On the copolymerization of monomers from renewable resources: L-lactide and ethylene carbonate in the presence of metal alkoxides

The copolymerization of L-lactide with ethylene carbonate carried out at 120 degrees C in the presence of organic derivatives of aluminum, zinc and tin as catalysts leads to the formation of linear polymers of relative molecular masses in the 10 000-30 000 range containing up to 12 mol. % of carbonate monomeric units (m.u.). A decrease in T g of copolymers down to 48 degrees C demonstrates the internal plasticization effect for polylactide matrix. In the first reaction step mainly lactide homopolymerization proceeds in the systems studied. Carbonate m.u. incorporate into the growing chain as a result of recurring transesterification processes. Backbiting type intramolecular transesterification reactions dominate in the system and macrocycle compounds containing 1 carbonate m.u. and 3-9 lactic acid m.u. are the main reaction products after prolonged time. On the basis of analysis of the low molecular weight products formed in the polymerization system and products of the model reaction of ethylene carbonate with metal alkoxides, the mechanism of elementary reactions leading to the incorporation of carbonate m.u. into the copolymer chains has been proposed.