Charlotte Katherine Williams

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.

Despite the importance of metal catalysts for cyclic ester and -carbonate ring-opening polymerizations, quantified and generally applicable catalyst structure-performance relationships remain elusive. Here, a generally applicable linear correlation between the metal active site Lewis acidity, as described by the metal cation pKh value, and the ring-opening polymerization rate is presented. The linear relationship applies to five different metal catalysts, each used to polymerize five different commercial monomers, including ε-caprolactone, δ-valerolactone, γ-methyl ε-caprolactone, trimethylene carbonate, and 2,2-dimethyltrimethylene carbonate. Using microscale calorimetry, the polymerization rate coefficients and transition state barriers are determined for polymerizations using commercial metal(2-ethylhexanoate)/benzyl alcohol catalyst systems, where the metal = Sn(II), Zn(II), Mg(II), Bi(III), Zr(IV). All polymerizations are quantified under industrially relevant conditions, i.e., at high temperatures (140–200 °C), low catalyst loadings (1:1000 metal:monomer), and either high concentrations ([monomer]0 = 6.25 M) or using neat monomer melts. The generally applicable linear free energy relationships, consistent with the coordination–insertion polymerization mechanism and supported by monomer spectroscopy characterization, reveal that Sn(II) is the best for cyclic ester polymerizations, while Zn(II) is the best for cyclic carbonate polymerizations. The linear free energy relationship predicts that l-lactide will be polymerized fastest using Sn(II) catalysts, a finding which is verified experimentally. It also enables improvements to the activity of Mg(II) catalysts, through ligand modifications, increasing polymerization rates by 3-fold. The generally applicable linear free energy relationships are important to identify the most effective monomer-metal catalyst partnerships, to select optimum polymerization processes, and to streamline future catalyst design targeting next-generation sustainable polymers.

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.

Designing polymers that combine performance with sustainability remains a critical challenge. Here, we report high-performance elastomers derived from CO₂ and biobased monomers that integrate both mechanical toughness and closed-loop chemical recyclability through a single material feature: dynamic metal–ionomer cross-links. These ABA block polymers, synthesized from ε-decalactone, δ-jasmolactone, CO₂, and bicyclic epoxides, incorporate abundant and inexpensive metal carboxylates (Na(I), Zn(II), and Al(III)) into the midblock, forming reversible networks that enhance tensile strength by 150% while maintaining high strain at break (>1500%) and elastic recovery (>85%). The same cross-links act as built-in catalysts, enabling energy-efficient depolymerization of both polyester and polycarbonate domains at 200 °C, recovering the original monomers. This dual-function approach advances circular polymer design by combining enhanced performance with efficient, low-energy, closed-loop recycling.

Poly( L ‐lactide) (PLLA) is the largest volume commercial bio‐derived plastic, but its brittleness and end‐of‐life recycling remain challenges. Here, glycerol ethoxylate (GEO), a branched ethylene glycol derivative, both toughens commercial PLLA and accelerates its catalyzed chemical recycling to L ‐lactide. A series of GEO–PLLA blends, containing 2–20 wt.% GEO, show significantly improved ductility and toughening compared to pure PLLA. The lead 10 wt.% GEO–PLLA sample achieves 9x higher elongation at break (191% ± 4%) and 6x higher tensile toughness (57.9 ± 1.9 MJ m ⁻³ ), while retaining desirable tensile strength (36.3 ± 1.5 MPa), thermal properties ( T _g = 39°C and T _m = 149°C) and crystallinity (25%). The GEO–PLLA samples are efficiently chemically recycled to L ‐lactide, showing both high recycling activity (TOF = 2240 ± 73 h ⁻¹ ) and quantitative selectivity for L ‐lactide (> 99%). The recycling is performed neat, at 180°C, using low loadings of commercial Sn(II)Oct ₂ catalyst. The 10 wt.% GEO–PLLA sample shows significantly faster chemical recycling than PLLA, with k _obs = 22.9 ± 0.8 h ⁻¹ versus k _obs = 1.8 ± 0.2 h ⁻¹ for PLLA. This recycling process is successful even with contamination from other commercial plastics, demonstrating its applicability to future postconsumer waste streams.

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.

Synthetic water-soluble polymers are ubiquitous in solution-based applications, but their petroleum origin and nondegradable bonds create environmental concerns. Here, CO₂- and glycerol-derived polycarbonates incorporating hydrophilic diglycerol motifs are prepared as a general-purpose water-soluble degradable polymer platform. A high-performance heterodinuclear [Co(III)/K(I)] catalyst enables controlled ring-opening copolymerization (ROCOP) of CO₂ with an acetal-protected epoxide, delivering well-defined polycarbonates with low dispersity (D̵ < 1.2) and predictable molecular weights (≈2000-20,000 g mol^-1). The catalysis is tolerant to protic initiators (chain transfer agents, CTAs), enabling control over both chain length and end-group chemistry. Deprotection of the acetals is quantitative and affords water-soluble polycarbonates incorporating hydrophilic diglycerol motifs. Using natural hydrophobic initiators yields amphiphilic polymers that self-assemble in water to form nanostructures of ≈7-11 nm with a critical micelle concentration of ≈30 mg L^-1. These polymers are stable at either neutral or acidic pH but depolymerize in alkaline solution to form nontoxic small molecules. Degradation proceeds by hydroxyl chain-end-initiated backbiting, i.e. by self-immolation, with pH- and end-cap-dependent kinetics, with complete degradation occurring over minutes to one month. Overall, this renewable polycarbonate chemistry, which is ∼23 wt % CO₂-derived; ∼77 wt % glycerol-derived, combines precise polymerization catalysis, spontaneous aqueous self-assembly and controllable aqueous degradability which are important for next-generation surfactants.

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.