cellulose chain (chain-end-type radical) and is produced by the mechanical destruction of cellulose is highly reactive, reacts easily with oxygen, and can be converted to peroxide radicals. Electron spin resonance (ESR) spectra of peroxide radicals show a characteristic amorphous pattern at 77 K, a temperature at which molecular motion ceases.8-11 Therefore, the original radical species of the peroxide radicals cannot be identified from the ESR spectra of the peroxide radicals. Therefore, the mechanism of the scission of the β-1,4 glycosidic linkages that occurs upon mechanical fracture of cellulose cannot be demonstrated. To reveal the scission mechanism of cellulose and the production of a chain-end-type radical, mechanical fracture of cellulose must be carried out in vacuum at 77 K. The results of our study on the mechanical destruction of synthetic polymers in vacuum at 77 K are as follows: (i) mechanical destruction of polymers induced main chain scission and resulted in the production of

10.1021/bm100879v  2010 American Chemical Society Published on Web 09/30/2010

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Figure 1. SEM images (a) before and (b) after fracture of BC.

chain-end-type radicals (mechanoradicals);12,13 (ii) the production of mechanoradicals requires a critical degree of polymerization,13 and materials with low molecular weights cannot be used to produce mechanoradicals;13 (iii) mechanoradicals were trapped on a fresh solid surface obtained by mechanical destruction of polymers;14-16 and (iv) mechanoradicals have high reactivity and can initiate radical polymerization and the subsequent production of diblock copolymers.17-24 To observe the scission mechanism of cellulose, we used bacterial cellulose (BC) and a vibration glass ball mill developed in-house12,13 to obtain mechanically fractured BC in vacuum at 77 K. In this study, we show that the mechanical fracture of BC in vacuum at 77 K resulted in the production of chain-end-type radicals of BC (BC mechanoradicals), which was induced by scission of β-1,4 glycosidic linkages. Furthermore, a novel diblock copolymer of BC and MMA (BC-block-PMMA) was synthesized on the BC surface by utilizing the BC mechanoradicals.

2. Materials and Methods 2.1. Bacterial Cellulose. Gluconacetobacter xylinus (Acetobacter xylinum) JCM9730 was used as a strain for the production of BC. Culture conditions were as follows: buffered Schramm and Hestrin’s (BSH)25 medium (2.0% [w/v] glucose, 0.5% [w/v] yeast extract, 0.5% [w/v] peptone, 0.27% [w/v] Na2HPO4, and 0.115% [w/v] citric acid, pH 5.0) inoculated with the strain was incubated statically at 28 °C for 1 week. After incubation, the BC produced was removed from the medium and washed with water and 4% (w/v) hot sodium hydroxide solution for 2 h. Then, the residue was thoroughly washed under running tap water for 2 days. The decolorized BC in water was pulverized in a blender for 1 min, resulting in the sol condition. Subsequently, the sol-BC was freeze-dried and sequentially dried under vacuum at 70 °C for 7 h. 2.2. Mechanical Destruction of BC. The dried BC (0.5 g) in a glass ball mill was evacuated under 0.6 Pa at 373 K for 7 h, sealed off, and placed in a Dewar filled with liquid nitrogen. The BC in the glass ball mill was mechanically fractured by a homemade vibration ball mill apparatus for 7 h at 77 K in vacuum.13 After milling, the fractured BC was dropped into the ESR sample tube attached to the top of the glass ball mill by turning it over in the liquid nitrogen within 1 s. 2.3. Synthesis of BC-block-PMMA. Methyl methacrylate (MMA; Wako Pure Chemical Industries, Ltd.) was purified by distilling twice in a vacuum line before use. Oxygen gas incorporated in MMA was eliminated by performing the freeze-pump-thaw method four times. The purified MMA was introduced into the glass ball mill containing the pre-vacuum-dried BC, and the glass ball mill was sealed off from the vacuum line, set to the homemade vibration ball mill, and milled in vacuum at 77 K for 7 h.

2.4. Acetylation of Fractured BC and BC-block-PMMA. Acetic acid (0.570 mol) and trifluoroacetic acid anhydride (0.436 mol) were ripened at 323 K for 20 min, BC (0.023 mol) was introduced, and the solution was acetylated at 323 K for 12 h. Acetylated BC was precipitated with methanol, filtered, and dried under vacuum at 343 K for 6 h. Acetylated BC-block-PMMA was produced by using the same procedure. 2.5. Scanning Electron Microscopy. Morphological analysis of BC before and after millings was carried out with a field-emission-type scanning electron microscope (FE-SEM; Hitachi S-4000) operated at 15 kV. Before the SEM observations, the samples were coated with a platinum film of 13 nm thickness by using Hitachi Ionsupatta E-1030. 2.6. Wide-Angle X-ray Diffraction. The wide-angle X-ray diffraction (WAXD) experiment was carried out using the BL45XU beamline with a wavelength of 0.09 nm at the SPring-8 synchrotron radiation facility in Harima, Japan. The degree of crystallinity was calculated from the ratio of integrals for the crystalline and amorphous regions to the overall intensity of the first dimensional profile according to Vonk’s method. 2.7. 1H NMR Measurement. 1H NMR spectra were recorded on a JEOL ALPHA-500 NMR spectrometer at 500 MHz with CDCl3. TMS was used as the internal reference for chemical shifts. 2.8. Electron Spin Resonance Measurement. Electron spin resonance (ESR) spectra were observed at a microwave power level of 2 µW to avoid power saturation and with 100 kHz field modulation using a Bruker EMX Plus spectrometer (X-band) equipped with a helium cryostat (Oxford ESR 900) and a temperature controller (Oxford ITC4). 2.9. ESR Spectral Simulation. ESR spectral simulation was carried out by using a computer program to calculate a line shape equation of the ESR spectra in solid state, having anisotropic g and hyperfine splitting tensor A, as shown in previous studies.10,11 2.10. BET Measurement. The nitrogen adsorption isotherm at 77 K was obtained on the Quantachrome Autosorb-1 system. All samples were degassed at 298 K over 5 h before measurement. Surface areas were calculated using the BET method in the relative pressure (P/P0) range from 0.13 to 0.27. 2.11. Gel Permeation Chromatography Measurement. The molecular weights of samples were observed by using GPC (Shimazu 10A GPC system) with column Shodex K802 and K806M.

3. Results and Discussion 3.1. Mechanical Fracture of BC and Radical Pair Formation. The SEM image of BC before mechanical fracture shows the characteristic profiles of its microfibrils (40-90 nm) (Figure 1a). After mechanical fracture in vacuum at 77 K, the SEM image shows that the microfibrils were destroyed and that nanoparticles (30-230 nm) appeared (Figure 1b). The WAXD spectrum of BC before mechanical fracture shows some sharp peaks due to the crystalline structures (Figure

Copolymer of Cellulose and Poly(methyl methacrylate)

Figure 2. Wide-angle X-ray diffraction spectra (a) before and (b) after fracture of BC.

2a). After mechanical fracture in vacuum at 77 K, these crystalline peaks decayed (Figure 2b). The degrees of crystallinity of BC before and after mechanical fractures were speculated to be 48.3 and 37.2%. These results indicate that the mechanical fracture of BC induced the destruction of microfibrils and of the crystalline structure and produced some nanoparticles. However, these results do not confirm a main chain scission in BC. Figure 3 shows that cellulose molecules are composed of β-1,4 glycosidic linkages of the glucopyranose ring. Upon application of mechanical energy to BC, its β-1,4 glycosidic


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linkages will be broken by two types of scissions: scissions I and II. Scissions I and II will produce pair formations of radicals Ia and Ib and radicals IIa and IIb, respectively. To avoid the production of peroxide radicals and side reactions of the radicals, the mechanical fracture of BC has to be carried out in vacuum at 77 K. Predried BC in a glass ball mill was further dried at 0.6 Pa at 373 K for 7 h, sealed off, and fractured in vacuum at 77 K for 7 h. The ESR spectrum of as-fractured BC was observed at 77 K (Figure 4a). This spectrum reveals that the mechanical fracture of BC produced free radicals. To elucidate the effect of temperature on the ESR spectrum, the fractured BC was annealed at 250 K for 10 min, cooled to 77 K, and observed at 77 K. The ESR spectrum (Figure 4b) of the annealed BC indicates that the humps were decreased by annealing at 250 K (Figure 4b, shown with arrows). Sequentially, the fractured BC was annealed at 290 K for 10 min, cooled to 77 K, and observed at 77 K. The ESR spectrum of BC annealed at 290 K (Figure 4c) indicates that the humps disappeared on the spectrum, and a broad singlet appeared. The profile of the broad singlet spectrum observed at 77 K was unaltered by annealing at 350 K for 10 min, but the ESR intensity decreased. These results indicate that the spectrum of fractured BC annealed at 290 K

Figure 3. Chemical structure of mechanoradicals (Ia and IIb) and alkoxyl radicals (Ib and IIa) produced by the mechanical scission of β-1,4 glycosidic linkages of BC.

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Figure 6. ESR spectrum of fractured BC annealed at 290 K (solid line) and the simulated spectrum of the alkoxyl radical (broken line).

Figure 4. ESR spectra of (a) as-fractured BC, (b) annealed at 250 K, and (c) annealed at 290 K. All ESR spectra were observed at 77 K. Figure 7. ESR spectra of fractured BC observed at 77 K (solid line) and at 5 K (broken line).

Figure 5. Relative intensities of radicals of fractured BC against annealing temperatures.

was composed of a single radical species, whereas the spectrum before annealing was a superposed spectrum composed of several radical species. Figure 5 shows the relative ESR intensities of the as-fractured BC against the annealing temperatures in which samples were annealed at each temperature for 10 min, stored at 77 K for 10 min, and observed at 77 K. The intensity was obtained by double integration of the ESR spectrum and normalization to the intensity of the as-fractured BC observed at 77 K. Figure 5 reveals that the radicals of the fractured BC were stable below 150 K. At more than 150 K, the number of radicals decreased with the increase in annealing temperature. The radical concentration was synchronized with the height of the humps on the ESR spectrum (Figure 4a). The height of the humps was unaltered below 150 K and was decreased with increasing annealing temperature. Then, the humps disappeared at 290 K. The singlet ESR spectrum appeared at 290 K (Figure 4c) and was unaltered on the profile at an annealing temperature at 350 K. The relative intensity of the singlet annealed at 290 K was 58%. These results indicate that the singlet spectrum (Figure 4c) is composed of a single radical species and its intensity is about half of the as-fractured BC. We have reported12,13 that the radical species resulting from the mechanical fracture of synthetic polymers in vacuum at 77 K are chain-end-type radicals (mechanoradicals). Production of the mechanoradicals is required to concentrate mechanical stress on the main chains of polymeric materials. Thus, low molecular weight materials cannot produce mechanoradicals, and a critical degree of polymerization is present.13 Accordingly, it is reasonable to assume that the mechanical action applied to BC in

vacuum at 77 K induces scission of the β-1,4 glycosidic linkages composing the BC main chain. Four types of radicals are produced by scissions I and II (Figure 3) of β-1,4 glycosidic linkages: scission I produces radicals Ia and Ib, whereas scission II produces radicals IIa and IIb. Radicals Ib and IIa are alkoxyltype radicals. They are an indistinguishable singlet ESR spectrum possibly due to the small hyperfine splitting constant (hfs) of the β proton (β-H), which is smeared out in the singlet spectrum. The simulation spectrum of alkoxyl radicals based on the above assumption was calculated by using a computer program10,11 with an isotropic g value (giso ) 2.0045 ( 0.005) and isotropic hyperfine splitting (Aiso ) 1.20 ( 0.08 mT) of one β-H. The simulation spectrum of the alkoxyl radical (broken line in Figure 6) closes on the observed spectrum of the fractured BC annealed at 290 K. This result indicates that β-1,4 glycosidic linkages composing the BC main chain were fractured by the mechanical actions, and this fracture produced alkoxyl-type radicals. Furthermore, the relative concentration of alkoxyl-type radicals was about half (58%). These results strongly suggest that the ESR spectrum of the as-fractured BC (Figure 4a) is a superposed spectrum composed of alkoxyl radicals (Ib and IIa) and alkyl-type radicals (Ia and IIb). The ESR spectrum observed at 77 K of the BC fractured in vacuum at 77 K (Figure 7, solid line) is in good agreement with that observed at 5 K (Figure 7, broken line). The spectral intensity of 5 K is normalized to that of 77 K. This result indicates that the molecular motion of radicals becomes rigid at 77 K. Spectral simulation was carried out by using a computer program on the assumption of the frozen molecular motion. Radical Ia produced by scission I has one R proton (R-H) and one β-H. By assuming the isotropic g value (giso ) 2.0043 ( 0.005), the principal value of anisotropic hyperfine splitting (Ax, Ay, Az) ) (2.10 ( 0.08, 3.10 ( 0.08, 1.10 ( 0.08 mT) of R-H, and isotropic hyperfine splitting (Aiso ) 2.80 ( 0.08 mT) of β-H, the simulated spectrum was obtained as a triplet line (shown in Figure 8, black chain line). Radical IIb produced by scission II has one R-H and two β-Hs. Assuming the isotropic g value (giso ) 2.0034 ( 0.005), the principal value of


Figure 8. Observed ESR spectrum of as-fractured BC (bold solid line) and simulated spectrum (broken line) composed of Ia (triplet line with black chain line), IIb (quartet line with red line), and alkoxyl radicals (blue line).

Figure 9. Observed ESR spectrum of BC fractured with MMA (solid line) and simulated spectrum of alkoxyl radicals (chain line). The simulated spectrum was composed of PMMA propagating radicals with a relative intensity of 0.8 and alkoxyl radicals with a relative intensity of 0.2 (broken line).

anisotropic hyperfine splitting (Ax, Ay, Az) ) (2.30 ( 0.08, 3.40 ( 0.08, 1.20 ( 0.08 mT) of R-H and, incidentally, nearly identical isotropic hyperfine splitting (Aiso ) 3.20 ( 0.08 mT) of two β-Hs, the simulated spectrum was obtained as a quartet line (shown in Figure 8, red line). The simulated singlet spectrum of alkoxyl radicals (Ib and IIa) is shown in Figure 8, blue line. We assumed the relative intensity of each radical on the ESR spectrum of the as-fractured BC: alkoxyl radicals (0.50), Ia (0.25), and IIb (0.25), respectively. The simulated spectrum (Figure 8, broken line) is fairly identical with the observed spectrum (Figure 8, bold solid line). From these results, we conclude that the β-1,4 glycosidic linkages composing the BC main chain were fractured by mechanical actions. This fracture produced pair formations of chain-end-type radicals Ia and IIb and alkoxyl radicals IIa and Ib. The ratio of scissions I and II may be nearly equal, which is based on the relative intensity ratio of each radical. The chain-


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end-type radicals Ia and IIb are called BC mechanoradicals without distinction in this article hereafter. 3.2. Block Copolymerization of BC with MMA Initiated by BC Mechanoradicals. We have reported that (i) mechanoradicals were trapped on the fresh surface produced by mechanical destruction of polymers14-16 and (ii) mechanoradicals had high reactivity, initiated radical polymerization (mechanochemical polymerization), and resulted in diblock copolymers on the surface.17-24 To produce the BC-PMMA diblock copolymer (BC-block-PMMA), we applied the mechanochemical polymerization technique to BC with MMA. BC with MMA was fractured in vacuum at 77 K. The ESR spectrum (Figure 9, solid line) observed at 77 K does not show any ESR spectrum of BC mechanoradicals and seems to be a superposed spectrum composed of characteristic PMMA propagating radicals: -CH2-C(CH3)(COOCH3)• 26-28 as a major part and alkoxyl radicals as a minor part of the spectrum (shown with chain line) that was simulated in the previous section. Iwasaki and Sakai29,30 reported the ESR simulation spectrum of the PMMA propagating radical that was calculated with Aiso,CH3 ) 2.22 mT for the freely rotating CH3 group, and Aβ,H1 ) 1.47 mT and Aβ,H2 ) 0.75 mT. In our calculation, the simulation spectrum of the PMMA propagating radical was calculated with Aiso,CH3 ) 2.22 mT for the freely rotating CH3 group and Aβ,H1 ) 1.51 mT and Aβ,H2 ) 0.82 mT. The profile of our simulation spectrum of the PMMA propagating radical is close to the spectrum reported by Iwasaki and Sakai.29,30 The simulated spectrum (shown in Figure 9, broken line) was calculated as a superposed spectrum of the PMMA propagating radicals with a relative concentration of 0.8 and the simulated spectrum of alkoxyl radicals with a relative concentration of 0.2. The simulated spectrum is fairly identical with the observed spectrum. On the other hand, no ESR signal was observed from the unfractured BC with MMA. Therefore, we can conclude that BC mechanoradicals (Ia and IIb), which were produced by scissions of β-1,4 glycosidic linkages and were trapped on the BC surface, initiated a radical polymerization of MMA and produced BC-block-PMMA on the BC surface. The reaction scheme of Ia is shown in Figure 10. The IIb has the same reaction scheme as Ia. Figure 11a shows the Fourier-transform infrared (FT-IR) spectrum of the BC fractured without MMA in vacuum at 77 K. The spectrum profile shows a characteristic cellulose pattern. No peak was observed at around 1729 cm-1. BC-block-PMMA was washed by Soxhlet extraction with chloroform for 24 h, and the residue was dried in vacuum at 343 K for 6 h. The FT-IR spectrum of BC-block-PMMA (Figure 11b) shows a similar pattern to that of BC fractured without MMA, except at

Figure 10. BC-block-PMMA propagating radicals initiated by BC mechanoradical Ia and sequentially reacted with MMA.

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Figure 11. FT-IR spectra of (a) the fractured BC and (b) the fractured BC with MMA.

the peak at 1729 cm-1 due to a carbonyl group on the PMMA chain. Because the MMA monomer was removed from the residue by the Soxhlet extraction treatment, this result is additional evidence for the production of BC-block-PMMA by the mechanochemical reaction of BC with MMA. The PMMA homopolymer (weight averaged molecular weight, Mw ) 272000; number averaged molecular weight, Mn ) 148000; Mw/Mn ) 1.837) was synthesized by radical polymerization of MMA with azobisisobutyronitrile (Wako Pure Chemical Industries, Ltd.) at 373 K for 2 h. The chloroform

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solution of PMMA was precipitated with methanol, and the resulting white-solid PMMA was dried in vacuum. The 1H NMR spectrum (Figure 12a) of PMMA with d-chloroform as solvent shows a characteristic spectrum of PMMA synthesized by radical polymerization.31,32 The signals at δ ) 0.85-1.24 (peak c), 1.81-2.07 (peak b), and 3.60 ppm (peak a) were assigned to the methyl protons (c), methylene protons (b), and methoxy protons (a), respectively. The fractured BC was acetylated with trifluoroacetic acid anhydride and acetic acid and resulted in acetylated BC (BCTA). The 1H NMR spectra (Figure 12b) of BCTA with d-chloroform as solvent shows the characteristic spectrum of cellulose triacetate.33,34 The signals at δ ) 1.94-2.12 ppm were assigned to the protons of acetyl groups. In an enlarged spectrum in the region from 3.4 to 5.3 ppm, the signals at δ ) 4.42, 4.79, 5.07, 3.71, 3.54, 4.38, and 4.06 ppm were assigned to the protons on the glucopyranose ring at positions 1-H, 2-H, 3-H, 4-H, 5-H, 6-H, and 6′-H, respectively. After ESR observation, BC-block-PMMA was washed by Soxhlet extraction with chloroform for 24 h and dried in vacuum at 343 K for 6 h. The washed and dried BC-block-PMMA was acetylated with trifluoroacetic acid anhydride and acetic acid (BCTA-block-PMMA). The BCTA-block-PMMA was dissolved

Figure 12. 1H NMR spectra of (a) PMMA, (b) BCTA, and (c) BCTA-block-PMMA.


Figure 13. Schematic of BC-block-PMMA on the BC surface.

in d-chloroform. The 1H NMR spectra (Figure 12c) of the BCTA-block-PMMA reveals both proton groups: one arises from protons on the glucopyranose ring of BCTA; 1-H (δ ) 4.42 ppm), 2-H (4.79 ppm), 3-H (5.07 ppm), 4-H (3.71 ppm), 5-H (3.54 ppm), 6-H (4.39 ppm), and 6′-H (4.06 ppm), while the other arises from the methyl protons (peaks c; δ ) 0.86-1.02 ppm), methylene protons (peak b; δ ) 1.81 ppm), and methoxy protons (peak a; δ ) 3.60 ppm) on the PMMA chains. It is reasonable to assume that the main chain composed of BCTA-block-PMMA is unaltered by the acetylation. Therefore, the 1H NMR spectra of BCTA-block-PMMA (Figure 12c) indicate that the BC-block-PMMA was produced by mechanochemical polymerization of MMA with BC mechanoradicals in vacuum at 77 K. From these results, we conclude that BC mechanoradicals produced by the mechanical scission of β-1,4 glycosidic linkages of BC initiated a radical polymerization of MMA, and BC-block-PMMA was produced on the BC surface. 3.3. Surface Modification of BC with BC-block-PMMA. We concluded in the previous section that the BC-block-PMMA on the surface of BC was produced by a mechanochemical polymerization of MMA in a solid state. In other words, BC particles were chemically modified with the PMMA chains of BC-block-PMMA. To reveal the profile of the surface modification of BC with BC-block-PMMA, we carried out the following experiments: To speculate the molecular weights of PMMA in BC-blockPMMA, BC-block-PMMA was acetylated with acetic acid and trifluoroacetic acid anhydride, which produced BCTA-blockPMMA. The GPC measurement of BCTA-block-PMMA was carried out with chloroform as solvent and polystyrene as standard. The molecular weights of BCTA-block-PMMA were as follows: Mw ) 5.15 × 105 g/mol, Mn ) 1.63 × 105 g/mol, and Mw/Mn ) 3.15 by GPC measurement. The molar ratios of BCTA and PMMA in BCTA-block-PMMA, calculated using the 1H NMR spectrum, were 86.4 and 13.6 mol %, respectively. Thus, the Mn values of BCTA and PMMA on the BCTA-blockPMMA chain can be calculated as 1.534 × 105 g/mol (polymerization degree [PD] ) 608.8, acetylated cellulose unit ) 252 g/mol) and 9.59 × 103 g/mol (PD ) 95.8, MMA unit ) 100.12 g/mol), respectively. Assuming an unaltered PD of PMMA from the acetylation of BC-block-PMMA, the Mn of the PMMA chain of BC-blockPMMA is 9.59 × 103 g/mol (PD ) 95.8). On the other hand, the specific surface area of the fractured BC (12.0 m2/g) was obtained by a BET method. The concentration of radicals in BC fractured in vacuum at 77 K for 7 h was 1.73 × 1018 spins/ g. An occupied surface area per spin was calculated to be 6.94 nm2/spin. Assuming the occupied surface area is a circle, the radius is assumed to be 1.49 nm. In other words, the radius (Rt) per tethered point (PMMA stem base) is 1.49 nm. The schematic is shown in Figure 13. The radius of gyration (Rg ) [C∞ (2PD - 1)ab2/6]0.5) of the PMMA chains on BC-block-PMMA was calculated as 2.27 nm


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based on a PD of 95.8, carbon-carbon bond length (ab ) 0.15351 nm),35 and characteristic ratio (C∞ ) 6.9) of atactic PMMA.36 The relative radius (RL ) Rg/Rt) of PMMA having PD ) 95.8 was 1.52 () 2.27/1.49). Thus, a tethered PMMA chain contacts and entangles with neighboring PMMA chains (illustrated in Figure 13). BC particles are covered with PMMA chains of BC-block-PMMA. In other words, a surface-modified BC with PMMA was produced. Because the PMMA chains of BC-block-PMMA are tethered on the BC surface by a covalent bond, the Rg of the PMMA chain of BC-block-PMMA should be larger than 2.27 nm. The PMMA chains on the surface are more in contact and entangled with the other chains, which means that BC particles are fully covered with PMMA chains of BC-block-PMMA.

4. Conclusions Chemical modifications of cellulose are limited to the hydroxide groups on the glucopyranose ring, because scission of β-1,4 glycosidic linkages that form the main chain of cellulose is very difficult. Modification of the main chain of cellulose, which involves diblock copolymerization, may provide a novel usage of cellulose. We intended to synthesize the diblock copolymer of cellulose by using radical polymerization initiated by chain-end-type radicals (mechanoradicals) of cellulose. In our case, we used BC as a model compound of cellulose and MMA as a monomer of radical polymerization. We conclude that the mechanical fracture of BC in vacuum at 77 K produced chain-end-type radicals of BC (BC mechanoradicals), which were induced by scissions of β-1,4 glycosidic linkage. The BCblock-PMMA was synthesized on the BC surface by radical polymerization of MMA, which was initiated by the BC mechanoradicals located on the surface. The BC surface was fully covered with the PMMA chains of BC-block-PMMA. The technique described in this article is not limited to MMA and BC; we are currently testing another combination in our laboratory. Acknowledgment. This research was partially supported by the Ministry of Education, Science, Sports and Culture, Grantin-Aid for Scientific Research (C) No. 21580208.

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