About formation of cycles in Sn(II) octanoate-catalyzed polymerizations of lactides

At first, formation of cycles in commercial poly(l-lactide)s is discussed and compared with benzyl alcohol-initiated polymerizations performed in this work. This comparison was extended to polymerizations initiated with 4-cyanophenol and pentafluorothiophenol which yielded cyclic polylactides via end-biting. The initiator/catalyst ratio and the acidity of the initiator were found to be decisive for the extent of cyclization. Further polymerizations of l-lactide were performed with various diphenols as initiators/co-catalysts. With most diphenols, cyclic polylactides were the main reaction products. Yet, only catechols yielded even-numbered cycles as main reaction products, a result which proves that their combination with SnOct₂ catalyzed a ring-expansion polymerization (REP). The influence of temperature, time, co-catalyst, and catalyst concentrations was studied. Four different transesterification reactions yielding cycles were identified. For the cyclic poly(l-lactide)s weight average molecular weights (M_w's) up to 120,000 were obtained, but ¹H NMR end group analyses indicated that the extent of cyclization was slightly below 100%. The influence of various parameters like structure of initiator and catalyst and temperature on the formation of cyclic poly(l-lactide)s has been investigated. Depending on the chosen conditions, the course of the polymerization can be varied from a process yielding exclusively linear polylactides to mainly cyclic polylactides. Three different reaction pathways for cyclization reactions have been identified.