Grafting Studies of Chitin

2.1. Materials and Instruments

Raw chitin, potassium tert-butoxide ((CH3)3CO-K+), methacryloyl chloride, 2, 2’azobisizobütironitril (AIBN) as initiator, 1-vinylimidazole (VIM), methacrylamide (MAm) and 2acrylamido-2-methyl-1-propanesulfonic acid (AMPS) as commercial monomer were purchased from Sigma-Aldrich. N, N-dimethylformamide, acetonitrile, ethanol, diethyl ether were used as solvent. The IR spectra of all samples were performed with a PerkinElmer Spectrum Two (UATR) IR spectrometer in the range of 4000-450 cm-1. Leco CHNS-932 was used for elemental analysis. Thermal analysis was performed with a Hitachi 7000 thermo balance at a heating rate of 10 °C min-1 under nitrogen flow of 10 ml min-1.

2.2. Synthesis of Chitin Graft Copolymers

To obtain chitin graft copolymers, chitin methacrylate was synthesized, primarily. Chitin methacrylate and its graft copolymers were synthesized by a method adapted from the literature [9-13]. Raw chitin was swollen in acetonitrile at the room temperature overnight, followed by adding a potassium-tert-butoxide ((CH3)3CO-K+) solution in acetonitrile. The mixture was stirred at the room temperature, and then methacryloyl chloride in solvent was added dropwise while stirring continued at the same temperature. The reaction mixture was refluxed for 48 hours, followed by a cooling and filtering procedure. Chitin methacrylate was washed away to remove salt and impurities with acetonitrile, water, ethanol, and diethyl ether. The synthesis reaction of the chitin methacrylate is shown in Figure 1.

Figure 1. Synthesis of chitin methacrylate 1 g chitin methacrylate, 5 g monomer that VIM, MAm and AMPS in N,N-dimethylformamide and 0.05 g AIBN as a free radical initiator were added into a polymerization tube, and argon was passed through. The monomer was allowed to graft onto chitin at ±72 °C for 36 h, stirring. Grafted copolymers were filtered and thoroughly washed with N,N-dimethylformamide, acetonitrile, ethanol, and diethyl ether to eliminate oligomer and homopolymer formed in the reaction as products. Synthesized graft copolymers were dried, and analyzes were performed. The synthesis reaction of the grafting of chitin methacrylate with VIM, MAm and AMPS monomers is shown in Figure 2.

3.1. Grafting of Chitin and its Characterization

The reaction of chitin with (CH3)3CO-K+ produces the alkoxide of chitin only on primary OH group even if an excess amount of (CH3)3CO-K+ was used. The treatment of the potassium chitin alkoxide with methacryloyl chloride results in the formation of chitin methacrylate (Chitin.met). As mentioned in the experimental section, a series of grafting studies on Chitin.met were carried out by using AIBN as a free radical initiator with VIM, MAm and AMPS (Fig 1 and 2). The FTIR technique is the first spectroscopic method used to identify functional groups within a molecule. The spectrum of chitin shows the characteristic bands at 3440 cm-1 (O-H strech), 3260 cm-1 (N-H strech), 2880 cm-1 (N–H symmetric stretch), 1654 cm-1 (C=O amide stretch), 1615 cm1 (N–H bending), 1552 cm-1 ( N–C stretch), 1375 cm-1 (C-H bending vibration), 1015 and 1050 cm-1 (C-O symmetric and asymmetric vibration at C-O-C bonds) were observed [7,14]. The presence of a new band 1620 (-C=O stretching), in which there is no band for chitin, indicates that the methacrylate group has attached to the chitin. Also, the most important evidence to demonstrate the grafting of methacrylate groups onto chitin is the increase in ester peak (-C = O) observed at 1720 cm-1 peaks (Figure 3). The elemental analysis of the polymers is shown in Table 1. The substitution degree in the glucose units of chitin was calculated as 29.24% by mole (y) from and 35.61% by weight (Y) the percentage of carbon [9-13]. The substitution percentage of the glucose units in chitin was calculated from the expression

Y = A - B x 100 C -D where A = % carbon found for modified chitin B = % carbon found for starting chitin C = % carbon calculated for 100% modification D = % carbon calculated for starting chitin. Table 1. Elemental analyses results of grafting on chitin with some monomers Polymer Chitin Chitin methacrylate Chitin.met-g-VIM Chitin.met -g-MAm Chitin.met -g-AMPS

Elemental Analysis H% N% 6.31 6.66 5.23 6.45 6.70 8.86 6.47 7.28 6.65 7.32

When the FT-IR spectra of the grafting studies on chitin methacrylate are examined, the following results are observed. FT-IR results of all chitin containing polymers show OH peaks at 3440 cm-1, NH peaks at 3260 cm-1 and amide peaks at 1654 cm-1 region clearly. For Chitin.met-gMAm and Chitin.met-g-AMPS, an increase in amide peaks due to polymerization of the monomer is observed. Furthermore, the weight increase in the synthesis of the graft copolymers as a result of the values calculated by gravimetric analysis is also evidence that graft copolymers are formed. FT-IR spectra of chitin, chitin methacrylate, the graft copolymers of chitin with poly(1vinylimidazole) (Chitin.met-g-VIM), poly(methacrylamide) (Chitin.met-g-MAm), and poly(2acrylamido-2-methyl-1-propanesulfonic acid) (Chitin.met-g-AMPS) are shown in Figure 3.

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Figure 3. FT-IR spectra of the a) Chitin b) Chitin.met c) Chitin.met-g-VIM d) Chitin.met-g-MAm e) Chitin.met-g-AMPS Thermal analysis measurements were made at a heating rate of 10 oC/min at a N2 gas flow of 10 ml/min. Referring to Figure 4, the thermal stability of the chitin is higher than that of graft copolymers. Initial decomposition temperature and temperature of a weight loss of 50%, decreased by grafting. All the polymers show that thermal degradation was completed in one

steps. It is observed that the synthetic polymers attached to chitin methacrylate decreased the thermal stability. Thermograms obtained for chitin, chitin methacrylate, and its graft copolymers are presented in Figure 4, while some thermal data are given in Table 2.

Figure 4. TGA curves of chitin, chitin methacrylate and its graft copolymers Table 2. Thermal analyses results of grafting on chitin with some monomers Polymer

Chitin Chitin methacrylate Chitin.met-g-VIM Chitin.met -g-MAm Chitin.met -gAMPS