Understanding Catalytic Synergy in Heterodinuclear Iron/s-Block Complexes for Epoxide/Anhydride Ring-Opening Copolymerization

Abstract Earth-abundant metal catalysts are important for future polymer production. Iron is both an earth-abundant and non-critical element and is attractive for catalysis. Here, a series of heterodinuclear catalysts incorporating Fe(III) with different s-block metals, M(I/II) = K(I), Na(I), Sr(II), Ba(II), an Fe(II)K(I) catalyst, and a monometallic Fe(III) catalyst are reported for the ring-opening copolymerization of cyclohexene oxide and phthalic anhydride to produce polyesters. The lead Fe(III)K(I) catalyst achieves a high turnover frequency of 3478 ± 51 h–1 (1:400:7000, [catalyst]0:[PA]0:[CHO]0, 140 °C) while retaining quantitative (>99%) polyester selectivity. Its performance is competitive with the best catalysts in the field. Using microscale calorimetry, the polymerization rate law, rate coefficients and transition state energies are determined. These data are rationalized by a dinuclear catalytic mechanism and allow for insight into the influence of varying catalyst structural features on the reaction enthalpy and entropy barriers. The results inform upon the physical origins of high catalytic activity and provide mechanistic insights useful for future catalyst design.