N-Heterocyclic Carbene-Based Zinc Complexes: Same Precursors for Different Lactide Ring-Opening Polymerization Mechanisms

Two zinc complexes bearing NHC ligands were synthesized and fully characterized by NMR spectroscopy. Their behavior in the Ring-Opening Polymerization (ROP) of lactides was investigated under several conditions of reaction. They showed to be efficient initiators, producing polylactides with molecular weights depending on the cocatalysts. Microstructural analysis of poly(rac-LA) by H-1 NMR spectroscopy revealed a slightly enriched-heterotactic PLAs with P-r values up to 0.66. Inspection of the kinetic parameters for L-LA and rac-LA allowed to compare the k(obs) of the studied complexes 1 and 2 under different conditions, and showed that propagations present the usual pseudo-first-order dependence on monomer concentration. Hence, some reactivity studies were carried out, addressed to unravel the nature of the active species in the polymerization processes conducted in the presence of different catalytic systems, namely zinc complexes alone and in combination with neutral or ionic cocatalysts.