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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 (Tm’s up to 201.2 °C) and melting enthalpies (𝚫Hm’s up to 106 J g−1)) are found confirming that 𝚫Hmmax, the 𝚫Hm of a perfect crystal, is around or above 115 J g−1 in contrast to literature data.