Cellulosic Graft Copolymer: Poly(methyl methacrylate) with Cellulose

Jul 8, 2009 - Phone: +81-75-753-6255. Fax: +81-75-753-6300. E-mail: [email protected]. Abstract. Abstract Image. A cellulose macromonomer, N...
0 downloads 8 Views 1MB Size
Download PDF
2110

Biomacromolecules 2009, 10, 2110–2117

Cellulosic Graft Copolymer: Poly(methyl methacrylate) with Cellulose Side Chains Yukiko Enomoto-Rogers, Hiroshi Kamitakahara,* Toshiyuki Takano, and Fumiaki Nakatsubo Division of Forest and Biomaterials Science, Graduate School of Agriculture, Kyoto University, Kitashirakawa-Oiwake-cho, Sakyo-ku, Kyoto 606-8502, Japan Received February 22, 2009; Revised Manuscript Received June 2, 2009

A cellulose macromonomer, N-(15-methacryloyloxypentadecanoyl)-tri-O-acetyl-β-cellulosylamine (CTA-C15MA (2); M2) with number averaged degree of polymerization (DPn) ) 13), was copolymerized with methyl methacrylate (MMA; M1) to give cellulosic copolymer with CTA side chains, PMMA-g-(CTA-C15) (3-A). An absolute molecular weight determined by multiangle laser light scattering (MALS; Mw,LS), degree of polymerization of MMA (X) and CTA-C15-MA (Y) of PMMA-g-(CTA-C15) (3-A) were determined to be Mw,LS ) 6.30 × 104, X ) 4.14 × 102, and Y ) 3.86. Cellulose graft copolymer with cellulose side chains, PMMA-g-(cellulose-C15) (3-H) was successfully obtained after deacetylation of the copolymer 3-A. Thermal analysis of copolymers 3-A and 3-H by means of differential scanning calorimetry (DSC) measurements revealed that a small amount of CTA and cellulose side chains affected thermal properties of the PMMA main chain.

Introduction Cellulose is a semirigid and linear polymer having many hydroxyl groups including the C1 (reducing end), C2, C3, C4 (nonreducing end), and C6 positions of the (1f4)-β-glucopyranose units. At the C1 position of the reducing end, the cellulose molecule has only one hemiacetal hydroxyl group, having different reactivity from the other hydroxyl groups, which can be preferentially substituted with other functional groups with high regioselectivity.1,2 Recently, we have reported the preparation of a novel A(cellulose)-b-B(long-chain alkyl groups) type cellulose diblock copolymer with well-controlled chemical structure, by introducing hydrophobic long-chain alkyl groups regioselectively to the reducing end of the cellulose chain via amide linkages.3-5 Based on our synthetic method to regioselectively introduce a functional group to the reducing end of the cellulose macromolecule, in this study we planned to prepare a combshaped cellulose copolymer with cellulose molecules grafted at the reducing end as side chains on a synthetic polymer main chain. The comb-shaped copolymer with cellulose side chains could achieve parallel orientation of the pendent cellulose molecules. This is significant, as cellulose crystals with parallel orientation have not yet been prepared in the solid state from regenerated cellulose or by chemical synthesis, although there have been some attempts using cello-oligosaccharide analogues.6,7 Generally, there are three synthetic routes that are well-known as effective ways to obtain comb-shaped copolymers. The first strategy is “grafting onto” (attachment of side chains to the backbone),8-10 the second is “grafting from” (grafting side chains from the backbone),11,12 and the third is “grafting through” (homo- or copolymerization of macromonomers).13,14 The former two strategies may be difficult, as the “grafting onto” strategy requires efficient attachment of the reducing end of the cellulose side chains onto a polymer main chain, and as the * To whom correspondence should be addressed. Phone: +81-75-7536255. Fax: +81-75-753-6300. E-mail: [email protected].

“grafting from” strategy requires efficient polymerization of the cellulose side chains from a polymer main chain. Based on “grafting through” strategy, we have recently succeeded in preparing the cellobiose monomers carrying a methacryloyl group at the reducing end and its homopolymers and copolymers with methyl methacrylate (MMA), as models of the cellulose macromonomer and the comb-shaped cellulose copolymer.15 In the present paper, the same strategy is applied to the corresponding cellulose derivatives, specifically, (1) synthesis of cellulose triacetate (CTA) macromonomer carrying a methacryloyl group at the reducing end, (2) polymerization of the cellulose macromonomer to obtain the graft copolymer with CTA side chains, and (3) deprotection of acetyl groups to obtain the desired graft copolymer having cellulose side chains with free hydroxyl groups. See Figure 1 for the structural image of the obtained copolymer. Herein, we describe the preparation and characterization of the cellulose macromonomer CTA-C15-MA (DPn ) 13) and the cellulosic graft copolymers PMMA-g-(CTA-C15) and PMMA-g-(cellulose-C15) and report their thermal properties.

Experimental Section General Measurements. 1H, 13C, and two-dimensional NMR spectra were recorded on a Varian INOVA300 FT-NMR (300 MHz) spectrometer, in CDCl3 with tetramethylsilane (TMS) as an internal standard. Chemical shifts (δ) and coupling constants (J) are reported in ppm and Hz, respectively. Gel filtration column chromatography was performed on P-1 Peristaltic Pump (GE Healthcare Bio-Science Co., USA) system using a SR 25/100 column (GE Healthcare Bio-Science Co., U.S.A.) packed with Sephadex LH-60. MALDI-TOF MS spectra were recorded on a Bruker REFLEX III with 2,5-dihydroxybenzoic acid (DHB) as a matrix in reflector mode. SEC-MALS Measurement. Size exclusion chromatographymultiangle laser light scattering (SEC-MALS) measurements were carried out at 25 °C using a Shimadzu SEC system (CBM-10A, SPD10A, SIL-10A, LC-10AT, FCV-10AL, CTO-10A, RID-10A, and FRC10, Shimadzu, Japan) and MALS detector (DAWN EOS, Wyatt Technology Co., Ltd., U.S.A.; λ ) 690 nm). Shodex columns (K802,

10.1021/bm900229g CCC: $40.75  2009 American Chemical Society Published on Web 07/08/2009

Cellulosic Graft Copolymer

Figure 1. Structural image of PMMA-g-(CTA-C15) (3-A).

K802.5, and K805) were used. Number and weight averaged molecular weights (Mn,PS, Mw,PS) and polydispersity indices (Mw,PS/Mn,PS) were estimated using polystyrene standards (Shodex). The photometer was calibrated with pure toluene. Chloroform was used as eluent. An absolute molecular weight determined by MALS was abbreviated to Mw,LS. The flow rate was 1.0 mL/min. Refractive index increments (dn/ dc) were measured at 25 °C by a DRM1021 (Otsuka Electronics Co, Ltd., Japan; λ ) 633 nm). The dn/dc values were 0.045 for PMMAg-(CTA-C15) (3-A) and 0.063 for PMMA. FT-IR Measurement. Fourier transform infrared (FT-IR) spectra were recorded on a FTIR-4000 spectrophotometer (Shimadzu, Japan). Samples were mixed with KBr and pressed into disks. The FT-IR spectra of PMMA, PMMA-g-(CTA-C15) (3-A), and PMMA-g-(cellulose-C15) (3-H) were normalized to the band at 1387 cm-1 for the symmetric methyl CH3 bending vibration mode of PMMA main chain (νCH3(PMMA)). The overlap between νCH3(PMMA) and the band for CH bending vibration mode of acetyl groups at 1371 cm-1 (νCH(acetyl))16 was ignored for the spectra of copolymers 3-A and 3-H. DSC Measurement. Differential scanning calorimetry (DSC) thermograms were recorded on a DSC823e (Mettler Toledo) under a nitrogen atmosphere. The samples were first cooled from 25 to -40 °C, heated to 210 °C (first heating scan) at a heating rate of 10 °C/ min, and then immediately quenched to -40 °C. The second heating scans were run from -40 to 490 °C at a heating rate of 10 °C/min, to record stable thermograms. The glass transition temperature was recorded as the midpoint temperature of the heat capacity transition of the second heating run. The reproducibility was confirmed by replicates.

Materials and Methods Tri-O-acetyl-β-cellulosylamine (DPn ) 13) was prepared, as described in our previous article.4 Methyl methacrylate (MMA) monomer was distilled under reduced pressure. 2,2′-Azobis(isobutyronitrile) (AIBN) was crystallized from ethanol before use. 15-Hydroxypentadecanoic acid, 1-ethylcarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ), 1,8-diazabicyclo[5,4,0]-7-undecene (DBU), and all other reagents were commercially obtained and used without further purification.

Biomacromolecules, Vol. 10, No. 8, 2009

2111

N-(15-Hydroxypentadecanoyl)-tri-O-acetyl-β-cellulosylamine (1). To a solution of tri-O-acetyl-β-cellulosylamine (DPn ) 13; 308.3 mg, 1 equiv) and 15-hydroxypentadecanoic acid (101.7 mg, 5 equiv) in chloroform/ DMF (3:2, v/v; 1.0 mL) was added EEDQ (101.4 mg, 5 equiv). The mixture was stirred for 7 days at 30 °C under nitrogen. The reaction mixture was poured into methanol, filtered, and dried in vacuo to give an amorphous solid (1; 286.0 mg, 92.8% yield). 1H NMR (CDCl3): δ 1.26 (m, aliphatic-H), 1.6 (m, aliphatic-H), 1.9-2.1 (CH3CO-), 3.55 (C5-H), 3.64 (t, J ) 6.6, -CH2-OH), 3.72 (t, J4,5 ) 9.3, C4-H), 4.06 (d, J ) 7.2, C6-Hb), 4.36-4.43 (C6-Ha, C1-H), 4.80 (t, J2,3 ) 8.6, C2-H), 5.07 (t, J3,4 ) 9.3, C3-H), 5.19 (C1-H at the reducing end), 6.11 (d, JNH,1 ) 9.3, C1-NH-CO-). 13C NMR (CDCl3): δ 20.4, 20.6, 20.8 (CH3-CO-), 25.1 (C1-NH-CO-CH2-CH2-), 25.7 (-CH2-CH2CH2-OH), 29.0, 29.2, 29.4, 29.5, 29.5 (aliphatic-C), 32.7 (-CH2-CH2OH), 36.6 (C1-NH-CO-CH2-), 61.9 (C6), 63.0 (-CH2-OH), 67.6 (C2 at the reducing end), 71.7 (C2), 72.5 (C5), 72.7 (C3), 74.4 (C4 at the reducing end), 76.0 (C4), 77.8 (C1 at the reducing end), 100.5 (C1), 169.3, 169.7, 170.2 (CH3-CO- of C2, C3, C6, respectively). MALDITOF MS (positive reflector mode; DHB as matrix): DP ) 7: C101H145NO59 Calcd, 2315.84; found [M + Na]+, 2338.57; [M + K]+, 2354.57. DP ) 8: C113H161NO67 Calcd, 2603.92; found [M + Na]+, 2626.73; [M + K]+, 2642.71. DP ) 9: C125H177NO75 Calcd, 2892.01; found [M + Na]+, 2914.85; [M + K]+, 2930.82. DP ) 10: C137H193NO83 Calcd, 3180.09; found [M + Na]+, 3202.95; [M + K]+, 3218.97. DP ) 11: C149H209NO91 Calcd, 3468.18; found [M + Na]+, 3491.06; [M + K]+, 3507.02. DP ) 12: C161H225NO99 Calcd, 3756.26; found [M + Na]+, 3779.18; [M + K]+, 3795.05. DP ) 13: C173H241NO107 Calcd, 4044.34; found [M + Na]+, 4067.22; [M + K]+, 4083.24. DP ) 14: C185H257NO115 Calcd, 4332.43; found [M + Na]+, 4355.09. DP ) 15: C197H273NO123 Calcd, 4620.51; found [M + Na]+, 4643.10. DP ) 16: C209H289NO131 Calcd, 4908.60; found [M + Na]+, 4931.22. DP ) 17: C221H305NO139 Calcd, 5196.68; found [M + Na]+, 5219.75. DP ) 18: C233H321NO147 Calcd, 5484.77; found [M + Na]+, 5507.78. DP ) 19: C245H337NO155 Calcd, 5772.85; found [M + Na]+ ) 5796.44. N-(15-Methacryloyloxypentadecanoyl)-tri-O-acetyl-β-cellulosylamine (CTA-C15-MA; 2). To a solution of N-(15-hydroxypentadecanoyl)-tri-O-acetyl-β-cellulosylamine (1; 326 mg, 1 equiv) in chloroform (5 mL) were added methacryloyl chloride (0.15 mL, 20 equiv) and Et3N (0.15 mL, 20 equiv). The mixture was stirred for 3 days at room temperature under nitrogen. The reaction mixture was poured into methanol, filtered, and dried in vacuo to give an amorphous solid (2, CTA-C15-MA; 300.2 mg, 92.1% yield). 1H NMR (CDCl3): δ 1.24 (m, aliphatic-H), 1.6 (m, aliphatic-H), 1.9-2.1 (CH3-CO-), 3.55 (C5H), 3.72 (t, J4,5 ) 9.3, C4-H), 4.06 (d, J ) 7.5, C6-Hb), 4.13 (t, J ) 6.9, -CH2-O-CO-), 4.37-4.43 (C6-Ha, C1-H), 4.80 (t, J2,3 ) 8.4, C2H), 5.08 (t, J3,4 ) 9.3, C3-H), 5.19 (C1-H at the reducing end), 5.55 (t, J ) 1.65, CH2dC-CH3), 6.10 (s, CH2dC-CH3), 6.10 (overlapped, C1NH-CO-). 13C NMR (CDCl3): δ 18.3 (CH2dC-CH3), 20.4, 20.6, 20.8 (CH3-CO-), 25.1 (C1-NH-CO-CH2-CH2-), 25.9 (-CH2-CH2-CH2OCO-), 28.5 (-CH2-CH2-O-CO-), 28.9-29.5 (aliphatic-C), 36.6 (C1-NHCO-CH2-), 61.9 (C6), 64.8 (-CH2-O-CO-), 67.6 (C2 at the reducing end), 71.7 (C2), 72.5 (C5), 72.7 (C3), 74.4 (C4 at the reducing end), 76.0 (C4), 77.8 (C1 at the reducing end), 100.5 (C1), 125.1 (CH2dCCH3), 136.5 (CH2dC-CH3), 169.3, 169.7, 170.2 (CH3-CO- of C2, C3, C6, respectively). MALDI-TOF MS (positive reflector mode; DHB as matrix): DP ) 7: C105H149NO60 Calcd, 2383.86; found [M + Na]+, 2406.84; [M + K]+, 2422.83. DP ) 8: C117H165NO68 Calcd, 2671.95; found [M + Na]+, 2695.03; [M + K]+, 2710.99. DP ) 9: C129H181NO76 Calcd, 2960.03; found [M + Na]+, 2983.18; [M + K]+, 2999.17. DP ) 10: C141H197NO84 Calcd, 3248.12; found [M + Na]+, 3271.29; [M + K]+, 3287.24. DP ) 11: C153H213NO92 Calcd, 3536.20; found [M + Na]+, 3559.48; [M + K]+, 3575.44. DP ) 12: C165H229NO100 Calcd, 3824.29; found [M + Na]+, 3847.56. DP ) 13: C177H245NO108 Calcd, 4112.37; found [M + Na]+, 4135.54. DP ) 14: C189H261NO116 Calcd, 4400.46; found [M + Na]+, 4424.22.

2112

Biomacromolecules, Vol. 10, No. 8, 2009

Enomoto-Rogers et al.

Scheme 1a

a

Reagents: (a) 15-Hydroxypentadecanoic Acid /EEDQ/DMF/CHCl3; (b) methacryloyl chloride /Et3N/CHCl3.

General Procedure for Homo- and Copolymerization of CTAC15-MA: PMMA-g-(CTA-C15) (3-A). For homopolymerization, CTA-C15-MA (30 mg) in ethylene glycol dimethyl ether (0.3 mL, 10 w/v % of CTA-C15-MA concentration) was weighed into a glass tube and degassed by freeze-pump-thaw cycles typically three times until oxygen is removed. Copolymerization of CTA-C15-MA with MMA was carried out in the same procedure with the appropriate amount of MMA. The glass tube was purged with nitrogen, and AIBN necessary to give the desired monomer-initiator ratio was loaded immediately into the tube. The tube was degassed again, sealed under vacuum, and placed in an oil bath at 75 °C for appropriate time. The tube was cooled to room temperature, and opened. Conversion (%) of MMA was calculated from the total weight of polymerization products at time t (Wt) after residual MMA was removed under vacuum, as follows: conv. (MMA; %) ) (Wt - W0 (CTA-C15-MA))/W0 (MMA). The polymerization products were analyzed by 1H NMR measurement. Conversion (%) of CTA-C15-MA was calculated from the peak area ratio value of remaining olefinic protons of the methacryloyl group to C3-H and C2-H ring protons and the corresponding value of the macromonomer. The polymerization products were purified to remove the remaining macromonomer by gel filtration column chromatography (LH-60) with methanol/ chloroform (1:4, v/v) to give an amorphous solid, PMMAg-(CTA-C15) (3-A). Degree of polymerization of each monomer was defined as X for MMA and Y for CTA-C15-MA. 1H NMR (CDCl3): δ 0.84, 1.02, 1.21 (-CH2-C-CH3), 1.82, 1.90 (-CH2-(MMA)), 1.9-2.1 (CH3-CO-), 3.61 (CH3-O-), 4.06 (C6-Hb), 4.37-4.43 (C6-Ha, C1-H), 4.81 (C2-H), 5.08 (C3-H). 13C NMR (CDCl3): δ 16.3, 18.6 (-CH2-CCH3), 20.8 (CH3-CO-), 29.7 (aliphatic-C), 44.5, 44.8 (-CH2-C-CH3), 51.8 (CH3-O-), 54.3 (-CH2-C-CH3), 62.0 (C6), 66.1 (-CH2-O-), 71.7 (C2), 72.4 (C5), 72.7 (C3), 76.0 (C4), 100.5 (C1), 169.3, 169.7, 170.2 (CH3-CO- of C2, C3, C6, respectively), 177.0, 177.8, 178.1 (-CO(MMA)). Preparation of PMMA-g-(cellulose-C15) (3-H). To a solution of the acetylated copolymer 3-A (25 mg) in methanol/ chloroform (1:4, v/v) (1.0 mL) was added DBU (0.03 mL, 0.20 mmol) at room temperature, and stirred for 3 h under the nitrogen. The mixture was precipitated into methanol, centrifuged, washed with methanol, and dried in vacuo to give an amorphous solid, PMMA-g-(cellulose-C15) (3-H; 16.1 mg, 74.2% yield).

Results and Discussion Synthesis of Cellulose Macromonomer CTA-C15-MA (2). Cellulose derivative N-(15-hydroxypentadecanoyl)-tri-O-acetylβ-cellulosylamine (1) and the cellulose macromonomer, N-(15-methacryloyloxypentadecanoyl)-tri-O-acetyl-β-cellulosyl-

Figure 2. MALDI-TOF MS spectrum of CTA-C15-MA (2), recorded with 2,5-dihydroxybenzoic acid (DHB) as a matrix in reflector mode.

amine (CTA-C15-MA; 2) were prepared from tri-O-acetyl-βcellulosylamine (DPn ) 13),4 following the model reactions with cellobiose derivatives, as described in Scheme 1.15 The molecular weights of 1 and 2 were confirmed by using MALDI-TOF MS analysis in reflector mode. MALDI-TOF MS spectrum of 2 is shown in Figure 2. The peaks of molecular weights with DP of 7-14 were detected. The observed molecular weights of 2 with each DP value agreed well with their calculated molecular weights, indicating quantitative methacryloylation. The number and weight averaged molecular weights determined by polystyrene standards (Mn,PS, Mw,PS) and the polydispersity index (Mw,PS/Mn,PS) of 2 were determined to be Mn,PS ) 4.03 × 103, Mw,PS ) 5.59 × 103, Mw,PS/Mn,PS ) 1.39, respectively. In the H-H gCOSY spectrum of 1 (Figure 3a), the correlation between the methylene protons at C14 (-CH2-CH2-OH, δ 1.6 ppm) and C15 (-CH2-OH, δ 3.64 ppm) of the long-chain alkyl group are observed. The correlation between the ring-proton (C1-H at the reducing end, δ 5.19 ppm) and the amide proton (C1-NH, δ 6.10 ppm) indicates the formation of the amide linkage. In the H-H gCOSY spectrum of 2 (Figure 3b), the methylene protons at C14 (-CH2-CH2-O-(CO)-, δ 1.6 ppm) and C15 (-CH2-O-(CO)-, δ 4.14 ppm) of the long-chain alkyl group are observed. The amide proton (C1-NH-, δ 6.10 ppm) and the two olefinic protons of the methacryloyl group (CH2dC-, δ 5.55 and 6.10 ppm) are also observed. The amide proton (C1-NH-) of 2 is overlapped with the olefinic proton at 6.10 ppm. The

Cellulosic Graft Copolymer

Figure 3. H-H gCOSY spectra of (a) compound 1 and (b) CTAC15-MA (2).

proton at C15 (CH2-O-) shifted downfield from 3.64 ppm for 1 to 4.14 ppm for 2 after methacryloylation. Homopolymerization of CTA-C15-MA. In our previous work, homopolymerization of a cellobiose monomer carrying a methacryloyl group proceeded successfully, and the weight averaged degree of polymerization determined by MALS (DPw,LS) reached up to 1.64 × 103.15 Therefore, the same polymerization strategy was applied to CTA-C15-MA, as described in Scheme 2. Homopolymerization of CTA-C15-MA was performed in ethylene glycol dimethyl ether using AIBN as an initiator. Ethylene glycol dimethyl ether was selected, because other general solvents for polymerization such as benzene or toluene did not dissolve CTA-C15-MA. The polymerization products were analyzed by 1H NMR and SEC measurements. The SEC (RI) curves of CTA-C15-MA and the polymerization products are shown in Figure 4a-c. The results of homopolymerization of CTA-C15-MA are listed in Table 1. In the case of run 1 with initial monomer-initiator ratio [CTAC15-MA]/ [AIBN] of 100/1, Conversion (%) was calculated to be 0% according to 1H NMR spectra analysis. In its SEC (RI) curve, the CTA-C15-MA peak was observed at 24.9 mL, as shown in Figure 4a,b. The number averaged molecular weight (Mn,PS) and degree of polymerization of CTA-C15-MA (Y) were

Biomacromolecules, Vol. 10, No. 8, 2009

2113

calculated to be 0.41 × 104 and 1.02, respectively. In the case of run 2 with initial monomer-initiator ratio [CTA-C15-MA]/ [AIBN] of 10/1, conversion (%) was calculated to be 36.1% higher than that of run 1. The Mn,PS and Y values increased slightly as indicated by the elution curve in Figure 4c and were calculated to be 0.55 × 104 and 1.36, respectively. Homopolymerization of CTA-C15-MA did not proceed efficiently. It has been reported that semiflexible or rigid rodlike macromonomers such as methacrylate-end-capped poly(nhexyl isocyanate)17,18 are hard to polymerize, whereas other flexible macromonomers such as poly(ethylene oxide)13 or poly(dimethylsiloxane)14 are not. This is likely because of thermodynamic repulsion between propagating polymers and monomers, steric hindrance of its long and rigid side chain at the radical end or increasing viscosity around the radical site during polymerization.18 The CTA chain of the CTA-C15-MA has a rigid structure,19,20 and the solubility of CTA-C15-MA in organic solvents is relatively poor, important as high monomer concentration is desirable for polymerization. The CTA-C15MA consists of CTA part that has many C-H bonds that are labile with respect to radical abstraction of the H-atom. There is a possibility that the CTA part works as a H-atom donor to terminate the polymerization at the radical ends, especially at the higher concentration of CTA-C15-MA. In addition, CTAC15-MA has the flexible spacer segment (C15) that might it easier for CTA-C15-MA to wrap back into the vicinity of the radical. Our experiments revealed that macromonomer including cellulosic moiety has low reactivity in contrast to the corresponding cellobiose monomer prepared in our previous work.15 Copolymerization of MMA and CTA-C15-MA. To incorporate CTA side chains into the desired graft copolymer, copolymerization of MMA (M1) and CTA-C15-MA (M2) was carried out. To understand the relative reactivity of the macromonomer CTA-C15-MA, the copolymerizations were carried out at different initial monomer ratio [M1]/[M2], and copolymerization parameters (r1 and r2) were determined. The polymerization was terminated at low monomer conversion (conv. (MMA) ) ca. 10%). The results of copolymerization are listed in Table 2. Representative SEC (RI) curves of polymerization products of runs 1-4 are shown in Figure 4d-g. A new peak with higher molecular weight appeared in addition to the peak of CTA-C15-MA. It was impossible to separate the high molecular weight fraction from the remaining CTA-C15-MA by gel filtration column chromatography, because of wide overlapping of two peaks and too small amount of the obtained copolymer for all runs. The polymerization products, which include the copolymer and the remaining macromonomer, were analyzed by 1H NMR. Conversion of M2 (%) was calculated from the peak area ratio of remaining olefinic protons of the methacryloyl group to C3-H and C2-H ring protons. The monomer ratio in the copolymer (d[M1]/d[M2]) was calculated from the conv. (%) values of M1 and M2. Copolymerization parameters of MMA (M1; r1) and CTAC15-MA (M2; r2) were calculated to be r1 ) 0.39 and r2 ) 1.7, respectively, by Fineman-Ross plots (see Supporting Information). Unexpectedly, the macromonomer, CTA-C15-MA showed higher reactivity compared to that of MMA not like other rigid macromonomers.17,18 Conv. (M2; %) values were calculated to be over 10% in all cases, by 1H NMR analysis. There is a possibility that CTA part works H-atom donor to terminate the polymerization at the radical end, as discussed in the former section. If this phenomena would happen in the copolymerization process, integral area of olefinic protons of CTA-C15-MA (M2) decreases

2114

Biomacromolecules, Vol. 10, No. 8, 2009

Enomoto-Rogers et al.

Scheme 2

a

(MMA)/AIBN/ethylene glycol dimethyl ether. b DBU/methanol/CHCl3.

in 1H NMR spectra of polymerization products, and Conv. (M2; %) value is calculated to be higher than the true value, and as a result, the monomer composition in the copolymer (d[M1]/ d[M2]) is also calculated to be lower than the true value to give higher r2 value. As shown in SEC (RI) curves of copolymerization products of runs 1-4, calculated integral area of macromonomer peak was nearly equal to the theoretical value estimated by actual injected mass of macromonomer fraction, also indicating that CTA-C15-MA was not incorporated to the copolymer. As discussed in the section of homopolymerization, CTA-C15-MA has rigid structure and low solubility, and this might cause steric hindrance around the radical-end in the copolymerization as well. From these data, it was indicated that macromonomer CTA-C15-MA has low reactivity even for the copolymerization.

Figure 4. SEC (RI) curves of (a) CTA-C15-MA, homopolymerization products from (b) run 1 and (c) run 2, copolymerization products from (d) run 1, (e) run 2, (f) run 3, (g) run 4, PMMA-g-(CTA-C15) (3-A) (h) before and (i) after the purification.

Preparation of PMMA-g-(CTA-C15) (3-A). Considering the monomer ratio d[M1]/d[M2] value in Table 2, we concluded that run 2 at the initial monomer ratio [M1]/[M2] of 97/ 1 is most appropriate condition to obtain the copolymer with the higher density of CTA side chains in high yield. Copolymerization of CTA-C15-MA (M2) with MMA (M1) was carried out with the initial monomer ratio [M1]/[M2] of 100/1 with the reaction time of 3 days to complete the reaction. In SEC (RI) elution curves of polymerization products, a new peak with higher molecular weight appeared in addition to the small shoulder peak of CTA-C15-MA, as shown in Figure 4h. The higher molecular weight polymer was purified by gel filtration column chromatography to remove the residual CTA-C15-MA, and the purified polymer PMMA-g-(CTA-C15) (3-A) was obtained in 56.1% yield. The SEC (RI) curves showed that the shoulder peak observed in the low-molecular-weight region disappeared after purification (Figure 4i). The 1H NMR spectrum of the purified copolymer 3-A is shown in Figure 5. The signals due to both PMMA and CTAC15-MA are observed. The monomer molecular composition of copolymer 3-A was calculated to be d[M1]/d[M2] ) 107 from the peak areas of the methyl protons (CH3-) of PMMA and ring protons at C1, C2, C3, and C6 positions of the CTA side chains. Structural Characterization of PMMA-g-(CTA-C15) (3-A). The molecular weight and structure of PMMA-g-(CTAC15) (3-A) was studied by SEC-MALS measurements to prove that the copolymer 3-A is the graft copolymer with CTA side chains. A PMMA homopolymer was also prepared at the same initial monomer-initiator ratio [MMA]/[AIBN] of 300 to compare its structure with copolymer 3-A. The SEC (RI) elution curves and plots of the molecular weight determined by MALS (Mw,LS) of copolymer 3-A and PMMA are shown in Figure 6. The characteristics of the polymers are listed in Table 3. The Mw,LS of copolymer 3-A was 6.30 × 104, and the Mw,PS/Mw,LS was 0.75. The molecular weight (Mw,PS) of copolymer 3-A was underestimated by SEC measurement with PS standards, compared to Mw,LS determined by MALS measurement. On the other hand, the Mw,LS of PMMA was 4.08 × 104 and the Mw,PS/ Mw,LS was 1.07. It is well-known that SEC, when calibrated only with linear standard polymers such as polystyrene (PS), severely underestimates the molecular weight of a branched polymer which has a more compact molecular volume in

Cellulosic Graft Copolymer

Biomacromolecules, Vol. 10, No. 8, 2009

2115

Table 1. Results of Homopolymerization of CTA-C15-MA run

[CTA-C15-MA]/ [AIBN]a,b

conv.c (%)

polymer yieldd (%)

Mn,PS (10-4)e

Mw,PS (10-4)e

Mw,PS/ Mn,PSe

(CTA-C15-MA)f

1 2

100 10

0 36.1

100 100

0.41 0.55

0.65 0.83

1.56 1.52

1.02 1.36

a Initial molar ratio of monomers and initiator. b [CTA-C15-MA]0 ) 2.5 × 10-2 mmol/mL (10 wt %) in ethylene glycol dimethyl ether. Reaction time ) 3 days. c Determined by 1H NMR spectra from the ratio of peak areas of olefinic and C2 and C3 protons. d Yield of polymerization products obtained after the reaction. e Estimated by polystyrene standards with chloroform as eluent. f Determined from Mn,PS of the polymer and CTA-C15-MA (4.03 × 103).

Table 2. Results of Copolymerization of MMA (M1) and CTA-C15-MA (M2) M1a M2a [M1] M2 [M1]/ AIBNb conv. conv. d[M1]e M2e d[M1]/ M n,PS Mw,PS Mw,PS/ run (mg) (mg) (mol %) (feed; wt %) [M2] (mg) (M1; %)c (M2; %)d (mol %) (copolymer; wt %) d[M2]e (10-4) (10-4)f,g,h Mn,PSf,g,h 1 2 3 4 5 6 7

90.0 46.6 20.0 8.60 1.17 0.50 0.13

10 20 20 20 20 20 20

99.7 98.9 97.6 94.5 70.2 50.2 20.7

10.0 30.0 50.0 70.0 94.5 97.6 99.4

362 94 40 17 2.4 1.0 0.3

0.49 0.25 0.12 0.05 0.009 0.005 0.003

12.8 9.8 10.4 4.1 10 10 10

33.1 16.2 30.7 23.1 28.7 22.4 1.0

99.3 98.3 93.2 75.5 45.1 31.0 73.1

22.3 41.5 74.7 92.9 98.0 98.9 93.7

140 57 14 3.1 0.8 0.4 2.7

4.94 2.89 3.23 3.49

9.04 3.92 4.08 4.21

1.83 1.36 1.26 1.21

a M(MMA) ) 100.12, M(CTA-C15-MA) ) 4.03 × 103. Initial macromonomer concentration was [CTA-C15-MA] ) 2.5 × 10-2 mmol/mL (10 wt %) in ethylene glycol dimethyl ether. Reaction time ) 0.5 h. b [M1]/[AIBN] ) 300 for runs 1-4, ([M1] + [M2])/[AIBN] ) 300 for runs 5-7. c Conv. (M1) (%) was determined as follows: Conv. (M1; %) ) [Wt - W0 (M2)]/W0 (M1) × 100. It was not measured for runs 5-7 because of too low initial concentration, and assumed to be 10%. d Determined by 1H NMR spectra from the ratio of peak areas of olefinic and C2 and C3 protons. e Calculated from conv. (M2; %) value dermined by 1H NMR analysis and initial monomer feeds. f Molecular weights and polydispersity index of the obtained polymer peak at high molecular weight region. g Estimated by polystyrene standards. h Unable to separate the copolymer because of too small amount in the cases of runs 5-7.

Figure 5. 1H NMR spectrum of PMMA-g-(CTA-C15; 3-A).

solution than that of a corresponding linear polymer with the same molecular weight.13 This means that copolymer 3-A has a more compact structure with higher density than linear polystyrene (PS) standards at the same elution volume. In addition, the Mw,LS of copolymer 3-A was higher than that of PMMA homopolymer at the same elution volume. From this data, it was revealed that copolymer 3-A had a branched structure with CTA side chains grafted on PMMA main chain. The monomer composition of PMMA-g-(CTA-C15) (3-A) was determined to be X (MMA) ) 4.14 × 102, Y (CTA-C15-MA) ) 3.86 from Mw,LS ) 6.30 × 104, and [MMA]/[CTA-C15-MA] ) 107 values. The structure is described in Figure 1. Thus, it was probed that the graft copolymer with CTA side chains, namely, PMMA-g-(CTA-C15) (3-A), was successfully prepared via copolymerization of CTA-C15-MA with MMA. Preparation of PMMA-g-(cellulose-C15) (3-H). Acetyl groups of the CTA side chains of PMMA-g-(CTA-C15) (3-A) were selectively removed with DBU5,21 to obtain the deprotected PMMA-g-(cellulose-C15) (3-H; 74.2% yield). Copolymer 3-H was insoluble in water or other solvents. As shown in the FTIR spectra of copolymer 3-A and 3-H (Figure 7b,c), a strong OH absorbance at ν ) 3446 cm-1 appeared for copolymer 3-H, indicating that the acetyl groups of copolymer 3-A were

Figure 6. SEC (RI) elution curves and molecular weight (Mw,LS) plots of (a) PMMA-g-(CTA-C15) (3-A) and (b) PMMA.

removed. The weak absorbance observed at about ν ) 3440 cm-1 in Figure 7a,b,d is likely due to absorbed water.

2116

Biomacromolecules, Vol. 10, No. 8, 2009

Enomoto-Rogers et al.

Table 3. Characteristics of PMMA-g-(CTA-C15) (3-A), Reacetylated Copolymer 3-reA and PMMA copolymers

[M1]/[M2]/ [AIBN]b

conv. (M1; %)

conv. (M2; %)c

PMMA-g-(CTA-C15) (3-A) 3-reAa PMMA

300/3/1 300/3/1 300/0/0

100

70.1

yield (%)

Mn,PS (10-4)d

Mw,PS (10-4)d

Mw,PS/ Mn,PSd

Mw,LS (10-4)e

Mw,PS/ Mw,LS

d[M1]/ d[M2]f

X (M1; 10-2)g

Y (M2)g

56.1

2.87 3.11 2.07

4.71 4.86 4.36

1.64 1.56 2.78

6.30 13.3 4.08

0.75 0.37 1.07

107 145

4.14 9.56 4.00

3.86 6.61

h

100

100

Reacetylated 3-H. b Initial molar ratio of monomers MMA (M1), CTA-C15-MA (M2), and initiator AIBN. [M1] ) 2.5 mmol/ mL, [M2] ) 2.5 × 10-2 mmol/mL (10 wt %) in ethylene glycol dimethyl ether. Reaction time ) 3 days. c Determined by 1H NMR spectra from the ratio of peak areas of olefinic and C2 and C3 protons. d Estimated by polystyrene standards with chloroform as eluent. e Determined by MALS measurements. f Molar ratio of the monomers determined by 1H NMR spectra from the peak areas of the methyl protons (CH3-) of PMMA and ring protons at C1, C2, C3, and C6 positions of CTA side chains. g Degree of polymerization of the monomers determined from Mw,LS of the copolymer, Mw,PS of CTA-C15-MA (5.59 × 103), and theoritical molecular weight of MMA (100.12), and d[M1]/d[M2] value. h Yield for acetylation was 100%. a

Figure 8. SEC (RI) elution curves and molecular weight (Mw,LS) plots of (a) PMMA-g-(CTA-C15) (3-A) and (b) PMMA.

Figure 7. FT-IR spectra of (a) PMMA, (b) PMMA-g-(CTA-C15) (3-A), (c) PMMA-g-(cellulose-C15) (3-H), (d) CTA, and (e) cellulose.

It was not clear whether copolymer 3-H maintained the cellulose side chains grafted on the PMMA main chain. Therefore, to show that the cellulose side chains, linked to the PMMA main chain via ester linkages remained and the copolymer still has a branched structure, copolymer 3-H was acetylated again with acetic anhydride and pyridine. The reacetylated copolymer 3-reA was obtained quantitatively. In the 1H NMR spectrum of the reacetylated copolymer 3-reA, the signals assigned to the ring protons of the CTA chains were present (see Supporting Information). The molecular weights and the polydispersity index of the reacetylated copolymer 3-reA were analyzed by SEC-MALS measurements, and are listed in Table 3. The Mw,LS, X, and Y values of the reacetylated copolymer 3-reA were Mw,LS ) 13.2 × 104, X ) 9.56 × 102, and Y ) 6.61. These values are higher in the reacetylated copolymer 3-reA as compared to those of copolymer 3-A, because the low-molecular-weight fraction of the deacetylated copolymer 3-H was lost in the workup procedure of the deprotection, for example, reprecipitation into methanol. The polydispersity index decreased from 1.64 to 1.56 after reacetylation, indicating loss of the low-molecular-weight fraction. The plots of Mw,LS versus elution volume in Figure 8 reveal that the reacetylated copolymer 3-reA had higher molecular weight as compared to the linear PMMA homopolymer at the same elution volume, indicating that it maintained a branched structure. These

results reveal that copolymer PMMA-g-(cellulose-C15) (3-H) was the copolymer with cellulose side chains grafted on the PMMA main chain. Furthermore, it has been proven that the methyl ester groups of the PMMA chain are stable under the conditions of deprotection with DBU, used in our previous work.15 Finally, the graft copolymer with cellulose side chains, PMMA-g-(cellulose-C15) (3-H) was successfully prepared. Thermal Properties of Copolymers PMMA-g-(CTA-C15) (3-A) and PMMA-g-(cellulose-C15) (3-H). Thermal properties of copolymers 3-A and 3-H were analyzed by means of DSC measurements to study the effect of CTA or cellulose side chains on the thermal properties of the copolymers. Copolymer 3-A gave a single glass transition temperature Tg at 106.4 °C as shown in Figure 9c. Interestingly, the Tg of copolymer 3-A was lower than that of PMMA (120.4 °C, Figure 9a) and CTA (142.1 °C, Figure 9b). It appears that the CTA side chains behave as a plasticizer to the PMMA main chain to give a Tg lower than that of both component blocks. Copolymer 3-H also gave a single Tg at 124.5 °C as shown in Figure 9d. PMMA is noncrystalline and hydrophobic, but cellulose is crystalline and hydrophilic. Thermal analysis of the cellulose (DPn ) 13) only showed a decomposition temperature at 283.2 °C, and had neither a Tg nor melting temperature (Tm) (data not shown). Therefore, the Tg of copolymer 3-H was assigned to the glass transition of the PMMA main chain. The Tg of the copolymer 3-H was about 4 °C higher than that of PMMA (120.4 °C, Figure 9a). This indicates that the cellulose side chains reduce the mobility of the PMMA main chain. Copolymer 3-H was also analyzed by means of X-ray diffraction measurements, to investigate the crystalline patterns of the

Cellulosic Graft Copolymer

Biomacromolecules, Vol. 10, No. 8, 2009

2117

Research from the Ministry of Education, Science, and Culture of Japan (No. 18688009). Supporting Information Available. Fineman-Ross plots for the copolymerization of MMA (M1) and CTA-C15-MA (M2), 1 H NMR spectra of (a) CTA-C15-MA, (b) reacetylated copolymer (3-reA), wide angle X-ray diffractograms of (a) PMMAg-(cellulose-C15) (3-H), (b) PMMA, and (c) cellulose II. This material is available free of charge via the Internet at http:// pubs.acs.org.

References and Notes Figure 9. DSC thermograms of (a) PMMA, (b) CTA, (c) PMMA-g(CTA-C15) (3-A), and (d) PMMA-g-(cellulose-C15) (3-H).

cellulose side chains, and the effect of the side chains on the structure of the copolymers. However, only amorphous diffraction patterns due to the PMMA main chain are observed, and the crystalline pattern due to cellulose side chains is not obviously observed (see Supporting Information) in the case of the copolymers studied in this work.

Conclusions The cellulose macromonomer, CTA-C15-MA (2) carrying a methacryloyl group at the reducing-end was prepared. The PMMA-g-(CTA-C15) (3-A) was obtained via copolymerization of CTA-C15-MA with MMA. The SEC-MALS measurements revealed that PMMA-g-(CTA-C15) (3-A) had a branched structure with the CTA side chains grafted on the PMMA main chain. The graft copolymer PMMA-g-(cellulose-C15) (3-H) with cellulose side chains on PMMA main chain was obtained after selective deprotection of PMMA-g-(CTA-C15) (3-A). DSC measurements indicate that the cellulose side chains remained in the solid state in copolymer 3-H and that they reduced the mobility of the PMMA main chain. X-ray analysis of copolymer 3-H did not show the crystalline pattern of cellulose, likely due to low composition of cellulose side chains in the copolymer. Acknowledgment. This study was supported in part by a Grant-in-Aid from a Research Fellowships of the Japan Society for the Promotion of Science (JSPS) for Young Scientists (Y.E.-R.), and by a Grant-in-Aid for Scientific

(1) Feger, C.; Cantow, H. J. Polym. Bull. 1980, 3, 407–413. (2) Nakatsubo, F.; Maeda, K.; Murakami, K. Bull. Kyoto UniV. Forests 1987, 59, 301. (3) Kamitakahara, H.; Nakatsubo, F. Cellulose 2005, 12, 209–219. (4) Kamitakahara, H.; Enomoto, Y.; Hasegawa, C.; Nakatsubo, F. Cellulose 2005, 12, 527–541. (5) Enomoto, Y.; Kamitakahara, H.; Takano, T.; Nakatsubo, F. Cellulose 2006, 13, 437–448. (6) Bernet, B.; Xu, J. W.; Vasella, A. HelV. Chim. Acta 2000, 83, 2072– 2114. (7) Murty, K. V. S. N.; Xie, T.; Bernet, B.; Vasella, A. HelV. Chim. Acta 2006, 89, 675–730. (8) Hourdet, D.; L’AIIoret, F.; Audebert, R. Polymer 1997, 38, 2535– 2547. (9) Poe, G. D.; Jarrett, W. L.; Scales, C. W.; McCormick, C. L. Macromolecules 2004, 37, 2603–2612. (10) Poe, G. D.; McCormick, C. L. J. Polym. Sci., Part A: Polym. Chem. 2004, 42, 2520–2533. (11) Beers, K. L.; Gaynor, S. G.; Matyjaszewski, K.; Sheiko, S. S.; Moller, M. Macromolecules 1998, 31, 9413–9415. (12) Borner, H. G.; Duran, D.; Matyjaszewski, K.; da Silva, M.; Sheiko, S. S. Macromolecules 2002, 35, 3387–3394. (13) Ito, K.; Tomi, Y.; Kawaguchi, S. Macromolecules 1992, 25, 1534– 1538. (14) Shinoda, H.; Miller, P. J.; Matyjaszewski, K. Macromolecules 2001, 34, 3186–3194. (15) Enomoto-Rogers, Y.; Kamitakahara, H.; Nakayama, K.; Takano, T.; Nakatsubo, F. Cellulose 2009, 16, 519–530. (16) Lee, S. J.; Altaner, C.; Puls, J.; Saake, B. Carbohydr. Polym. 2003, 54, 353–362. (17) Se, K.; Aoyama, K. Polymer 2004, 45, 79–85. (18) Kawaguchi, S.; Mihara, T.; Kikuchi, M.; Lien, L. T. N.; Nagai, K. Macromolecules 2007, 40, 950–958. (19) Howard, P.; Parikh, R. S. J. Polym. Sci., Part A: Polym. Chem. 1968, 6, 537–546. (20) Fort, R. J.; Hutchinson, R. J.; Moore, W. R.; Murphy, M. Polymer 1963, 4, 35–46. (21) Baptistella, L. H. B.; dos Santos, J. F.; Ballabio, K. C.; Marsaioli, A. J. Synthesis 1989, 21, 436–438.

BM900229G