Polylactones, 8. Mechanism of the cationic polymerization of L,L‐dilactide

Abstract Various acidic compounds were tested as potential initiators of the cationic polymerization of L,L ‐dilactide. Regardless of solvent and temperature only trifluoromethanesulfonic acid and methyl triflate proved to be useful initiators. With methyl triflate as initiator L,L ‐dilactide polymerizes faster than diglycolide, whereas diglycolide is more reactive when anionic or complexing initiators are used. Initiation with methyl triflate yields poly( L ‐lactide) with methyl ester endgroups indicating that the chain growth proceeds via cleavage of the alkyl‐oxygen bond. At polymerization temperatures > 100°C the resulting poly( L ‐lactide) is 100% optically active indicating that the chain growth does not proceed via free carbenium ions, but via triflate ester endgroups. All attemps to identify cyclic dioxocarbenium ions as reaction intermediates failed, because propagation is much faster than initiation. However, in nitromethane oligomers with triflate ester endgroups were detected. For comparison the reaction of γ‐valerolactone with methyl triflate was studied by means of IR and 1 H NMR spectroscopy. Methylation of the exocyclic oxygen yielding a cyclic dioxocarbenium ion was found to be the sole reaction.