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 (Delta H-P degrees = 2.0 and 0.5 kJ mol(-1), respectively) and endoentropic (Delta S-P degrees = 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 degrees C. poly(LA-co-3HP) exhibits a glass transition temperature up to 13 degrees C and, depending on the regioregularity, exhibits a melting temperature up to 96 degrees C.