Dalton Transactions

The structural and kinetic studies of polymerization reactions of ε-caprolactone (ε-CL) using (pyrazolylmethyl)pyridine Cu(II) and Zn(II) complexes as initiators is described. Reactions of 2-(3,5-dimethylpyrazol-1-ylmethyl)pyridine (L1) and 2-(3,5-diphenylpyrazol-1-ylmethyl)pyridine (L2) with Zn(Ac)₂·2H₂O or Cu(Ac)₂·2H₂O produced the corresponding complexes [Zn(Ac)₂(L1)] (1), [Cu(Ac)₂(L1)] (2), [Zn(Ac)₂(L2)] (3) and [Cu₂(Ac)₄(L2)₂] (4) respectively. Solid state structures of 1 and 4 confirmed that complexes 1 and 4 are monomeric and dimeric respectively and that L1 is bidentate in 1 while L2 is monodentate in 4. X-band EPR spectra of 2 and 4 revealed that complex 2 is monomeric both in solid and solution state, while the paddle-wheel structure of 4 is retained in solution. Complexes 1–4 formed active initiators in the ring opening polymerization of ε-CL. Zn(II) complexes 1 and 3 exhibited higher rate constants of 0.044 h⁻¹ and 0.096 h⁻¹ respectively compared to rate constants of 0.017 h⁻¹ and 0.031 h⁻¹ observed for the corresponding Cu(II) complexes 2 and 4 respectively at 110 °C. All the polymerization reactions follow pseudo first-order kinetic with respect to ε-CL monomer. Initiator 1 showed first-order dependency on the polymerization reactions and utilizes only one active site as the initiating group. The molecular weights of the polymers produced range from 1982 g mol⁻¹ to 14568 g mol⁻¹ and exhibited relatively broad molecular weight distributions associated with transesterification reactions.