Poly(α-hydroxy acids) are important biodegradable polymers with wide applications. Attempts to synthesize them from O-carboxyanhydrides (OCAs) with pendent functional groups have been plagued by uncontrolled polymerization. We have recently developed an effective protocol that combines photoredox Ni/Ir catalysis with the use of a Zn-alkoxide for efficient ring-opening polymerization (ROP), allowing for the synthesis of isotactic polyesters with expected molecular weights (&gt;140 kDa) and narrow molecular weight distributions (Mw/Mn &lt; 1.1).1 Herein, expanding from our initial discovery, we present a general approach for the synthesis of new stereoblock polyesters using photoredox ROP of OCAs (Fig. 1). We investigated zinc-based complexes to high activity of zinc alkoxide for photoredox ROP polymerizations of OCAs. We previously found that sterically bulky β-diiminate Zn complexes did not initiate controlled polymerization of L-1 at a DP of 300, whereas a less bulky tridentate Schiff base ligand (NNO-1) did (Fig. 1a). We discovered that within 4 h at -15 oC under irradiation, the photoredox ROP mediated by (NNO-1)ZnEt ([L-1]/[D-1]= 150/150) resulted in a product with a Mn of 45.7 kDa, which is close to the expected MW (44.5 kDa), and a narrow polydispersity of 1.05; whereas the similar reaction mediated by (NNO-2)ZnEt had less controlled polymerization. Analysis of the microstructure of this polymer using homonuclear decoupled 1H NMR in the methine region (δ ~ 5.2 ppm) revealed a high degree of isotacticity as evidenced by the large mmm tetrad peak (Fig. 1b). The probability of meso dyads (i.e., isotactic enchainment , Pm) was up to 0.97 calculated from homonuclear decoupled 1H NMR .

Photoredox ring-opening polymerization of O-carboxyanhydrides to synthesize stereoblock polyesters

255th National Meeting (2018)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.