Ring-opening polymerization of lactide, ε-caprolactone and their copolymerization catalyzed by β-diketiminate zinc complexes

A series of zinc silylamido complexes bearing non-symmetric beta-diketiminate ligands were synthesized and structurally characterized. Ring-opening polymerization (ROP) of rac-lactide catalyzed by these zinc complexes afforded heterotactic polylactides at room temperature (P-r = 0.79 similar to 0.83 in THF). The steric and electronic characteristics of the ancillary ligands showed significant influence on the polymerization performance of the corresponding zinc complexes. All these zinc complexes also showed moderate activities toward the polymerization of epsilon-caprolactone at ambient temperature in toluene, producing polycaprolactones (PCLs) with high molecular weights and moderate polydispersities. PCL-b-PLLA copolymers could be obtained via three different copolymerization strategies (one-pot polymerization, and sequential addition of the two monomers in either order) by adopting complex 6 as the initiator through the adjustment of reaction temperatures. The diblock nature of the copolymers was confirmed by C-13 NMR spectroscopy and DSC analysis.