Matching Monomers to Metals: Quantified Linear Free Energy Relationships in Metal-Catalyzed Ring-Opening Polymerizations

Abstract

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.