Controlled Cationic Polymerization of Lactide

The combination of trifluoromethanesulfonic acid (as a catalyst) and a protic reagent (such as water, 2-propanol, or 1-pentanol as an initiator) efficiently initiates the cationic polymerization of lactide in dichloromethane solution at room temperature. Polylactides (PLAs) with molar masses up to 20 000 were obtained via an activated-monomer mechanism. 1H NMR spectroscopy and electrospray-ionization mass spectrometry of PLA oligomers demonstrated the quantitative incorporation of the protic initiator in the polymer chains and showed that transesterification reactions did not occur to a significant extent. The controlled character of the polymerization was indicated by the linear relationships of the number-average molar mass vs monomer conversion and monomer-to-initiator ratio. Kinetic measurements revealed a first-order dependence on monomer concentration. Moreover, no noticeable epimerization of the stereogenic carbon atom was observed in these polymerization conditions, as deduced from 1H NMR analysis of a poly(l-lactide) sample.