Polyesters and poly(ester carbonates) were synthesized via ring-opening polymerization with new tin(II) macroinitiator adducts containing oligomeric (L)-lactide (LLA,), rac-lactide (rac-LA), and is an element of-caprolactone (CL). The novel initiating species were synthesized by the reaction of LLA, rac-LA, or CL with Sn(OEt)(2) (monomer concentration/initiator concentration less than or equal to20) and then were dissolved in methylene chloride or toluene and stored in a stoppered flask for the subsequent ring-opening polymerization of cyclic esters and carbonates. The soluble tin alkoxide macroinitiators yielded predictable and quantitative initiation of polymerization for up to 1 month of storage time at room temperature. The resulting polymers displayed low polydispersity (less than or equal to 1.5), and a high monomer conversion (greater than or equal to95%) was obtained within relatively short polymerization times (less than or equal to2 h). Adjusting the monomer/macroinitiator ratio effectively controlled the molecular weights of the polymers. NMR was used to characterize the initiating species and polymer microstructure, and size exclusion chromatography was used to determine the molecular weight properties of the polymers.