Quantum chemistry studies on the cationic polymerization of p-methylstyrene in ionic liquid [BMIM][NTf2]

Abstract

Context

Ionic liquids (ILs) have garnered significant attention as eco-friendly media for living/controlled cationic polymerization due to their tunable solvation properties and enhanced reaction control. However, the fundamental principles governing cationic polymerization and the specific role of ionic liquids in these reactions remain poorly understood, leading to the continued reliance on a trial-and-error approach for ILs selection. To address this fundamental challenge, we conducted computational investigations of the CumOH/BF₃OEt₂-initiated living cationic polymerization of p-methylstyrene within the prototypical hydrophobic ionic liquid 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([BMIM][NTf₂]). Additionally, a comparative analysis of solvent effects was conducted, contrasting the performance of [BMIM][NTf₂] with the conventional organic solvent dichloromethane (CH₂Cl₂). Quantum chemical calculations revealed that both the BMIM⁺ cation and CH₂Cl₂ significantly lower the activation barrier of the initiation step. More importantly, the NTf₂⁻ anion was found to play a dual catalytic role by stabilizing key cationic intermediates and modulating the transition state geometry during chain initiation and dimerization. These mechanistic insights quantitatively account for the the observed differences in reaction rates and yields of p-MeSt polymerization in [BMIM][NTf₂] compared to CH₂Cl₂.

Methods

Employing density functional theory (DFT) at the B3LYP/6–311 +  + G(d,p) level of theory using Gaussian-03, we conducted a comprehensive mechanistic investigation of three pivotal elementary steps governing the early-stage polymerization: (i) initiator activation, (ii) monomer chain initiation, and (iii) dimer formation.