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.

本文报道一项机理研究，探究了2-R-恶唑啉（R为甲基、苯基）与ε-己内酯（CL）在茂金属介导的阳离子开环均聚与共聚（cationic ring-opening homo- and copolymerization, CROP）中的反应活性差异。我们采用M06-2X级别密度泛函理论，考察了单体顺序加成路径，重点关注C-O键插入机理。吉布斯活化能结果揭示了明确的反应活性趋势：CL-co-MeOX > PhOX-co-MeOX ≈ CL-co-PhOX > (PhOX)₂ ≈ (MeOX)₂ > MeOX-co-PhOX ≈ PhOX-co-CL > MeOX-co-CL，该趋势与实验观测结果一致。这一序列偏好源于单体电子结构与催化剂-助催化剂相互作用的协同效应。苯基恶唑啉（PhOX）的引发过程通过增强的锆-环戊二烯基（Zr-Cp）键合与阳离子-π相互作用，实现了快速开环；而连续的苯基恶唑啉插入步骤则受到π-π堆积效应的阻碍。初始己内酯（CL）插入步骤展现出最优反应活性，此时锆-硼（Zr-B）距离较短且催化剂-助催化剂接触面积较大；而反向的单体添加序列则面临极高的能垒，难以进行。乙腈溶剂中的三苯碳鎓四(五氟苯基)硼酸盐（[Ph₃C]⁺[B(C₆F₅)₄]⁻）助催化剂体系，可对反应中间体提供最优的稳定化作用。构效关系分析表明，吉布斯活化能与一系列电子及结构参数（包括Zr-B距离、原子电荷及电子密度分布）之间存在较强的相关性（决定系数R²>0.6）。本研究所得结论为优化茂金属介导的CROP工艺提供了具体指导，并为设计结构可控的精准嵌段共聚物奠定了理论基础。