However, sequence-selective copolymerization involving lactide is challenging because lactide coordinatively inhibits the polymerization of other lactones. Consequently, many lactide polymerization catalysts do not copolymerize monomer mixtures and do not polymerize lactones after lactide. Multistep methods that avoid monomer mixtures have been used to prepare block copolymers. But statistical copolymerization needs to overcome the unfavorable kinetics of lactide/lactone mixtures. Catalysts with partial selectivity for random copolymerization deliver blocky or gradient polymer macrostructures. In a major breakthrough, Nomura reported nearly random copolymerization of lactide and ecaprolactone with an aluminum catalyst in 2009. Subsequent work on random lactide/lactone copolymerization has focused on steric tuning of metal catalysts. Nevertheless, alternating copolymerization of lactide with lactones remains challenging, although alternating copolymers have unique and useful properties. Alternating copolymerization has been extensively studied for other classes of monomers, resulting in informative and useful materials. Among copolyesters, precisely alternating poly(lactic-co-glycolic acid) exhibits more controlled hydrolysis kinetics than random or blocky copolymers, and its thermal properties are consistent with a more homogeneous solid phase. These materials were prepared by stepwise segmer growth polymerization and selective polymerization of methyl glycolide or other specialty monomers (Figure ). Each of these methods has their drawbacks, as segmer growth offers limited molecular weight control and specialty monomers are more expensive than commodity lactide and lactones. Moreover, there are few examples of precisely alternating copolymers of lactide with other monomers. Meyer reported alternating copolymers of lactic acid and caprolactic acid made by segmer growth. Likewise, Tolman and Hillmyer reported alternating lactic acid/ethylene oxide copolymers made from a specialty monomer. Despite the practical advantages of preparing these materials by ring-opening polymerization of lactide and lactones, there are no reports of perfectly alternating copolymers obtained this way. The only example of highly alternating selectivity in lactide/e-caprolactone copolymerization was reported by Visseaux and Bonnet, using rare earth allyl borohydride catalysts (Figure ). Nevertheless, main-group metals are the preferred catalysts for lactone ring-opening polymerization because of their colorlessness, nontoxicity, and earth abundance. Yet main group catalysts have only ever provided modest alternating selectivity in lactide/lactone copolymerization, including Nomura's aluminum complex discussed above. In this report, we describe the most alternating lactide/e-caprolactone copolymer obtained with a maingroup catalyst, a dizinc triiodide complex of binucleating bis(pyrazolyl)alkanes (see comparison in Section S7.11). Dizinc complexes have provided great insight into maingroup catalyzed lactide polymerization, especially the mechanism of metal-metal cooperativity. Coates and Diaconescu showed that apparently mononuclear zinc complexes will aggregate during polymerization and that chain growth occurs through a binuclear mechanism. Rieger 45 and Williams each reported dizinc complexes of binucleating macrocycles that showed the highest activities reported for lactide ring-opening polymerization. Other notable dizinc polymerization catalysts using binucleating phenol ligands were studied by Tolman and Hillmyer, by Williams, and by Garden, providing further examples of bimetallic cooperativity and bifunctional insertion (Figure ). Nevertheless, these complexes show limited covalent flexibility, and rarely have a compelling monometallic analog for analysis of metallicity. Despite insight into lactide homopolymerization, these catalysts have not provided new polymerization selectivity or new materials. Our dizinc catalyst 1 provides exceptional alternating selectivity in lactone/lactide copolymerization (Figure ). Moreover, we report two instructive monometallic analogs of 1 that provide complementary insight into metal-metal cooperativity, and a useful model for the active catalyst.