Ring Opening Polymerization of ω-Pentadecalactone by Weakly Oxophilic Fe(II)-based Catalytic Systems Bearing Bulky α-Diimine Ligands

High catalytic efficiencies in ring opening polymerization (ROP) of a large ring-sized macrolactone, ω-pentadecalactone (PDL), by using transition metal Fe(II)-based catalysts were achieved for the first time in this study. Benefited from the bulky nature of the ligated α-diimine ligands, as evidenced from single-crystal structures, as well as the weakly oxophilic nature of the metal centers, chain transesterification reactions could be partially suppressed, allowing the polymerization proceed in a living-like and semi-controllable manner, i.e. good linear dependence of propagation rates on catalyst concentration and PDL concentration as observed in the detailed kinetics studies. The whole polymerization proceeds via a “coordination-insertion” mechanism, and with the aid of density functional theory (DFT) calculation studies, a “slow insertion → fast elimination” manner was demonstrated for the monomer propagation step, suggesting the insertion of Fe-OR into the carbonyl group C=O as the rate-determining step. The present catalytic system also showed fast chain transfer reactions to alcohol compounds, affording quasi-immortal characteristics. DFT calculations showed that such a transfer reaction only required an energy barrier of 6.4 kcal/mol, performing a good consistency with the fast chain transfer rates.