Transesterification in Alcohol‐Initiated ROPs of l‐ and meso‐Lactide Catalyzed by Sn(II) and Sn(IV) Compounds at Low Temperatures

Abstract

The purpose of this study is to shed more light on the transesterification processes in alcohol‐initiated and tin(II) 2‐ethylhexanoate (SnOct2)‐catalyzed polymerizations of lactides at low or moderate temperatures. Ethanol‐initiated polymerizations are conducted in concentrated solutions at 80 °C and a strong dependence of even/odd equilibration on the alcohol/Sn ratio. Around or above 120 °C cyclization of poly(l‐lactide) via “backbiting” occurs as a third mechanism. However, poly(m‐lactide) shows a higher cyclization tendency and yields cyclics even at 100 °C. Combinations of ethanol and certain cyclic dibutyltin(IV) catalysts also yield cyclic oligomers of l‐lactide at 80 °C. Reaction conditions allowing for a total suppression of all transesterification reactions are not found, but even‐numbered poly(m‐lactide)s with a purity >95% are obtained at 70 or 60 °C.

Reversible polycondensations outside the Jacobson–Stockmayer theory and a new concept of reversible polycondensations

“…However, intermolecular transesterification reactions that broaden the MWD occur more frequently and at lower temperatures than back-biting, which was already reported previously. 36 That model reaction was performed at 120°C in such a way that L-lactide was polymerized by means of SnOct 2 + benzyl alcohol in the presence of preformed polylactide having blocked end groups (Ac-PLA-Et). Three series of additional experiments was performed in the present work.…”

Stereocomplexation of cyclic polylactides with each other and with linear poly(l-lactide)s

“…2. 48,49 Since the hydrodynamic volume of linear (but not of cyclic) polylactides is higher than that of polystyrene, polystyrene calibrated SEC measurements overestimate the real molecular weights by approximately 50%. When the measured number average molecular weights (M n 's) were multiplied with the correction factor 0.68, they were quite similar to those of the CI cycles.…”

About the transformation of low Tm into high Tm poly(l-lactide)s by annealing under the influence of transesterification catalysts

“…Two examples for such an application have recently been published by the authors. 39,40 The mass spectra of the SnBiph-catalyzed polylactides revealed the following trends. At relatively low temperatures of 130 C and 140 C the mass spectra remain unchanged up to 48 h but display a considerable transformation of cyclic into linear chains aer annealing for 8 d (Fig.…”