Ring-opening polymerization is used to prepare polyesters with precisely controlled molecular weights, molecular weight distributions, and tacticities. Herein, we report a newly developed Co/Zn catalytic system that can be activated by an electrical current to mediate efficient ring-opening polymerization of enantiopure O-carboxyanhydrides, allowing for the synthesis of isotactic functionalized polyesters with high molecular weights (&gt;140 kDa) and narrow molecular weight distributions (Mw/Mn &lt; 1.1). We began our studies of electrochemical ring-opening polymerization (eROP) of l-1 under galvanostatic (constant current) conditions in an undivided cell and thoroughly evaluated the electrode materials, electrolyte, current, and temperature, aiming to avoid a cumbersome electrochemical reaction setup from an industrial scale-up standpoint (Figure 1a). We found that Mg was optimal for the anode (working electrode) and that Pt was best for the cathode (counter electrode). THF proved to be the ideal solvent. The use of tetrabutylammonium hexafluorophosphate (TBAH, 0.1 M in THF) as the electrolyte led to better control of the polymerization than other tetraalkylammonium salts. Better polymerization results (i.e. Mn value close to the calculated MW) were obtained at 0 °C than at room temperature. Under these conditions, the Mn of the poly(l-1) product increased linearly with initial [l-1]/[Co-1]/[Zn-1] ratio up to 1000/1/1 (Figure 1b) We also demonstrate that the Co/Zn catalysts can be used for stereoselective ring-opening polymerization of racemic O-carboxyanhydrides to synthesize syndiotactic or stereoblock copolymers with different glass-transition temperatures (Figure 1c). The difference in microstructure between the two obtained polymers was confirmed by differential scanning calorimetry: poly(sb-1) had a glass transition temperature (Tg) of 46 °C, whereas poly(sd-1) exhibited a Tg of 58 °C . Note that the atactic poly(rac-1) had a Tg of 51 °C.

Stereoselective electrochemical ring opening polymerization to synthesize functionalized polyesters

ACS Fall 2021

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