A Stable Chromium(III) Catalyst for Ring-Opening Alternating Copolymerization of Cyclic Anhydrides/Epoxides under Air

The catalytic ring-opening copolymerization (ROCOP) of epoxides and anhydrides offers a promising method for synthesizing diverse polyesters with alternating repeating units and various properties. However, to address critical issues like the limited activity and selectivity of the required catalysts at low loadings, it is crucial to explore more effective catalysts for better control over the production of high-molecular-weight polyesters with narrow molecular weight distributions. Herein, we describe the synthesis and characterization of an air-stable Cr(III) chloride complex, [Cr(L)Cl₂]Cl, where L = 2,2'-([2,2'-bipyridine]-6,6'-diyl)bis(propan-2-ol). This complex has been effectively utilized as a catalyst for the copolymerization of propylene oxide (PO) with succinic anhydride (SA) or phthalic anhydride (PA) under mild conditions. When combined with 1.0 equiv of (PPN)Cl as a cocatalyst, the process yielded perfectly alternating polyesters (M_n up to 61 kDa, D̵ below 1.2) with highly selective ester linkages (ester linkages >99%) at high conversion of the cyclic anhydride. Additionally, the polymerization process maintained its effectiveness without requiring extensive drying and purification of commercial monomers or an inert atmosphere, resulting in good performance and preservation of end-group fidelity, even with low catalyst loadings. The study also discussed kinetic studies, copolymer microstructure, and the proposed catalytic mechanism involved in this process.