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 14,568 g mol(-1) and exhibited relatively broad molecular weight distributions associated with transesterification reactions.

Compounds N‐(1H‐benzimidazol‐2‐ylmethyl)aniline (L1), N‐(1H‐benzimidazol‐2‐ylmethyl)‐2‐bromoaniline (L2), and N‐(1H‐benzimidazol‐2‐ylmethyl)‐2‐aminothiophenol (L3) react with ZnII and CuII carboxylates to form complexes [Zn2(L1)2(OBn)4] (1), [Zn2(L2)2(OBn)4] (2), [Zn2(L3)2(OBn)4] (3), [Cu2(L2)2(OBn)4] (4), [Zn(L1)2(OAc)2] (5), [Zn(L2)2(OAc)2] (6) and [Cu2(L1)2(OAc)4] (7). Structures of 2, 4 and 6 revealed that L1–L3 are monodentate, binding through the imidazolyl N‐atom. The X‐band EPR spectrum of 4 in the solid state is consistent with an antiferromagnetically‐coupled (singlet) ground state and a low‐lying EPR‐active triplet excited state characterised by two main transitions. In dimethyl sulfoxide (DMSO) solution, a single resonance confirmed the retention of the dinuclear paddlewheel core. Complexes 1–7 formed active catalysts towards ring‐opening polymerisation of ϵ‐caprolactone. The polymerisation reactions follow first‐order kinetics with respect to the monomer and occur through a coordination–insertion pathway.

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