N‐alkylation of chitosan by β‐halopropionic acids in the presence of various acceptors

N‐carboxyethylation of chitosan by β‐halopropionic acids in the presence of various proton and halogen ion acceptors was investigated. It has been observed that carboxyethylation of chitosan in aqueous medium is accompanied by the by‐processes of hydrolysis and dehydrohalogenation of the β‐halopropionic acids yielding β‐hydroxypropionic acid, bis(2‐carboxyethyl) ether, and acrylic acid. Degree of carboxyethyl substitution (DS) of chitosan and the relative rates of the by‐processes varied significantly depending on the conditions used and nature of the proton or halogen ion acceptor. At carboxyethylation of chitosan with the alkaline β‐bromopropionates, the DS increased in the order Cs⁺ < Rb⁺ < K⁺ ～ Na⁺ < Li⁺. For alkaline earth salts BrCH₂CH₂COOM_0.5 (M = Be²⁺, Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺), the highest DS was obtained with strontium and barium salts, which could be subsequently removed from the reaction mixture by precipitation as sulfates. Among the organic bases applied (tetrabutylammonium hydroxide, triethylamine, trimethylamine, pyridine, 4‐N,N‐dimethylaminopyridine, 2,6‐lutidine, and 1,5‐diazabicyclo[4.3.0] non‐5‐ene), the highest DS was obtained using a moderately strong base triethylamine. For the halogen acceptors (Pb²⁺, Ag⁺, Tl⁺), the stoichiometrically highest DS was achieved in a system comprising iodopropionic acid plus Tl⁺ and a comparable conversion rate was obtained using also a combination of chloropropionic acid and Ag⁺. A novel alternative preparative approach—gel‐state synthesis—was suggested that provides for the highest DS at the optimum reaction conditions. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

羧乙基化和碱预处理玉米芯渣制备纳米纤维素的研究

玉米芯提取木糖后剩余的残渣富含纤维素和木质素,采用羧乙基化反应和质量分数1%的NaOH溶液分别对玉米芯渣进行预处理,再经机械解离制备了纳米纤维素,最后经棒涂法制得纳米纤维素膜,并对预处理前后玉米芯渣、纳米纤维素及其膜的化学结构、组分含量、微观形貌、尺寸分布、水接触角和热稳定性进行了分析和表征。结果表明,羧乙基化预处理可增加玉米芯渣的羧基含量,同时脱除部分木质素,而碱预处理可脱除大部分木质素。经预处理后,玉米芯渣纤维尺寸明显降低、可及度增加。经机械解离后,羧乙基化预处理所得纳米纤维素的直径更小、分布更均匀,其经棒涂法所得膜也更加致密平滑,水接触角较大。另外,两种预处理方法均降低了所得纳米纤维素膜的热稳定性。