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(η 5 -C 5 H 4 SiMe 2 OPh 7 Si 7 O 11 - κ 2 O 2 )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 ( k app = 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 ( M n = 1.4–13.9 KDa, Đ = 1.11–1.33) and a significant preference for heterotactic enrichment ( P r 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 ( M n = 10.7–16.2 KDa, Đ = 1.21–1.72) with a tapered gradient microstructure and tunable thermal properties based on the feed composition.