Ring-opening copolymerization of CHO and MA catalyzed by mononuclear [Zn(L2)(H2O)] or trinuclear [Zn3(L2)2(OAc)2] complex based on the asymmetrical bis-Schiff-base ligand precursor

• Mononuclear or trinuclear Zn2+-containing catalysts based on asymmetrical bis-Schiff-base precursor.
• Polyester or poly(ester-co-ether) polymers from CHO and MA.
• Effect of coordination environments of metal active species on catalytic behaviors.

Based on the half-unit Schiff-base ligand precursor HL1 and the asymmetrical bis-Schiff-base ligand precursor H2L2 synthesized from the reaction of 1-phenyl-3-methyl-4-benzoyl-5-pyrazolone (PMBP), o-phenylenediamine and/or o-vanillin, three complexes containing low toxicity Zn2+ ions, mononuclear [Zn(L1)2] (1), [Zn(L2)(H2O)] (2) and trinuclear [Zn3(L2)2(OAc)2] (3), are obtained, respectively. Complex 1 proves to be inactive, resulting from its saturated octahedral coordination environment around the central Zn2+ ion, while in complex 2 or 3, the unsaturated five and/or four-coordinate coordination environment for the catalytic active centers (Zn2+ ions) permits the monomer insertion for the effective bulk or solution copolymerization of CHO (cyclohexene oxide) and MA (maleic anhydride). All the bulk copolymerizations afford poly(ester-co-ether)s, while some of the solution copolymerizations produce perfectly alternating polyester copolymers. Moreover, higher polymerization temperature, lower catalyst and co-catalyst concentration and shorter reaction time are helpful for the formation of alternating copolymers in bulk or solution copolymerization. Of the three co-catalysts, DMAP (4-(dimethylamino)pyridine) is found to be the most efficient, while an excess thereof is detrimental for chain growth of the copolymers.

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