Synthesis of Thermoplastic Mixed Chitin Esters

Abstract

Mixed chitin esters, that is, chitin benzoate stearates, exhibiting thermoplasticity, were synthesized by the acylation of chitin using benzoyl and stearoyl chlorides in the presence of pyridine and N,N-dimethyl-4-aminopyridine for 1 h + 24 h at 100 °C in an ionic liquid, 1-allyl-3-methylimidazolium bromide. IR and 1H NMR spectroscopic analyses confirmed the formation of the desired chitin benzoate stearates. Powder X-ray diffraction analysis of the products indicated that the crystalline structures of the chitin main-chains and stearoyl side-chains were strongly affected by the benzoyl/stearoyl substituent ratios. Introducing a small number of benzoyl groups, in addition to a large ratio of stearoyl groups, contributed to disrupting the intrinsic chitin crystals and enabling the chitin main chains and stearoyl side chains to form regularly controlled layered and parallel arrays, respectively. The resulting products exhibited meting points, associated with regular stearoyl packings, and formed melt-pressed films during the melt-pressing process. These results suggest that chitin benzoate stearates with appropriate benzoyl/stearoyl substituent ratios exhibit thermoplasticity.

Synthesis of Mixed Chitin Esters via Acylation of Chitin in Deep Eutectic Solvents

“…The IR spectrum of the product, obtained by a feed ratio of benzoyl/stearoyl chlorides = 9:1 (entry 1, Table 1), observed two new carbonyl absorptions at 1715 and 1740 cm −1 ascribed to aromatic and aliphatic esters, respectively, compared to that of chitin powder (Figure 2a,b). The spectroscopic pattern was the same as that of the chitin benzoate stearate prepared in AMIMBr in our previous study [40]. The IR result indicated the progress of acylation under the above conditions in the DES to produce the chitin benzoate stearate.…”

“…The 1 H NMR spectrum of the product in CDCl 3 /CF 3 CO 2 H (2/1 in volume) also supported its structure because of the detection of both benzoyl and stearoyl signals, as shown in Figure 3a. However, the signals corresponding to H1-H6 in the N-acetyl-D-glucosamine repeating units were not clearly detected; this is owing to the shielding of the chitin chain by bulky benzoyl groups, as discussed in our previous study [40]. As the DS value of acyl groups was not exactly calculated due to this reason, the substitution ratio (SR) was alternatively estimated from the integrated ratio of the methyl signals of the stearoyl group to the aromatic signals of the benzoyl group (1.17:0.79), which was 1:0.41.…”

“…The substituent ratios in the derivatives were changed according to the feed ratios of the two acyl chlorides. We have already found, in a previous study, that chitin benzoate stearates with appropriate substituent ratios show thermoplasticity, associated with the regular parallel stearoyl packings [40]. Because we have developed the efficient acylation method of chitin in the DES in this study, we will investigate new properties and functions from the other mixed chitin esters with different acyl substituents, such as heterocyclic acyl groups, obtained via the present acylation method, and their potential to be practically applied as chitin-based materials in the future.…”

“…We selected benzoyl and stearoyl groups as the substituents to initially investigate the acylation of chitin for the synthesis of mixed chitin esters because such derivatives with varying substituent ratios were precisely characterized after their synthesis in AMIMBr [40]. Therefore, chitin benzoate stearates were synthesized using benzoyl and stearoyl chlorides according to the acylation method in the AMIMCl/TMG-DES as follows (Figure 1).…”

“…Particularly, acylation of chitin was found to efficiently occur in the absence of DMAP/pyridine in a DES consisting of AMIMCl/1,1,3,3-tetramethylguanidine (TMG) owing to the high basicity and low nucleophilicity of TMG, which produces chitin ester derivatives with high DSs [39]. We also found that thermoplasticity was conferred to chitin benzoate stearates with appropriate substituent ratios, which were synthesized in AMIMBr, as mentioned above [40]. In the present study, based on the above backgrounds, we attempt to synthesize mixed chitin esters with long fatty and bulky acyl groups by means of the above acylation method in the DES composed of AMIMCl/TMG (Figure 1).…”