SnOct2-catalyzed and alcohol-initiated ROPs of L-lactide – About the influence of initiators on chemical reactions in the melt and the solid state

[Display omitted] •The established mechanism of SnOct2-catalyzed and alcohol-initiated ROPs of L-lactide needs modification.•The structure of the initiator has strong influence on transesterification in the melt and thus on the molecular weight distribution.•The end groups formed by the initiators also have an influence on transesterification reactions in the solid state.•These transesterification reactions involve the mobile and immobile amorphous phases, but not the interior of the crystallites. SnOct2 (Sn(II) 2-ethylhexanoate) catalyzed ROPs of L-lactide were performed in bulk with eight different alcohols as initiators. The time was varied between 1 h and 24 h for all initiators. For two initiators the temperature was also lowered to 115 °C. Even-numbered chains were predominantly formed in all polymerizations at short times, but the rate of transesterification (e.g. even/odd equilibration) and the molecular weight distribution were found to depend significantly on the nature of the initiator. Observed transesterification reactions also continued in solid poly(L-lactide), and with the most active initiator, almost total equilibration was achieved even at 130 °C. This means that all chains including those of the crystallites were involved in transesterification reactions proceeding across the flat surfaces of the crystallites. The more or less equilibrated crystalline polylactides were characterized by DSC and SAXS measurements with regard to their melting temperature (Tm), crystallinity and crystal thickness.