Hybrid amino/iminopyridines with auxiliary phosphines enhancing their zinc complex catalysts for the ring-opening polymerization of lactide

Abstract

The series of 2-(dipehnylethyl)amino-6-iminopyridines, 2-((Ph2PCH2CH2)N=CMe)-6-(ArN=CMe)C5H3N (Ar=2,6-iPr2C6H3 (L1), 2,6-Et2C6H3 (L2), 2,6-Me2C6H3 (L3), 2,4,6-Me3C6H2 (L4), 2,6-(CHPh2)2−4-MeC6H2 (L5)), and its analogue 2-(n-butyl)amino-6-iminopyridine L6 were synthesized and used to form their dichlorozinc complexes, LZnCl2 (Zn1-Zn6). All organic compounds and zinc complexes were well characterized by 1H/13C/31P NMR spectroscopy and elemental analysis along with the single crystal X-ray diffraction of Zn1 and Zn4 indicating bistrigonal geometry around zinc. By treated with two equivalents of LiN(SiMe3)2, all Zn1-Zn6 performed high activities toward ring-opening polymerization (ROP) of L-lactide (L-LA), indicating the positive influences of substituents as well as auxiliary phosphine. Notably, the excellent activity was achieved even at high temperature; such as, Zn1/2LiN(SiMe3)2 catalyzed 2000 equiv. L-LA with 86% conversion in 10 minutes at 100 ℃, observing its TOF as 10320 h−1. The activities were observed in the order of substituents as: isopropyl > ethyl > methyl, so were higher activities for more alkyl substituents. The auxiliary phosphine enhanced the activities of zinc complexes; all Zn1-Zn5 showed higher activities than Zn6 (with butyl), which formed the PLA with relatively lower molecular weight. The coordination–insertion and ring expansion mechanism has been proposed for the ROP of L-lactide.