Katharina H. S. Eisenhardt

Catalyst design and understanding are improved by quantified structure–performance relationships, but these are underutilized in polymerization catalysis. In epoxide/carbon dioxide ring-opening copolymerization (ROCOP), an important route to make polymers from bio- and waste-based feedstocks, significant success has been achieved in understanding cobalt catalysts, but understanding of other metal catalysts is much less developed. This work targets earth-abundant bimetallic catalysts featuring iron and potassium. A series of seven Fe(III)K(I) catalysts, coordinated by different ancillary ligands, are compared with analogues Co(III)K(I) catalysts; the iron catalysts demonstrate both high activity (TOF = 675 h–1) and polymer selectivity (98%) in cyclohexene oxide/carbon dioxide copolymerization (20 bar CO2 pressure, 100 °C). The Fe(III)K(I) catalysts also show a linear free energy relationship between the copolymerization rates and the catalyst ligand-to-metal charge transfer (LMCT) wavelength, with a volcano relationship identifying the best catalysts. The most active catalysts have an Fe(III) center with an intermediate LMCT wavelength, and these features are consistent with the proposed copolymerization mechanism. In this field of catalysis, quantified structure-performance relationships are very rare; thus, the findings are fundamentally interesting and should help to accelerate the design of better-performing iron-based polymerization catalysts.

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.

Despite many CO ₂ use strategies being reliant on fast and selective CO ₂ insertion reactions into metal-alkoxide bonds, in-depth studies into this chemistry remain rare. Here the effect of CO ₂ pressure on the CO ₂ insertion chemistry is studied using epoxide–CO ₂ copolymerizations. Five high-performance literature catalysts are investigated under systematically varied CO ₂ pressures, revealing kinetic profiles indicative of CO ₂ insertion equilibria. For each catalyst, two key parameters describing the CO ₂ insertion chemistry are determined: the equilibrium constant, K _eq , and the saturation CO ₂ pressure above which catalytic performance is maximized, P _threshold . Generalizable correlations between copolymerization activity, K _eq and P _threshold are uncovered and used to predict performances for four further catalyst–monomer combinations. These correlations are a direct link between CO ₂ insertion chemistry and process operating conditions, providing a mechanistic framework and testing protocols to accelerate future catalyst development. These results should help deliver efficient scalable CO ₂ use technologies, operating with minimal energy.

Oxygenated block polymers, comprising esters and carbonates, are priority materials to replace petrochemical polymers in a circular plastics economy. These materials should repopulate the thermomechanical property space mapped by current plastics and elastomers. Here, a novel machine learning approach, PolyReco, predicts structures of oxygenated block polymers meeting the mechanical performance thresholds for widely used and hard-to-replace petroleum derived hydrocarbon polymers. Triblock oxygenated polymers are represented as graphs, and a link prediction algorithm enables feature extraction to identify new block polymer combinations, and associated degrees of polymerization, to meet the target properties. PolyReco is paired with a visualization tool for further material down selection based on user requirements. Three case studies highlight and experimentally validate its predictive power for identifying high-performance oxygenated block polymers, with new block polymers prepared and tested. These new block polymers exhibit tensile mechanical properties in the range of high-impact polystyrene, poly(dimethylsiloxane), and styrenic elastomers; the experimental results indicate that PolyReco may help support the identification of sustainable materials that could reduce dependence on fossil-based polymer incumbents.

Understanding and predicting the effect of epoxide structure on the rate of polymerization in epoxide/CO₂ ring opening copolymerization catalysis is a long-standing challenge. Here, a known highly active Co(III)K(I) catalyst is used to investigate the influences of six different epoxides' binding strengths on their rates of copolymerization. Since calculations and experiments indicate that studying the catalytically relevant Co(III)-epoxide adduct directly is experimentally challenging, epoxide-catalyst binding interactions are quantified using a Co(II)K(I) complex to model the key catalytic intermediate. Epoxide-catalyst coordination is investigated using UV-vis spectroscopy titrations which provide fast and effective determination of association or binding constants. The epoxide-catalyst equilibrium constants show a clear exponential correlation with copolymerization rates and a new catalyst performance linear free energy relationship is revealed. Epoxides exhibiting stronger catalyst binding constants show higher copolymerization rates. The structure-activity correlation is consistent with the polymerization kinetics, mechanism and DFT calculations. Both the methods to investigate epoxide-catalyst coordination and the linear free energy relationship are shown to apply to the series of six epoxides and a second Co(III)K(I) catalyst. These structure-performance relationships are likely applicable to other transition metal catalysts and should expedite future epoxide and catalyst selection to make useful poly(carbonate) materials.

Intermetallic synergy can be useful to both enhance catalytic performance and access diverse chemistries, but understanding and predicting the most effective metal combinations remains a challenge. This perspective focuses on a common class of homogeneous heteronuclear catalysts which combine transition metals (M(II/III)) with Group III, f-, or s-block metals (M’(I–III)) in phenoxy-imine-ether donor ligands. Independent investigations into these heteronuclear complexes across different fields of catalysis, including electrocatalysis, polymerization catalysis, and small-molecule asymmetric catalysis, have all demonstrated the benefits of intermetallic cooperativity. In both electrocatalysis and polymerization catalysis, similar quantified structure-performance relationships, relating catalytic performance to metal Lewis acidity, have been discovered. This selective perspective article focuses on highlighting the most recent insights and quantified structure–activity relationships in electro- and polymerization catalysis. We then apply the insights and methods, developed separately in these two fields of catalysis, to reanalyze previously reported data for catalytic asymmetric organic transformations. The same quantified structure-performance (i.e., activity or selectivity) trends appear to apply in this third field of homogeneous catalysis. These generally applicable, quantified structure–activity relationships apply to related synergistic heterometallic catalysts across all three disparate fields of chemistry ─ this finding is both unexpected and very helpful in providing a clearer understanding of how to design catalysts for successful intermetallic synergy. This perspective highlights both the fundamental understanding of such catalytic synergy and the analytical methods used to investigate it. The objective is to provide a guide for both current and future researchers to measure synergistic interactions and to apply those principles to rational catalyst design.

The ring-opening copolymerization of propene oxide and carbon dioxide is an effective route to make polycarbonates. Catalysts showing high activities, high polymer selectivities, molecular weight control, and tolerance to impurities are rare. Here, a series of four Co(III)Na(I) catalysts are tested for the ring-opening copolymerization of propene oxide and carbon dioxide. The complexes have systematic variations in the ligand structure in which both Co(III) and Na(I) binding sites are modified. Significant differences in catalyst performance are observed, with the lead catalyst featuring six oxygen donors and an ethylene diamine linker. This catalyst shows excellent activity and high poly(propene carbonate) selectivity at elevated temperatures (TOF = 1428 h^–1, poly(propene carbonate) selectivity = 98%, 20 bar CO₂, 70 °C), outperforming analogous Co(III)K(I) catalysts. These results inform on the optimal coordination environment and operating conditions for the Co(III)Na(I) catalysts and highlight the benefits of using Na(I) relative to heavier s-block metals in the heterodinuclear catalysts.

In homogeneous catalysis, uncovering structure–activity relationships remains very rare but invaluable to understand and rationally improve performances. Here, generalizable structure–activity relationships apply to a series of heterodinuclear polymerization catalysts featuring Co(III) and s‐block metals M(I/II) (M=Na(I), K(I), Ca(II), Sr(II), Ba(II)). These are shown to apply to polycarbonate production by the ring‐opening copolymerizations (ROCOP) of cyclohexene oxide (CHO) and carbon dioxide (CO₂), conducted at high (20 bar) and low (1 bar) CO₂ pressures, and to polyester production by copolymerization of cyclohexene oxide and phthalic anhydride (PA). For the CHO/PA and high‐pressure CHO/CO₂ copolymerizations, activity increases exponentially with s‐block metal acidity peaking at the Co(III)K(I) catalyst, whilst for the low‐pressure CHO/CO₂ copolymerization it increases linearly to the same metal combination. The polymerization kinetics fit second order rate laws and the correlations support dinuclear metallate mechanistic hypotheses. These relationships help understand and identify key metal complex structural features in synergic polymerization catalysis.

In homogeneous catalysis, uncovering structure–activity relationships remains very rare but invaluable to understand and rationally improve performances. Here, generalizable structure–activity relationships apply to a series of heterodinuclear polymerization catalysts featuring Co(III) and s‐block metals M(I/II) (M=Na(I), K(I), Ca(II), Sr(II), Ba(II)). These are shown to apply to polycarbonate production by the ring‐opening copolymerizations (ROCOP) of cyclohexene oxide (CHO) and carbon dioxide (CO₂), conducted at high (20 bar) and low (1 bar) CO₂ pressures, and to polyester production by copolymerization of cyclohexene oxide and phthalic anhydride (PA). For the CHO/PA and high‐pressure CHO/CO₂ copolymerizations, activity increases exponentially with s‐block metal acidity peaking at the Co(III)K(I) catalyst, whilst for the low‐pressure CHO/CO₂ copolymerization it increases linearly to the same metal combination. The polymerization kinetics fit second order rate laws and the correlations support dinuclear metallate mechanistic hypotheses. These relationships help understand and identify key metal complex structural features in synergic polymerization catalysis.

The ring-opening copolymerization (ROCOP) of epoxides with CO₂ or anhydrides is a promising strategy to produce sustainable polycarbonates and polyesters. Currently, most catalysts are reliant on scarce and expensive cobalt as the active center, while more abundant aluminum and iron catalysts often suffer from lower activities. Here, two novel heterodinuclear catalysts, featuring abundant Al(III), Fe(III), and K(I) active centers, are synthesized, and their performance in the polymerization of four different monomer combinations is compared to that of their Co(III) analogue. The novel Al(III)K(I) catalyst exhibits outstanding activities in the cyclohexane oxide (CHO)/CO₂ ROCOP, and at 1 bar CO₂ pressure it is the fastest aluminum-based catalyst reported to date. The M(III) site electronics for all three catalysts, Al(III)K(I), Fe(III)K(I), and Co(III)K(I), are measured using IR and NMR spectroscopy, cyclic voltammetry, and single crystal X-ray diffraction. A correlation between M(III) electron density and catalytic activity is revealed and, based on the established structure-activity relationship, recommendations for the future catalyst design of abundant Al(III)- and Fe(III)-based catalysts are made. The catalytic performances of both Al(III)K(I) and Fe(III)K(I) are further contextualized against the relative elemental abundance and cost. On the balance of performance, abundance, and cost, the Al(III)K(I) complex is the better catalyst for the carbon dioxide/epoxide ROCOP, while Fe(III)K(I) is preferable for anhydride/epoxide ROCOP.