Bis(3,5-dimethylpyrazole) copper(II) and zinc(II) complexes as efficient initiators for the ring opening polymerization of ε-caprolactone and d,l-lactide

Abstract

3,5-Dimethylpyrazole (L), reacts with [Cu(OAc)2] or [Zn(OAc)2]; and either C6H5COOH, 4-OH–C6H5COOH, 3,5-NO2–C6H5COOH or 2-Cl–C6H5COOH to afford the corresponding metal complexes: [Cu(C6H5COO)2(L)2] (1), [Cu(2-Cl–C6H4COO)2(L)2] (2), [Cu(3,5-NO2–C6H3COO)2(L)2] (3), [Cu(4-OH–C6H4COO)2(L)2] (4), [Zn(C6H5COO)2(L)2] (5), [Zn(2-Cl–C6H4COO)2(L)2] (6), [Zn(3,5-NO2–C6H3COO)2(L)2] (7), [Zn(4-OH–C6H4COO)2(L)2] (8). The diamagnetic zinc complexes were characterized by IR, NMR spectroscopy and elemental analysis; while the paramagnetic copper complexes were only characterized by IR spectroscopy, elemental analysis and 1 and 2 were further characterized by single crystal X-ray diffraction studies. All eight complexes are very good initiators for the ring opening polymerization (ROP) of ε-caprolactone and d,l-lactide at elevated temperatures, under solvent-free conditions and in toluene respectively; producing polycaprolactone (PCL) and poly(d,l-lactide) (PLA) of moderate molecular weights (ca 4757Da for PCL and 2484Da for PLA) and narrow polydispersity indices (1.36 for PCL and 1.42 for PLA). The 13C{1H} NMR spectra indicate the stereochemistry of PLA is heterotactic.