Ring-opening polymerization and epimerization of L-lactide catalyzed by indium(III) acetate: synthesis, characterization, and computational validation

Abstract

We evaluated the catalytic performance of indium(III) acetate [In(OAc)3] catalyst and its selectivity towards the ROP of L-lactide (L-LA). The arious parameters examined were the initiator (water, 1-octanol, or ethylene glycol), catalyst concentration, reaction time, and bulk or solvent conditions. Indium catalyst showed excellent efficiency catalyzing the bulk ROP of L-LA and D,L-lactide (D,L-LA). The conversion percentage was calculated using 1H nuclear magnetic resonance (NMR) data, which revealed higher performance by the catalyst under bulk conditions than in solvent, and excellent conversions were maintained at higher catalyst concentration. Instrumental analyses evidenced the best semicrystallinity, stereoregular control, and mechanical properties of the oligoesters obtained under bulk conditions using water as an initiator. The worst molecular weight control occurred as a result of intermolecular transesterification reactions. In the ROP of L-LA using 1-octanol as an initiator, the performance of alcohol and moisture (water) as initiators was compared using the MALDI-TOF technique. Epimerization was monitored through tetrad analysis in the methine region of the 13C NMR spectra, which exhibited high epimerization in early stages of bulk ROP of L-LA. Kinetic studies show that the initiator does not impact the rate of bulk ROP of L-LA catalyzed by In(OAc)3. Additional experiments revealed a potential use of In(OAc)3 in the presence of 1-octanol to degrade or epimerize commercial PLLA. Mechanistic studies based on density functional theory (DFT) validated the coordination-insertion mechanism of the initiation step of ROP. The results revealed a preference of epimerization of L-LA over a representative oligomer model (open monomer).