Bipyridine-phenolate based aluminum complexes mediated ring-opening polymerization of ε-caprolactone and lactides with a high stereoselectivity

Aluminum complexes coordinated by bipyridine-phenolate (BpyPh) ligands of 2-([2,2′-bipyridin]-6-yl)-4,6-di-tert-butylphenol (BpyPh2,4-tBu-H), 2-([2,2′-bipyridin]-6-yl)-4-(tert-butyl)phenol (BpyPh4-tBu-H), and 2-([2,2′-bipyridin]-6-yl)-4,6-dimethylphenol (BpyPh2,4-Me-H) have been synthesized and applied to mediate the ring-opening polymerization (ROP) of ε-caprolactone (ε-CL) and l-lactide (L-LA) in the presence of benzyl alcohol. The living characters of predictable molecular weight, narrow molecular weight distribution, and the formation of block copolymer were observed in the polymerization mediated by (BpyPh2,4-tBu)AlMe2 and all three complexes associated with benzyl alcohol could initiate the ROP of ε-CL and L-LA. The iso-selectivity of rac-lactide polymerization could be enhanced from Pm = 0.38 to Pm = 0.75 by increasing the steric hindrance of ortho-substituent on phenol group of the ligand.