Polymer International Polym Int 53:1491–1494 (2004)DOI: 10.1002/pi.1570

Homogeneous graft copolymerizationof chitosan with methyl methacrylateby γ -irradiation via a phthaloylchitosanintermediateLi Liu, Yu Li, Wei-an Zhang, Guoqing Zhang and Yue-e Fang∗Department of Polymer Science and Engineering, University of Science and Technology of China, Hefei, Anhui 230026, People’s Republicof China

Abstract: The graft copolymerization of methyl methacrylate (MMA) onto chitosan was tried via anew protection-graft-deprotection procedure. Because the intermediate phthaloylchitosan was solublein organic solvents, the graft copolymerization was carried out in a homogeneous system. Grafting wasinitiated by γ -irradiation. The graft percentage extent was dependent on the irradiation dose and theconcentration of MMA monomer, and copolymers with grafting above 100 % were readily prepared. Thegraft copolymers exhibited a high affinity not only for aqueous acid but also for some organic solvents.Differential scanning calorimetry measurements revealed the presence of a glass transition phenomenon,which could be ascribed to the poly(methyl methacrylate) side-chains. 2004 Society of Chemical Industry

Keywords: chitosan; phthaloylchitosan; methyl methacrylate; graft copolymers; γ -irradiation

INTRODUCTIONChitosan, the N-deacetylated derivative of chitin, isa well-known abundant natural polymer having asimilar structure to cellulose. The presence of anamino group in chitosan has imparted to it variousfunctions, including biological activity and cationicpolymer properties.1 Despite the high potential ofchitosan expected in the field of biomaterials, itsinsolubility in common organic solvents and its non-thermal plasticity have delayed its utilization andbasic research.2 Chemical modification of this rigidaminopolysaccharide should lead to an interestingnovel type of polymeric material.

Of the possible chemical modifications of chitosan,graft copolymerization is attractive to expand appli-cations as functional materials through the appro-priate selection of molecular characteristics of theside-chains to be grafted.3 Some syntheses of chitosan-graft-polystyrene, chitosan-graft-poly(vinyl acetate)and chitosan-graft-poly(methyl methacrylate) haveappeared in the literature by the use of initiators suchas the ceric ion,4 Fenton’s reagent,5 or γ -irradiation.6,7

However, thus far these graft polymerizations on chi-tosan, especially those with some oleic monomers,have usually been performed in heterogeneous sys-tems, for example, in the emulsion or solid state.

Phthaloylchitosan has proved to be a derivative withimproved solubility in organic solvents, whilst thephthaloyl group can be deprotected easily to regeneratea free amino group.8,9 Because of these advantagesit has been utilized as a versatile key intermediatefor some regioselective chemical modifications.10,11

In this work, we intended to carry out the graftcopolymerization of chitosan with methyl methacrylate(MMA) via phthaloychitosan as intermediate. Thisnot only enabled the grafting reaction to be carriedout in a homogeneous system but also retained theabundant amino groups in the chitosan-graft-PMMAcopolymers.

EXPERIMENTALMaterialsChitosan (degree of deacetylation = 85 %, deter-mined by 1H NMR spectra in D2O/CF3COOD 95:5v/v) was purchased from Oceanary Biology Companyof Zhejiang, China. Methyl methacrylate (MMA) fromthe First Reagent Factory of Shanghai (China) was dis-tilled under reduced pressure. Phthalic anhydride andhydrazine monohydrate were supplied by the FirstReagent Factory of Shanghai (China) and used asreceived without further purification. Dimethylfor-mamide (DMF) was distilled under reduced pressure

∗ Correspondence to: Yue-e Fang, Department of Polymer Science and Engineering, University of Science and Technology of China, Hefei,Anhui 230026, People’s Republic of ChinaE-mail: fye@ustc.edu.cnContract/grant sponsor: National Natural Science Foundation of China; contract/grant number: 20274044(Received 18 September 2003; revised version received 9 December 2003; accepted 20 February 2004)Published online 22 June 2004

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from calcium hydride and stored over molecular sieves(4A).

MeasurementsAll infrared spectra were obtained from samples inKBr pellets using a Bruker VECTOR-22 FTIR spec-trophotometer. Elemental analyses were performedwith an Elementar Vario EL-III elemental analyzer.Thermal properties of the graft copolymers were elu-cidated by thermogravimetric analysis (TGA) with aShimadzu TGA-50H under N2 at a heating rate of20 ◦C min−1 and with differential scanning calorime-try (DSC) by a Shimadzu DSC-50 under N2 at aheating rate of 20 ◦C min−1.

PhthaloylchitosanChitosan was heated with excess phthalic anhydride indried DMF to give phthaloylchitosan, according to thepreviously reported procedure.8 It was obtained as ayellow powdery material and the degree of substitution(DS) of phthaloyl groups was determined by elementalanalysis.

Graft copolymerizationPulverized phthaloylchitosan (DS = 1.4, 0.5 g) wasdissolved in 5 ml of DMF, and a mixture of 2.5 gMMA and the right amount DMF was added. Afterthe mixture was deoxygenated by slow bubbling ofnitrogen gas through the solution for 5 min, thegrafting reaction was carried out in the 60Co sourceunder various grafting conditions. Then the productwas poured into methanol, and the precipitate wasfiltered and extracted with acetone in a Soxhletapparatus for 24 h.

The phthaloyl-protected graft copolymer obtainedwas stirred in 10 ml of DMF and heated to 100 ◦Cunder nitrogen. Hydrazine monohydrate was added,and the reaction was continued for 1 h to deprotectthe phthaloyl group. The yellow solution was allowedto cool to room temperature to precipitate, and thenthe precipitate was collected, washed thoroughly withethanol, and dried.

RESULTS AND DISCUSSIONGraft copolymerizationPhthaloylchitosan is characterized by considerablesolubility in organic solvents, such as DMF anddimethylsulfoxide (DMSO), and the phthaloyl groupcan be deprotected easily to regenerate a freeamino group. It is thus possible to carry out graftcopolymerization with MMA in a homogeneoussystem. The DS value of phthaloylchitosan preparedhere was about 1.40, calculated from elementalanalysis data. Further, γ -irradiation, as the initiatingmethod of graft copolymerization, was chosen becauseof its high efficiency and because no initiator neededto be added.

The obtained copolymers were isolated by thor-oughly extracting with acetone to remove the

4000 3500 3000 2500 2000 1500 1000 500

(d)

(c)

(b)

(a)

Wavenumber (cm-1)

Figure 1. IR spectra of chitosan (a), phthaloylchitosan (b), graftcopolymer with 123 % grafting (c) and graft copolymer with 245 %grafting (d).

homopolymer PMMA. Figure 1 shows IR spectraof the product in several steps. Nevertheless, thestrong characteristic absorptions due to the phthal-imido group at 1712 cm−1 and 1777 cm−1 made itdifficult to discern the characteristic peaks of theester groups belonged to PMMA branches in the IRspectrum before deprotecting the phthaloyl groups.The deprotection of phthaloyl groups was carried outby incubation with hydrazine monohydrate and con-firmed by the disappearance of the IR signals of thephthalimido group. Then, as a result of the graftcopolymerization, the IR spectra of the copolymersshowed strong bands characteristic of the ester groups,particularly at 1730 cm−1 and 1270–1150 cm−1.

The grafting percentage, defined as follows, couldbe evaluated by using the IR absorbance ratios ofthe characteristic bands at 1240 cm−1 due to PMMAand at 1070 cm−1 due to chitosan.12 A calibrationcurve was obtained by IR spectroscopy on some mix-tures of PMMA and chitosan.Grafting percentage =[(weight of introduced PMMA branches)/(weight ofchitosan main chain)] × 100

The results of the polymerization trials withvarying irradiation dose and varying concentrationof MMA monomer are listed in Table 1. Generally,grafting percentage tended to increase with increasingirradiation dose and reached a saturated value at

Table 1. Results of graft copolymerization of methyl methacrylate

onto chitosan

Run noConcentration of

MMA (g ml−1) Dose (kGy)Grafting

percentage (%)

1 0.36 11.31 702 0.36 18.86 1213 0.36 26.39 1234 0.36 33.94 1135 0.21 26.39 966 0.50 26.39 245

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Graft polymerization of chitosan with MMA

Table 2. Solubility of the graft copolymers

GraftingSolventa

Sample percentage (%) DMSO DMF CHCl3 Toluene Ethanol Acetic acid

Chitosan N/A — — — — — +Chitosan-graft-PMMA 123 ±± ±± — — — +Chitosan-graft-PMMA 245 + + ± ± ±b ±±b

a +, soluble; ±±, partially soluble or highly swollen; ±, swollen; –, insoluble.b Milky-like dispersible when heating.

18.9 kGy (runs 1–3). Then, when the dose continuedto increase above 33.9 kGy, the grafting percentagebegan to decrease a little, probably because ofthe degradation of PMMA segments at high doses(run 4). The concentration of MMA monomerwas another important factor in this homogeneousgrafting system. As is evident in Table 1, thegrafting percentage increased markedly with increasingmonomer concentration (runs 3, 5 and 6) andreached 245 % with a monomer concentration atabout 0.5 g ml−1 (run 6).

Solubility of graft copolymersThe resulting graft copolymers, chitosan-graft-PMMA, were obtained as earthy yellow powderymaterials having lustre. They exhibited an improvedaffinity for organic solvents, unlike the original chi-tosan. As summarized in Table 2, a higher extentof grafting brought about higher solubility owing tothe introduction of hydrophobic PMMA chains. Forinstance, copolymers of 245 % grafting were almostsoluble in DMSO and DMF, and swelled even inlow-boiling solvents such as chloroform. In addition,there were a large number of free C-2 amino groups inthe graft copolymers after the deprotection describedabove, which made it possible for the graft copoly-mer to maintain its basic hydrophilic property and thebioactivity of chitosan. This could be proved by theevidence that the graft copolymers were partly solubleor swelled highly in diluted aqueous solutions of aceticacid.

Thermal propertiesThe graft copolymers prepared in these studies were anew type of natural/synthetic (polysaccharide/PMMA)hybrid material. Thermal properties, in particularthe glass transition temperature (Tg), of the graftcopolymers are interesting in view of the struc-ture–property relationship and also for practical appli-cations. Although no glass transition phenomenon wasobserved with chitosan, the graft copolymers showedinflection points in their DSC profiles at 127–130 ◦C,depending on the grafting percentage, suggesting theoccurrence of a glass transition. The results are listed inTable 3. The glass transition of PMMA homopolymerwas detected at 120 ◦C, and the transition phenom-ena of the copolymers would thus be associated withthe transition of the introduced PMMA side-chains.

Table 3. Thermal properties of the graft copolymers

Sample (Grafting percentage) Tg (◦C) Tda (◦C)

PMMA 120 289Chitosan-graft-PMMA (245 %) 127 299Chitosan-graft-PMMA (123 %) 130 282Chitosan —b 305

a Decomposition temperature at which 10 % weight loss was observedduring TGA.b Not observed.

Thermal decomposition was also examined. As sum-marized in Table 3, the 10 % weight-loss temperaturesof graft copolymers and PMMA were 280–300 ◦C inTGA, whereas that of chitosan was 305 ◦C.

CONCLUSIONSPhthaloylchitosan has been found to be a versatilekey intermediate for regioselective chemical modifica-tions, since it is soluble in common organic solventsand easily deprotected to regenerate the free aminogroups. We synthesized a new type of amphoteric nat-ural/synthetic hybrid material composed of chitosanand poly(methyl methacrylate) through this phthaloyl-protected technique. The graft copolymerization wascarried out in a homogeneous system. The graftpercentage extent was dependent on the irradiationdose and the concentration of MMA monomer, andcopolymers with grafting above 100 % were readilyprepared. Solubilities of the graft copolymers couldbe regulated by the grafting extent and they exhib-ited a high affinity not only for aqueous acid butalso for some organic solvents. It is more importantthat the glass transition phenomenon was observedwith chitosan-graft-PMMA in view of the intractablenature of the original chitosan. The graft copolymerbased on chitosan and commodity polymers are ofinterest because of their wide potential applications inbiodegradable and biomedical materials. Such hybridmaterial would open further extension in the field ofresearch and development of biopolymer-based func-tional materials.

ACKNOWLEDGEMENTThis work was supported by a grant from the NationalNatural Science Foundation of China (20274044).

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