Synthesis, Characterization, and Gas Permeation Properties

Chapter 5
135
Chapter5
Synthesis and Properties of Amino Acid Esters of Hydroxypropyl
Cellulose
Abstract
The amino acid esters of hydroxypropyl cellulose [R' = H (2a), CH3 (2b),
CH2CH(CH3)2 (2c), CH2CONH2 (2d), CH2CH2CONH2 (2e),
CH2CH2CH2CH2NHOCOC(CH3)3 (2f)] were synthesized in good yield by the reaction
of t-butoxycarbonyl (t-Boc)-protected amino acids with hydroxy groups of
hydroxypropyl cellulose (1; molar substitution (MS), 4.61). The amino acid
functionalities displaying varied chemical nature, shape, and bulk were utilized and the
bulk of the substituent on the α-carbon of amino acids was elucidated to be of vital
significance for the observed degree of incorporation (DSEst). The 1 H NMR spectra
and elemental analysis were employed to determine the degree of incorporation of
amino acid moiety (DSEst) and almost complete substitution of the hydroxy protons
was revealed for 2a, 2b, and 2f. The presence of the peaks characteristic of the
carbonyl group in the FTIR spectra furnished further evidence for the successful
esterification of hydroxypropyl cellulose. The starting as well as the resulting
polymers (1 and 2a–f) were soluble in polar organic solvents; however, the
esterification of 1 with bulky organic moieties resulted in an increased hydrophobicity
as all of the amino acid-functionalized polymers (2a–f) were insoluble in water. The
onset temperatures of weight loss of 2a–f were 175–230 °C, indicating fair thermal
stability. The amino acid functionalization led to the enhanced polymer chain
stiffness, and the glass transition temperatures of the derivatized polymers were
30–40 °C higher than that of 1 (Tg 3.9 °C; cf. Tg of 2a–f, 35.1–43.3 °C).