Indian Journal of Chemical Technology Vol. 18, May 2011, pp. 234-243

Homogeneous grafting of PMMA onto cellulose in presence of Ce4+ as initiator

B Tosh* & C R Routray Department of Chemistry, Orissa Engineering College, Bhubaneswar 751 007, India

Received 14 June 2010; accepted 22 March 2011

Poly (methyl methacrylate) (PMMA) is successfully grafted onto cellulose in homogeneous medium in N,N-dimethyl acetamide/LiCl solvent system. The method is based upon ring opening reaction of cellulose with Ce4+ ion. Ceric ammonium nitrate (CAN) forms free radical on cellulose back bone in presence of DMSO via ring opening mechanism and PMMA is grafted through homogeneous breaking of the acrylic double bond. Methylene blue was used as an inhibitor to check the formation of homopolymer. The graft yield and grafting efficiency was studied by varying reaction time, temperature, and monomer concentration. The products are characterized by FTIR and 1H-NMR analysis and a possible reaction mechanism is deduced. Thermal degradation of the grafted products is also studied by thermo-gravimetric analysis (TG).

Keywords: Methyl methacrylate (MMA), Homogeneous medium, N,N–Dimethyl Acetamide/LiCl, Ceric ammonium nitrate (CAN), DMSO, Grafting, Ring opening, FTIR, Grafting efficiency, Methylene blue, Inhibitor

Cellulose is the most abundant naturally occurring polymer and its derivatives have many important applications in fiber, paper and paint industries. The polymeric material with desired properties is a current need of the society. To control the properties such as hydrophobicity, adhesivity, selectivity, drug delivery, wettability and thermosencitivity, graft copolymerization of suitable monomer is the versatile technique for cellulose modification1. Although cellulose has good properties it has some undesirable ones such as low tensile strength, high moisture regain, and low strength against microbial attack and hence grafting of synthetic polymers on cellulose eliminates these drawbacks and allows the acquisition of additional properties of grafted polymers without destroying its own properties2. Heterogeneous grafting of synthetic polymers onto cellulose back bone has been carried out by many researchers1-10.

The discovery of new solvents for cellulose dissolution opened the possibility of performing derivatization and/or grafting reactions in homogeneous conditions, thus assuring important advantages, such as a better control of the degree of substitution11, a more uniform distribution of substituents along the polymer and a higher conversion yield12,13 .

During past few years a number of cellulose derivatives has already been synthesized under

homogeneous conditions11,14-16 and work on homogeneous grafting of vinyl monomers onto cellulose and some cellulose derivatives in non-degradable solvent systems such as DMSO/ PF14,15,17,18, DMSO/PhMe19,20 and DMAc/ LiCl12,13,21,22 have been tried using different initiators like ammonium persulfate (APS), azobisisobutyronitrile(AIBN), and benzoyl peroxide, but so far work on homogeneous graft copolymerization of cellulose dissolved in N,N-dimethyl acetamide/lithium chloride (DMAc/LiCl) solvent system using ceric ammonium nitrate(CAN) has not been investigated.

CAN in presence of nitric acid is an efficient initiator for graft copolymerization of vinyl monomers onto cellulose1-3 in heterogeneous medium but in homogeneous conditions this will produce gel confirming the regeneration of cellulose from DMAc/LiCl solvent system. It is only reported that CAN in presence of dimethyl sulfoxide (DMSO) can produce Ce4+ ion12 and can be a suitable redox system to initiate graft copolymerization process, but hardly any work is reported on this system. It is also reported that presence of methylene blue in the reaction system reduces the formation of homopolymers in the graft copolymerization process23.

Therefore, in the present study we describe the homogeneous graft copolymerization of methylmethacrylate (MMA) onto cellulose in DMAc/LiCl solvent system taking CAN in presence

—————— Corresponding author (E-mail: bntosh@yahoo.com)

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of DMSO as initiator. The effect of varying in reaction time, temperature, concentration of initiators and monomer were studied to optimize the conditions under which grafting would occur most effectively. The effect of methylene blue on homopolymers formation is also studied. The grafted products obtained were characterized by Fourier transformation infrared (FTIR) and proton nuclear magnetic resonance (1H-NMR) spectroscopy and their molecular weight and number of grafts per cellulose backbone were determined. Finally, thermal degradation of the grafted products was studied by thermo-gravimetric (TG) analysis.

Experimental Procedure Materials

Cellulose powder from cotton linters, having viscosity 50-150 cP (Brookfield RTV, Spindle # 1, 20 rpm (lit)) was obtained from Sigma Aldrich Chemicals Pvt. Ltd. It was purified by washing with methanol, acetone, and de-ionized water and finally dried in an oven at 50°C for 7 days. The viscosity average molecular weight of cellulose was calculated by nitrating the sample and using Mark-Houwink-Sakurada equation24 and was found to be 33,500. Methyl methacrylate purchased from Sigma Aldrich was purified from the inhibitor (hydroquinone monomethyl ether) by extracting with aqueous sodium chloride–sodium hydroxide solution and dried over sodium sulfate. The stabilizer free monomer was vacuum distilled and stored below 5°C. DMAc (sd-fine chemicals) was distilled under CaH2 and stored on molecular sieve (4 Å) under nitrogen atmosphere.

Ceric ammonium nitrate and DMSO (E-Merck) were of reagent grade and used without further purification. Methylene blue was procured from E-Merck and is a 0.05% (w/v) aqueous solution. N2 gas was passed through alkaline pyrogallol, sulfuric acid and potassium hydroxide solution before it was passed into the reaction mixture. Preparation of cellulose solution

A 2% solution of cellulose was prepared by taking 8 g of cellulose in 400 mL of DMAc, heated at 150°C for 26 min in a round bottom flask equipped with a short path condenser. Then 40 g LiCl was added and heated up to 165°C for 8 min. It was stirred overnight to get a clear solution11,13. The solution was made 1% during grafting.

Grafting

25 mL of 1% cellulose solution (1.45 mmol of the corresponding anhydroglucose unit) was taken in a three necked round bottom flask, equipped with a magnetic stirrer and temperature controlled oil bath. To this different amount of CAN ranging from 0.5 to 0.7 g (0.91-1.28 mmol) dissolved in 10 mL DMSO was added followed by addition of 1.25- 2.0 mL (11.7-18.7 mmol) of MMA. All the reactions were carried out in a dry nitrogen atmosphere. The reaction was carried out at different temperatures between 60 and 80°C for 2-6 h. The reaction was terminated by addition of hydroquinone14. The polymerization mixture was poured into cold distilled water with vigorous stirring and kept overnight at 5°C and then filtered, washed thoroughly in cold distilled water and dried at 50°C and weighed. Then the products were soxhlet extracted with acetone for 24 h to remove any adherent homopolymer. The extracted cellulose-grafted products were then dried at 50°C and stored over P2O5.

A comparative study was also carried out to study the effect of methylene blue in the formation of homopolymer by adding 0.7 mL (1.09 ppm) of methylene blue to the reaction at 80°C having monomer concentration 18.7 mmol and initiator concentration 1.28 mmol.

The graft yield (GY), total conversion of monomer to polymer (TC), grafting efficiency (GE) and number of graft per cellulose chain were calculated on the basis of oven-dried weight of the cellulose from the increase in weight after grafting by using the following relations25.

GY (%) = 100×−

A

AC

GE (%) = 100×−

−

AB

AC

TC (%) = 100×−

D

AB

Number of grafts per cellulose chain =

100GY

PMMAgraftedofweightMoleculrcelluloseofweightMolecular

×

Where A is the weight in grams of the original cellulose taken for the reaction; B is the weight in grams of the grafted cellulose before extraction; C is

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the weight in grams of the grafted product after extraction and D is the weight in grams of monomer charged. Molecular weight

Cellulose grafted with PMMA was hydrolyzed with 72% H2SO4 to isolate PMMA14. The intrinsic viscosities [η] (cm3g-1) of isolated graft polymers were measured at 25°C, taking acetone as a solvent to estimate the viscosity average molecular weight by using the following Mark-Houwink-Sakurada equation14.

[η]Acetone = 5.3 × 10-3 M0.73 FTIR analysis

IR spectra of the grafted and ungrafted cellulose samples were recorded on a PerkinElmer spectrometer (Spectrum RX1, PerkinElmer, Singapore) using KBr pellet technique, in the range 4000-400 cm-1, with a resolution of 2 cm-1, using 4 scans per sample. NMR analysis

The 1H-NMR spectra of the grafted products were collected on a Bruker WM – 400 spectrometer operating at 300 MHz for proton. All the chemical shifts were reported in parts per million (ppm) using tetramethylsilane (TMS) as the internal standard and CDCl3 as the solvent for the samples.

Thermal analysis

Thermo gravimetric (TG) analysis of the grafted products was carried out using a Simadzu thermal analyzer 30, in the temperature ranges from 40-800°C at a heating rate 10°C.min-1, in air atmosphere. Alpha-alumina was used as reference material for the study. 7 - 9 mg of the samples were taken for analysis.

Results and Discussion

Effect of reaction time

The graft co-polymerization of MMA onto cellulose is carried out at 60, 70, and 80°C with reaction time ranging from 2-6 h with 1 h interval. The data on weight gain with respect to reaction time at different temperatures are shown in Tables 1-4. Figures 1-3 represent the percentage weight gain versus reaction time at temperatures 60, 70 and 80°C, respectively with MMA concentration 11.7 mmol and CAN concentration 0.91 mmol. Figures 4 and 5 represent the reactions at 80°C with CAN concentration 0.91 mmol and monomer concentrations 14.0 and 18.7 mmol. Figures 7 and 8 show the percentage weight gain versus reaction time at 80°C with MMA concentration 18.7 mmol and CAN concentration 1.1 and 1.28 mmol. It is observed that the %GY and %TC is increased with increase in reaction time in all the cases.

Effect of temperature

Grafting reactions are carried out by varying the temperature from 60 to 80°C and the data of weight

Table 1—Graft co-polymerization of MMA onto cellulose at different temperatures with MMA concentration 5% (11.7 mmol) and CAN concentration 5% (0.91 mmol) in 10 mL DMSO

Reaction temperature (°C)

Sample code Reaction time (h)

% GY

% GE

% TC

Mw of PMMA

No of grafts/cellulose chain

60

Cell-g-PMMA-01 Cell-g-PMMA-02 Cell-g-PMMA-03 Cell-g-PMMA-04 Cell-g-PMMA-05

2 3 4 5 6

8 12 16 20 28

28.6 30.0 23.5 26.3 29.2

5.6 8.0 12.8 15.2 19.2

2172 2760 3135 3564 4404

1.23 1.45 1.71 1.88 2.13

70

Cell-g-PMMA-06 Cell-g-PMMA-07 Cell-g-PMMA-08 Cell-g-PMMA-09 Cell-g-PMMA-10

2 3 4 5 6

12 16 20 28 36

60.0 57.1 62.5 77.8 47.4

4.2 6.0 6.8 7.7 16.2

3268 4014 4189 4258 4385

1.23 1.34 1.60 2.21 2.75

80

Cell-g-PMMA-11 Cell-g-PMMA-12 Cell-g-PMMA-13 Cell-g-PMMA-14 Cell-g-PMMA-15

2 3 4 5 6

20 24 28 32 40

57.1 55.6 50.0 57.2 58.8

6.0 7.7 10.2 12.0 14.5

2974 2956 3695 3829 4573

1.80 2.27 2.60 2.80 2.93

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gain are given in Table 1 and Figures 1-3. It is observed that at a particular reaction time the %GY of the MMA grafted product is also increased with increase in reaction temperature.

Effect of monomer concentration

At a reaction temperature of 80°C, keeping the CAN concentration at 5% (0.91 mmol) the concentration of MMA is changed from 5% (11.7 mmol) (Samples; Cell-g-PMMA-11 to Cell-g-PMMA-15; Table 1) to 6% (14.0 mmol) (Samples; Cell-g-PMMA-16 to Cell-g-PMMA-20; Table 2)

and 8% (18.7 mmol) (Samples; Cell-g-PMMA-21 to Cell-g-PMMA-25; Table 2). Figures 3-5 show the %GY, %GE and %TC of the grafted products. %GY and %TC increase with increase in reaction time and monomer concentration.

Effect of initiator concentration

Grafting reactions are carried out at 80°C at MMA concentration 8% (18.7 mmol) and CAN concentration is changed from 5% (0.91 mmol) (Samples; Cell-g-PMMA-21 to Cell-g-PMMA-25; Table 2) to 6% (1.1 mmol) (Samples; Cell-g-PMMA-

Table 2—Graft co-polymerization of MMA onto cellulose with different monomer concentration at 80°C and CAN concentration 5% (0.91 mmol) in 10 mL DMSO

Monomer concentration

Sample code Reaction time (h)

% GY

% GE

% TC

Mw of PMMA

No of grafts/cellulose chain

6 % (14.0 mmol)

Cell-g-PMMA-16 Cell-g-PMMA-17 Cell-g-PMMA-18 Cell-g-PMMA-19 Cell-g-PMMA-20

2 3 4 5 6

20 24 28 36 44

62.5 50.0 70.0 60.0 61.1

5.7 5.7 7.1 10.7 12.8

6280 6664 9553 6546 6065

1.06 1.20 0.98 1.84 2.43

8 % (18.7 mmol)

Cell-g-PMMA-21 Cell-g-PMMA-22 Cell-g-PMMA-23 Cell-g-PMMA-24 Cell-g-PMMA-25

2 3 4 5 6

36 40 44 52 56

69.2 62.5 61.1 46.4 40.0

7.5 8.5 9.6 14.9 18.7

6305 8441 8340 6888 6496

1.45 1.58 1.76 2.53 2.89

Table 3—Graft co-polymerization of MMA onto cellulose with different initiator concentration at 80°C and MMA concentration 8% (18.7 mmol)

Initiator concentration

Sample code Reaction time (h)

% GY

% GE

% TC

Mw of PMMA

No of grafts/cellulose chain

6 % (1.1 mmol)

Cell-g-PMMA-26 Cell-g-PMMA-27 Cell-g-PMMA-28 Cell-g-PMMA-29 Cell-g-PMMA-30

2 3 4 5 6

40 44 48 52 60

58.8 50.0 48.0 39.4 40.5

9.1 11.8 13.4 17.6 19.8

6826 9246 7703 7653 5685

1.96 1.88 1.91 2.10 2.35

7 % (1.28 mmol)

Cell-g-PMMA-31 Cell-g-PMMA-32 Cell-g-PMMA-33 Cell-g-PMMA-34 Cell-g-PMMA-35

2 3 4 5 6

44 48 52 56 60

55.0 45.8 7.1 28.0 29.4

10.7 12.8 18.6 26.7 27.2

7566 9665 9128 7096 6406

1.94 1.66 1.90 2.83 2.93

Table 4—Graft co-polymerization of MMA onto cellulose at 80°C with CAN concentration 7% (1.28 mmol) and MMA concentration 8% (18.7 mmol) and methylene blue 0.7 mL (1.09 ppm).

Sample code Reaction time (h)

% GY

% GE

% TC

Mw of PMMA

No of grafts/cellulose chain

Cell-g-PMMA-36 Cell-g-PMMA-37 Cell-g-PMMA-38 Cell-g-PMMA-39 Cell-g-PMMA-40

2 3 4 5 6

44 48 52 56 60

52.4 54.5 56.5 63.6 62.5

11.2 11.8 12.3 12.8 13.4

3495 3550 3403 3303 3137

4.2 4.5 5.1 5.7 6.1

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26 to Cell-g-PMMA-30; Table 3) and 7% (1.28 mmol) (Samples; Cell-g-PMMA-31 to Cell-g-PMMA-35; (Table 3). Figures 5-7 indicate the effect of CAN concentration on the weight gain with respect to reaction time. As evident from the tables and figures, the %GY and %TC increase with increase in the initiator concentration at a particular reaction time.

Effect of inhibitor

To study the effect of methylene blue as the inhibitor for the formation of homopolymer, the reaction is carried out by taking 25 mL of 1% cellulose solution, CAN concentration 7% (1.28 mmol), MMA concentration 8% (18.7 mmol) at 80°C, with addition of 0.7 mL (1.09 ppm) methylene blue and the data are shown in Table 4 and Fig. 8.A comparative study of Table 3 and 4 shows that, the %GY of the grafted product remains the same but the %TC of the grafted product without methylene

Fig. 4—Effect of reaction time on grafting of MMA onto cellulose in presence of CAN at 80°C: cellulose, 1% (1.45 mmol); MMA, 14.0 mmol; CAN, 0.91 mmol

Fig. 5—Effect of reaction time on grafting of MMA onto cellulose in presence of CAN at 80°C: cellulose, 1% (1.45 mmol); MMA, 18.7 mmol; CAN, 0.91 mmol

Fig. 1—Effect of reaction time on grafting of MMA onto cellulose in presence of CAN at 60°C: cellulose, 1% (1.45 mmol); MMA, 11.7 mmol; CAN, 0.91 mmol

Fig. 2—Effect of reaction time on grafting of MMA onto cellulose in presence of CAN at 70°C: cellulose, 1% (1.45 mmol); MMA, 11.7 mmol; CAN, 0.91 mmol

Fig. 3—Effect of reaction time on grafting of MMA onto cellulose in presence of CAN at 80°C: cellulose, 1% (1.45 mmol); MMA, 11.7 mmol; CAN, 0.91 mmol

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blue goes on increasing rapidly with reaction time (Table 3), where as in the presence of the inhibitor the %TC increases very slowly. There is not much more variation in the %GE in the later case (Table 4).

Molecular weight of PMMA and number of grafts per

cellulose chain

The molecular weight of PMMA extracted from the grafted samples prepared under different reaction conditions were determined and reported in the respective tables. As seen in Table 1, the grafting reactions with monomer concentration 11.7 mmol and CAN concentration 0.91 mmol, in the three temperature conditions give the grafted product having well control on the molecular weight of the PMMA and number of grafts per cellulose chain. At monomer concentration 14.0 and 18.7 mmol (Table 2) the molecular weight of PMMA goes on increasing up to a reaction time of 3-4 h and then decreases slowly. But the number of grafts per cellulose chain goes on increasing with increase in reaction time. The same trend is also observed at a monomer concentration of 18.7 mmol with MMA concentration 1.1 and 1.28 mmol (Table 3). For the reactions with methylene blue as the data show (Table 4), there is almost no change in the extracted polymer molecular weight and the number of grafts per cellulose chain increases slowly in comparison to other conditions. This shows that, methylene blue acts as a good inhibitor for the formation of homopolymer and controls the molecular weight of the graft copolymer and increases the grafting sites in the cellulose chain as evident from Table 4.

Moreover, as seen from the Tables 1-3, the grafting efficiency is maximum of 77.8% for sample Cell-g-PMMA-09, but for this sample the GE is only 28% and also the molecular weight of the homo polymer is 4258. The samples prepared in the presence of methylene blue as inhibitor (Table 4) give better result than the former. By observing the tables, the conditions for getting the sample Cell-g-PMMA-39 is considered as the optimum conditions, which is grafting at 80°C for 5 h with a monomer concentration 18.7 mmol, initiator concentration 1.28 mmol and inhibitor concentration 1.09 ppm. For this sample the % GE is 56,% GE is 63.6, molecular weight of the homo polymer is 3303 and number of grafts per cellulose chain is 5.7.

FTIR studies

FTIR spectra of the grafted cellulose in the absence of methylene blue (Cell-g-PMMA-35) and presence

Fig. 6—Effect of reaction time on grafting of MMA onto cellulose in presence of CAN at 80°C: cellulose, 1% (1.45 mmol); MMA, 18.7 mmol; CAN, 1.1 mmol

Fig. 7—Effect of reaction time on grafting of MMA onto cellulose in presence of CAN at 80°C: cellulose, 1% (1.45 mmol); MMA, 18.7 mmol; CAN, 1.28 mmol

Fig. 8—Effect of reaction time on grafting of MMA onto cellulose in presence of CAN at 80°C: cellulose, 1% (1.45 mmol); MMA, 18.7 mmol; CAN, 1.28 mmol; Methylene blue, 0.7 mL (1.09 ppm)

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of methylene blue (Cell-g-PMMA-39) are shown in Fig. 9. Both the products show identical peaks at 3432 cm-1 (OH str of cellulose), 2948 cm-1 (–CH2- of PMMA), 1728cm-1 (>C=O str. of PMMA), 1632 cm-1 (C-C str of PMMA), 1484 cm-1 (OH bending of cellulose), 1447 cm-1 (CH bending of cellulose), 1397 cm-1 (CH deformation of cellulose), 1261 cm-1 ( –C-O-C- str of PMMA), 1190 and 1147 cm-1 (C-C str of cellulose and PMMA), 1014 cm-1 (–CH2- wagging of cellulose), 801 and 745 cm-1 (CH rocking vibrations of cellulose and PMMA), thereby indicating the formation of MMA-grafted cellulose.

NMR studies

1H-NMR spectra of the grafted cellulose with PMMA (Cell-g-PMMA-35 and Cell-g-PMMA-39)

are shown in Fig. 10. Both the products show identical peaks and the peak at 3.53 ppm is due to the –O–CH3 group of the grafted polymer. The –CH2– group shows peaks at 2.02, 1.83 and 1.75 ppm and the peaks at 1.18, 0.95 and 0.78 ppm are for the –CH3 group26. The peak at 4.0 ppm is due to the –OH group of the cellulose chain27.

Mechanism of polymerization

It is known that metallic cations form complexes with carbon hydrates. After complexation with cellulose, ceric ion is reduced to cerous ion, the bond between C2 and C3 is broken and a free radical appears on C2 or C3

28. Then this free radical initiates the monomer grafting and the polymerization reaction of MMA. The FTIR and 1H-NMR spectra of the

Fig. 9—FTIR spectra of PMMA grafted cellulose (a) in presence of methylene blue and (B)

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grafted products also shows the peacks for the –OH group which proves that the grafting occurs by breaking of a C-C bond and not at the –OH group. Hence the mechanism is the one which is shown in Scheme 1.

Methylene blue acts as inhibitor and affects the termination step and the rate of the reaction will be affected29. This is also evident from Table 4 that all the samples prepared in the presence of methylene blue show uniformity in the homo polymer molecular weight.

Fig. 10—1H-NMR spectra of (a) PMMA grafted cellulose and (b) PMMA grafted cellulose prepared in presence of methylene blue

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Thermogravimetric analysis

Figure 11 shows the TG curves of grafted samples Cell-g-PMMA-35 and Cell-g-PMMA-39. The percentage weight loss of these samples with temperature are given in Table 5. The initial decomposition for Cell-g-PMMA-35 started at 170°C where as that for Cell-g-PMMA-39 is 210°C. This may be due to the increase in the percentage of grafting and molecular weight of the homopolymer for the former case. The results reveal a decrease in the thermal stability with an increase in the percentage of grafting30 and molecular weight of the grafted polymer. Up to a temperature 250°C the thermal degradation of Cell-g-PMMA-39 is less in comparison to Cell-g-PMMA-35 (Table 5) and from

Fig. 11—Thermogravimetric (TG) analysis of (a) PMMA grafted cellulose and (b) PMMA grafted cellulose prepared in presence of methylene blue

Scheme 1—Mechanism of grafting of PMMA onto cellulose by Ce4+ ion

Table 5—Thermal stability of cellulose grafted products in air at heating rate 10 °C.min-1.

Wt loss (%) Sample code IDTa (°C)

200°C 250°C 300°C 350°C 400°C 450°C 500°C

Cell-g-PMMA-35 Cell-g-PMMA-39

170 210

6.5 0.7

11.2 8.5

21.4 28.2

32.0 40.1

58.8 61.3

81.2 82.5

81.2 82.5

a IDT is the initial decomposition temperature.

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300°C and above both samples show nearly equal degradation profile.

Conclusions Homogeneous graft copolymerization of MMA

onto cellulose in DMAc/LiCl solvent system was carried out by using CAN as an initiator in presence of DMSO. The formation of grafted products is confirmed by FTIR spectroscopy. The effect of reaction time and temperature, monomer and initiator concentration on the %GY, %GE and %TC is evaluated. Graft copolymerization of cellulose in presence of Ce4+ proceeds through ring opening of cellulose and formation of free radical in the cellulose chain, which initiates the polymerization by free radical mechanism. It is concluded that in presence of methylene blue as the inhibitor for homopolymer formation, the grafted products shows uniform molecular weight of the grafted chain. It is also concluded that the increase in the percentage of grafting and molecular weight of the homopolymer decrease the thermal stability of the compound.

Acknowledgement The authors are thankful to the Department of

Science and Technology, New Delhi, for financial support to carry out the work and to the Principal and President G.B., Orissa Engineering College, Bhubaneswar, India, for their encouragement and support.

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