Improved route to PAHA polyesters

Poly(α-hydroxy acid), or PAHA, polyesters can form products such as biodegradable sutures and implants, as well as cell-penetrating nanoparticles for drug and gene delivery. But synthesizing PAHA polyesters hasn’t been easy. Now researchers report a new way to prepare PAHA polyesters with desirable sets of properties that have been difficult or impossible to access before.

The stiffness, stretchability, tensile strength, and other physical properties of these polymers can be customized by varying the sidechains of the lactide and glycolide monomers used to make the materials. The repertoire of PAHA polyesters has been limited because synthesizing these monomers requires multistep and low-yield reactions, and adding sidechains to them is difficult.

In 2006, Didier Bourissou of Paul Sabatier University and coworkers showed that PAHAs could instead be made from O-carboxyanhydride (OCA) monomers, which are much easier to prepare and modify. Nevertheless, the organocatalytic reactions used to polymerize OCAs are slow and have undesired side reactions. Also some of the polymer products have uncontrolled stereochemistry and broad and unpredictable molecular weight distributions.

Rong Tong and Quanyou Feng at Virginia Tech have now addressed these issues with a new method for rapid and controlled OCA polymerization (J. Am. Chem. Soc. 2017, DOI: 10.1021/jacs.7b01462). In the two-step approach, a photoredox nickel-iridium catalyst first decarboxylates OCAs. Then zinc alkoxide catalyzes a ring-opening polymerization of the decarboxylated OCAs to yield the PAHAs. The process is fast, minimizes undesired side reactions, and produces PAHAs with controlled stereochemistry and narrow and predictable molecular weight distributions.

“Our chemistry will allow us to prepare polyester materials with customized macroscopic properties such as rigidity, elasticity, and biodegradability,” Tong says. Virginia Tech has applied for a patent on the technique.

Bourissou agrees that the technique should make “a variety of PAHA polymers with different functionalities and microstructures readily accessible.” He points out that “one practical limitation is that the ring-opening polymerization has to be performed at low temperature, –15 °C or below, to achieve good control and avoid side-reactions.” Fortunately, he says, OCAs are highly reactive and polymerize rapidly even at low temperature.