Electronic and steric effects of substituents in series of Zn2+ asymmetrical bis-Schiff-base ligands complexes on catalytic ring-opening copolymerization of CHO and MA

Abstract

Based on the half-unit Schiff-base ligand HL and series of asymmetrical bis-Schiff-base ligand H2Ln (n = 1–6) in situ synthesized from the reaction of 1-phenyl-3-methyl-4-benzoyl-5-pyrazolone (PMBP), o-phenylenediamine, and/or different salicylaldehyde derivatives, complexes [Zn(L)2] (1) and series of [Zn(Ln)] (n = 1–6, 2–7) are obtained, respectively. Complex 1 proves to be inactive, resulting from its saturated octahedral coordination environment around the central Zn2+ ion, while in complexes 2–7, the unsaturated 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). As to the electron and steric effect in complexes 2–7 on the catalytic behaviors, the copolymerization results show that the introduction of withdrawing –Br substituent para to the phenoxide group endows higher reactive activity, while the bulkiness effect of the encumbering substituent (–Br, –OMe or tert-butyl) ortho to the phenoxide groups is in favor of the chain growth of the copolymers. Moreover, all the bulk copolymerizations in 2-7/DMAP (4-(dimethylamino)pyridine) systems afford poly(ester-co-ether)s, while lower catalyst and co-catalyst concentration for 3/DMAP system is helpful for the formation of perfectly alternating polyester in solution. Of the three co-catalysts, DMAP is found to be the most efficient, while an excess thereof is detrimental for the chain growth of the copolymers.