Zn(II) and Cu(II) formamidine complexes: structural, kinetics and polymer tacticity studies in the ring-opening polymerization of ε-caprolactone and lactides

Treatment of N,N'-bis(2,6-dimethylphenyl)formamidine (L1), N, N'-bis(2,6-diisopropylphenyl)formamidine (L2), and N,N'-dimesitylformamidine (L3) with Zn(OAc)(2)center dot 2H(2)O or Cu(OAc)(2)center dot H2O produced the corresponding Zn(II) and Cu(II) N,N'-diarylformamidine complexes [Zn-3(L1)(2)(OAc)(6)] (1), [Zn-2(L2)(2)(OAc)(4)] (2), [Zn-2(L3)(2)(OAc)(4)] (3) and [Cu-2(L2)(2)(OAc)(4)] (4), respectively. While complex 1 is trinuclear, compounds 2-4 are dimeric in the solid state. The X-band EPR spectra of complex 4 in solid and solution states are consistent with perfect axial symmetry and confirm retention of the dinuclear paddle-wheel core in the solution state. Complexes 1-4 formed active catalysts in the ring opening polymerization (ROP) of epsilon-caprolactone (epsilon-CL) and lactides (LA). Complexes 1 and 3 exhibited higher rate constants of 0.1009 h(-1) and 0.0963 h(-1) compared to the rate constants of 0.0479 h(-1) and 0.0477 h(-1) observed for 2 and 4, respectively, in the ROP of epsilon-CL at 110 degrees C. Higher rate constants of 0.5963 h(-1) and 1.2962 h(-1) were obtained for complexes 1 and 3 in the ROP of LAs compared to those reported in the ROP of epsilon-CL at 110 degrees C. Activation parameters were determined as Delta H-double dagger = 25.08 kJ mol(-1) and Delta S-double dagger = -201.7 J K-1 mol(-1) for the ROP of epsilon-CL using 3. Investigation of the kinetics of polymerization of epsilon-CL and LAs revealed first order dependence of the polymerization reactions on monomer concentration. Moderate molecular weight polymers of up to 21 286 g mol(-1) exhibiting relatively moderate molecular weight distributions and moderately heterotactic PLAs with Pr up to 0.65 were obtained.