Structural and kinetic studies of the ring opening polymerization of lactides and ε-caprolactone by (Benzimidazolylmethyl)amine ZN(II) and Cu(II) complexes.

Abstract

The syntheses and structural characterization of Zn(II) and Cu(II) carboxylate complexes supported on benzimidazolyl amine ligands and their application in the ring opening polymerization of lactides and ɛ-caprolactone are reported. Synthesized derivatives of benzimidazole, N-(1H-benzimidazol-2-ylmethyl)aniline (L1), N-(1H-benzimidazol-2-ylmethyl)-2-methoxyaniline (L2), N-(1H-benzimidazol-2-ylmethyl)-2-bromoaniline (L3) and N-(1H-benzimidazol-2-ylmethyl)-2-aminothiophenol (L4) were reacted with Zn(II) and Cu(II) carboxylates to generate bimetallic complexes [Zn2(L1)2(OBn)4] (1), [Zn2(L2)2(OBn)4] (2), [Zn2(L3)2(OBn)4] (3), [Zn2(L4)2(OBn)4] (4), [Cu2(L1)2(OBn)4] (5), [Cu2(L3)2(OBn)4] (6), [Cu2(L1)2(OAc)2] (9) and monometallic complexes [Zn(L1)2(OAc)2] (7) and [Zn(L3)2(OAc)2] (8) in good yields. The complexes were characterized by mass spectrometry, IR spectroscopy, elemental analysis and single crystal X-ray diffraction for 3, 6 and 8. In addition, Zn(II) complexes (1-4, 7 and 8) were characterized by NMR spectroscopy while the copper complexes (5, 6 and 9) were characterized by magnetic moment measurements due to their paramagnetic nature. The magnetic moments measured for the copper complexes 5, 6 and 9 were obtained as 1.85 BM, 1.86 BM and 2.12 BM respectively. The molecular structures of complexes 3, 6 and 8 as established by single-crystal X-ray crystallography revealed ligands L1-L4 are monodentate, binding via the N-atom of the benzimidazole. EPR spectra of the copper complexes 5 and 6 revealed that the paddle-wheel dimeric structures are retained in solution. The catalytic activities of the complexes in the ring opening polymerization of ɛ-caprolactone (ɛ–Cl), D,L-lactide (D,L-LA) and L-lactide (L-LA) were investigated. All the complexes tested were active in the polymerization of both ɛ-caprolactone and lactides. Generally, zinc complexes were more active than their analogous copper complexes. The nature of the carboxylate group also influenced the catalytic activities of the initiators. The acetate complexes showed higher activities than the corresponding benzoates. Catalyst activities of mononuclear complexes were also observed to be relatively higher compared to the binuclear complexes. Ligand architecture also influenced the activities of the complexes. The complexes bearing electron-withdrawing groups on their ligand motif showed lower activities compared to their corresponding complexes containing electron-donating groups. The lactide monomers showed greater reactivity compared to ɛ-caprolactone. The polymerization of the monomers proceeded via a pseudo-first order reaction pathway with respect to the monomer. All the tested complexes gave rise to moderate molecular weight polymers in the range of 2 000 to 12 700 g/mol and broad polydispersity index in the range of 1.29-3.64. The methine region of the 1H and 13C NMR of the polylactides revealed the polymerization of L-lactide yielded isotactic polylactide while D,L-lactide afforded predominantly atactic polymers. The tacticity of poly(D,L-lactide) indicates that the ligand architecture does not regulate the stereoselectivity of the complexes. From the analysis of the 1H NMR spectra and MS-ESI data of ɛ-caprolactone and lactide polymers, the polymerization reaction proceeded via an activated-monomer mechanism.