Improved route to PAHA polyesters

Poly(α-hydroxy acid) (PAHA) polyesters form products such as cell-penetrating nanoparticles for drug and gene delivery and biodegradable sutures and implants, but they’ve been hard to synthesize. Rong Tong and Quanyou Feng at Virginia Tech now report a polymerization that could make them more accessible (J. Am. Chem. Soc. 2017, DOI: 10.1021/jacs.7b01462). PAHAs can be customized by varying the side chains of the lactide and glycolide monomers used to make them. But synthesizing PAHAs from lactides and glycolides requires multistep, low-yield reactions, and adding side chains is difficult. In 2006, Didier Bourissou of Paul Sabatier University and coworkers showed that PAHAs could also be made from O-carboxyanhydride (OCA) monomers, which are much easier to prepare and modify. But OCA polymerizations were slow and had undesired side reactions, uncontrolled stereochemistry, and broad and unpredictable molecular weight distributions. In Tong and Feng’s new approach to OCA polymerization, a photoredox nickel-iridium catalyst first decarboxylates OCAs. Zinc alkoxide then catalyzes the ring-opening polymerization of decarboxylated OCAs to PAHAs. The process is fast, minimizes undesired side reactions, and produces PAHAs with controlled stereochemistry and narrow and predictable molecular weight distributions.