Origin of Comonomer Reactivity in Zirconocene-Mediated Polymerization of 2-Oxazoline and ε-Caprolactone: Insights from DFT

Abstract

A mechanistic study investigating reactivity differences in metallocene-mediated cationic ring-opening homo- and copolymerization (CROP) of 2-R-oxazolines (R = Me, Ph) and ε-caprolactone (CL) is reported. Using density functional theory at the M06-2X level, we examined sequential monomer addition pathways, focusing on the C–O bond insertion mechanisms. Gibbs activation energies reveal a clear reactivity trend: CLcoMeOX < PhOXcoMeOX < CLcoPhOX < (PhOX)₂ ≈ (MeOX)₂ < MeOXcoPhOX ≈ PhOXcoCL < MeOXcoCL, aligning with experimental observations. This sequence preference stems from the interplay between the monomer electronic structure and catalyst–cocatalyst interactions. PhOX initiation promotes a rapid ring opening through enhanced Zr-Cp bonding and cation−π interactions, while consecutive PhOX insertions are hindered by π–π stacking effects. Initial CL insertion shows optimal reactivity with short Zr–B distances and extensive catalyst–cocatalyst contact areas, whereas reverse sequences face prohibitively high barriers. The [Ph₃C]⁺[B(C₆F₅)₄]⁻ cocatalyst system in acetonitrile provides optimal stabilization of reaction intermediates. Structure–property analysis reveals strong correlations (R² > 0.6) between Gibbs activation energies and electronic and structural parameters, including Zr–B distances, atomic charges, and electron density distributions. These insights provide specific guidelines for optimizing metallocene-mediated CROP and offer a theoretical foundation for designing well-defined block copolymers with controlled architectures.