Highly Regioselective Ring-Opening Polymerization of an Asymmetric Diester

Controlling the monomer sequence in hydroxy acid copolymers remains a significant challenge, yet it is essential for fine-tuning the properties of copolyesters. This study presents the regioselective ring-opening polymerization (ROP) of l-3-tert-butyl-6-methyl-1,4-dioxane-2,5-dione (l-tBMG), resulting in the synthesis of alternating polylactic acid-co-2-tert-butylglycolic acid copolymers. The process is facilitated by enantiomerically pure Schiff base aluminum catalysts featuring either a binaphthyl framework or a 2,2-dimethylpropylenediamine and 3,5-dichlorosalicylaldehyde framework. These catalysts promote regioselective ring-opening of the asymmetric cyclic diester specifically at the acyl-oxygen bond of the lactic acid moiety. The molecular chain structure of the resulting polymers was elucidated through heteronuclear multiple bond correlation spectroscopy, while chain regularity was assessed using ¹H NMR and quantitative ¹³C NMR (q-¹³C NMR). According to the ¹H NMR and q-¹³C NMR test results, the molecular chain regularity is about 0.93. Incorporation of 2-tert-butylglycolic acid units led to a glass transition temperature (T_g) of the random copolymer to 65 °C. Notably, the T_g of the isotactic polymer, synthesized via ROP catalyzed by the aforementioned catalysts, was further elevated to 70 °C.