Synthesis of aliphatic polyesters by controlled ring-opening polymerization of cyclic esters. Characterization,properties, transesterification reactions

The polymerization of (D,L)-Lactide at room temperature in solution using Y(OCH₂CH₂OiPr)₃ and Ln₅(μ-O)(OiPr)₁₃ with Ln = La, Sm, Y, Yb as initiators was studied. According to kinetic data, a controlled type polymerization is observed for most of the systems. La μ-oxoisopropoxide is the most reactive initiator, but leads to a substantial broadening of molecular weight distribution at high conversions. The nature of transesterification reactions was studied by SEC, ¹³C NMR and MALDI-TOF MS techniques. For Sm and Y-μ-oxo initiators only limited intermolecular ester exchange occurs, while with La μ-oxoinitiator and with Y(OCH₂CH₂OiPr)₃ initiator, but after a long time of the polymerization reaction, both inter and intramolecular transesterification occur with formation of cyclics.