Cyclopentadienyl–Silsesquioxane Titanium Complexes in the Polymerizations of Styrene and L-Lactide

In this contribution, two silsesquioxane-cyclopentadienyl titanium complexes featuring one or two chloride ancillary ligands, [Ti(η⁵-C₅H₄SiMeO₂Ph₇Si₇O₁₀-<i>κ</i>O)Cl₂] (<b>1</b>) and [Ti(η⁵-C₅H₄SiMe₂OPh₇Si₇O₁₁-<i>κ</i>²O₂)Cl] (<b>2</b>), were synthesized and evaluated in the Ziegler-Natta polymerization of styrene and the ring-opening polymerization (ROP) of L-lactide, respectively. Complex <b>1</b>, activated with methylaluminoxane (MAO), catalyzed the syndiotactic polymerization of styrene with turnover frequencies up to 28 h^-1, affording polymers with narrow dispersity, low number-average molecular weights (<i>M</i>_n = 5.2-8.2 kDa), and high stereoregularity, as confirmed by ¹³C NMR. Complex <b>2</b>, in combination with benzyl alcohol, promoted the ring-opening polymerization of L-lactide in solution at 100 °C, achieving conversions up to 95% with good molecular weight control (<i>M</i>_n close to theoretical, <i>Đ</i> = 1.19-1.32). Under melt conditions at 175 °C, it converted up to 3000 equiv. of monomer within 1 h. Kinetic analysis revealed first-order dependence on monomer concentration. The results highlight the ability of these complexes to produce syndiotactic polystyrene with narrow molecular weight distributions and to catalyze controlled ROP of L-lactide under both solution and melt conditions. Computational studies provided insight into key structural and energetic features influencing reactivity, offering a framework for further catalyst optimization. This work broadens the application scope of silsesquioxane-cyclopentadienyl titanium complexes and supports their potential as sustainable and versatile catalysts for both commodity and biodegradable polymer synthesis.