Titanium Catalyst Turns Racemic Lactide into Heterotactic Polylactide and Tapered Lactide-Caprolactone Copolymers

Chemists have shown that a single titanium complex bearing a cyclopentadienyl-silsesquioxane ligand can polymerize racemic lactide with an unusual preference for heterotactic linkages and can also copolymerize L-lactide with ε-caprolactone into tapered-gradient copolymers with tunable thermal properties. The study, published as an open-access research article in Discover Polymers on 15 October 2026, extends the catalytic repertoire of a titanium(IV) complex previously tested only on the pure L enantiomer of lactide, and combines kinetic measurements, end-group analysis and density functional theory calculations to explain why the catalyst favors one stereochemical pathway over the other.

Polylactide, or PLA, has attracted sustained attention as a biodegradable alternative to petroleum-based plastics, and it is synthesized through the ring-opening polymerization of lactide catalyzed by organometallic initiators. Compared with other polymerization methods, ring-opening polymerization offers greater control over molecular weight and stereochemistry and allows the preparation of polymers with superior mechanical and thermal properties. The stereochemical composition of PLA strongly influences its crystallinity, thermal behavior and mechanical strength, so the ability of a catalyst to control how D- and L-lactyl units are strung together is central to tailoring the final material.

The stereochemical stakes are high. Polymerization of L-lactide typically yields isotactic polymers with a uniform S-configuration at the chiral centers, producing semicrystalline PLA with enhanced thermal and mechanical properties. Random incorporation of D- and L-units from racemic lactide gives atactic PLA, which is largely amorphous, soft and flexible. Controlled insertions of the two enantiomers, by contrast, give isotactic or heterotactic PLA, which are more crystalline, with higher thermal stability and mechanical strength. Achieving heterotactic PLA formation with titanium(IV) catalysts remains a significant challenge, which is precisely the outcome the new complex delivers.

The catalyst, denoted complex D, carries a silsesquioxane framework covalently linked to the cyclopentadienyl ligand, creating a constrained coordination environment around the titanium center that can influence both activity and stereoselectivity. In the presence of benzyl alcohol as a co-initiator, the complex showed good activity in the ring-opening polymerization of rac-lactide, with an apparent propagation constant of 0.320 ± 0.005 h−1 under conditions of 1.0 M monomer, a monomer-to-titanium ratio of 150, 100 °C in toluene. Benzyl alcohol was essential: without it, the complex showed low activity, likely because the chloride ligand is a poor initiator. With three equivalents of benzyl alcohol, high monomer conversions were achieved within 8 hours, and the authors report that the complex achieved 96% conversion within 1 hour at 100 °C at a titanium concentration of 0.83 mM and a monomer-to-titanium ratio of 100, placing it among the most active titanium-based catalysts reported to date for this reaction.

The kinetic comparison with the pure enantiomer is the most striking result. Under identical conditions, the apparent propagation constant for L-lactide was 0.182 ± 0.001 h−1, giving a ratio of 1.75 and confirming that rac-lactide polymerizes at nearly double the rate. The reaction followed first-order kinetics with respect to monomer, starting instantaneously. The resulting polylactides displayed monomodal molecular weight distributions, with number-average molecular weights closely matching theoretical values calculated assuming one growing chain per equivalent of benzyl alcohol, spanning 1.4 to 13.9 kDa with dispersities of 1.11 to 1.33. These dispersities suggest the occurrence of intra- and intermolecular chain-transfer side reactions, yet the authors conclude that the polymerization proceeds in a relatively well-controlled manner.

Microstructure analysis by homonuclear decoupled proton NMR revealed a significant preference for racemic enchainment, with probabilities of racemic enchainment (Pr) reaching up to 0.86, indicating heterotactic enrichment. Increasing the alcohol concentration led to a slight decrease in heterotacticity, from 0.81 to 0.79, and the higher reaction rate observed for rac-lactide correlates well with the preference for racemic linkages seen by NMR. End-group analysis of a low-molecular-weight sample showed a benzyloxy initiating group and a lactyl end group, consistent with a coordination–insertion mechanism in which the benzyloxy group attacks the carbonyl carbon of the monomer, with termination proposed to occur via hydrolytic cleavage of the titanium–oxygen bond. A mass increment of 72 units in MALDI-TOF mass spectra pointed to transesterification side reactions during chain propagation. Under reduced catalyst loading at monomer-to-titanium ratios of 500 and 1000, conversions remained high but molar masses fell below theoretical values and dispersities broadened slightly. In bulk at 175 °C with technical-grade monomers, the complex converted up to 1500–3000 equivalents of rac-lactide within one hour, though with diminished control.

Density functional theory calculations explained the stereochemical outcome. The authors modeled insertion of either L- or D-lactide into the titanium–oxygen bond of a pre-opened L-lactide unit, where insertion of L-lactide leads to an isotactic diad and D-lactide to a heterotactic diad. Direct coordination of the monomer to titanium could not be located; instead, weakly bound adducts were identified at 1.8 and 4.3 kcal/mol. The first step, nucleophilic attack on the incoming lactide carbonyl, proceeds through transition states at 25.9 and 24.9 kcal/mol for the isotactic and heterotactic routes, a difference of only 1.0 kcal/mol that makes this step essentially non-stereoselective. The decisive step is ring opening: the transition state for the heterotactic pathway sits at 23.1 kcal/mol, well below the 29.3 kcal/mol of the isotactic counterpart, so the heterotactic route is kinetically favored at the rate-determining step.

The structural origin of that preference is subtle.

Subject of Research: Ring-opening polymerization of rac-lactide and copolymerization with ε-caprolactone using a cyclopentadienyl-silsesquioxane titanium catalyst

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

Article References: Anwar, S., Vinueza-Vaca, J., Impemba, S., Jiménez, G., Tabernero, V., Marín-Díaz, A., & Milione, S. (2026). Polymerization of rac-lactide and copolymerization with ε-caprolactone catalyzed by a cyclopentadienyl-silsesquioxane titanium complex. Discover Polymers, 3(1), Article 31. https://doi.org/10.1007/s44347-026-00070-8

Image Credits: AI Generated

DOI: 10.1007/s44347-026-00070-8

Keywords: ring-opening polymerization, rac-lactide, polylactide, titanium catalyst, silsesquioxane, heterotactic PLA, ε-caprolactone, copolymerization, DFT calculations, biodegradable polyester, stereoselectivity, tapered gradient copolymer