The research focused on polylactide (PLA) type materials is a very active and expanding area of polymer science as these polymers are environmentally sustainable and have attractive material properties. The lactide (LA) monomer can be derived from renewable agricultural sources, and PLAs have many potential agricultural and packaging applications because of their inherent biodegradability and mechanical properties. PLA materials are also indispensable and widely used in biomedical and pharmaceutical applications such as sutures, artificial tissue networks, and drug-delivery agents as they undergo controllable hydrolysis/decomposition reactions without release of toxic byproducts. The degradation rate of the polymer matrix can be controlled by blending or by copolymerization of various LAs with, for example, poly(ε-caprolactone) or glycolide. Low-molar-mass PLAs can be directly prepared from lactic acid, but the most efficient way to produce high molar mass polymers in a controllable manner is by catalyst-assisted, ring-opening polymerization (ROP) of six-membered cyclic diesters. Sn(II) alkanoates and their derivatives are frequently used in LA polymerization, and they can be considered as standard initiator precursors for ROP. Sn(II) alkanoates have many advantages as they are easy to prepare and handle, and they polymerize LAs and lactones with moderate activity in the presence of additional alcohols. In recent studies concerning the polymerization mechanism of Sn(II) alkanoates, it was shown that alcohol activators create Sn–alkoxide bonds, which are prerequisite to initiate polymerization and that ROP proceeds via a coordination-insertion mechanism. The disadvantage of Sn(II) alkanoate-based initiators is that the required alcohols can also act as a chain transfer agent during the polymerization, which complicates the catalytic control. SnOct2 can also act as a strong transesterification agent, and copolymers with random microstructures are obtained. The amount and toxicity of initiator residues in the polymer are crucial parameters when certain material is aimed for biomedical applications. Therefore, initiators combining high activity with nontoxic decomposition products are highly desirable In this presentation we introduce the use of Sn(IV) and Bismuth (III) complexes as highly active initiators for the ROP of heterocyclic monomers.KEYWORDS: Initiators; polyesters; ring-opening polymerization; Biodegradable Acknowledgment: The author thanks the financial supports to Alexander von Humboldt Foundation and also the Hassan II Academy of Science and Technology (Morocco).