Copolymerization of l-lactide and ε-caprolactone catalyzed by mono-and dinuclear salen aluminum complexes bearing bulky 6,6′-dimethylbipheyl-bridge: random and tapered copolymer

Abstract

The mono- and dinuclear salen aluminum complexes L^1–3AlMe (1, 2, 5), L²AlOⁱPr (3), L²AlOBn (4) and L^1–3Al₂Me₄ (6–8) (L = 6,6′-[(6,6′-dimethyl-[1,1′-biphenyl]-2,2′-diyl)bis(nitrylomethilidyne)]-bis(2-R¹-4-R²-phenol): L¹, R¹ = R² = Me; L², R¹ = ^tBu, R² = Me; L³, R¹ = R² = cumyl) with a racemic 6,6′-dimethylbiphenyl bridge were tested as initiators for the copolymerization of L-LA and ε-CL. In the presence of alcohol, the mononuclear aluminum complexes catalyzed the controlled copolymerization in the melt at 110 °C, producing practically random copolymers with narrow polydispersity indices (Đ = 1.03–1.07). Notably, the component of the copolymer produced by complex 3 strictly correlated with the feed ratio of the monomers at different stages of the copolymerization. In a 50/50 feed ratio of L-LA/ε-CL, the reactivity ratios of the two monomers are r_LA = 1.17, r_CL = 0.80. The average lengths of the lactidyl and caproyl sequences in the resultant copolymer are L_LA = 1.91, L_CL = 1.93. Even at 180 °C, the copolymerization initiated by complex 3 was still well-controlled, producing random copolymers with narrow dispersity indices (Đ = 1.07–1.09). Compared with the mononuclear counterparts, the dinuclear aluminum methyl complexes enable a few fold boost in activity because of the lower steric hindrance around the aluminum centers, but gave tapered copolymers with broadened Đ values (Đ = 1.12–1.22).