Ring-opening polymerization of ε-caprolactone mediated by a di-zinc complex bearing a macrocyclic thioether-phenolate [OSSO]-type ligand

Abstract

A unique example of zinc bromide complexes bearing a macrocyclic [OSSO]-type thioetherphenolate ligand (Di-[OSSO]ZnBr) has been successfully explored toward ring-opening polymerization (ROP) of ε-caprolactone (ε-CL) in the presence of epoxides and onium salts. The catalytic efficiency is calculated from the turnover frequency (TOF) up to 81 840 per h per Zn (equiv. to 6.86 kg g⁻¹ Di-[OSSO]ZnBr, 20 min), markedly surpassing those of conventional zinc analogues. A plausible coordination–insertion mechanism is proposed in line with the kinetic studies and the spectroscopic analyses such as NMR, ESI-MS, and GC-MS. The key step is the formation of Zn-alkoxide (Zn-OR) species resulting from the nucleophilic ring opening of a coordinated epoxide by the halide ion of onium salts (cocatalysts), where Zn-OR is a real active center. The induction period of ROP of ε-CL remarkably depends on the epoxides and onium salts used. Of interest is that benign ethylene carbonate could also be used as a co-initiator which forms EO via decarboxylation in situ. This study provides insights into the development of more efficient catalysts.