Finally, we investigated other monomers in the copolymerization with l-lactide at 50°C with PD 4-tBu Zn2I3 (1, Table ). Despite the lack of homopolymerization, we did see moderate incorporation of g-butyrolactone and dvalerolactone and high incorporation of glycolide (entries 2, 4, and 5). Sequence analysis by C NMR resulted in a nearly random copolymer with modest alternating selectivity (Section S7.7). Glycolide/lactide copolymers are useful as biomedical materials. As above, DSC and NMR analyses of isolated copolymers are highly consistent with statistical copolyesters. Our mechanistic analysis clearly shows that copolymerization occurs by first lactide homopolymerizaton and then e-caprolactone incorporation occurs by transesterification. Thus, a time-conversion study on the copolymerization of e-caprolactone and l-lactide with PD 4-tBu Zn2I3 (1) at 50°C gave us 100% conversion of l-lactide before the first aliquot at 30 minutes, while e-caprolactone was more slowly consumed over 24 hours (Figure , Section S7.10). Transesterification also explains the moderate dispersities and molecular weights obtained during copolymerization. Nevertheless, we can rule our transesterification between homopolymers of l-lactide and e-caprolactone; we performed a control experiment with poly(lactic acid), poly(caprolactic acid), and PD 4-tBu Zn2I3 (1) under the same conditions and did not obtain a copolymer (Section S7.9). Instead, transesterification must be between lactone copolymer and poly(lactic acid). To further characterize this process, we performed transesterification between commercial poly(l-lactic acid) and comonomers using PD 4- tBu Zn2I3 (1) at 50°C (Table .4). With e-caprolactone, this procedure gave high comonomer incorporation (88%) and a high R-value (1.34, entry 1). This sequence selectivity also distinguishes our system from others that produce copolymers by transesterification. Although many other catalysts were reported to give poly(lactic-cocaprolactic acid) copolymers by transesterification, 81 their sequence selectivity is either blocky or random but never highly alternating. Accordingly, the simple catalyst ZnI2 gave us a nearly random lactide/caprolactone copolymer (Table , entry 10). Characterizing the basis for this unusual sequence selectivity in our dizinc complexes remains an ongoing interest of our laboratory. We did not obtain a copolymer with b-butyrolactone or wpentadecalactone by this method (entries 2 and 4). But we did obtain copolymers with moderate incorporation of gbutyrolactone (16%), d-valerolactone (6%), and glycolide (15%; entries 3, 5, and 6). This process could be used to tune the properties of PLA after large-scale production.