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Kricheldorf, Hans R.; Weidner, Steffen M.; Scheliga, Felix
Macromolecular chemistry and physics, March 2022, 20220301, Volume: 223, Issue: 6Journal Article
Ring‐opening polymerizations (ROP) of l‐lactide (LA) are conducted in bulk at 130, 160, or 180 °C and are initiated with two different alcohols at 160 °C. The lactide/initiator ratio (LA/In) is varied from 50/1 (20/1 at 180 °C) to 900/1 and the lactide/catalyst ratio (LA/Cat) between 2000/1 and 8000/1. At all temperatures a nearly perfect control of number average molecular weight (Mn) via the LA/In ratio is feasible up to LA/In = 200/1, but at higher ratios the Mn value lags behind the theoretical values and the discrepancy increases with higher LA/Cat ratios. Variation of the LA/Cat ratio influences the formation of cycles but does not significantly influence Mn, when the LA/In ratio is kept constant. The formation of cycles is favored by lower In/Cat ratios and is the main reason for the unsatisfactory control of Mn at high LA/In ratios. The results also suggest that the cycles are mainly or exclusively formed by end‐to‐end cyclization and not, as believed previously, by back‐biting. Alcohol‐initiated ROPs of l‐lactide in bulk at 130–180 °C allow for perfect control of the molecular weight up to lactide/initiator ratios around 200/1, whereas at higher ratios the real molecular weights are lower due to increasing cyclization.
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