Synthesis and Anionic Ring-Opening Polymerization Behavior of Amino Acid-Derived Cyclic Carbonates

Abstract

Synthesis and anionic ring-opening polymerization of l -serine (Ser)- and l -threonine (Thr)-based cyclic carbonates, (2-oxo[1,3]dioxan-5-yl)carbamic acid benzyl ester (Z-S-CC), (4-methyl-2-oxo[1,3]dioxan-5-yl)carbamic acid benzyl ester (Z-T-CC), and (4-methyl-2-oxo[1,3]dioxan-5-yl)carbamic acid tert -butyl ester (Boc-T-CC), were examined. The monomers were synthesized by reaction of triphosgene with the corresponding diols obtained by reduction of N -benzyloxycarbonyl- l -serine (Z-Ser), N -benzyloxycarbonyl- l -threonine (Z-Thr), and N - tert -butoxycarbonyl- l -threonine (Boc-Thr). Their ring-opening polymerizations were carried out with tert -BuOK, tert -BuOLi, and n -BuLi as initiators (2−10 mol %) in tetrahydrofuran (THF) at −78 to −30 °C for 1 h to obtain the corresponding polycarbonates. Poly(Z-T-CC) and poly(Boc-T-CC) showed inverted specific rotations with respective to the monomers. The small increase of the absolute values suggested the absence of higher-order structures. The polymers showed three different diads in the ratio of 1:8:1, which agreed well with the value expected from the equimolar reaction of the monomer with tert -BuOK. The polymers underwent cleavage of the Z and Boc groups by hydrogenation and acid treatment, respectively to afford the corresponding polycarbonates with free amino groups.