Ring opening polymerization of D,L-lactide in the presence of zinc metal and zinc lactate

The polymerization of D ,L -lactide in the presence of zinc metal was investigated for low values of the monomer/catalyst ratio in order to allow end-group characterization of the growing chains by high resolution 360MHz ¹H NMR. The results showed that the polymerization was moisture sensitive and that only a fraction of zinc was active. Small quantities of a side-product were detected and identified as zinc lactate. This compound appeared to be an efficient initiator of the ring-opening polymerization in the bulk. Zinc lactate is thus proposed as the actual initiator of the polymerization of 1,4-dioxane-2,5-diones in the presence of zinc metal. A polymerization mechanism based on a cationic process co-catalysed by lactic acid is also proposed, which accounted very satisfactorily for experimental data. Initiation by zinc lactate yielded high molecular weight polymers with a high degree of conversion and rather high polymerization rates. Therefore, zinc lactate appears to be an attractive alternative to the initiations by zinc metal and stannous octoate, the latter being suspected of some toxicity. © 1998 SCI.