Synthesis and Characterization of Functionalized Polylactides Containing Acetal Units

Abstract

New functionalized lactide copolymers containing acetal units were prepared for the first time in a controlled manner that enabled the regulation of the number of reactive groups introduced into the polyester chain. The presence of functional groups in the copolymer backbone provided chemical modification sites, and the nature of the acetal unit affected the material degradability. First, paraformaldehyde was reacted with selected diols containing reactive pendant groups (3-allyloxypropane-1,2-diol and 3-chloropropane-1,2-diol), which was catalyzed by p-toluenesulfonic acid, to synthesize new cyclic acetals with different functionalities (allyl- or chloro-). In addition, using butane-1,4-diol, a nonfunctionalized seven-membered cyclic acetal (dioxepane) was obtained for comparative studies. In the next step, the prepared cyclic acetals were used for cationic copolymerization with lactide in the presence of glycol as an initiator and triflic acid as a catalyst. Different temperatures (−15, 2, and 30 °C) and copolymerization times (24, 48, 72, and 192 h) were investigated to produce copolyesters with variable contents of acetal units in the range of 5–27%. The copolymers’ structure and molar masses were carefully investigated using 1H, 13C NMR, 2D NMR, and size-exclusion chromatography. Moreover, the ability of functionalized copolymers to perform post modifications was also proven by the reaction with sodium azide and propanethiol. Finally, we speculate that structurally diverse groups can be attached to the copolyester chain, fine-tuning the on-demand properties, which could rapidly expand the library of polylactide-based materials.