We first evaluated the polymerization activity of our zinc complexes against l-lactide, at room temperature using propylene oxide as solvent (Table ). We assessed conversion at 10 minutes, 1 hour, and 4 hours and used the simple zinc salts ZnCl2, ZnBr2, and ZnI2 as controls. The polymerization of l-lactide showed a clear halide ion dependence; the chloride and bromide salts were unreactive under these conditions (entries 2-7) whereas the iodides showed varying levels of activity (entries 8-16). Among the iodides, the neutral complexes PD Me2 Zn2I3 (8) and PD 4-tBu Zn2I3 (1) were significantly more active than their cationic counterparts [PD Me2 Zn2I2][BArF] ( ) and [PD Zn2I2][BArF] (15) (entries 8-11). This result is consistent with a coordination/insertion mechanism for ROP, in which less nucleophilic cationic complexes are expected to be less reactive. Among the neutral iodide catalysts, PD 4-tBu Zn2I3 (1) was more reactive than its more sterically hindered analogues bearing 3-substituted pyrazoles PD Me2 Zn2I3 (8), PD Et2 Zn2I3 (9) and PD iPr Zn2I3 (10; entries 8, 12, and 13). This activity/substituent trend agrees with what we reported with our [PD R Zn2Et2] + series of catalysts, but not with our BINOL-based catalysts. The success of PD 4-tBu Zn2I3 (1) also shows that ligand PD 4-tBu H (4) can serve as a more soluble alternative to PD H H (2-H). All of our neutral dizinc triiodide complexes were more reactive than ZnI2 (entry 14), confirming that they are not simple sources of free Zn 2+ in these reactions. By contrast, the monometallic zinc iodide analogs P 4-tBu ZnI2 ( ) and [P 4-tBu HZnI][BArF] (16) were nearly unreactive under these conditions (entries 15 and 16), supporting our design principle for bimetallic cooperativity.