Bis(pyrazolylmethyl)pyridine Zn(II) and Cu(II) complexes: Molecular structures and kinetic studies of ring-opening polymerization of ε-caprolactone

• Zn(II) and Cu(II) complexes synthesized and structurally characterized.
• All the complexes form active catalyst for the ROP of ε-CL.
• ROP reactions follow first-order kinetics.
• The polymers end groups contain OH from hydrolysis.

We report the synthesis, molecular structures and applications of bis(pyrazolylmethyl)pyridine Zn(II) and Cu(II) complexes in the ring-opening polymerization (ROP) of ε-carprolactone (ε-CL). Reactions of bis(3,5-dimethyl-pyrazol-1-ylmethyl)pyridine (L1) with the appropriate Zn(II) and Cu(II) carboxylates afforded the corresponding complexes; [Zn(L1)(C6H5COO)2] (1), [Zn(L1)(2-Br-C6H4COO)2] (2), [Cu(L1)(C6H5COO)2] (3) and [Cu(L1)(2-Br-C6H4COO)2] (4) in good yields. Solid state structures of complexes 1 and 2 reveal that the complexes are polymeric and monomeric respectively. EPR analyses of the paramagnetic Cu(II) complexes 3 and 4 were consistent with anisotropic coupling and confirm that the compounds are monomeric both in solution and solid states. Complexes 1–4 formed active catalysts for the polymerization of ε-CL at 110 °C to give polymers with moderate molecular weight and molecular weight distributions. Kinetic analyses of the polymerization reactions indicate pseudo first-order dependency on both monomer and catalyst.

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