We report a new class of dizinc halide complexes PD R Zn2X3 and [PD R Zn2X2] + based around binucleating bis(pyrazolyl)alkane ligands PD R H (2-R and 4). The iodide complexes give active l-lactide polymerization catalysts, with PD 4-tBu Zn2I3 (1) giving the highest activity. PD 4-tBu Zn2I3 (1) provides copolymers of l-lactide and ecaprolactone with the highest degree of alternation (R = 1.36) reported with a main-group catalyst. Comparison with model compounds demonstrates cooperativity in polymerization and provides insight into the active catalyst. Copolymerization occurs by transesterification of ecaprolactone (along with g-butyrolactone, d-valerolactone, and glycolide) into PLA. This process is notable in its sequence selectivity, as all other catalysts known to give lactide/caprolactone copolymers by this mechanism show blocky or random selectivity. These results will inform the development of new biodegradable polyesters, potentially as improved biomedical materials or superior replacement for petrochemical polymers.