Amino acid-functionalized ethyl cellulose: Synthesis, characterization, and gas permeation properties

Amino acid esters of ethyl cellulose [R′ = H (1), CH₃ (2), CH₂CH(CH₃)₂ (3), CH₂CONH₂ (4), CH₂OCH₂C₆H₅ (5, 5′), CH₂CH₂CH₂CH₂NHOCOC(CH₃)₃ (6)] were synthesized in moderate to quantitative yields (30-99%) by the reaction of t-butoxycarbonyl (t-Boc)-protected amino acids or an activated ester derivative with hydroxy groups of ethyl cellulose [EC; degree of substitution (DSE t), 2.69]. The amino acid functionalities displaying varied chemical nature, shape, and bulk were used, and bulk of the substituent on the α-carbon of amino acids was elucidated to be of vital significance for the observed degree of incorporation (DSE st). ¹H NMR spectra were used to determine the degree of incorporation of amino acid moiety (DSE st), and almost complete substitution of the hydroxy protons was revealed in 1, 2, and 5′. The onset temperatures of weight loss of 1-6 were 198-218 °C, indicating fair thermal stability. The glass transition temperatures of the derivatized polymers were 30-40 °C lower than that of EC (Tg 131 °C; cf. Tg of 1-6, 93.5-103 °C). Free-standing membranes of EC and its amino acid esters (1, 2, 5, 5′, and 6) were fabricated, and enhanced permselectivity for CO₂/N₂ and CO₂/CH₄ gas pairs was discerned, when compared with EC.