The homoleptic bis(β-quinolylenolate) zinc catalysts for the ring-opening polymerization of ε-caprolactone: Kinetics and mechanism

Abstract

A series of bis(β-quinolylenolate) zinc complexes (L2Zn) 1-5 (L = [(2-C9H6N)–CH=C(R)–O–], R = tBu (1), Ph (2), o-tolyl (3), p-tolyl (4), p-OMePh (5)), have been structurally characterized and used as initiators in the ring-opening polymerization (ROP) of ε-caprolactone (ε-CL). The molecular structures of 3 and 4 were defined by X-ray diffriaction analyses, showing a distorted-tetrahedral geometry around the zinc center. All complexes are stable in air and high temperature, and they efficiently catalyzed the ROP of ε-CL with high conversions in a controlled manner. Kinetic studies showed that polymerization reaction catalyzed by 1-5 proceeded with first-order dependence on the monomer and their catalytic activity is correlated with the substituents on the Ar moieties of the ligand. Complex 3 displayed the higher activity than others, might be due to its stronger electron-donating nature of the ortho-Me group on the aryl ring (Ar) of the enamino framework than that of orther sites, however, complex 2 without substituent on the Ar group exhibited poor activity in the polymerization reaction. The resultant PCL was a mixture of linear BnO- and MeO-capped structures.