TY - JOUR A1 - Weidner, Steffen A1 - Kricheldorf, H. R. T1 - Spirocyclic bisphenoxides of Ge, Zr, and Sn as catalysts for ring-expansion polymerizations of L- and meso-lactide T2 - Journal of Polymer Science A: Polymer Chemistry N2 - Spirocyclic phenoxides of germanium, zirconium, and tin were prepared from 2,20-dihydroxybiphenyl and 2,20-dihydroxy-1,10-binaphthyl. Ring-expansion polymerizations of L-lactide are mainly studied at 160 or 180 °C. The reactivity of the catalysts increases in the order: Zr < Ge < Sn. Regardless of catalyst, the weight-average molecular weights (Mw) never exceed 50,000 g mol−1. The resulting poly(L-lactide)s are optically pure and have a cyclic architecture. Decreasing temperature and time favor Formation of even-numbered cycles, and at 102 ° cyclics, almost free of odd-numbered rings are obtained. Analogous polymerizations of meso-lactide give similar results >120 °C, but different results at 100 or 80 °C. Surprisingly, bell-shaped narrow molecular weight distributions are obtained <140 °C, resembling the pattern of living polymerizations found for alcohol-initiated polymerizations. An unusual transesterification mechanism yielding narrow distributions of odd-numbered cycles is discovered too. PB - Wiley Periodicals KW - Cyclization KW - Polylactides KW - MALDI-TOF MS KW - Ring-opening polymerization KW - Spirocyclic PY - 2018 UR - https://opus4.kobv.de/opus4-bam/frontdoor/index/index/docId/46498 AN - OPUS4-46498 SN - 0887-624X SN - 1099-0518 VL - 56 IS - 24 SP - 2730 EP - 2738 AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany