Aluminum salen and salan complexes in the ring-opening polymerization of cyclic esters: Controlled immortal and copolymerization of rac-β-butyrolactone and rac-lactide

Aluminum-based salen and salan complexes mediate the ring-opening polymerization (ROP) of rac-β-butyrolactone (β-BL), rac-lactide, and ε-caprolactone. Al-salen and Al-salan complexes exhibit excellent control over the ROP of rac-β-butyrolactone, yielding atactic poly(3-hydroxybutyrate) (PHB) with narrow PDIs of <1.15 for Al-salen and <1.05 for Al-salan. Kinetic studies reveal pseudo-first-order polymerization kinetics and a linear relationship between molecular weight and percent conversion. These complexes also mediate the immortal ROP of rac-β-BL and rac-lactide, through the addition of excess benzyl alcohol of up to 50 mol eq., with excellent control observed. A novel methyl/adamantyl-substituted Al-salen system further improves control over the ROP of rac-lactide and rac-β-BL, yielding atactic PHB and highly isotactic poly(lactic acid) (P_m = 0.88). Control over the copolymerization of rac-lactide and rac-β-BL was also achieved, yielding poly(lactic acid)-co-poly(3-hydroxybutyrate) with narrow PDIs of <1.10. ¹H NMR spectra of the copolymers indicate a strong bias for the insertion of rac-lactide over rac-β-BL.