Structural and Mechanistic Studies of Bis(phenolato)amine Zinc(II) Catalysts for the Polymerization of ε-Caprolactone

The syntheses, characterization, ε-caprolactone (CL) polymerization activity, and kinetics investigation of two zinc(II)
bis(phenolato)amine complexes L2Zn2 are reported (L = methylamino-N,N-bis(2-methylene-4,6-di-tert-butylphenolato)
and/or methylamino-N,N-bis(2-methylene-4-adamantyl-6-tert-butylphenolato)). X-ray crystallographic and 1H NMR
studies, including NOESY and PGSE experiments, provided insight into the solid and solution state structures,
respectively, as well as evidence for the catalytically active species responsible for the ring-opening polymerization
of CL. Additionally, solution polymerizations and kinetic experiments involving (L1)2Zn2 in the presence of benzyl
alcohol (BnOH) were performed to elucidate the influence of catalyst structure, solvent, and the concentration
dependence of the catalytically active species, CL, and BnOH on the rate and control of poly-ε-caprolactone (PCL)
formation. The structural, polymerization, and kinetic data support equilibria involving both mononuclear and dinuclear
forms of (L1)2Zn2 as well as a monomer-activated route to PCL.