Entropically Driven Macrolide Polymerizations for the Synthesis of Aliphatic Polyester Copolymers Using Titanium Isopropoxide

Thermal and mechanical properties of sustainable aliphatic polyesters can be tuned through the synthesis of copolymers. The synthesis of two 14-membered macrolides are reported: a cyclic tetraester with alternating lactic acid (LA) and 3-hydroxypropionic acid (3HP) units and a cyclic diester with alternating glycolic acid (GA) and 2-methyl-1,3-propanediol (2MD) units. Ring-opening transesterification polymerization (ROTEP) of these macrolides to yield poly(LA-stat-3HP) and poly(GA-alt-2MD), respectively, were found to be modestly endothermic (ΔHp ^° = 2.0 and 0.5 kJ mol ^-1 , respectively) and endoentropic (ΔSp ^° = 27 and 23 J mol ^-1 K ^-1 , respectively). Inexpensive and nontoxic titanium isopropoxide Ti(Oi-Pr) ₄ functions as an active catalyst for these entropically driven ROTEPs, achieving high conversions (>90%) in under 1 h. The polymerizations exhibit control over molar mass with dispersity values <1.7. poly(GA-alt-2MD) is an amorphous polymer with a low glass transition temperature near -30 °C. poly(LA-co-3HP) exhibits a glass transition temperature up to 13 °C and, depending on the regioregularity, exhibits a melting temperature up to 96 °C.