Exploring the effect of Zr/B ratio on the stability and reactivity of activated ε-caprolactone complexes: A DFT, QTAIM and NCI study

Abstract

Monomer insertion, leading to the formation of an activated monomer complex, is a critical step in cationic ring-opening polymerization (CROP) of cyclic monomers, such as ε-caprolactone (CL). In this study, Density Functional Theory (DFT) calculations were employed to investigate the structural and electronic properties of four activated complexes at two Zr:B ratios (1:2 and 1:1), where Zr is the cationic zirconocene catalyst, Cp₂ZrMe⁺, and B is the borate cocatalyst, [MeB(C₆F₅)₃]^‒ or [B(C₆F₅)₄]^‒. Steric hindrance at the reactive site was analyzed using topographic steric maps, while inter- and intramolecular interactions of the complex systems were examined through the Quantum Theory of Atoms in Molecules (QTAIM) and non-covalent interaction (NCI) analyses. The 1:2 ratio exhibited significant steric hindrance above and below the monomer plane, restricting access to the Cp₂ZrMe⁺ catalytic site and potentially limiting monomer insertion. In contrast, the 1:1 ratio displayed reduced steric congestion and stronger localized attractive forces at the catalytic site, facilitating better interactions with monomers and solvents. Conceptual DFT descriptors revealed that 1:1 systems had smaller HOMO-LUMO energy gaps, lower hardness, and higher electrophilicity, with 1:1@[B(C₆F₅)₄]⁻ identified as the most reactive complex. QTAIM identified key hydrogen bonding interactions, and the Zr-O_CL bonds, distinguishing stability and reactivity across Zr:B ratios. These findings provide valuable insights into the steric and electronic effects on monomer-activated species, enabling the optimization of Zr:B ratios and cocatalyst conditions for improved polymerization efficiency.