Polymerization of rac-lactide and copolymerization with ε-caprolactone catalyzed by a cyclopentadienyl-silsesquioxane titanium complex

The catalytic application of a cyclopentadienyl-silsesquioxane titanium complex Ti(5-C5H4SiMe2OPh7Si7O11-2O2)Cl] in the ring-opening polymerization (ROP) of rac-lactide (rac-LA) and in the copolymerization of L-lactide (L-LA) with ε-caprolactone (ε-CL) is reported. In the presence of benzyl alcohol (BnOH) as a co-initiator, this complex demonstrated good activity (kapp= 0.320 ± 0.005 h-1, [LA]0= 1.0 M; [LA]0/[Ti]0= 150, T= 100 °C, toluene, 2.0 mL) and satisfactory control, yielding polylactide with controlled molecular weights (Mn = 1.4-13.9 KDa, Đ = 1.11-1.33) and a significant preference for heterotactic enrichment (Pr up to 0.86) Kinetic studies revealed that the polymerization of rac-LA proceeds at nearly double the rate of the pure enantiomer L-LA, attributed to the kinetic preference for racemic linkages. Density Functional Theory (DFT) calculations confirmed that the heterotactic pathway is favored by a significantly lower activation barrier at the rate-determining ring-opening steps. Furthermore, the system was successfully employed in the copolymerization of L-LA and ε-CL, leading to the formation of copolymers (Mn = 10.7-16.2 KDa, Đ = 1.21-1.72) with a tapered gradient microstructure and tunable thermal properties based on the feed composition.