Abstract
The model precatalyst sp(3)- and sp(2)-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 (LZnCl(2)) bearing N-diphenylphosphinoethyl pendants. Activated with two equivalents of LiN(SiMe(3))(2), the title zinc complexes efficiently promote the ROP of L-lactide (L-LA) in situ; among them, Zn4/2Li(NSiMe(3))(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(NSiMe(3))(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 CH(3)O as major and cyclic structure as minor. In this work, all the ligands and zinc complexes were well characterized by (1)H/(13)C/(31)P NMR, FT-IR spectroscopy as well as elemental analysis.