High T m Poly( l ‐lactide)s by Means of Bismuth Catalysts?

One series of BiSub‐catalyzed ring‐opening polymerizations (ROPs) is performed at 160 °C for 3 days with addition of difunctional cocatalysts to find out, if poly( l ‐lactide) crystallizes directly from the reaction mixture. An analogous series is performed with monofunctional cocatalysts. High T m crystallites (T m  > 190 °C) are obtained from all bifunctional cocatalysts, but not from all monofunctional ones. It is shown by means of SAXS measurements that the high T m values are mainly a consequence of a transesterification–homogenization process across the lamellar surfaces resulting in thickness and smoothing of the surfaces. An unusual enthalpy‐driven modification of the molecular weight distribution is found for samples that have crystallized during the polymerization. A third series of ROPs is performed at 170 °C for 2 h followed by annealing at 120 °C (2 h) to induce crystallization. Complete transformation of the resulting low T m crystallites (T m  < 180 °C) into the high T m crystallites by annealing at 170 °C for 1 d is not achieved, despite variation of the cocatalyst.