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

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.