Synthesis, characterization and chemical degradation of poly(ester-triazole)s derived from d-galactose

Abstract

α-Azide-ω-alkynyl ester monomers were designed and synthesized in order to obtain hydrolytically degradable polymers. The monomers were prepared from d-galactose, as a renewable resource. Environmentally benign azido-alkyne cycloaddition polymerizations were conducted to afford poly(ester-triazole)s, with complete atom economy. Although polymer formation prevailed under optimized polymerization conditions, variable proportions of cyclic oligomer byproducts were detected. The Cu-catalyzed click polymerization led regioselectively to 1,4-disubstituted triazole linkages, while the thermal, metal-free polymerization produced a random distribution of 1,4- and 1,5-disubstituted triazoles in the polymer backbone. The poly(ester-triazole)s exhibited high molecular weights (M _w in the range 35-85 kDa). They were soluble in organic solvents but highly insoluble in water, thus removal of the Cu(i) catalyst was simplified. The polymers were stable up to 300 °C, and had T _g values in the range 90-100 °C. The materials were hydrolysed under either basic or strong acid conditions, and the degradation products have been characterized.

Scheme 1. Synthesis of monomer 5 from d-galactono-1.4-lactone (1).

Fig. 1. ¹H NMR (500 MHz, CDCl₃) spectra of (a) monomer 5, (b) poly(ester-triazole) 6, (c) poly(ester-triazole) 7, and (d) cyclotetramer 10.

Scheme 2. Poly(ester-triazole) synthesis.

Scheme 3. Transesterification products formed during polymerization in methanol.

Fig. 2. DSC curves for poly(ester-triazole) 6 obtained according to Table 1, entry 1, (a) first heating, (b) second heating; and cyclic tetramer 10 (c) first heating, (d) second heating.

Fig. 3. SEM images of poly(ester-triazole)s from Table 1: (a) and (b) entry 1; (c) and (d) entry 2; (e) and (f) entry 9; (g) and (h) entry 11.

Scheme 4. Products of hydrolytic degradation of poly(ester-triazole) 6.

Fig. 4. ¹H NMR (500 MHz) spectra of (a) poly(ester-triazole) 6 (in CDCl₃) and degradation products (b) 13 at 25 °C (in D₂O) or (c) 14 at 80 °C (in DMSO-d₆).