About the Influence of (Non‐)Solvents on the Ring Expansion Polymerization of <scp>l</scp>‐Lactide and the Formation of Extended Ring Crystals
Hans R. Kricheldorf, Steffen M. Weidner, Andreas Meyer
Abstract
Abstract Ring‐expansion polymerizations (REPs) catalyzed by two cyclic tin catalysts (2‐stanna‐1.3‐dioxa‐4,5,6,7‐dibenzazepine [SnBiph] and 2,2‐dibutyl‐2‐stanna‐1,3‐dithiolane [DSTL]) are performed at 140 °C in bulk. Small amounts (4 vol%) of chlorobenzene or other solvents are added to facilitate transesterification reactions (ring–ring equilibration) in the solid poly( l ‐lactide)s. In the mass range up to m/z 13 000 crystalline PLAs displaying a so‐called saw‐tooth pattern in the MALDI‐TOF mass spectra are obtained indicating the formation of extended‐ring crystals. The characteristics of extended‐ring crystallites and folded‐ring crystallites are discussed. Furthermore, extremely high melting temperatures ( T m 's up to 201.2 °C) and melting enthalpies (Δ H m 's up to 106 J g −1) ) are found confirming that Δ H m max, the Δ H m of a perfect crystal, is around or above 115 J g − 1 in contrast to literature data.