Next, we interrogated the reaction between PD Me2 Zn2I3 (8) and propylene oxide to gain insight into the active catalyst. We did not use PD 4-tBu Zn2I3 (1) for this study because its NMR data was less easily interpreted. Unfortunately, we did not isolate any new complex by vacuum drying a mixture of PD Me2 Zn2I3 (8) and propylene oxide, merely recovering pure PD Me2 Zn2I3 (8). Instead, NMR analysis of a solution of PD Me2 Zn2I3 (8), propylene oxide, and CD2Cl2 did show a possible initiator, which had the same symmetry as 8 but included two equivalents of ringopened epoxide (Scheme 2B, Section S4.3). This 2:1 stoichiometry of epoxide to complex is consistent with our GPC results which showed two active chains per zinc complex. To rule out the formation of a simple zinc alkoxide like Zn(OCH(CH3)CH2I)2, we also performed a control experiment with ZnI2, propylene oxide, and CD2Cl2 under the same conditions. The latter mixture did not give the same epoxide-opened signals as with PD Me2 Zn2I3 (8) and instead gave poly(propylene oxide) (Section S4.4). Scheme 2. Alkoxide models of the active catalyst. A) Preparation and validation of stable dizinc alkoxide 18 as an active catalyst for lactide polymerization. B) Plausible active catalyst under reaction conditions based on NMR and modeling.