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 in vestigated for low values of the monomer/catalyst ratio in order to al low end-group characterization of the growing chains by high resolutio n 360 MHz H-1 NMR. The results showed that the polymerization was mois ture sensitive and that only a fraction of zinc was active. Small quan tities of a side-product were detected and identified as zinc lactate. This compound appeared to be an efficient initiator of the ring-openi ng polymerization in the bulk. Zinc lactate is thus proposed as the ac tual 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 conversi on and rather high polymerization rates. Therefore, zinc lactate appea rs to be an attractive alternative to the initiations by zinc metal an d stannous octoate, the latter being suspected of some toxicity. (C) 1 998 SCI.