Synthesis and anionic ring-opening polymerization behavior of amino acid-derived cyclic carbonates

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 degreesC 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.