Intensive Cycloalkyl-Fused Pyridines for Aminopyridyl–Zinc–Heteroimidazoles Achieving High Efficiency toward the Ring-Opening Polymerization of Lactides

The model precatalyst sp³- and sp²-N dinitrogen-coordinated zinc–heteroimidazole has been used as an efficient catalyst for the ring-opening polymerization of cyclic esters. Subsequent to our exceptional active 5,6,7-trihydroquinolin-8-amine-zinc catalysts for the ring-opening polymerization (ROP) of ε-caprolactone, various pyridine-fused cycloalkanones (ring size from five to eight) are developed for the correspondent fused amine–pyridine derivatives and their zinc–heteroimidazole chloride complexes Zn1–Zn8 (LZnCl₂) bearing N-diphenylphosphinoethyl pendants. Activated with two equivalents of LiN(SiMe₃)₂, the title zinc complexes efficiently promote the ROP of L-lactide (L-LA) in situ; among them, Zn4/2Li(NSiMe₃)₂ catalyzed 500 equivalent L-LA at 80 °C with 92% conversion in 5 min (TOF: 5520 h⁻¹). Under the same conditions, the catalytic efficiency for the ROP of rac-LA by Zn1–Zn8/2Li(NSiMe₃)₂ was slightly lower than that for L-LA (highest TOF: 4440 h⁻¹). In both cases, cyclooctyl-fused pyridyl–zinc complexes exhibited higher activity than others, while the cycloheptyl-fused zinc complexes showed the lowest activity. The microstructure analysis of the polymers showed they possessed a linear structure capped with CH₃O as major and cyclic structure as minor. In this work, all the ligands and zinc complexes were well characterized by ¹H/¹³C/³¹P NMR, FT-IR spectroscopy as well as elemental analysis.